This Smart BMS CL 12-100 is designed for use with Victron's 12 volt Smart Lithium Batteries. Its purpose is to manage the battery bank so that the batteries don't get overcharged or run flat.  It has a pair of data ports that connect to the electronics within the battery and it monitors the battery temperature and the voltage of each cell.  As long as the battery is within its allowed parameters it will permit the battery to be charged or discharged.   If you don't have a Victron Smart Lithium Battery then this device is not for you, sorry.

This BMS was specifically designed for use in RV's and Camper Vans where there is a desire to tap into the vehicle alternator and take a measured amount of power to charge the Lithium battery.  Because we don't want to try and modify the engine or alternator in any way we need some way to limit the power taken from the alternator to prevent it from overheating.  This BMS has a way to fine tune how much charging current is taken from the vehicle up to a maximum of 100 amps.  In more complicated systems where there is a second alternator, or on boats where there is an externally regulated alternator, there is another BMS that might be more appropriate for the job.  

Basic Connections

The BMS data cables connect to the Smart Lithium Battery.  If you have more than one battery the data cables daisy chain from one battery to the next and then each end of the chain comes back to the BMS.  As you get more batteries you will need extension cables to allow you to mount the BMS in a more convenient place.

Daisy Chain M8 Data Cables          M8 Extension Cables

The BMS then has a connection to the start battery and to the Lithium battery, and a negative wire to the common ground. The basic layout looks like this:

Both batteries must share the same negative, which in a van would be the Chassis Ground.  Because the BMS is connected to two batteries you must put circuit protection at both ends of the circuit, one fuse or circuit breaker at the Lithium end and one fuse or circuit breaker at the Start Battery end.  Note that the BMS is powered from the Start Battery end only.

Setting up the alternator charge current

The amount of current that the BMS will take from the start battery to the Lithium battery is adjusted in two places.  The value of the fuse installed on the face of the BMS is what determines the amount of current flowing.  You must also enter that value in the programming menu as shown below.   The BMS will pass approximately 90% of the value of the installed fuse, up to a maximum of 100 Amps. The supplied fuse is rated at 100 Amps and so the approximate current it will pass is about 90 Amps.  For proper calibration you should use one of the Victron Mega Fuses which are available in values from 60 amps up, or use the included adaptor for smaller values.  The maximum fuse allowed is 125 amps which gets you a max charge current of 100 Amps.  The minimum fuse size is with the included adaptor with twin 7.5 Amp fuses which gets you a charge current of 9 Amps.  We suggest that for your initial installation you set this up with the minimum fuse size and once your system is  commissioned you can switch it to the final size and reprogram accordingly.

Charge Current  


Whichever fuse you install you must also set the program to match.  This is a screenshot from the programming page of the BMS in the Victron Connect App:

Programming Page

Wire size and circuit protection

We suggest that if you are planning on setting the alternator charge current at or near the maximum you use 4 awg wire for the positive connection to both batteries.  If you have an extended wire run you could bump this up a size. We suggest that you put a 130 amp fuse next to each battery to protect this wire from overcurrent.  If you end up using a lower current setting you can reduce the wire size and circuit protection accordingly.  The circuit protection is to protect the wire from carrying more than it is capable, see the Ampacity Tables to select the appropriate value.   The wires size for the negative connection can be 16 awg,  a short length is included with the BMS.

Controlling external devices

The BMS has two terminals used for controlling external devices, they are labelled "charge disconnect" and "load disconnect"    Charge disconnect is activated if any cell gets to up to 4 volts, Load disconnect is activated if any cell gets down to 2.8 volts.  Charge disconnect is also activated if the battery gets down to a temperature of 5 Deg C. When the BMS is in normal operating mode both these terminals will be +12 volts.  When the battery gets full the "charge disconnect" terminal will turn off (and become 0 volts)  When the battery is almost empty the "load disconnect" will do the same.      That's all that the BMS does, you have to take those signals and use them to activate external devices.

You may well have more than one device you want to control from each terminal so you would need to make a terminal strip to split the signal to multiple devices.

These signal cables can be 16 awg.  Typically they would go to the H terminal of a BatteryProtect.

