Update February 2020

This article was written back in 2017 before the Victron Orion Smart DC DC Chargers were available.  These chargers were specifically designed to take charge from one battery to another.  They are fully programmable so you can give a three stage charge or set a lithium program.  You can also set the conditions on which they turn on and off based on voltage, or a signal from the ignition switch, or from a manual switch, or any combination of the above.   That's the update, the rest of this article is from 2017.

A discussion on the best way to incorporate an alternator into a Lithium-Ion battery system.

There are a few things to consider when incorporating an alternator into a lithium-ion battery system.  The first is:  "Is the alternator up to the job?"  A Lithium Ion battery can put a lot of load on an alternator and have it working flat out for an extended time.  There might need to be some way to limit the demand to prevent the alternator from self destructing.  Other considerations include making sure the alternator will charge the lithium battery at the correct voltage and figuring out what happens when the lithium battery is full.

The relationship between battery capacity and alternator output

Lithium batteries have a maximum charge current and a recommended charge current.  For maximum battery life the alternator charge should be sized to not exceed the recommended charge current. Here is the table for Victron 12.8 volt Lithium Ion batteries:

You can see in the table above that the 300 Amp Hour battery can accept a charge at up to 750 Amps but the recommended continuous value is 150 Amps or less.

Charge Voltage

As you can see in the table above the recommended charge voltage is 14.0 - 14.4 Volts    Most alternators will not be able to get the battery to that voltage until the battery is nearly full. When it does get to the full voltage the BMS (Battery Management System) needs to shut off the charge to the battery.

What happens when the battery gets full?

When the battery gets full the BMS needs to shut off the charge.  One way to achieve that is to interrupt the connection between the alternator and the lithium battery.  In this case there needs to be another conventional battery, usually a start battery, for the alternator to charge. You cant disconnect an alternator's output while it is running because with nowhere for the power to go its internal voltage would soar and destroy the diodes.

Another way to achieve the same result is to have the BMS turn off the alternator regulator. This is normally only possible with an externally regulated alternator.  When its turned off the alternator isn't available to charge any other batteries, such as a starting battery.

Ways to limit the load on the alternator

If you have a monster alternator then it may not need limiting.  That would be the kind of heavy duty truck alternator that is designed to run at high output all day.  For any other alternator it would be wise to consider some kind of limiting to prevent it from self destructing when charging Lithium Batteries at high output for an extended time.

Alternators that have an external regulator with alternator temperature sensing offer the first and easiest way to limit the current.  When the alternator gets too hot the regulator dials it down a bit to keep it within the allowed temperature.  The latest version of the Balmar MC-614-H regulator offers a continuously adjustable output to keep the alternator within temperature.  

Other alternators may need some additional external limiting device.  The Victron 12/1200 BMS has a built in feature that limits the alternator to a pre-determined output up to 100 Amps.  It works by putting a fuse of the desired limit in the BMS.  If you want to limit the alternator to 80 Amps you put in an 80 Amp fuse.  As the fuse nears its rated capacity it begins to heat up and its resistance changes.  The BMS recognizes that and reduces the current accordingly.   Using this device requires you to have an alternate battery for the alternator to charge.  This 12/1200 BMS can be used as a current limiting device even if it isnt being used as the BMS that controls the batteries.

Another high tech device used to limit the alternator is the Victron Buck Boost DC DC Converter.  This is especially appropriate to Sprinter and EuroVans and other vehicles with high tech alternators.  The Buck Boost DC DC converter has a vibration sensor to turn it on automatically when the engine is running.  It takes the input voltage and changes it to whatever voltage is required by the Lithium Battery, and in the process limits it to the programmed value, up to a maximum of 50 Amps.  With this you can use a 24 volt alternator to charge a 12 volt battery or vice versa

Circuit Diagrams

In the layout above the alternator is charging a conventional start battery at all times.  The start battery is connected to the Lithium Ion battery via the Cyrix-Li-Ct which is a battery combiner controlled by the BMS.  When the Lithium Battery is full the combiner gets shut off but the alternator continues to charge the start battery.  There is no current limiting shown, the alternator had better be up to the job or have active temperature control.

Victron 12/1200 BMS
In this next diagram (above) the Victron 12/1200 BMS is used and this has a built-in way to limit the alternator to up to 100 Amps depending on what size fuse is placed in the left hand fuse holder.  Note that the alternator always has a conventional battery to charge even when its output to the lithium battery is interrupted.

