We've been having issues and I need some advice. Started a couple of months ago, SOC sits at 50%+ and then drops to 10%, no heavy loads at the time the drop occurs. Drop happens within minutes. After the third incident the installer came and replaced the battery (same make and model). Been running fine until last week, same issue. SOC dropped from 64% to 10%.

So installer had the old battery tested and the battery checks out. New battery we've only had one instance but I'm expecting more....

Battery is connected to a Deye 5kw inverter. Made sure that the inverter firmware is on the latest version. These incidents happen randomly and every time the SOC was above 50%. No other issues with the battery. Charges / discharges fine under normal circumstances.

Any ideas? Installer is checking with suppliers if there is an updated BMS available.

LiFePo4 has a very flat charge/discharge curve, if the battery has not absorbed fully then that 50% or 60% state of charge will not be a good indication of how much energy is actually in the battery and would then explain the sudden drop-off. Your battery suppliers should know this stuff and advise you accordingly

@Langeraat advised that there were a batch of shoto batteries with faulty BMS that would be replaced under warranty. Perhaps he can provide further input, maybe these are from that batch. 

20 minutes ago, Hansa said:

@Langeraat advised that there were a batch of shoto batteries with faulty BMS that would be replaced under warranty. Perhaps he can provide further input, maybe these are from that batch. 

From a reliable source it seems it's only some diodes that need replacing.

7 hours ago, Scorp007 said:

From a reliable source it seems it's only some diodes that need replacing.

Is it a Seplos BMS in them?

4 hours ago, Tinbum said:

Is it a Seplos BMS in them?

I wouldn't know. Those that had their batteries repaired might be able to answer.

16 hours ago, WannabeSolarSparky said:

LiFePo4 has a very flat charge/discharge curve, if the battery has not absorbed fully then that 50% or 60% state of charge will not be a good indication of how much energy is actually in the battery and would then explain the sudden drop-off. Your battery suppliers should know this stuff and advise you accordingly

Please explain? So if my battery is discharging (as it should during loadshedding) and shows 64% after 90min of use and then suddenly drops to 10% that is "ok"?

Note that the drop occurred during as well as outside loadshedding hours.... 

29 minutes ago, RyC said:

So if my battery is discharging (as it should during loadshedding) and shows 64% after 90min of use and then suddenly drops to 10% that is "ok"?

That will depend how how well the battery has been absorbed with energy/amps.

% and voltage is not really a very good indication of available energy/amps available with LiFePo4 batteries.
You need a proper coulomb counter/meter to get the max value out of your batteries.
Relying on the inverter and or bms is not a reliable way to understand what your batteries are doing.

Here is just one video explaining some of the charge/discharge characteristics of LiFePo4

LiFePo4 Explained - Sort Of :)

16 hours ago, Scorp007 said:

From a reliable source it seems it's only some diodes that need replacing.

This is how I have it as well...

3 hours ago, RyC said:

Please explain? So if my battery is discharging (as it should during loadshedding) and shows 64% after 90min of use and then suddenly drops to 10% that is "ok"?

Note that the drop occurred during as well as outside loadshedding hours.... 

That doesnt sound like correct performance to me. My shoto runs from  70% SOC at midnight to around 35% SOC at 7pm. Pretty much in line with the consumption during the night. 

Mine is also hooked up to a deye. Are you using the CAN cable or voltages as a comms with the inverter?

Perhaps refer to this topic about a bad batch as was indicated earlier.

 

11 hours ago, Scorp007 said:

I wouldn't know. Those that had their batteries repaired might be able to answer.

Looking at the photos of the front of the batteries the layout is the same as the Seplos BMS. That BMS had a problem with a diode (D4) being sub standard in a batch manufactured before August 2021. Some boards manufactured before that date are still being sold, I bought a few earlier on this year but you can see by looking carefully at the PCB that the diode has been replaced.

7 hours ago, GTP said:

That doesnt sound like correct performance to me. My shoto runs from  70% SOC at midnight to around 35% SOC at 7pm. Pretty much in line with the consumption during the night. 

Mine is also hooked up to a deye. Are you using the CAN cable or voltages as a comms with the inverter?

CAN cable

i bought a second hand shoto a month or two ago , can't recall the date

i bought it cause i could not get cells to expand my diy 8s to 15s/16s

 

it worked fine but would drop much quicker on the last two lights ie it would drop from 4 lihghts ie full to 2 lights approx 1.5 hrs

and then the last 2 light would barely make the last 30 min during load shedding

 

now i dont know if this bms has coulomb counter ie lights may be inacurate 

 

when i got cells to expand the diy , and struggled to ballance my arse off without a proper bench power supply

and had a battery to use, i disconnected the shoto and opened it up to check its balance status

 

i found one cell was lower volatege than the others ie lower SOC

 

charged this cell up to get it balanced and low and behold now i get more out of the battery

by a quick thumb suck i was missing about 1-1.5kw of capacity 

have not run it all the way down to see how close i am to the rated capacity yet

 

i was also getting alarm light around 47.5 volts (due to cell low voltage ) this is supposed to happen much lower as these are suppose to cut of closer to 40v

 

if the capcity drops quick its most often due to the rapid drop in voltage as some cells on the bottom end of the curve

the balance current of these bms's aren't capable to deal with truly unbalenced cell, but only to prevent/correct small differences/drift as they cycle due to resistance differences 

 

 

@Leondavibe Can you please explain more about how you charged it, did you disconnect the cell or sense leads from the BMS or leave it connected/on while applying a charge to the cell? I have a similar issue with my DIY battery bank and a low cell, and think I could get more Ah if I was able to get the bank better balanced.

