Need battery voltage disconnect device : cut off at 50.5- 51.5v and reconnect at 52 - 52.5v

[new off grid]

Hello everyone,
system with 2 battery ,one 10khw with 15 cells and the other 5kwh with 16 cells , 48v
both sharing the same set up, in parallel. Problem is, the bigger one has lower voltage
compare to the the smaller one, due to 1 cell less
I be able to charge them up ok, but discharging cause some trouble
when the 10kwh still have 90% SOC , the 5kwh down to 0% and its BMS shut it down
I wanted the smaller one to keep at least 10 - 15% SOC before it turn off, by installing a
voltage disconnect device or any thing like that, so it can cut off at around 51v,
reconnect around 52v and be able handle 80-100 Amp
searching everywhere but no luck
Any one here having similar issues like mine ? and what's your solution for it ?
any device out there you can recommend ?
thanks a lot for your input
good day
Dien

 

[HighTechLab]

You should not be connecting a 15-cell battery with a 16-cell battery. Pick a voltage, in my opinion 16 cells / 51.2v, and stick with that - otherwise you have a fire waiting to happen when your 15S BMS fails due to repeated overvoltage.

 

[new off grid]

You should not be connecting a 15-cell battery with a 16-cell battery. Pick a voltage, in my opinion 16 cells / 51.2v, and stick with that - otherwise you have a fire waiting to happen when your 15S BMS fails due to repeated overvoltage.

copy that
not a good idea to mix them up
been running 4 years on the 10kwh/15 cells
but I got a super nice deal on the 16 cells
and decided to add it up, then fix the issue later
but it seem harder than I thought

 

[HighTechLab]

copy that
not a good idea to mix them up
been running 4 years on the 10kwh/15 cells
but I got a super nice deal on the 16 cells
and decided to add it up, then fix the issue later
but it seem harder than I thought

Can you take a cell out and reprogram the BMS? What is the make/model?

 

[new off grid]

Can you take a cell out and reprogram the BMS? What is the make/model?

wow, that job feel like an Engineer levels, can't imagine to try lol...,
it's the Sungoldpower

48V 100AH SERVER RACK LIFEPO4 LITHIUM BATTERY SG48100P​

thanks

 

[new off grid]

You should not be connecting a 15-cell battery with a 16-cell battery. Pick a voltage, in my opinion 16 cells / 51.2v, and stick with that - otherwise you have a fire waiting to happen when your 15S BMS fails due to repeated overvoltage.

15s vs 16s = 6.67% difference
when I set the max charge voltage of the 16s at 3.5v or 56v per pack
that 56v goes into the 15s, it become 56 : 15 = 3.73v
3.73v is a bit high than I would like, but as far as we know
the max voltage for Lifepo4 batt to reach the dangerous level is 4.2
at 3.73, it might shorten its life a bit, but no where near the exploding point yet
even the 15s running without any BMS, continuing taking 3.73v will not catch any fire right ?
what do you say friend ?

 

[再生可能 エネルギー]

>> Can you take a cell out and reprogram the BMS?
wow, that job feel like an Engineer levels, can't imagine to try lol...,

I was wondering if using a separate DC/AC inverter for each battery could be possible?
The issue I was thinking is that the two inverters should be in phase, but I think that something that can be set.

 

[HighTechLab]

15s vs 16s = 6.67% difference
when I set the max charge voltage of the 16s at 3.5v or 56v per pack
that 56v goes into the 15s, it become 56 : 15 = 3.73v
3.73v is a bit high than I would like, but as far as we know
the max voltage for Lifepo4 batt to reach the dangerous level is 4.2
at 3.73, it might shorten its life a bit, but no where near the exploding point yet
even the 15s running without any BMS, continuing taking 3.73v will not catch any fire right ?
what do you say friend ?

