question

mikegleasonjr avatar image
mikegleasonjr asked

BlueSolar PWM Pro Lithium Li-ion Charge Parameters

Hi, I am planning to charge a 7S?P battery of 18650 cells with a charge controller normally intended for led acid battery.


I created a spreadsheet with my parameters, I would like a second opinion. I have a hard time deciding on the float/absorption voltages. I live in Morocco, plenty of sun and dead laptop batteries.


In the spreadsheet we can see the defaults for 12 and 24 volts and the values I decided for the battery (column in grey):



EDIT: adding new values:



Thanks!

Lithium BatteryBlueSolar PWM
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1 Answer
boekel avatar image
boekel answered ·

I'd set float voltage close to or the same as absorption (4,1V?)

new charge cycle: not important if absorption is high enough

I also think 4,15V is a bit high (4,1 would be my choice)

What (kind of) BMS are you using? does it communicate with the charger?

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mikegleasonjr avatar image mikegleasonjr commented ·

Thanks @Boekel,


I am relying on the solar charge controller for the over-charge and over-discharge protection. As for the cell balancing, I am using an active balancer which kicks in whenever the voltage difference exceeds 100mv and stops when it is 30mv or less. It does it on any stage of charge.


So I don't have a BMS.


The pack is not done yet, so I will do my tests with 7S1P for now, then add up cells as the system is stable.


I will re-upload another screenshot after adjustments tomorrow!


EDIT: added new parameters in original question...

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boekel avatar image boekel ♦ mikegleasonjr commented ·

Please don't use without a BMS that cuts of the pack / charger / load, it's not if, but when you will have a fire, the charge controller is -not- capable of preventing overcharge / discharge events.

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mikegleasonjr avatar image mikegleasonjr boekel ♦ commented ·

Got it thanks, any recommendations?

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boekel avatar image boekel ♦ mikegleasonjr commented ·

Ahhh hard question.


Problem is the cheap BMS options almost all disconnect at 4,25V and 'balance' at a fixed voltage (4,2V), there are some configurable options, but I personally don't have much trust in them.

Also balancing current is too small for used 18650 cells.

So that leaves you the expensive but trustworthy options like REC, Emus, etc.

In general I can only advice to use good cells (from EV's, you can often use the factory bms modules also) for systems that are online 24/7.
18650's can work if they're all the same chemistry, age, SOH, etc. but too much work to make a safe battery out of them.

For experimenting you can use just about anything...but don't leave connected without supervision...lot's of people have burned their workshop / shed / house...don't be one of them!!

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mikegleasonjr avatar image mikegleasonjr boekel ♦ commented ·

Thanks, I think I will experiment first, then if more serious and want to leave unattended, I will add a proper BMS. The Emus BMS Mini looks great although pricey.


In any case I think I will do a 2-stage charge which means, use a standard controller to charge a lead-acid battery, then the load connected to the charge controller would be a charger for the 18650 which would have a BMS. I would loose a bit on the DC-DC conversions.


I wonder why you do not trust the over-voltage, under-voltage and charge voltage/current of the charge controller, assuming I have a proper balancing setup in place?


Thanks again!

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boekel avatar image boekel ♦ mikegleasonjr commented ·

Because the charge controller doesn't know individual cell voltages. if one is above max it will be damaged, and can catch fire.
a BMS detects these events.
'a proper balancing setup' cannot prevent this (a very high current balancer does about 1A of balance current, on a total charge current 10A - 100A this cannot prevent overcharging.

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mikegleasonjr avatar image mikegleasonjr boekel ♦ commented ·

'a proper balancing setup' cannot prevent this (a very high current balancer does about 1A of balance current, on a total charge current 10A - 100A this cannot prevent overcharging.

Ok now I get it thanks !

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