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olivier-hecht avatar image
olivier-hecht asked

Voltage drop with BMS low temp cutoff

Trying to understand whether this is normal or not. As I’ve watched the low-temp charging cutoff from the BMS within my (3) BigBattery lithium batteries engage, I’ve noticed the voltage as reported in the BMS-712 and Cerbo GX drop from 13.2 to 12.6 instantly. SOC stayed the same at around 90% and the small load on the batteries continued uninterrupted.

When this happened, there was nothing charging the batteries as it was the middle of the night so the MPPT 150/70 was off, I wasn’t running the engine with the twin Orions, and the Multiplus 3000 was off. The only thing going on was about a 1.8a load on the 12v system when the (idle) charging side was disconnected. Is this normal? Even if it is, I’d like to better understand why disconnecting the charging circuit at the batteries would cause such a large drop in voltage even when no charging was taking place. Thanks!

BMS
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2 Answers
wkirby avatar image
wkirby answered ·

With no other charging sources, I would have expected the Voltage to drop from 13.2V to 0V when the batteries disconnect.
The only thing that springs to my mind is that perhaps the battery is only disabling charge and discharge is still allowed, it exhibits this behaviour when charge is disabled. Perhaps Bigbattery will have more knowledge on the expected behaviour of low temperature conditions.

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olivier-hecht avatar image olivier-hecht commented ·
I would also expect 0 volts if the batteries were totally disconnected, but I have confirmed that only the charging is disabled by the BMS at these temperatures…discharging is still enabled and the batteries have continued to support the ~1.8a constant load throughout this time. So I’m still confused by the voltage drop from 13.2 to 12.6 when only charging is disabled.
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wkirby avatar image wkirby ♦♦ olivier-hecht commented ·

It's unknown to us here how that battery's BMS is designed. This may be expected behaviour or maybe not. The battery manufacturer could advise on how the internal components of the battery are supposed to work.
You could verify with a multimeter at the terminals of the battery to be sure, but I'm pretty certain that is what is actually happening inside the battery and the BMV-712 isn't just making it up.

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olivier-hecht answered ·

I am in contact with the manufacturer trying to understand exactly what’s happening here - they’ve confirmed that the BMS only disabled charging at these temperatures so I’m also just trying to understand the concepts of what’s going on with the voltage in these various states. The first chart below shows the voltage drop when the BMS first disabled charging. This was when no charging sources were active. The second chart shows what the voltage does when the BMS is still preventing charging at the batteries but the solar charger is active and only offsetting the ~2a load over the course of one sunny day. The third chart shows what the voltage does when the BMS allowed charging again as the temperature gets high enough (middle of the chart - to the left is another day of solar active but BMS preventing charging the batteries so solar is just offsetting the active load). The part near the middle when the voltage goes from ~12.6 to ~13.2 is when no charging devices are active…I’m sure the voltage will rise again to 14.6 as soon as the solar charger starts again later this morning, which is normal.

What I’m really confused about is the difference in voltage readings based on whether charging is allowed by the BMS when no charging is happening that has me confused…12.6 when the BMS is preventing charging and 13.2 when the BMS is allowing charging.


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Alexandra avatar image Alexandra ♦ commented ·
@Olivier Hecht

By way of finding out if it is systen related. When it has the low readings disconnect it from the system and then test the battery.

Then you will know if it is the battery itself or a system issue.

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olivier-hecht avatar image olivier-hecht Alexandra ♦ commented ·
Ok. I’ll try that. I’m not certain there’s an issue at all, though, the low temp disconnect is working and the system is allowing discharge during that time as designed…I’m more interested in understanding why/how the voltages would vary like this just because charging is blocked.
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