 

there is a tedious long process or short process 

 

long process, what i did initially i left balance leads connected ie the battery was still connected to my inverter when i did it

 

got a few 12v 50w halogen downlights ,with crocodile clamps

some car headlight 50/55w bulbs will work the same way

 

i tried to pull down the voltage on the cells that was high ie they would cause the bms to switch of charging to prevent over charge

but since my battery is 15s 48v it was taking ages to lift the overall charge voltage, 

 

so i connected one 12v bulb over 1 cell the highest voltage ones pull them down one by one to the same voltage as the lowest voltage cell

but this had to be done repeatedly since on the next charge they would lift quicker than the empty cell

 

i  connected the bulb on the ones that started to run away from the others the moment i picked up on it while it was charging 

i found that 1-2 bulbs would keep them in check while the lower volatge ones start lifting or catching up

 

i had 4 bulbs but with the 15s the musical chairs was just getting to much, ie if yours is 4s 12v it might be manageable

 

then i tried another approach

i used my car battery charger and connected it over 3 cells ie the low one and two next to it with the bulbs i ran down the 2 with hiogher voltage until it matched the low one , then preoceded to charge them again , repeated again as soon as the 2 was running away again

 

i did all of this charging on the cells bypassing all bms protections, so its not something you can connect and leave was constantly monitoring the cells voltages 

since i did not want to lose any cells, and the moment they jumped to 3.6 i would stop charging and use the bulb on the runaway voltage ones

 

in the end this method saved lots of time

 

my car charger was quite old so it popped i diode in the process, as i was putting quite a bit of load on it over a short period of time and when i opned it i spotted that the overload button was bypassed, hence why it popped the diode

 

 

I've seen the bulb thing on youtube or online that you are describing in the first approach. I've got an 8s pack, so was going to try the same thing, just using a 20ohm, 50W load resistor. I'm mostly trying to raise the voltage of one cell in the middle of the pack, so might be able to put a resistor across all the cells from it to the -ve and from the other side of this cell to the +ve. But, yeah, if I'm hoping to get all cells exactly even it may take a long time with moving resistors around.

Did you do the second/quicker approach after the BMS considered the pack fully charged and with the BMS and inverter etc still connected and on? I get how it was quicker, but I would be very nervous about attaching my car charger to the 3 cells in case I accidentally connected something wrong! Would certainly have to also keep a close eye on it like you said.

yea i was ultra carefull with the bulbs initially since this was all connected to the inverter

you can do this while the sense wires are connected

 

i even connected the car charger while the inverter was charging it just means that those low cells charge quicker

 

if the capcity isn't an issue for you , you dont have to go that route.

 

i was only rushing cause i needed the capcity

 

dropping the SOC via the resistor of the cells on either side will

 

my situation required a bit more oomph as i expanded a 8s to a 15s and the new cells was at a way different state of charge

 

what is the voltage of your lowest cell when the inverter stops charging on total voltage

no i went the second scenario

cause after a whole day of doing the first approach it seemed like i aint seeing the tunnel

never mind the light at the end of it

So after reading up more and I made a decision to fully charge to 100% once per week. What I see happening does not make sense. Battery would be at 85% and SOC will get to 98% fairly quickly (90 min). After this the battery takes for ages to get to 100%. For instance today SOC has been at 98% for 2 hours. My app shows that I've produced 2kWh and most of it went to charging. The numbers do not add up.......

All I can get from this is that the % reading on the battery is not accurate? This would explain why I get drops from 60% to 10% in a short time. Could be due to imbalance in cells?

My plan going forward is to charge to 100% and keep it there to balance the cells once a week. Hope this solves my issues going forward as it should help with overall battery health.

If you would like to see exactly what your battery is doing when all this is happening then you can use the seplos software via the RS485A port which is very helpful, details are in this post: 

 

From reading elsewhere it seems the seplos can become uncalibrated when the batteries are not charged properly. Seplos suggests discharging the battery until bms cutoff and then charging it up till bms cutoff again, all without the comms cable and that should recalibrate things. Charge up slowly (10-20A) or a cell imbalance might cause an overvoltage alarm to stop charging before the battery is 100%.

 

An update on this. Since I started charging the battery to 100% at least once per week I've had no issues. Works like a bomb

2 hours ago, RyC said:

An update on this. Since I started charging the battery to 100% at least once per week I've had no issues. Works like a bomb

That's the way to do it to keep the cells balanced and get your full capacity from your battery. 

Seplos BMS only balances when cells are full