The Victron 12/1200 BMS has its limitations though so you might prefer to use the VE Bus BMS.  Any system with a Multi-Plus or Quattro inverter charger is going to want to use the VE Bus BMS.  The 12/1200 is really for smaller systems and has a limit of 100 Amps on the output side.  In that case you can still use the VE Bus BMS as the BMS that is controlling the batteries but use the 12/1200 solely in its function as a current limiter.  The wiring diagram would look like the one below.  Again, as before, the alternator always has a backup battery to charge.

In the next drawing, below, the alternator is only charging the Lithium battery.  The alternator is externally regulated by a Balmar MC-614-H regulator.  The setup includes a temperature sensor on the alternator which limits its output by keeping the alternator within the allowed temperature.  It is also possible to limit the alternator to a certain percentage of its rating in the regulator software.  The ignition circuit of the regulator is controlled by the "allow to charge" signal from the BMS.  When the battery is full the "allow to charge" signal turns off and so does the regulator and therefore the alternator.  A small relay is required to turn on and off the regulator since the BMS "allow to charge" signal is not capable of supporting any real load. This needs to be a solid state relay because of its super low power consumption.  We have found that one of the Victron Battery Protects inserted in the power supply to the regulator can serve this purpose.  You might also like to take a look at my Blog post where I discuss the settings for the alternator regulator.

Alternator regulator
The last way shown below is the most hi tech (and the most expensive)  It was designed for use with the type of smart alternators and dynamos used on vehicles such as the Sprinter Van  and Euro Van where the output voltage can vary dramatically. To avoid interfering with the vehicles electronics, which might cause warranty issues, the buck boost converter can be programmed to use its built in vibration sensor to only operate when the engine is running. The output produced by the converter can be regulated to suit the requirements of the Lithium batteries it can prevent excessive load on the alternator by limiting how much charge is available for the Lithium battery.  This sophisticated device is a clever workaround to all kinds of difficult problems that can be encountered when making modifications to these vehicles. The wiring diagram looks like this:

Buck Boost DC DC Converter

A note about these diagrams

The diagrams on this page are block diagrams to illustrate a concept.  They were all taken from published Victron Energy drawings available on the company website www.victronenergy.com  They were edited to remove extraneous information not relevant to the immediate topic in hand. In general they don't show circuit protection, battery switches or other similar items.

I did have a quesrion about all these systems, what happens when the engine is stopped?  Does the lithium battery drain into the conventional battery?  Here is the official answer:  "The Cyrix-Li-ct will disconnect when the engine isn’t running and the starter voltage starts to drop much like the regular Cyrix. The BMS 12/200 blocks energy from flowing back to the starter battery altogether."

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Date 4/4/2018

Great post! I'm interested in putting lithium on my yacht. I am looking at a brand of lithium which has an internal BMS but no way of communicating information to the outside (as it looks like the Victron batteries do... no CAN bus or extra wires hanging out, just terminal post). In fact i notice this with a lot of LFP lithium batteries on the market. My question is specific to this post and designing a alternator charging system which can accommodate LFP. How does the victron VE BMS integrate with batteries without any communication from the internal BMS to the battery? I notice on the bottom of the victron BMS there are some input sources which come from victron lithium batteries, are these a requirement? Any thoughts would be very helpful.

Peter Kennedy

Date 4/4/2018

Since I wrote this post I have had some second thoughts about using the 12/1200 BMS in boats. It works by controlling on the negative side of the circuit, and if you get into the small details it requires you to isolate the grounds of everything else on the boat. The bigger the boat the harder that is to achieve and I have come to the conclusion that it isn't a very good option for anything except boats with outboards. To answer to comment above: Victron Batteries with their external BMS give you much more options to build a fully integrated system. When you try to replicate this using other batteries that have an internal BMS it is much harder to achieve. I spend a lot of time trying to help people out with this and some of the Victron components are very useful but they still don't give the same level of control as when they have access to the BMS. Below are a few ways you can use Victron products with Non-Victron batteries: The Victron BMV energy monitors can be programmed with alarms and an external relay circuit to either operate on voltage alone or on state of charge. You can use their output to control loads and charge sources. You would require some external relays in addition. Because they are just looking at the voltage of the bank as a whole and not at an individual cell level it doesn't give quite the same level of control as you get with an external BMS. That would require you to set the parameters to give you more cushion. The effect of that is you have to shut off the charge a bit before the battery is full and shut off the load a bit before it it empty, giving a bit less capacity than the rated while still protecting the battery. The Victron Battery Protects can be programmed to be relays controlled by a signal from the BMV Energy Monitor, so they can be the relays described above to shut off the load or the charge. They can also be programmed to act on their own and shut off when the voltage drops below a preset level.