First, over 3.65v/cell is bad and will start wearing out the battery quicker. Sure, something like 3.70 may not do a huge amount of damage if it's only brief, but the thing is the 16S pack will keep the cells in the 15S pack pushed to this voltage - not like a normal situation where the charger stops once too high of a voltage is reached.

Second, you are assuming perfect cell balancing...but in the real world we don't have perfect balance. One (or a few cells) will be the runners that hit the catastrophic failure point before others.

@Steve_S are you out there?

 

[new off grid]

First, over 3.65v/cell is bad and will start wearing out the battery quicker. Sure, something like 3.70 may not do a huge amount of damage if it's only brief, but the thing is the 16S pack will keep the cells in the 15S pack pushed to this voltage - not like a normal situation where the charger stops once too high of a voltage is reached.

Second, you are assuming perfect cell balancing...but in the real world we don't have perfect balance. One (or a few cells) will be the runners that hit the catastrophic failure point before others.

@Steve_S are you out there?

thank you so much for for input sir
I will strongly reconsider my next move
1st step is down the charging voltage to 3.65 right away
thanks again friend

 

[new off grid]

I was wondering if using a separate DC/AC inverter for each battery could be possible?
The issue I was thinking is that the two inverters should be in phase, but I think that something that can be set.

not an ideal to add another Inverter friend
economically is no good
better sacrifice the battery life

 

[yabert]

Don't mess with disconnect device and simply use both batteries in their voltage range.
Charge at 54V (3.6V/cells for 15S and 3.38V for 16S), stop the discharge at 45V and you will use almost the full capacity of each battery.

​

 

[new off grid]

Don't mess with disconnect device and simply use both batteries in their voltage range.
Charge at 54V (3.6V/cells for 15S and 3.38V for 16S), stop the discharge at 45V and you will use almost the full capacity of each battery.

​

add another charge controller into the system ?
I think I can do that and not so expensive
but letting the 2 different batt connecting to each other to the same inverter is the question
how to make it happen friend ?
thx

 

[Vigo]

Charge at 54V (3.6V/cells for 15S and 3.38V for 16S), stop the discharge at 45V and you will use almost the full capacity of each battery.

I personally agree with this. If you can stand to give up some amount of your battery capacity, you can simply leave them in parallel with 'safe' settings. One battery will get cycled harder than the other and.. in all likelihood it wont matter unless you're actually planning to successfully 'cycle out' a lifepo4.

Even then, 3.38v/cell is enough to fully charge a lifepo4, it's just that the charge rate will be VERY slow for much of the SOC curve. Depending on how you use the system this may be completely fine!

 

[new off grid]

I personally agree with this. If you can stand to give up some amount of your battery capacity, you can simply leave them in parallel with 'safe' settings. One battery will get cycled harder than the other and.. in all likelihood it wont matter unless you're actually planning to successfully 'cycle out' a lifepo4.

Even then, 3.38v/cell is enough to fully charge a lifepo4, it's just that the charge rate will be VERY slow for much of the SOC curve. Depending on how you use the system this may be completely fine!

agree and easy to do, for cheap
but how to feed 2 diff batt into the same inverter friend ?
best solution ?
thx guys

 

[Vigo]

I guess you are asking about how to physically wire them into a parallel setup?

 

[new off grid]

I guess you are asking about how to physically wire them into a parallel setup?

correct friend
how to wire them 2 diff batt in parallel into the same inverter ?
thx

 

[Vigo]

Generally you are advised to keep wire lengths and sizes close to identical but your setup is inherently imbalanced at which point it doesn't matter much! Simply make sure both positives from battery are hooked to positive of inverter, and both negatives of battery hooked to negative of inverter. It's preferable to run both sets of positives and negatives to a set of bus bars and then run a single positive and single negative to the inverter, but that kind of depends what the inverter's battery terminals look like. Some would be near impossible to put two wires onto, some make it easy. For example I have a 5kw inverter in my RV that makes it very easy to hook at least two wires to each terminal, but the 5kw inverters in my house would make it almost impossible.