BMS rating - beginner question help please

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technolaser

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Dec 20, 2021
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Hi all,

I'm trying to figure out what BMS to use with a 4s 100Ah LiFePo4 pack.

1. If I will only ever draw a maximum of 50A, is a 100Ah BMS ok?
2. For my cells, if the max charging current is 0.1C, how do I know that the BMS I choose will limit the charging current to 10A if the charger is rated at 20A?

Thanks for all input.
 

Korishan

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maximum of 50A, is a 100Ah
Don't get your "A" and "Ah" mixed. They are not the same thing. "A" is potential, "Ah" is capacity.

It is generally always a good idea have a bms that can handle far more amps than what the general purpose of the battery is for. So as you state, you plan on charge of 50A max, and the bms is rated for 100A, this is plenty fine enough. The limit of the bms is there to show what it is capable of handling.
how do I know that the BMS I choose will limit the charging
BMS's are not designed to "limit" charge current. That is not their job. A "Charger" limits charge current.

The values of a BMS rating, say 100A, means that the BMS will be able to successfully disconnect a load/charger if the current flowing through the BMS is greater than 100A. Just like your breaker in your power box for your house doesn't control your electrical outlets and lights, unless there's a fault.
 

technolaser

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Dec 20, 2021
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Thanks Korishan.

So given my 100Ah cells that I want to limit the charge to 10A, I need a current limiting charger before the BMS or simply a charger that can't supply more than 10A?
 

OffGridInTheCity

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Thanks Korishan.

So given my 100Ah cells that I want to limit the charge to 10A, I need a current limiting charger before the BMS or simply a charger that can't supply more than 10A?
Yes... You don't say the voltage but assuming its a 4s (12v nominal) battery, then as an example this one shows 12v @ 15a which is higher than you're goal.
1640013307068.png


Just out of curiosity, why are you interested in a 10a limit?
 

technolaser

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Dec 20, 2021
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The 10A I gave was just as an example.

I want to replace 2 lead acid batteries in a boat - one is used to provide power to lights, instruments, etc and seems to be very simple to do. The other battery is the starter for the small diesel engine, currently a 40Ah battery.

Both are charged from an alternator/regulator on the engine. This charging is what's bothering me as the alternator is rated at 40A. I'd rather limit the charging to between 10 and 20a.
 

Korishan

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Thanks OffGrid for covering the previous information.

Both are charged from an alternator/regulator on the engine
Will the LiFePO4 batteries be connected directly alternator, or is there a charger-controller in between?
If they are connected directly, you really should look at getting an alternator that can handle Lithium based batteries. Otherwise a partially depleted battery will pull 100% current from the alternator until they are charged. This is extremely hard on alternators. At least vehicular alternators, not entirely sure on marine alternators.
If there's a charger-controller in the middle, then that'd be fine, no extra changes needed as long as the controller is lithium charging capable.

Lithium batteries should be charged based on a CC/CV (Constant Current/Constant Voltage) curve profile. This insures for a long life on the cells.
 

technolaser

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Dec 20, 2021
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Thanks to all for the input - much appreciated.

Can someone recommend the type of charge controller I could use just to limit charge current? I'll also be using the BMS for proper CC/CV limits for both charge and discharge.
 

technolaser

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Dec 20, 2021
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I have a charge controller which monitors my lead acid battery. When full, it closes the charge control relay - all other times it's open.

If I used a charge controller like this one on aliexpress, would my attached circuit work?

Thanks guys.
 

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