Connecting to a MultiPlus requires a special cable and is covered in another Blog Post Controlling the MultiPlus from the CL 12-100 BMS.   The official Victron Instruction Manual gives some additional examples of ways to control external devices.


For optimal cooling the BMS should be mounted in an upright orientation on a vertical surface.  We recommend that you use Blue Sea 4010 rubber terminal boots to cover the exposed connection points.  I haven't yet come up with a neat way to cover up the one remaining exposed positive terminal but you could always use tape.

The ON-OFF switch

The BMS must be turned on to work.  The jumper in the green plug in the photo below acts as the switch.  If you unplug the plug or remove the jumper the BMS will be off.  You can replace the jumper with a remote switch, any small low current switch can be used, for example the Blue Sea 4150 or Blue Sea 8230.  The exact function of the switch can be adjusted in the programming menu, it can either turn the entire device on and off or it can just turn the alternator charging on and off, its up to you.

Checklist and troubleshooting
This installation checklist might help you finish off your installation:

  • Connect to both Start and Lithium batteries using suitable circuit protection at each battery
  • Both batteries must share same ground
  • Connect ground terminal of BMS to common ground
  • Connect data cables to batteries
  • Select fuse size for alternator charging and program BMS to match
  • Connect external devices to the Charge and Load Disconnect terminals.
  • Program external devices to act on command of the BMS
  • Select the function of the ON-OFF switch in the program and make sure it is in the on position
  • Connect optional relay or alarm terminals.  A suitable alarm is the Blue Sea 1070 Floyd Bell Turbo Series Alarm which has adjustable volume
  • Test and document your system thoroughly as detailed in my Blog Post Testing your Lithium Ion Battery System
The BMS has some basic LED's to indicate status, green for on, blue for Bluetooth etc.  Additional fault indication shows up in the app and is detailed in the instruction manual.

Typical system wiring diagram for a van.

Right click on the image to open it in a new tab and see it full size.  This is only one page of a multi-page set.   If you buy a system from us here at PKYS we can customize a drawing to your requirements.  

The drawing that comes in the instruction manual is reproduced below.  It omits any circuit protection.  If you right click it you can open it in a new tab to view full size.

Additional information on operation

Your alternator can only charge the Lithium battery if it is producing power.  The start battery must be at least 13 volts for charging to occur.  Smart alternators are designed to save fuel and so they only work intermittently based on demand.  If your vehicle has a smart alternator your alternator will only charge the Lithium battery when the alternator is working.  In this case it makes sense to make the charge current to the maximum possible since it will only be for some of the time.  The alternator should respond to the fact that the battery is being discharged and respond by working harder.   With a conventional alternator it will be working all the time and charging can be set for a lower value.

Because the BMS is powered from the start battery it will continue to operate even if the Lithium battery is turned off.  As long as the data cables are still connected to the Lithium battery the BMS will remain on, and depending on how the system is wired it may be possible to power items directly from the start battery.  For this reason if you are doing any system maintenance you should make sure that the engine is off and the BMS is disabled.

Current cannot flow through the BMS from the Lithium Battery to the Start Battery, it is a one way device.

Final words

This came from the Victron Community Forum

Feel free to roll around on the floor laughing if you choose. I solved the problem of the lower than expected charge current as follows:


I buffed and thoroughly cleaned with alcohol the tabs of the fuse links, studs on the BMS and cable lugs. After that I got the charging current I had been expecting with both 100A and 125A fuse links.

I had changed fuse links (new units) several times, updating the fuse capacity within settings of the Bluetooth app. I had tightened the connections. They definitely were not loose. And every time the current was less than expected.

So, the knowledge I would like to pass onto others is that the cleanliness and integrity of the connections to the fuse link for this type of BMS, that uses the fuse link as a current sensing shunt, are paramount. Even a few millivolts dropped at either connection will make the BMS current limit prematurely.

Further Reading

This personalized instruction manual is designed to be read in conjunction with the official Victron Datasheet and Instruction Manual.  If you find any discrepancies the official documents take precedence, please let me know if you find anything wrong in this article.


Howard Turpin

Date 7/4/2021
Date 3/25/2024

Peter Kennedy

Date 3/25/2024

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