Date 1/14/2019

Regarding the second diagram, and Ryans question. Will it be possible to limit the flow of current from the altenator with the Victron 12/200 BMS with Non-Victron batteries (no battery control wires connected), or is the Victron 12/200 exclusively for Victron batteries? I need it as a current limiter to solve altenator heating problem when charging LiFePo4. Thanks in advance.

Robert L.

Date 5/31/2018

Thank you so much for this post! Still I cannot find an answer to the following question: Like Ryan I am using LiPo-Packs with integrated a BMS, charge cut-off voltage 12.6V. Now, if I connect my 14.4V alternator, would the voltage then drop to the battery voltage until it's charged? (because when I measure the voltage of my car battery with the engine running I recall measuring always the nominal alternator voltage - and in this case the charge cut-off of the BMS would kick in, correct?) I would be really happy if you enlighten me in this question, cannot wrap my head around it...

Peter Kennedy

Date 1/16/2019

Yes, when you connect a Lithium Battery to your alternator the charge voltage will drop to just above the battery voltage and will only rise slowly until the battery is nearly full. The battery in this case is such a big load that it determines the outcome. You can liken the effect to connecting strings of lights to a portable generator. The more lights you add the dimmer they get. Eventually if you add too many it will give up!


Date 7/1/2018

Thank you!!! Exactly the information I was looking for!!!


Date 9/1/2018

I want to use a buck boost dcdc On a 2500 Ram Promaster and it has a 180amp alternator in it. Would I need to upgrade the alternator to a 220amp? and if i can use the 180amp would it hurt it at all.

Peter Kennedy

Date 1/16/2019

The Buck Boost DC DC Converter is self limited to 50 Amps although you can use more than one. Its unrealisting to expect a nominal 180 Amp alternator to actually put that much out on a continuous basis. Alternators are rated under the most favorable circumstances (when they are cold) but once they get hot the performance drops off considerably.

Thomas tamplain

Date 9/3/2018

I want to monitor the status of two banks. 1- 3-12v lithium 50ah troll motor batta-36v 2-2-12v lead cell house batts 12v system Can this be one with one gauge -system or do I need two gauges -systems Please explain if u can sir I know what I want but not sure how to achieve and keep simple, boat system

Gordon Naylor

Date 9/5/2018

Thank you Peter for this excellent post. In a system using the Buck Boost controller (Sprinter van), would you also need an automatic charge relay (such as the Blue Sea systems ACR) or does the Buck Boost controller manage that function? Thank you

Peter Kennedy

Date 1/16/2019

The Buck Boost DC DC Converter acts like a charge relay in this case. You can program how it behaves, for instance you can use its built-in vibration sensor so it only turns on when it senses that the engine is running. This feature was added so you could install it with a minimum of interference to the vehicles wiring (and thus avoid any warranty issues).

Paul 10/4/18

Date 10/4/2018

To; Mr Peter Kennedy Thank you so much for all the precious informations. I am building a camper van now. I am thinking about using Battle Born Lithium Battery 100-200 Ah and BB1260 Sterling BB1260-12 Volt, 60 Amps Battery Charger. (or 30 Amps) 1. When Lithium battery gets full, does the built-in BMS will shut off further charging from alternator? They said using the above Sterling charger, it would be fine, but I want to be sure. 2. When you said "... need another start battery to take charging from the alternator", did you mean ANOTHER Battery (3rd one after van starting battery and Lithium battery)? Or the van start battery will be just getting charged after Lithium battery is full? 3. I plan to set up solar panel 320 W; but am curious if I can just start alternator alone first, and later adding solar? Or just using alternator alone is bad idea? Thanks you. Paul

Dr Andy Godbehere

Date 10/20/2018

Hi. Tere is lots of info but my application is simple - to install lithium into a coach being converted to a motorhome. Tis has an indépendant alternator to the bus systems, which is used to provide power to heating systems, TVs etc. It is a 24V system and very hight alternator capacity and I'm looking to install about 500Ah of capacity. Which o fate above do you suggest. Lots of options but O'm not sure which to choose. Ta Andy


Date 1/16/2019

What a great article. Is there a way I can contact you for (paid) input on an install?

Joseph Pica

Date 2/22/2019

Hello Peter, I was an early 2012/13 adopter of LF04's with an external custom BMS by Demitri when he was still building them. My system incorporated starting batteries as buffers for alternators in the event the bank was isolated form an LVC or HVC event. alternators were controlled via MC614s . ONe issue I hadn't realized until doing some testing was that when I isolated the bank and the alternators kept boosting voltage as the regulator could not sense any battery voltage so kept increasing voltage trying to charge the batteries blindly. The carefully dedicated voltage sense wires (of equal length) were run directly to the LFP04 bank (with twin Victron multiplus 3000's)and once bank was isolated could not provide charging(effect) feedback information. I am not an electrician but was told that accurate voltage sense is critical to accurate charging as just and few tenths voltage sense differential has a huge impact. What would you recommend as a work around to have best most accurate. voltage sensing for both the main bank and the buffer(starting batteries especially if an isolation event occurs underway. I no longer have that boat .but am upgrading batteries in a new outboard powered boat (to me Rosborough). I shied away from my first choice of lithiums because of lack of knowledge with how to control the alternator issues with the Etec Outboards. Thanks in advance for your wisdom and advice.

Peter Kennedy

Date 2/25/2019

That is an interesting comment. Where to connect the voltage sensor of the regulator requires some thought. In the top drawing it would go on the start battery because that is the primary battery that is being charged. If the only battery being charged is the Lithium battery then the BMS should turn off the regulator when it is full so there is nothing to sense as it is turned off. If you had some other arrangement you might want to put the voltage sense of the regulator on the load side of the battery system so that the battery protect would shut it off in the event of low battery. I believe the regulator can consume power through that voltage sense connection and you dont want that to bypass the control mechanisms. My understanding was if the voltage sense of the MC-614 was off or disconnected it wouldnt work at all. I dont have a test bench here with an actual alternator so I could use a bit of feedback from others who have tried this.

Gerald Miller

Date 3/20/2019

I have a sprinter rv van and will be updating to lithium (I already updated solar using most of the Victron parts and accessories from PKYs last fall). I may go up to 600 amp lithium but am concerned that the possible high current draw my fry the 280 amp Sprinter alternator. I am considering instead of the Victron Cryrix-li-ct to use a 50amp dc to dc battery charger to limit the draw to keep the alternator from being overtaxed. Is that a good alternative? Seems to me the only drawback would be a longer period to recharge. On the plus side, I won't need as heavy a gauge of wire. Advice?

Peter Kennedy

Date 3/21/2019

I am pretty sure that with 600 Amp hours of batteries you will need to limit the alternator to prevent it overheating Victron have the Buck Boost DC DC Converter for just that purpose. It acts as a battery to battery charger, you can program how many amps you want it to pass throug, you can program the delivery voltage, and you can program it to only work when the vehicle is running, and to shut off when the Lithium battery is full. The 25 and 50 Amp Buck Boost models have been out for a while, the 100 amp model should be available in Spring 2019

Steve Johnson

Date 4/1/2019

In the first scenario (start and house bank separated by Cyrix-Li-Ct) how do you program the voltage regulator on the alternator? It will be charging two different battery types (AGM and lithium) times. I could see using a lithium program due to the fact that most of the charging time will be on the house bank but will that program harm the start battery?

Peter K

Date 4/2/2019

The Lithium battery has so much less internal resistance than the AGM battery so most of the charge will go to the Lithium battery. You might as well program the system to suit the Lithium battery as it will dominate what happens. Just at the end of the charge cycle the voltage will rise and you want to make sure you dont allow it to rise above what is allowed for the AGM battery.

Chris C

Date 4/17/2019

The last diagram in this post shows the Buck-Boost connected to the VE Bus BMS with a purple trace. How is this connection made? The Buck-Boost documentation shows: 1) a USB connection, which as far as I can tell, is only used to program and monitor the Buck-Boost via the TSConfig Windows software. And 2) Pins 1 and 2. Pin 1 can be used to switch the unit on (in place of the vibration sensor), but the Victron documentation makes no mention of pin 2. Bigger picture, how does the Buck-Boost communicate with the rest of the Victon components? Can the Buck-Boost be turned on and off by the Victron BMS, and if so, how is this connection made, or is charging controlled exclusively by monitoring the house battery voltage? How is the vibration sensor connected to the Buck-Boost (this is not mentioned in the Victron documentation)? What is pin 2 on the Buck-Boost used for? Thanks!

