question

You'd need a BMS to accomplish this. And you what charge sources do you have?

As stated above, managing lithium cells is more complicated than just measuring SOC. I also have four Winston LiFEPO4 cells in a mainly Victron system.

You need battery management system to monitor the voltage/temperature of each of the four cells which cuts the charge sources when any one cell voltage/temperature is too high and cuts the loads if cell voltage is too low. The BMV702 will not do the for you.

I like many others, use a product from another Dutch company which I'll not plug too hard on a Victron forum however I use two Victron Battery Protect units, BP220 and a BMV712. One BP220 has all the charge sources connected and open on high cell voltage/temperature and the other BP220 has all the loads and opens on low cell voltage. The BMV712 does the normal 'calorie counting' of energy-in/energy-out for SOC.

If you have a Victron Multiplus, there is an excellent support document on Victron's site of how to shut it down in the appropriate circumstances using control signals as the Multiplus should always be connected to the battery unswitched.

Unfortunately this is the level of complexity you need to keep a set off Winston or similar Lithium cells healthy.

Hope that helps.

Ian, SV-Gabrielle.

Thanks for all of your feedback. To further describe our installation which we have been running a little over 2 years including a 14-month journey from Sweden to the Caribbeans and back:

For charging, we have two 90A standard alternators mounted on our two Volvo Penta D1-30s with a Sterling Alternator-to-battery-charger plus about 800w of solar controlled using two Victron Smart MPPT-controllers.

All charging is currently connected to a single charge-point from where we distribute the power to: (1) Old buffer/backup Lead-Acid battery (buffer for when switching off the charging and backup if the LiFePo4 failed during our live-aboard) and (2) the 400Ah LiFePo4-batterybank. The engine batteries have individual charging from another output on the Sterling which uses a standard lead-acid battery charge level.

For controlling the charging and consumption we have used:

(1) BMV 702 controlling a BP220 as high voltage protection using the voltage and SOC to decide when to shut down the charging. The 702 is also complemented with a manual switch so we can shut down the charging manually if needed.

(2) A BP220 as a low voltage disconnect.

(3) A simple cell-monitor measuring the voltages on each cell to allow me to manually keep an eye on the cell voltage levels.

(4) The inverter is only used intermittently when boiling water (to save gas), run power tools, charge gaming computers etc and is controlled manually with a switch.

Our experience after our two years with this system is that:

(1) LiFePo4-batteries are awesome for both normal cruising and bluewater cruising (we can discuss this in another post)

(2) The LiFePo4-cells have worked perfectly and the difference after two years is roughly 30 mV between the highest / lowest battery during storage (currently not connected as the temperature is still <0'C in parts of Sweden. I'll do a manual balancing with a long piece of wire before installing them this year.

(3) The Sterling A2B-charger does its job by pulling down the (IN) voltage to the charger to trick the alternators to run at 100%. This makes our engine panels send out a warning for low battery voltage... Haven't bothered fixing that... but it is annoying

(4) The solution with the BMV702+BP220 has worked OK but the solution shuts down the charging a fair bit too early (based on looking at the cell voltages).

The thing we are looking into fixing is (3) and (4).

"(3)" will probably just be a firmware upgrade and configuration.

"(4)" will be a bit more tricky. Based on the recommendations here I would have to replace the BMV 702 with a BMS like the REC BMS or 123BMS and use it to control the BP220:s.

Question: Just to verify: Victron has, as I now understand it, no product that can either better monitor the SOC than the 702 (or monitor the cell voltages) that can be used for controlling the BP220s. Correct?

Grateful for your support! The answer from the local supplier here in Sweden was "No - noting can be done to improve this".

@sy-barit-se - as I will be building a similar system to yours, I was wondering if you found a solution for the BMS? Would you mind sharing it?

Thanks!
Philipp, @svamae

Battery Management System