DIY Li-Ion Battery BMS cuts off due to voltage drop, then recovers

raresserban

10 µW
Joined
Jun 17, 2020
Messages
6
Hello guys,
So I have a 5S2P pack I made for a bluetooth speaker (uses TPA3166D2 amp), protected with a BMS board with balancing.
I first used the pack directly to connect to the amplifier, and it worked fine, and the battery life was really good, since pack starts at 21V and then goes down to 14-15V, but this means I was losing amp power.
So I added a boost converter to 24V to get a boost to amp power.

The problem is that now the battery life is much shorter (expected), but the battery doesn't seem to go down to quite "0%" anymore.

I also have one of those cheap battery indicators, the ones that have 4 colored lines and a red outline.
Without the boost converter, I could see the indicator slowly losing lines and then just remained with the red outline (so voltage was below 16.5V for the whole pack, <3.3V per cell).
With the boost converter, the indicator remains at one line (voltage between 16.5V-17.5V for whole pack, between 3.3V-3.5V per cell) and then the battery shuts off, probably because the BMS cuts off. Then, if I remove the load and leave it like 5 minutes, the battery is back, again the indicator displays one line remaining. The indicator never goes to a red outline, so the cells never have <3.3V in them.

I am assuming this is because of a voltage drop when there is a load applied, but I want to know why this is the case and why it impacts the battery so much. Let's say that at half volume the amp has peak power consumption at 24V*3A=72W, so that's ~4.4A at 16.5V. The battery I use is LG INR18650-M36, which should supply up to 5A/cell, so a 2P configuration should provide 10A, so 4.4A doesn't seem that much.

Can somebody better explain to me why the big voltage drop and why the BMS cuts off (even though it should cut off at 2.8V per cell), and how I can improve the battery life?Should I just remove the boost converter, or dial it down to 22V or something else that I can try?

Thanks!
 
Bit of a guess but from reading your post sounds like the 24v booster is inefficient ,there are losses during voltage boost, do you know its power consumption? Maybe causes to much drain on battery , then voltage sag like you said,
using the output of amp to calculate its power usage gives rough estimate as the amp not 100% efficient either, could uses more amps than you expected, might have been just OK before on the LGs amp limit, then add draw from booster maybe it pushes amps needed over the 10amps ,not checked lg specs but don't think that's continuous either, also one cell might be pulling pack down by not coping with draw as well as others :mrgreen:
 
I don't think it's drawing over 10A, since I have a 10A fuse after the BMS :)
But I get your points, I think the boost converter has like 90% efficiency, so that means that 10% more load on the battery, plus the extra load from the amp running on 24V, plus the fact that at a higher amperage, the batteries have less capacity, might all contribute to the result.
Didn't really think the converter efficiency could affect it this much, that is not the only variable at play here. Thanks for the reply, I might just revert to not using the converter and getting less power from the amp, but at least I can keep the party going for longer :)
Might also try a 6S2P configuration in the future when I have time, since that fully charge should be ~25V and discharged like ~19V, so that will improve power without sacrificing so much battery life since I won't need a boost converter.
 
The amp will draw more current at 24v. Maybe a lot more. Combine this with the losses in the boost converter and you may have significantly higher battery current. Ideally you would want to try measuring the battery current when it is running.

Higher current draw is also going to cause the cells to reach cutoff sooner due to battery sag. This is sort of a double-whammy on your run time.

If you need more voltage, going to 7s would be better than using a boost converter (but I guess it may not fit).
 
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