diy solar

diy solar

one battery draining completely, the other is ok?

st_

New Member
Joined
Jul 24, 2021
Messages
5
kind of a saga but I don't know what to do so...
built system end of may/early june.
victron mppt 100|20
2 x 12v 100ah lithium in series
450 w panel (Voc-49V)
3000w 24 v inverter
seemed to be working fine for a few weeks with voltage steady at around 26-27v max a day.
Then I left for an hour and a half and in that time the inverter alarm had gone on and then shut off AND the batteries had gone off completely so that even the little battery monitor I have hooked up to them shut down and the charge controller shut down. Everything off.
I disconnected the batteries from series, checked the voltage. One was around 12v the other around 0.4v. I didn't even know that was possible. The low voltage battery was the first in the series.
I connected them in parallel to wake the second one up and managed to equalize and reconnected them into the system. Lasted about half a day before I started discharging the batteries with more than my fridge (just a 60w fan and 5w max fan, maybe a phone charger) and then i could watch them lose voltage in real time on the victron app. I ended up trying to charge with a Lithium Battery charger and the low battery was so low that the charger wouldn't recognize it.
I assumed it wasn't a problem with my system as everything had been working for weeks seemingly ok.
So i got the battery manufacturer to replace the battery that had been at .4V
Fast forward to me getting the battery (and 3 weeks with no power in July)
i measured each battery and charged one so they were the same voltage before re-connecting them in series as I had before. This time the new battery was the first in the series. It's been 5 days with no problems until this morning I woke up and no power. The only thing I did differently yesterday was add my fridge (45 w) back into the solar system and off the car battery. Last I checked mid-afternoon/evening yesterday voltage was around 26.something. Was running my fridge and both fans all night until at most 4:30 am when i realized no power. I don't think that is enough to drain the batteries. This time I measured and the new battery is 13v, the old one that had been alright is at .25v. This time it was the second battery in the series that has the low voltage.
Again I connected them to equalize and got the low voltage up to 11.9v and will try charging them separately before connecting again, but there is something I'm missing. I don't know if the battery company will replace this one and I'm not made of money and I also want power. So, there's got to be another problem/solution but I don't know what to try.
Any one have ideas? Thanks
 
Most likely scenario is you are drawing more from batteries then you think. 26v across batteries is nearly fully discharged.

0.4v indicated the battery internal BMS has disconnected. Low voltage <10v, high voltage >14.8v, high current >100A, or high internal temp >50 degs C, will cause BMS to shut down. The battery allows for small current charge rate to reset a BMS that has tripped off. You can use a normal auto charger to wake a 12v LFP BMS back up again. It may take a little while to recharge it enough at low current to reawaken BMS. Some of the 'smart' auto chargers will not charge if they don't detect a minimum battery voltage first.

Cheap 12v LFP battery BMS are not designed to be series stacked but you usually get away with only two series stacked 12v LFP battery. Their BMS is designed with lower cost 20v to 30v breakdown voltage MOSFET switch which should not be series stacked as the stack voltage will exceed MOSFET breakdown voltage. If the battery spec's do not state the maximum series stacked batteries allowed then it likely should not be series stacked.

Do you know how much idle no load current your inverter draws? Does your monitor report consumed AH's?
 
Last edited:
100ah at 24 volts ... my guess is you don't have enough battery and are hitting low voltage disconnect. Maybe fridge compressor startup is causing it to sag.

Victron recommended a minumum of 200ah at 24 volts for similar sized inverter. I ran with that for 6 months, but recently doubled it.
 
Most likely scenario is you are drawing more from batteries then you think. 26v across batteries is nearly fully discharged.

0.4v indicated the battery internal BMS has disconnected. Low voltage <10v, high voltage >14.8v, high current >100A, or high internal temp >50 degs C, will cause BMS to shut down. The battery allows for small current charge rate to reset a BMS that has tripped off. You can use a normal auto charger to wake a 12v LFP BMS back up again. It may take a little while to recharge it enough at low current to reawaken BMS. Some of the 'smart' auto chargers will not charge if they don't detect a minimum battery voltage first.

Cheap 12v LFP battery BMS are not designed to be series stacked but you usually get away with only two series stacked 12v LFP battery. Their BMS is designed with lower cost 20v to 30v breakdown voltage MOSFET switch which should not be series stacked as the stack voltage will exceed MOSFET breakdown voltage. If the battery spec's do not state the maximum series stacked batteries allowed then it likely should not be series stacked.

Do you know how much idle no load current your inverter draws? Does your monitor report consumed AH's?
Thanks for your reply. I think you are right re: how much power I'm actually using. I know my solar setup isn't enough to fully charge my battery bank in one day but I still thought I was using minimal power.

I paralleled them this morning to wake up the low voltage one and now it is charging with the lithium charger.

The battery manual indicates that you can series connect but the examples are always using more batteries than just 2.

Just checked my inverter manual and (see photo) I'm assuming "standby input power" would be <30w for this 3000w inverter. Which adds up for sure. So perhaps that is the culprit.

Seems I need to keep an eye on the charge more (my shunt and monitor was not properly installed at first but last I saw consumed energy was around 1200wh I think) and keep inverter idle time to a minimum.

Do you have any thoughts though on why only one of the batteries BMS was activated? I would think it would have been both which is why I initially thought battery malfunction.IMG_2052.JPG
 
100ah at 24 volts ... my guess is you don't have enough battery and are hitting low voltage disconnect. Maybe fridge compressor startup is causing it to sag.

Victron recommended a minumum of 200ah at 24 volts for similar sized inverter. I ran with that for 6 months, but recently doubled it.
can i just connect these 2 batteries in parallel as well then? A friend of mine suggested connected the 2 12v 100ah in both series and parallel but i haven't seen anyone do that with just two batteries.
 
I run a compressor cooler, TV, fan, lights and charge a few phones off 200w of panels on my sailboat. No inverter. Your Inverter is probably using to much.
 
Weakest link goes first. Just a little less capacity in one battery causes it to trip out first.

There is also tolerance variations between the two BMS trigger points for cell voltage trip, battery current trip, and temp trip. Your going to have this when you have multiple BMS's involved.
 
can i just connect these 2 batteries in parallel as well then? A friend of mine suggested connected the 2 12v 100ah in both series and parallel but i haven't seen anyone do that with just two batteries.
No, series or parallel only. Don't listen to your friend any more. You cannot connect in parallel and series at the same time ... that would create a dead short, which could be fun/scary for a little while, unless the BMSs have good short circuit protection.
 
Back
Top