I am sure I have read a post on this topic somewhere but it was a long time ago and possibly on the old forum system. Anyway, I can’t find it now so I’d like to try to find an answer now. I have an AC3 inverter with 2 x 9.5 kWh batteries. Over the summer, they have mostly been charged to 100% daily by solar and the battery percentages have usually been within 10-15% of each other. Lately though, with less solar, they are being charged through WonderWatt to between 60 & 80% and then (sometimes) topped up with solar. My primary battery is currently showing as 59% charged and voltage of 53.5v. The secondary battery is at 84% and 53.7v. Is this something I need to do something about or do I just trust the BMS knows what it is doing? If I do need to do something, would one of you more knowledgeable people be able to offer some advice, please?
Thanks for your time.
Battery SoC percentage is a very fraught thing to work out as there is no direct way to measure it. All the BMS can do is to measure the voltage and to inter what the SoC % is based upon the voltage figures it got when you last did a battery calibration.
If you have multiple batteries then the SoC can drift apart on them, the newer BMS’s are better at keeping them in track, so if possible upgrade your BMS.
Also, if you have not been using the batteries much over summer, SoC has been near full all the time then it’s very likely the batteries have lost track of what the actual SoC is. A full discharge to empty and fill to full will enable the BMS to regain track of it.
But in general, don’t worry too much about the correlation of the SoC % and voltage, that’s the BMS’s job to worry about, but the above advice can give it the best chance of doing so
Thanks for the quick reply, Geoffrey. My portal reports that my battery firmware is 3020 and no updates are shown available so I’ll just do as you suggest and let the BMS do its thing!
If you are on BMS 3020 then that’s the most recent publicly released BMS. BMS3022 is available in beta but what you’ve got it pretty good
Just a thought. It’s now 4:30pm and my primary battery is at 34% and the secondary is 69%. I have a timed charge set for tonight to 80%. If I was to turn the secondary battery off tonight, would that result in getting the primary back up to a bit nearer the secondary or is it likely to cause all sorts of problems with the BMS or anything else in the system? It’s just curiosity - I don’t think I would actually do it!
if you were to turn the secondary off then only the primary would charge, yes
what happens is that the batteries are daisy chained together, the primary takes the load and any charges first, so it will usually have the higher (when charging) and lower (when discharging) soc than the secondary. The BMS should deal with this though
Thanks Geoffrey. Maybe now that I know that it’s unlikely to break anything, I might give it a go and see how long it lasts.
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