Peter Kennedy

Date 4/18/2019

The purple wire in the drawing is the "allow to charge" wire from the BMS, it is a +voltage signal that it is OK to be on. This signal only allows it to be on, something else has to actually turn it on. The input terminals of the Buck Boost are programmable so you get to choose what you want them to do. In the scenario described above both the "allow to charge" and whatever "turn on" signal you have programmed have to be active for it to run.

Alec R.

Date 5/24/2019

I have a new 12V 100A Battle Born deep-cycle lithium iron battery that I want to charge using the 90A alternator (via the 12V lead-acid starting battery) in my VW Vanagon camper. Despite endless research, I am still baffled as to what I should use to isolate the two batteries. A Blue Sea ACR? A plain old solenoid? A battery-to-battery charger? I have heard arguments for and against each of them. I am not an auto mechanic or an electrician, so I'd greatly appreciate some plain-language advice on how to solve this (what I mistakenly thought would be simple) problem. Thank you!

Peter Kennedy

Date 5/24/2019

When you buy a Battle Borne or Relion battery they dont tell you how difficult it will be to hook it up to an alternator, this is a very difficult problem and there is no easy answer. That is why batteries such as the Victron Smart Lithium Ion batteries, with their external BMS, give you so much more flexibility in how you can connect them. This flexibility more than outweighs their slightly higher sticker price. ACR's such as those by Blue Sea dont work well or at all with Lithium batteries because the voltage of the battery barely changes between being on charge or not, and so there is no cue to the ACR on when to combine or separate.

Steve Johnson

Date 6/12/2019

I am looking to put in a system that looks a lot like your first diagram except my alternator has a controller (Balmar). If I chose a AGM start battery I think I should be able to program the alternator with a LiPo profile and just let the start battery live with the results. I should be able to limit the current there as well I hope. If you have any input on that setup I would be glad to hear it.

Colin Smith

Date 7/3/2019

Hi Peter, excellent information, thank you! I'm setting up 2 systems as per the first drawing using the Victron Cyrix. In the first installation the engine – a low-revving marine diesel- has a dedicated starter battery and 50A alternator and a separate 160A alternator to charge the service batteries. It’s a 12V system. I plan to have a “buffer” 12v 110Ah sealed lead acid battery which will have a couple of light loads attached and to use 3xVictron 100Ah LiFeP04 “SuperPack” batteries (with the built-in BMS & safety switch) in parallel to give a 300Ah ‘pack’. So the external VEBus BMS would not be required. Which Cyrix would you use – the Li-ct, the Li-Load or the Li-charge? The second installation is as above but using the VEBus BMS and 3xVictron 150Ah LiFeP04 Smart batteries in parallel. Again, which Cyrix would you recommend? Thank you for your advice.

David Villers

Date 8/20/2019

Hi Peter, I purchased 8 Valence U27-12xp lifepo4 batteries([email protected] each) for a Promaster van conversion intended for full-time off grid use. Depending on which configuration I go with (12v or 24v)I may only use 6. The Promaster has a 180amp alternator which will be the main source of charging for the batteries with 200-300w of solar backing it up. Valence documentation says they are particular and require their hardware, which im having trouble locating. I was hoping you could recommend an alternator charging system. I think the last diagram applies to me but not sure. Thanks for the help, David

Peter Kennedy

Date 8/20/2019

That's a lot of batteries for a Van! The Buck Boost DC DC Converter is now available in 100 Amps and you could use that to connect the start and house batteries and have it turn on when the engine is on, either using the vibration sensor or the ignition wire to be the trigger.

David Villers

Date 9/4/2019

Thank you for that suggestion. I'm leaning towards a 24v system using a "VICTRON MULTIPLUS 24/3000/70-50 - 120V VE.BUS INVERTER/CHARGER" but still looking. The promaster has a 180amp alternator. The buck a boost 100amp does the job but is more than I want to pay, so is the Multiplus for that matter and I don't think the alternator is fussy like the sprinters are but I could be wrong. What would you pair with the multiplus to convert from 12v to 24v that would be able to take advantage of the big alt and keep everything healthy? Or what combination would you recommend?

