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stanpete avatar image
stanpete asked

Voltage Spikes with LiFePo4

Hey everybody,

I just set up my first small solar system. 2x200W panels, a 24V LiFePo4 battery (29,2V fully charged) and a Victron 100/20 charging controller. Now I'm a bit worried because of a strange behavior - when it's sunny the 100/20 controller shows the battery charging for a minute, at let's say a voltage of 28V and then suddenly the battery voltage shows 33V and the charging stops. Then suddenly it's back to 28V, charging again, and this cycle keeps repeating.

FIY: The controller is of course set to LiFePo4, 24V.

So might this be the controller's fault or the battery? And do I have to be worried something might blow up?

battery chargingLithium Battery
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7 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Sounds like the batteries BMS is disconnecting the battery due to cell imbalance.

Did you fully charge each battery before placing them in series?

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Alexandra avatar image
Alexandra answered ·

@stanpete

Usually it is the bms of the battery bms that causes this. Although strange if is is at a lower soc with only 20A charging. Most can charge at that rate.

Try setting a lower charge current on the mppt and try again?

Maybe some more information on the battery and panles config?

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stanpete avatar image
stanpete answered ·

Ok, thanks for the quick response!

The battery is a black box from china with a "SUPNOVA Battery Capacity Voltage" display. I charged it initially to 29,2V with a mains LiFePo4 charger.

The two panels are each 36V, wired in series and they show somewhere around 85/90V when it's sunny. So there aren't many amps going into the battery, as you can imagine.


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Trevor Bird avatar image
Trevor Bird answered ·

The battery BMS may be struggling with over voltage. Try reducing the absorption voltage to something like 27.5 volts and see if the voltage stabilises. If it does you will know it is the battery BMS causing the problem. I have seen similar symptoms because of this problem.

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stanpete avatar image stanpete commented ·
I set it to 28.0 V and that looks better already.

So what is the catch now? Can I use only X% of the capacity, Do I need to get a better BMS or should I be worried something catches fire? ;-)


Thank you so much for your advice, btw. Even if it seems not to be mppt related.

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klim8skeptic avatar image klim8skeptic ♦ stanpete commented ·

So what is the catch now? Can I use only X% of the capacity?

If you charge to 3.5v per cell with a little absorption time, that would be ~99%.

Cell capacity test. <<LINK!

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Trevor Bird avatar image Trevor Bird stanpete commented ·
Dont be concerned about fire. The BMS is designed to reduce the possibility of a cell geting too hot. My rough understanding that when the cell gets to 3.7volts, the energy can no longer be absorbed by the cell and it is dissipated as heat. That is why the BMS becomes active and stops it charging.

If you reduce the voltage a bit the battteries will still charge pretty close to 100%. Just be happy the voltage has settled down and enjoy your lithiums.

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stanpete avatar image
stanpete answered ·

klim8skeptic and trevor-bird ❤

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dunnp avatar image
dunnp answered ·

Just a simple suggestion: check the blade fuse in the MPPT. I had a situation where the battery voltage it was reporting would intermittently go really high and charging would stop. It turned out to be a faulty blade fuse and/or bad connection at the fuse terminals.

Perhaps yours is a different cause but worth a look at the fuse for signs of overheating.

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stanpete avatar image
stanpete answered ·

So in the end it looks like it really was some weird “mechanical“ problem. After playing around with the absorption voltage it turned out there was still something wrong. So, as suggested, I replaced the blade fuse and also redid all the wiring. I had some cables still without crimped ends, so I crimped them properly and then tightened down all the screw terminals in the mmpt controller as well as in all my fuses/switches.

Now it seems to work as it should!

Thanks to all of you!

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