What happens when I have multiple charge sources charging a single battery bank?  I get this question asked almost every day.  "I have a battery charger, an alternator, and solar panels charging the same battery. Does it get overcharged?"    The other worry that I hear is that the battery might get undercharged because one charge source sees the voltage of the other charge source and shuts off.

A Victron MPPT Charge ControllerLets take a typical scenario.  We have a lead acid battery bank hooked up to a shore power charger, an alternator and solar panels.  The recommended bulk charge voltage for the battery bank is 14.4 volts.

At the dock the boat is typically plugged in to shore power and the battery soon gets fully charged.  So the worry is that if solar panels are running at the same time will they overcharge the battery because it now has two sources charging it.   Well assuming there is a charge controller fitted on the solar installation then both charge sources, the battery charger and the solar system, are regulated.  They read the battery voltage and are programmed to not allow it go beyond a certain limit, they are also programmed to go into float mode after a certain time or when the charge current reduces by a certain amount.  So the battery doesn't get overcharged.  

The alternator isn't normally running when the boat is also connected to shore power but if it is it doesn't matter. The alternator is also a regulated power supply and the program will limit the maximum voltage as well as the time spent at that voltage.

But what about when the boat is away from the dock: Can having multiple charge sources mean that one sees the voltage produced by the other and none of them work properly?

Lets assume the battery is quite depleted.  The solar panels haven't been working all night and at dawn they turn A Balmar alternator and regulator comboon the engine and the sun comes out at the same time.  If the battery is big and empty then they might not get it to 14.4 volts right away.  While this is happening they are all working at full power.  Once the battery gets to 14.4 volts they are all limited and cannot let it get above that voltage.  That is the maximum charge that the battery can accept so nothing is being wasted. As the battery gets fuller the charge voltage will remain at 14.4 but the current will reduce because the batteries internal resistance increases.  At this point although all the charge sources are sharing the work it is quite likely that one is doing more work than the other.  It doesn't matter which one is doing the work, the batteries are still being charged at their maximum rate.  Eventually the charge sources will go into float mode and the voltage will reduce and we are back to the situation at the start of this article. 

That is the broad outline of what happens.  As usual the Devil is in the details.  Of course the way the individual devices are programmed, or even how accurately they read voltage, will not be the same.  During the course of a charge cycle one may do all the work and then the other, they may share it equally, or it may fluctuate.  The overall effect may not be quite as efficient as if they were truly working in harmony.  It works though and batteries hooked up like this where all the regulation devices are working correctly do not get undercharged or overcharged. 


The latest version of the Victron Connect app allows you to network together multiple solar charge controllers so they all work in unison.  The procedure is explained in the Victron Connect Smart Networking Manual  Below is an excerpt from the manual.  I kid you not, the Victron Smart Solar Charge Controllers elect a leader to be the master and the rest become slaves. This is achieved automatically when you set up the smart network.

How synchronising works on solar chargers

Synchronising the chargers works in a master-slave manner. The chargers will elect a master among them and that master will be the one to dictate the charge algorithm. As the master cannot be determined by the user, it is important to make sure all chargers belonging to the same network have the same battery settings. To know more about the battery settings and some other information, check the VictronConnect manual.

After being elected, the master will make sure all chargers are on the same charge state and with the same voltage setpoint. As mentioned before, battery charge current is not controlled by the master, but by each of the chargers individually.

At the beginning of the day, the master will measure the battery voltage before any of the other chargers in the network start charging (to find battery idle voltage). This information is used to decide what should be the total absorption time for some types of batteries. The battery idle voltage is shared with the other chargers, as well as the total absorption time, and the elapsed time on the current charge state. That information is important so the charge algorithm can be resumed by the chargers if, for any reason, the master stops charging (i.e. sun went down on its panels, charger was shut down, charger loses contact with the network, etc).

In the absence of battery current sensor, such as the BMV, the chargers on the network will have their output current combined to estimate a better battery charge current. This improves the precision of the tail current setting, a feature intended to finish the charge cycle earlier if necessary


Robert Weatherby

Date 6/28/2018

So with all of this in mind, there isn't any chance of damage form one charging source to another of the charging sources?

Peter Kennedy

Date 6/29/2018

When source A is charging the battery and raises the voltage to 14.4 volts all that happens to source B is it sees a full battery. Nothing bad is going to happen because of that, Source A and source B can work it out between themselves as to who does the work based on how aggressive their charging profiles are.

Darryl S.

Date 10/24/2018

But what if something goes wrong? For example: If Source A is shore power (ie: 120VAC through a converter to battery) and Source B is Solar through a charger controller to battery, should I have isolating diodes to prevent a high current short or reversed polatity in one source from damaging the other source since they are effectively connected to each other at the battery?

Peter Kennedy

Date 10/25/2018

All your charge sources should have circuit protection at the battery end so if something goes very wrong the circuit will be interrupted. Under normal circumstances though the charging devices are designed to be attached to the battery and designed with the knowledge that the battery voltage will vary with state of charge, so just because multiple charge sources are connected doesnt change the equation in any way. The charge sources are one way devices, they charge the battery, not the other way around.

Pat Murphy

Date 6/14/2019

Its raining hard and the solar isn't charging. I plugged into shore power but the display showed negative current flowing out of the battery and negative sign flowing into the battery. The "state of charge" readout continued dropping. When plugged into shore power shouldn't the negative sign be positive and the "state of charge" increase?

Peter Kennedy

Date 7/23/2019

Sounds like your shore power charger isn't working.


Date 7/22/2019

Does any of the logic you mentioned in this article change when using Lithium batteries? Their C rate differs quite a bit from regular lead-acid batteries but does it make a difference at all?

Peter Kennedy

Date 7/23/2019

The same thing applies with Lithium batteries. The effect will become more apparent that the charging devices will not be able to raise the batteries to their target voltages until the batteries are almost full. For almost the entire charge cycle all the devices can work flat out.


Date 9/5/2019

Does this all still apply if the charging sources are all connected to a positive bus bar instead of directly to the battery's positive terminal?

Peter Kennedy

Date 9/6/2019

Yes it does.

John McMillan

Date 9/10/2019

So simple,so clear, thank you.


Date 9/12/2019

Great article. Thank you.

Simon Sea

Date 10/17/2019

Simple clear explanation. I think the equipment suppliers try to add mystique.


Date 11/5/2019

My shore power charger can control charging to the house bank and the starting battery giving each what it needs. So can the (external) alternator regulator. I f I add solar to the mix, should the solar regulator also be able to charge each battery bank separately?

Peter Kennedy

Date 11/5/2019

Most charging devices have a single output. (alternators, solar chargers, wind generators, inverter chargers etc) A few devices like battery chargers have multiple outputs, but they are very seldom individually controlled. You may think your charger is independently controlling the output because you see different current going into different batteries, but that is just because more current flows into the empty battery than into the full one, not because the charger is orchestrating that in any way..

Thomas Bennett

Date 11/8/2019

What is the effect on current. Say each charger has a set limit of 10 amps output for battery protection, wouldn't the battery be able to draw up to 30A if discharged?


Date 12/15/2019

If the recommended charging current for a 12v 200AH battery is max of 30% of the capacity of the battery that's 60amps. If the solar panel is bringing 60amps and the other source is bringing say 30amps, does it mean that the battery is getting 90amps of charge which is way above its safe 30% mark? Will this not destroy the battery as it is taking more current to charge than is necessary?

Peter Kennedy

Date 12/16/2019

Interesting question. The scenario you are describing really only occurs when you have lithium batteries with an internal BMS. The situation doesn't happen with conventional batteries because the resistance of the battery automatically limits the current flow and so if your charge devices are limited to the appropriate charge voltages nothing bad can happen.

When lithium batteries have an internal BMS they have an internal relay designed to shut them off when they get full or as they approach empty. Because these relays are expensive the manufacturers make them on the small side, which means the batteries have an artificially low allowed charge rate. If you exceed the rate its not that the battery cant accept the charge, its that the relay might burn up or not be able to shut off the charge. With these kind of batteries you have to come up with some additional external limitation on the charge devices you connect to them. This might mean having smaller chargers.

Note that with the Victron Smart Batteries we sell there is no such artificial limitation on the charge rate. The Victron Smart 200 amp hour battery has a recommended continuous charge current of 100 amps and a maximum charge current of 400 amps. On the discharge side batteries with an internal BMS are similarly limited in their discharge rate. Just another reason to be more sophisticated when selecting Lithium batteries.


Date 12/29/2019

Thank you for this informative thread. Based on it, as I understand it, I should be able to wire (on a bus) all four of my charge sources (off-shore>converter, vehicle alternator (after in-line relay), solar panels, and generator (through converter)) 'DIRECTLY' to my deep cycle battery (or parallel setup of my bank of batteries)). Is there other device needed from either of the sources (i.e. charge controller, regulator, etc).. a wiring diagram/sketch would be very helpful. Thank you in advance for your time and consideration.