David Villers

Date 9/4/2019

There's another option that growing on me. Replacing my lead starter battery with one of the 138Ah Lifepo4's. That might be enough to run my van AC with the engine off for an hour or two. If two or more can be linked doubling the AC run time at that point it seems advantageous to have the entire aux and starter system combined so the alternator only has one output to something like a multiplus. I like that it's a 12v solution and eliminates the need to buy another AC that guzzles energy. Does this simplify things or over complicate it? What components would this kind of system require? Thanks again

Peter Kennedy

Date 9/4/2019

Your alternator always has to have somewhere for its power to go or it will self destruct. When Lithium batteries get full they have to stop charging. If the Lithium battery is full and stops charging the alternator will be toast. You can't use a Lithium battery in this situation without jumping through all kinds of hoops to protect the alternator.

rodney abbott-buchanan

Date 9/18/2019

I am running two AGM batteries with ML-ACR on 140A alternator. I want a separate Lithium battery in truck camper with solar charge input. Will I be able to connect a Cyrix circuit from Lithium BMS to the main starter battery?

Peter Kennedy

Date 9/18/2019

if you have a battery with an external BMS then you can use the Cyrix-Li-Ct to be the combiner between your start battery and your lithium battery. This will only work if your battery is the type that has an external BMS

Shawn K.

Date 9/26/2019

The BMS 12/200 datasheet mentions that some form of voltage regulation is offered, but it doesn't go into any detail about it: "3. Excessive input voltage and transients are regulated down to a safe level." Can you say more about how voltage is regulated, the limits, and if it's programmable? Thanks!

Peter Kennedy

Date 9/26/2019

The 12/200 BMS is for Victron batteries only. People try to buy this for use with other batteries but it will not work in that application. It is not programmable in any way. I don't know all the details of how it works but I would imagine that when it says it limits transient voltages all it means is that the unit contains a capacitor which would have the effect of smoothing out spikes. This BMS uses the properties of a fuse to set a current limit on the circuit. As the fuse reaches its current limit it begins to heat up and its resistance changes. The BMS uses this change in properties as a signal that the current it reaching its limit and it uses a semiconductor to limit the flow in the circuit. The current is limited to approx 80% of the fuse size. With a 100 Amp fuse the current is limited to about 80 Amps This BMS operates on the negative wire of the circuit and to avoid bypassing the BMS loads in the circuit need to have isolated negatives, either by not sharing a common ground (vehicle chassis) or by being connected via an isolated DC DC Converter.

Mikael Nilsson

Date 10/19/2019

I'm planning a boat system where the alternator is only charging the Lithium battery. The alternator is externally regulated by a Balmar MC-614. Would it be possible to have the alternator positive connected to an isolation diode feeding both the starter battery and the LiFeP04

Peter Kennedy

Date 10/21/2019

I haven't personally done it this way but I don't see what it wouldn't work.

Mark Campbell

Date 11/6/2019

In the option with an Alternator and only Lithium battery, you are using an external BMS to trigger the Regulator to shut down the Alternator. 1) What are the options if the batteries have an internal BMS? How do I tell the Regulator to shut down? Is there a signal I could wire from say a BMV-712 or CCGX? 2) Does the regulator truly shut down the alternator as you don't want further power going into the LIFEPO4 batteries or is there still current coming from the alternator? Does this need to be diverted elsewhere?


Date 11/21/2019

<DIV> Im trying to find a way to charge a 24v tesla battery with an alternator. It seems easy enough, but tesla batteres are NCA, and the only batteriues mentioned are LiFePO. Im not sure if the diagrams would be directly compatible or what modifications would be needed. </DIV> <DIV> <BR> PKYS Asks: Has anyone tried this and would like to add a comment?</DIV>

Joe Swartz

Date 12/10/2019

Hi Peter, I'm starting to develop a 1000Ah Lifepo for my van conversion. my question, I want to understand how to size the alternator for this size of a system? my goal is to be able to charge the system in 2-3 hours of driving. Thank you Joe

Peter Kennedy

Date 12/11/2019

That's a lot of batteries for a van. If you had a 250 Amp alternator it would take four hours at full output to charge them from empty to full. I don't know if its possible to find an alternator that powerful and figure out a way to mount it on a van, you might need more than one alternator.

