Battery charge / discharge profile

sijones2010

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May 31, 2021
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9
Hi all,

Am trying to make an intelligent battery / inverter management software, i.e, it does the comms between the BMS and the Inverter and manage it all better than no comms between, including multiple BMS's, dynamic charge and discharge current limiting, and also want to include the peak/off peak tariff calculation so if you have low battery you can charge up on cheaper overnight electric.

What I need to know is a few things on the Lifepo4 batteries:

1. how to tell when the batteries are getting full, I've read that at a certain amps of the C rate you need to switch to CV,
2. what amount of C rate should the amp-age be for topping off after bulk charge, i.e after CC to CV.
3. what limit of C rate loading at what percentage of battery life left, i.e is it at 25% you should only use 10% of C rating to not drop them off to quick
4. any load compensation to cell voltages, i.e at 0.5 C load allow 100mv drop?

Anything else you think is good info would be welcome.
 

smoothJoey

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Nov 30, 2019
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9,869
Any charge voltage over ~3.45 per cell will eventually fully charge a typical LFP battery.
 

smoothJoey

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Thank you for the info, am after a bit more detail on the algorithms.
The only thing a charger has to do for constant current is adjust the voltage down to maintain the target current.
For the constant voltage phase the battery is in control.
Is there something else you want to know?
Perhaps you want the charger to reduce charge current to allow the bms more balance time?
 
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sijones2010

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May 31, 2021
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9
The maximum charge rate of these is 1C? Would you say to keep constant current to take the voltage the battery is at and add 1v is right or would it required to be higher? Also after knowing how to know the load to voltage drop compensation and is there a point where you should limit your discharge current? I.e. if the batteries are 20% charged, should you not load them more than 0.2C, that kind of thing.
 

smoothJoey

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The maximum charge rate of these is 1C?
Depends on the cells.
Typically .5c is max charge rate, .2c to .33c is optimal.
For the big blue cells that are popular here .05c is the prescribed tail current which is actually dependant on the charge current.
Are you talking about over-potential?
Did you read the doco I linked?
Would you say to keep constant current to take the voltage the battery is at and add 1v is right or would it required to be higher?
No.
Also after knowing how to know the load to voltage drop compensation and is there a point where you should limit your discharge current?
Depends on the cells.
How would you limit the discharge current?
 
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