Peter Kennedy

Date 12/30/2019

The ABYC allows charge sources to be connected directly to the battery without going through a battery switch. All wires connected to the battery are required to have circuit protection (with the exception of the starter motor circuit which is exempt). The level of circuit protection will vary according to the circuit and is dependent on both the amount of current expected from the device and the ampacity of the wire.

One economical way of providing circuit protection if you have three charge sources is the Blue Sea 5196 Terminal Fuse Block for 3 Circuits This terminal block can accommodate fuses from 30 to 300 Amps. When using this block one wire goes from each fuse to its corresponding charge source and then a single wire goes to the battery positive. On the negative part of the circuit you would want to use a busbar to gather the wires together in a similar way so that you don't have multiple wires going direct to the battery terminal.

Damon Cruz

Date 1/2/2020

I have been told I cannot connect the PWM/relay regulator for my wind and water DC generator to the same battery bank as my MPPT solar controller because one sees the other as a massive sink, so on a previous boat I wired the water generator to a separate battery used only for the refrigeration load. Is this a necessary precaution to prevent damage to one or the other system?

Peter Kennedy

Date 1/2/2020

Often a wind generator has a dummy load as part of the control system so that when the battery is full there is still somewhere for the power to go. It would require a full understanding of how this was set up in detail to be able to answer the question.


Date 1/9/2020

Hey, Great article, thanks. I have a little more complicated question. I have alternator power through sterling 30 DC to dc connected to bus bar, as well as victron 100/30 from panel(295wat). When both are on, my battery monitor shows negative draw from battery(amps), but with high charging voltage(14v+). After a couple of hours battery(300 Ah bank) gets to 100% accordingly to monitor(voltage goes up on it), so it's charged. Why during charing from alternator(through proper DC to dc) battery monitor shows drawing amps, not charging? Thanks, Slawek

Peter Kennedy

Date 1/10/2020

Is this on a motor vehicle? My guess is that DC Negative from the DC DC Charger is bypassing the shunt somehow via the chassis ground and not being measured.

Marek K

Date 1/15/2020

Hey, This is good article, but it's assuming all devices are stupid blunt and just give a charge to batteries on the same level I have DC/DC charger (3 stage), MPPT (3 stage), and wind turbine (stupid one), and 3/5 stage 220V charger. We all want to have our batteries charged in 3/5 stages. I (probably) can disable 3 staging i MPPT and DC-DC - but do u know a device that can take 3/4 (also the multistage 220V charger) input sources and charge my house lithium battery in multistage option?

Peter Kennedy

Date 1/16/2020

When the battery is low all the devices will be trying to charge it at full power. As it reaches its target voltage whichever charging device has the most aggressive charge profile will end up winning and the other devices will take a back seat.


Date 3/24/2020

Is there a way to protect my lithium battery from receiving to much current when the Alternator and Solar are on? My battery has a simple over current protection BMS integrated but id rather it be protected a different way. My alternator can supply 30 amp and my solar can provide 30amp, potentially charging the battery at 60 amps. However the battery should never be charge at a rate of over 40 amps. What is the easiest way or best way to provide protection. Thanks

Peter Kennedy

Date 3/24/2020

The situation you describe can happen when you are using a lithium battery such as one from Battleborn or Relion with a built in BMS. These batteries have an internal relay that will switch off the charge when the battery is full. Having a relay built in to the battery increases the cost so the manufacturers put as small a one as they can get away with. The charge rate limitation is a function of the relay and not the battery.

One solution would be to add more batteries and thus spread the power wider. This keeps the charge rate per battery within the limits. The other solution would be to limit the power by throttling back the devices that are doing the charging.

In your case if you have a Victron MPPT Solar Charge Controller with Bluetooth you can adjust the output amps of the solar using the App on your phone. If your alternator has an external voltage regulator such as one by Balmar you can also throttle back its charge using the Belt Load Manager part of the program.

James Nemeth

Date 5/17/2020

I have added solar panels and a solar charger, attached to battery bank 1 to be expanded to battery bank 2 and to the dedicated starter battery with two battery combiners. My question is: Why can't I just run the positive lead of my solar output to the common on the battery bank selector switch, the same place the alternator output charging current cable is attached? I would avoid the parallel connection of the battery combiners and retain the practicality of my selector switches. I can't find any diagrams to support that and the 1 tech guy I spoke to felt battery combiners or the like was the only way. My second question is could a smart battery isolator be used to run all my charging inputs with little lost of power? I am looking for the most direct and simple way to run my solar charger, single output to my 3 battery banks.

