diy solar

diy solar

LFP cells gone bad

The point of the BMS is not the pack voltage, it's the cell voltages...
In your 20 amp hour pack, what are the cell voltages when the pack is at 14 volts? Any of the individual cells over 3.6 or 3.7 volts? it's the individual cell safety and health that is the point of using a BMS.
3.6V max. Use a lipo balance charger, don’t always use the balance function. It’s an A123 pack that was barely ever cycled. Worst I’ve seen the cells is 14mV when one hit 3.60. The others weren’t too far behind.

Edit. Oh you were asking at 14V. 3.50V with typically an 8mV deviation
 
3.6V max. Use a lipo balance charger, don’t always use the balance function. It’s an A123 pack that was barely ever cycled. Worst I’ve seen the cells is 14mV when one hit 3.60. The others weren’t too far behind.

Edit. Oh you were asking at 14V. 3.50V with typically an 8mV deviation

"A BMS is not needed" (but I'll let you know later that I use a balance charger)

A balance charger is not a regular piece of equipment for most on this forum, and it's a very different thing. Even if you don't use the balance function, they still charge based on set per-cell limits. If you can see the cell voltages, the balance leads are connected, and it's regulating the charge based on the peak cell voltage, not pack voltage.

When you balance charge, it top balances, and there's not an inverter/charger or MPPT that will do that for you at the cell level, so it's 100% irrelevant to a "DIY Solar Forum," and "A BMS is not needed" is just irresponsible diarrhea of the mouth.

@MarcoWasRightIsWrong
 
The legend continues he is now on the forums:
 
A BMS is not needed. But some common sense is. If your system is running without a human present to check on it at least twice a day and has the capability of recharging the cells in cold weather then you've set yourself up for failure.
If you have cells that could be exposed to freezing temps you need to either get a BMS that has low temp protection/built in heating, install your own heating system with external temperature controllers or have a way to disable the charger/prevent the cells from receiving charging current if the temperature drops below a certain point.
I'd like to know how his cells dropped below 2 volts. What inverter was he using? Did he have it correctly programmed with a low voltage setting? I'm going to bet the answer to that is no.
More importantly, what drained his cells beyond the low voltage threshold of the inverter? What was drawing a load after the voltage in the pack was too low to power any kind of AC load? I'm assuming his inverter had completely powered off and there was no idle current draw from it...so??
From what i have been told, he was using a Growatt 3k unit. He said all the settings were set properly. At first he blamed the inverter as faulty. I took it home and tested it and it worked fine. I checked his settings and it was configured correctly.

My coworker went on to say that there was 2 ft of snow on the panels all winter and a battery blanket was always drawing power to keep the batteries warm. So i figure he drained the batteries completely, they froze at some point and were probably overcharged at some point.
The pack voltage (16 cells) registered 5.67V total.

We did some testing here at work and 13/16 cells are done. wont hold any charge. The remaining 3 cells will take about 1V each, but after 24 hrs the voltage sits at 0.45v.

So he has a new set up batteries now and decided after 3 weeks of the system running on its own (unattented) to finally put the bms in. Bms was installed June 4 and cells 1-12 were 3.3-3.4V, Cell 13, 15 were at 2.2V, Cell 14 and 16 were at 4.51V and 4.25V and bloated.

So he has decided to use AGM batteries because he states they are more reliable then Lithium lol.
 
From what i have been told, he was using a Growatt 3k unit. He said all the settings were set properly. At first he blamed the inverter as faulty. I took it home and tested it and it worked fine. I checked his settings and it was configured correctly.

My coworker went on to say that there was 2 ft of snow on the panels all winter and a battery blanket was always drawing power to keep the batteries warm. So i figure he drained the batteries completely, they froze at some point and were probably overcharged at some point.
The pack voltage (16 cells) registered 5.67V total.

We did some testing here at work and 13/16 cells are done. wont hold any charge. The remaining 3 cells will take about 1V each, but after 24 hrs the voltage sits at 0.45v.

So he has a new set up batteries now and decided after 3 weeks of the system running on its own (unattented) to finally put the bms in. Bms was installed June 4 and cells 1-12 were 3.3-3.4V, Cell 13, 15 were at 2.2V, Cell 14 and 16 were at 4.51V and 4.25V and bloated.

So he has decided to use AGM batteries because he states they are more reliable then Lithium lol.

did you test the 13 for capacity, internal resistance and self-discharge? if the answer is "no" to any, you don't know they're good.
 
In my very 1st few months (6 yrs ago) I had a 7s50p NMC battery on my 1st solar system (200w PV, EPEVER, Reliable Inverter).
I thought - I'll just measure the pack voltages each morning..... don't need a BMS.... but I learned :)

After a few weeks, the morning measurement showed significant different pack voltages at rest. So I started measuring A LOT more and found that some packs were dropping to 2.5v (or lower - who knows?) and then bouncing up to 3.3v after the inverter turned off. Had no clue with 1 measurement (or even several) in 24hrs... over the charge/discharge sequence.

YIKES - a BMS is absolutely needed as one needs to monitor voltages of the lithium-ion cells/packs in series all the time.. when part of an automated charge/discharge application such as a solar system which cannot be done by a human + action (alerts as a minimum, battery current cut-off is good) is valuable based on the measurements if they get out of line - e.g. 2.5v for NMC is not good!.
 
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We don't use a bms on radio control lipo packs, any number of cells. However we DO use a balance charger - specific to the chemistry type. That's what the individual cell leads are for. Although packs are pretty durable these days and many can stand very high discharge and charge rates, no more than 1C charging is still recommended. The chargers are configurable. LIPO WILL go up in flames - and the fire WILL NOT go out until they completely burn. Plenty of kids have burned down their family's garage. Charging in a fireproof bag is still recommended. I've been messing with them for a half-dozen years without a problem, but I follow the protocols.

For lifepo4 cells a bms is required to manage and protect the individual cells. This forum turned me onto Overkill Solar, which are JBD bms's. Maybe get one of those before something bad happens again. There are plenty of fire threads, even on here. One was a bus, spectacular.
 
We don't use a bms on radio control lipo packs, any number of cells.
Agreed, there is a category of battery where you can control the charge via balanced chargers.... and RC is often single use discharge followed by a balanced charge - done manually.

I use this approach for my 12,000lb winch battery made with headways.... needs to support up to 600a? I balance charged it and have a BattGo 8s to monitor manually if desired but if I use it, it will be single use (10mins?) to get unstuck and then balanced charged manually later on. I'll likely never use it but take it boondocking sometimes just in case.

Powerwall / daily automated charge/discharge situations or EVs or things like this are fundamentally different and really need a BMS.
 
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did you test the 13 for capacity, internal resistance and self-discharge? if the answer is "no" to any, you don't know they're good.
Internal resistance is basically equivalent to a short. Everytime i try to charge the cells, the voltage hovers around .74V and as soon as you remove the power source it drops to zero. Im pretty sure these cells are garbage.
 
Internal resistance is basically equivalent to a short. Everytime i try to charge the cells, the voltage hovers around .74V and as soon as you remove the power source it drops to zero. Im pretty sure these cells are garbage.

Cells that won't hold voltage are clearly bad. I'm talking about the "good" cells that do.
 
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