BSL 5.1kWh min/max cell voltage + charge voltage

So I have a BSL bull 5.1kWh battery and I read the spec sheet says charge voltage 55V, however the max charge voltage (I think this is sent over CANbus to the inverter is always 54.5V (have started to log to see if it ever changes)), and where the max I have seen it 54.7V on VRM, my main concern is once 100% for 10 to 20 min the min/max cell voltages go from 0.02V difference to 0.23V (min 3.37/ max 3.60V).

Min-Max cell voltage

During the inverter setup, in the charger tab it says 55V charge (absorb) and 54.8 float.

Charger tab
victron_bsl_manual_-_2021-04-14.pdf (3.6 MB)

I believe its a 16cell battery (says nominal voltage 51.2V (16S)) and have no idea what all the individual cell voltages are, they could be 3.49 for all I know.

Should I be concerned or change something. Have left it for a few hours at 100% (solar only charging super sunny Somerset West) and switched all my loads to Eskom however no change, must I put the inverter on charge only via the switch?

I am though only a month into this all so its 100% likely I am misunderstanding something. Okay110%

Okay so just went from battery life to keep charged and in 0.1seconds the voltage went to 55V, I shall keep watching it.

So back down to 54.7V

You have nothing to worry about. I tested that battery quite extensively and you can push it to over 60V with absolutely no ill effects.

The battery has separate charge/discharge blocking mechanisms (likely two series MOSFETs or banks of MOSFETs). If any cell exceeds 3.75V (that is if my memory services) it will block charge. You will then see that the inverter reports a higher voltage than the battery does. That way the battery protects itself. It can handle quite a bit more than the advertised 54.5V, although the advertised voltage is more than adequate. In testing, I had to push it to 61V to get it to activate charge blocking. You can comfortably push it to 57V without issues, but of course prolonged high-voltage conditions do shorten the life slightly.

It is indeed a 16-cell battery. In terms of the BMS, I currently think this is a better battery than South Africa’s present favourite (another 15-cell battery I’m not going to name explicitly), but that other 15-cell battery is getting very good long-term results in the Australian battery tests, so it’s a bit of a toss-up at the moment :slight_smile:

Edit: Forgot to say, your highest/lowest cell voltage graph is absolutely fine. One cell is always a bit more trigger happy than the rest, and yours seems to peak around 3.6V. That’s perfect. Over time, you will probably see the battery settle and the highest and lowest cell will grow closer together. But right now, you are seeing exactly what I would expect of a battery that’s still fairly new.


It’s got potential, just lack the track record, but definitely a battery to keep in mind when the need for replacement arises.

A few months back I could buy them for 75% of the cost of a Pylontech. (Trade Prices) . I the time I was considering getting rid of My Pylontechs and replacing them with BSL.s

The very day @plonkster told me that they were done with the test and that Victron decided to add BSl to their list of “Tested” battery list, the price shot up to exactly the same as Pylon US3000’s per kwh. In the meanwhile, pylons just continued dropping their prices leaving the gap between them getting bigger and bigger. Currently I cant supply BSl at a better price anymore, although I wished that BSl can drop the price a bit. It would be interesting to see how many clients will opt for the BSL.

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Thanks plonkster, I too am hoping its the right battery choice.

As for the different cell voltages, do you think I should increase the charge voltage a bit every so often to correct this or just 100% trust the BMS to fix the imbalance if it ever becomes a problem by its own internal processes.

I did turn on keep batteries charged this morning and it peaked at 55V and has slightly reduced the difference to 0.14V (0.06V down) which I am happy with so maybe play with that when the sun comes back.

If the battery is plugged into the GX with a CAN cable, then the system will follow the voltage the battery is asking for in any case, and you don’t have much control over it. You can adjust it downwards (on the DVCC menu), but not upwards.

The reason it peaks a little above 54.5V is because the solar chargers are told to aim a little bit higher for various reasons I’m not getting into right now, and which really doesn’t cause any problems. The system is technically aiming for 54.9V, and due to various calibration differences it probably ends up just above it. It doesn’t matter, it is more than close enough.

The balancer will kick in if there is a cell above 3.5V. The exact voltage where the balancer starts working will differ between batteries, but it is fairly common practice to balance above 85% SoC, when the cell voltages are around 3.35V or higher. Pylontech starts needs around 3.485V per cell to really get going. So suffice it to say, as long as you have a cell above 3.5V, the balancer is working. Leave it and let it do its job :slight_smile: