Chinese Xiaoxing BMS with Calb cells

dragonfly

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I finally got delivery of all my parts from china, partly due to me been busy, and partly due to delivery times :)

So I have 4 x 100ah CALB cells, which I've top balanced over the last couple of days. and a Bluetooth\USB enabled BMS. All of which are wired up and working quite nicely with my old 12v UPS.

So now time to start fine tuning. I'm using the android app and the default settings are


Overvoltage: 3.75v
Overvoltage Release: 3.6v
UnderVoltage: 2.7
UnderVoltage Release: 3.0
Discharge Overcurrent 30.0a
Cycle Capacity : 16ah
Nominal Capicity : 20ah

Open Balance - open
Balance Mode - charge balance

Balance turn-on 3.4v
Balance Precision 0.015v

Hardware Overcurrent protection 70a
Hardware short circuit protection 445a


Okay so straight away the nominal capacity is 100ah, but I'm thinking of charging 20-80 so does the Cycle Capacity need to be 60ah?

The BMS is a 20amp model so it makes sense to change the Discharge Overcurrent to 20amp??

Both the undervoltage settings are as high as they can go which is a little disappointing, however I can change the Overvoltage to 3.6v and 3.5v ?



While on the subject I vaguely remember someone logging data with a Pi Zero connected to either the Bluetooth or USB, can anyone point me to the project, ideally I want to log BMS information along with a sonoff POW before and after the UPS to see what sort of efficiency I'm getting.

The sonoff units can also then be used to control the load and charging to preserver the battery.
 

ajw22

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dragonfly

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Thanks for that, that looks like the stuff I was thinking of.

Next problem, looks like the ups is only charging at 13.8v to 14.0 at 0.3A so isn't giving the batteries much charge.
 

ajw22

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Isn't 13.8V~14.0V about 80% charge for LiFePO4, what you were aiming for?
Perhaps if you post which UPS you're using, someone might be able to suggest settings/modifications to set higher charge voltages.
 

dragonfly

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The UPS is Cit UPS-650B it's old, was cheap and I had it lying around, so it got used.

The BMS reported 68% charge, and never really shifted
I suspect the 0.3A didn't really help. I'm going to run them down to a something like 20% and try charging from there see if it makes a difference.
 

daromer

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14V is float voltage for most UPS systems. The BMS base their SOC on voltage.

LiFe is generally pretty full at 3.4-3.5V per cell. I bet they have a rather linear SOC vs Voltage curve and therefore dont think the cells are full untill they have reached 3.65V. LA chargers work a bit different with Bulk and float settings and most of the UPSs out there you cant change those settings.
 

dragonfly

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daromer said:
14V is float voltage for most UPS systems. The BMS base their SOC on voltage.

LiFe is generally pretty full at 3.4-3.5V per cell. I bet they have a rather linear SOC vs Voltage curve and therefore dont think the cells are full untill they have reached 3.65V. LA chargers work a bit different with Bulk and float settings and most of the UPSs out there you cant change those settings.

Sorry I'm a little confused by that, are you saying the batteries are pretty well charged but the BMS thinks they aren't, or that the UPS thinks they are charged, so goes to a 0.3amp trickle when infact they could do with a little more.
 

daromer

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The ups you picked was for Lead acid batteries and you did add Lithium batteries right?
Unless you hacked the code the UPS still think it have Lead acid batteries and will charge them like that. Therefore the voltage is like it is :)

The BMS is as i said a cheaper variant and most of them base their SOC on rather linear voltage curve vs SOC. They often trade their SOC value based on Voltage. So 14V might be 68% meanwhile 14.6 is 100% and 3.0 is 10%.. Or whatever...

So at 14V yo uhave 3.5V per cell and max on LiFe is generally 3.65 that is 14.6. So the BMS wont ever get to 100% unless the charger goes above 14V.

La Charger depending on UPS may top to 14.5 for a short while and then go down to idle at 13.5-14V for the rest of the time.



As me and other said before LiFePo4 is pretty well charged above 3.45V.

Note that im guessing a bit so dont take all my numbers above. I havent read up on that UPS as an example.
 

dragonfly

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daromer said:
The ups you picked was for Lead acid batteries and you did add Lithium batteries right?
Unless you hacked the code the UPS still think it have Lead acid batteries and will charge them like that. Therefore the voltage is like it is :)

The BMS is as i said a cheaper variant and most of them base their SOC on rather linear voltage curve vs SOC. They often trade their SOC value based on Voltage. So 14V might be 68% meanwhile 14.6 is 100% and 3.0 is 10%.. Or whatever...

So at 14V yo uhave 3.5V per cell and max on LiFe is generally 3.65 that is 14.6. So the BMS wont ever get to 100% unless the charger goes above 14V.

La Charger depending on UPS may top to 14.5 for a short while and then go down to idle at 13.5-14V for the rest of the time.



As me and other said before LiFePo4 is pretty well charged above 3.45V.

Note that im guessing a bit so dont take all my numbers above. I havent read up on that UPS as an example.
Thanks for the explanation. I'm currently running the battery down ready to start a recharge in the morning, it may well work in my favour and give me longer battery life. I don't think it is worth the effort of trying to trick\hack the UPS to behave more lifepo4 than lead acid. So if it doesn't charge properly my options are either us it as an inverter only, and find a better charger or even a charger\inverter.
 

daromer

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I would say if it doesn't go above 14V thats pretty fair and good top voltage :)

If you can monitor the voltage during charging do that. Some UPS systems do a bump where it goes to 14.5 or even 15+v before settling.

Testing is a good way of knowing how it behaves and if that suits you.
 

dragonfly

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Well it looks like it charges at 13.8v and refuses to supply 0.3A, disconnecting the mains, and putting a power supply in parallel at 14.6v seems to charge without interfering with the UPS. So making it an inverter and separate charger effectively
 

daromer

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But it charges up to 13.8 thats not bad at all. You get longer life out of the batteries if not :)
 

dragonfly

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daromer said:
But it charges up to 13.8 thats not bad at all. You get longer life out of the batteries if not :)

Yes but at 0.3A, soa full charge of 100ah would takeabout a fortnight. So, the batteries would last lots longer cos they'd never get used, or atleast not very often :(

Had a thought overnight, try a car battery charger that should supply 14.4v which would be ideal, and a readily available.
 

ajw22

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dragonfly said:
Had a thought overnight, try a car battery charger that should supply 14.4v which would be ideal, and a readily available.

Depending on the circuitry/logic inside the UPS, it might start burning energy to reduce the voltage to what it thinks is the proper level. Possibly overheating components inside.Monitor the current flow with a DC clamp meter to make sure nothing funny is going on.
 

dragonfly

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I guess the alternative is to find an inverter/charger that supplies the correct charge voltage. Only need about 200w of supply, at the mains, as it's a backup for a central heating pump.
 

daromer

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But 0.3A at 13.8V is since its fully charged... It will reduce the Current the closer it comes to full

What is the charge during lower voltage? And no, smaller UPS systems are not ment to have 100x bigger battery if thats what you are trying to archive. The charge time will then take many hours more. Most UPS systems have very very very small charge current :)
 

ajw22

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Oh, that reminds me... most cheap small UPS have virtually no active mechanism to cool itself, the thinking being that the battery will run out (15min?) before components inside get too hot. So it may overheat when you attach a much larger battery with much longer runtime.

I did a similar project years ago, and cut cooling vents into the sides with a 80mm fan blowing air through.
 

dragonfly

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Yes it was meant to be 7ah and is now 100ah, maybe I have reached the limit of this little experiment. Think I really need to find something better suited. BTW It appears to charge at a constant 0.3 amps
 

daromer

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I ran 150Ah in my 600VA ups when i started 25 years ago or whatever it was... Yeah im a tad old.

That took 1.5 day to charge atleast when i drained it. But i had power for a day with it.. It ran my old router, server and other gear :)

Now im running a 120kWh battery bank with full solar :D
 

Redpacket

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+1 keep the LiFePo4 cell voltage max 3.45 - 3.5V = full , 3.65V is overvoltage level not daily charge too voltage.
 
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