George P Jessup

Date 12/14/2019

How you suggest to charge 24v alternator to 24v truck wet batteries (maybe lithium in future) to 24v house lithium bank that is also tied to solar and or wind (expedition boooooon dock camper)

Peter Kennedy

Date 12/16/2019

The new range of Victron Smart DC DC Chargers will be available shortly to take care of charging one battery from another, whether the voltage is 12 to 12, 24 to 24, 12 to 24 or 24 to 12 there will be a programmable three stage DC to DC battery charger available to do the job. At the time of writing (December 2019) the first few are now available and the rest should be here within a couple of months

Philippe M.

Date 5/20/2020

Hi everyone, Just bought a Victron SuperPack 12,8v 100Ah battery, this battery has the BMS integrated this is the reason why I chose it. This battery will be installed in my camper van as a leisure battery, but a big question question come to mind, will my 140A alternator sustain the charging load. From the battery's technical datasheet, it only pulls up to 50Amps when charging, but is this really the case ? Since this battery has a BMS of its own, does it control the current pulled when charging ? Will I need a DC to DC charger in-between the alternator and the battery ? By the way, the alternator is a Valeo 439753 which is regulated and I currently have an 80Ah battery as my main battery and also a split charger relay. Thank you in advance for you response Philippe

Peter Kennedy

Date 5/20/2020

The superpack battery you mention is similar to all lithium batteries that have an internal BMS. It has a relay to shut off the battery if it gets too close to full or too close to empty. Because putting a relay like that in the battery takes space and costs money they tend to be on the small side. So the limitation on current is not the battery itself but a limitation of the switch. The battery will not limit the current in any way and behaves like every other lithium battery. It will take all the charge you can provide. You need to fit some other external regulating device such as a battery to battery charger which can limit the current.

Lawrence Blanchette

Date 5/27/2020

I'm looking at the Victron BMS plus Battery Protect with the Alternator directly charging the Victron Lithium House Batteries. I'm a bit confused on Alt shutdown with Battery protect. With the MC-614-H Balmar says to turn off by opening the ignition circuit (http://www.balmar.net/wp-content/uploads/2019/01/SERVICE-BULLETIN-Lithium-Battery-Charge-Programming-12V-January-2019.pdf bottom of page 3) and in emergency (whatever that is) open both ignition and power circuit. You say open the power circuit. The Alt B+ to the battery while fused will always be connected. Does it matter which, power or ignition, is used to stop charging?

Peter Kennedy

Date 5/28/2020

If you are using the Victron Battery Protect it has to go on the power wire. That is because the Victron Battery Protect intentionally lets a tiny amount of power through (for use in another application) and that tiny amount is enough to keep the regulator in the on state all the time. Maybe some other solid state relay would work better but I haven't gotten around to trying it. My customers have successfully used the Battery Protect on the power wire.

Colin Budich

Date 6/16/2020

Hi Peter I am another person considering the possibility of using several batteries with built in BMS in parallel up to approx 600 AH. I have a catamaran with existing programmable AC chargers, solar with MPP controllers. 1800 W inverter, 840 AH existing AGM . There are two 120 amp alternators with no external regulation at present connected to two AGM start batteries This ,as has been mentioned before,is the sticking point for an adequate install. Is there any non external BMS solution to the alternator problem. I can go for a full custom built system but as I can't install it myself it will be at least twice as expensive as using drop ins with some modification to existing setup. Thanks for any advice

Peter Kennedy

Date 6/16/2020

As you can see by the number of comments this is a very vexing problem for many people. There is no easy answer however. You can use your alternator to charge the start battery and then use a DC DC Charger to charge the Lithium. Or you can take your alternator output to a diode isolator and have it charge both batteries simultaneously. Those are your options.
The DC DC Charger puts some limitation on how much power is drawn so it will help stop your alternator from self destructing due to over work. The isolator will allow your alternator to work itself to death if it is not up to the job or you can't limit it in some way.


Date 4/12/2021

Hi Peter, Like many in this thread I’d like to add a liFeP04 bank. Regarding above post how can you be sure alternator is up to job or how can it be limited/controlled so not to self destruct? Thank u ...j

Peter Kennedy

Date 4/12/2021

I think you can only connect an alternator to a lithium battery directly if it has an external regulator with the ability to tweak the output (like the Balmar regulators do using the belt load manager) and automatic temperature adjustments so the alternator backs off before it gets too hot. Regardless of any of the above you cant connect an alternator directly to a lithium battery that has an internal BMS because if the Battery shuts off the alternator would be fried.

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