Peter Kennedy

Date 5/18/2020

Yes you can run your solar output to the battery switch output terminal and then it will go to whichever battery the switch is set to. That means you have to leave the switch on all the time. It sounds like your battery system is getting a bit complicated, maybe its time to review why you are doing all this.

Robert Laidely

Date 7/26/2020

Hi Peter Thanks for the article. As someone who is trying to learn about all of this, having someone 'plain English it' is really helpful. I am still struggling through the issues re: simultaneous charging of lithium batteries given that a good one can be ~80% depleted in terms of AH, but nevertheless still be reading a relatively high voltage. Therefore if the master charger system is determining the SOC on voltage alone, then I think it will be fooled into thinking the lithium battery is nice and full, when in fact it is not. Whereas, it seems like combining the voltage with the Amps the battery will accept at a point in time gives a better indication of the true SOC of the lithium battery. Any thoughts on this? Redarc say they have a system that will simultaneously charge by taking as much as it can from Solar first (as priority) and then 'topping up' from shore power and, if shore power isn't there or not giving enough, from engine alternator power. E.g. Assuming you use their 30Amp charging system and the system is in a Bulk charge stage, if Solar is giving, say, 15 amps then it will 'top up' the charge with another 15 amps from shore power or the alternator. I believe Redarc to be an amazing brand/product, but they are on the expensive side and they don't tend to let you mix and match like Victron does (which I'm a huge fan of) and also give you the App based control of the system like Victron does (which I am also a huge fan of). Reading your article and some of your responses above, it seems like you are saying that Victron doesn't necessVictron arily 'top up' from a secondary source to get the max possible charge. Is that right? Also, do you know whether/what Victron components are needed in the system to ensure that it properly measures the SOC for a lithium (i.e. as opposed to just going off voltage?). I am looking at putting a Victron MPPT 100-50 together with a DC-DC 12/12-30 to a parallel bank of 12v lithiums (200Ah). To provide the system with data from the battery I was looking at either the BMV712 or the Smart Shunt, but I can't work out whether they will actually give the chargers accurate info re: the Lithium SOC. Rob

Peter Kennedy

Date 7/27/2020

I didnt get into writing about prioritizing charge sources in this article. That is a whole subject of its own that you will find discussed in more detail on the Victron Community site. Here is a link to get you started: https://community.victronenergy.com/questions/14751/prioritize-pv-solar-over-grid.html

Robert Garner

Date 3/15/2021

Well done!!!!


Date 6/5/2021

This type of election based consensus algorithm should use the terms "leader" and "follower" as being a more accurate way to describe the relationship between nodes. Master/Slave is actually not a proper metaphor for this type of digital synchronization.

david milne

Date 8/18/2021

Hi Peter i have a Victron MPPT 150/70 -tr VE.Can, [solar] Victron tr smart non isolated dc dc charger[alt] , Victron blue smart ip22 charger[AC240V] all chargers have bluetooth. WITH 4 X 12V 200 A/H LITHIUM W/BMS not Victron connected in series/parralell to give 24v [VoltX Life PO4 BATTERIES] I have read your words about all the chargers talking to each other to regulate the charging and also about the lithium batteries like mine having small internal shut off relays. Also that a larger string of batteries can absorb any higher charge rates without overloading these relays. Does my system require a seperate charge controller that all the chargers connect to before going into the batteries. Thankyou David Milne

Tom Potts

Date 9/6/2021

Hi, great article! I think I have a slightly different situation! My RV has 4 deep cell house batteries than can be charged via shore power, alternator, generator, or solar. My 2 AGM chassis batteries seemed to only be charging via alternator while engine was running! Consequently, yesterday I added a 30 watt solar panel w/3-stage controller to charge and maintain my cranking batteries! I currently have it connected to the batteries through the cigarette adapter and it appears to be working great! My question is, since I have 2 separate solar systems charging 2 separate battery banks, is there a chance of damage to anything when the engine is running and the alternator is distributing power to both systems? Thank you!

Lon Gowen

Date 9/7/2021

Peter, great article and Q&A...thank you! I have a Lagoon 500 catamaran, so I have two engine alternators, a genset, shore power (when connected), and two 420W solar panels. My alternators are Hitachi LR180-03C, 12V, and 80A. I want to connect all of these sources to my house bank, which is a 500A 12V LFP (LiFePO4) with an external Daly 4S 500A BMS. I'm using a Victron 100/30 MPPT for my two panels, a Magnum I/C for all 120V AC sources, and then I have the two alternators with no external regulator. Do I need to do anything special? Or, can I just connect all of these to my house bank? Thank you!

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