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LiPO battery cell voltage and temperature

treez

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Hi,
Supposing you have a 16Ah 12 cell LiPO battery.

Supposing we charge it up to 47V (3.91V/cell)

We will simply charge it with constant current of 13A up to 47V, then keep it at 47V (Our charger is current limited to 13A).
When it reaches 47V, a comparator simply disables the charger………is it true that the battery voltage will then sink to 44.4v (3.7V/cell)?.....if so, then our end-of-charge comparator will be tripping on and off repeatedly.

Also, supposing the charging is done at zero degrees C…and the battery is charged up to 47V….then the temperature rises to 40 degrees c….what will the voltage of the battery then become?

Sorry I tried to answer this on batteryuniversity etc but couldnt find the info.
 

betwixt

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That's what comparators are for. Turn the charging on when the voltage goes below 44.4V and turn it off when it reaches 47V (whatever it would be at 40C).

Brian.
 
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BradtheRad

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I designed my home-brew charge controller to work the same way, on and off. It's simpler than tapering charge current.
(My system was 24v, made from 6V golf cart batteries.)

I adjusted hysteresis at an op amp which controlled a mosfet. The batteries took a few hours charging to 29V, whereupon charging ceased. Battery voltage gradually dropped a couple volts, then charging resumed. Etc.
I installed led's to notify me what was going on.

As these up-down cycles grew shorter and shorter, it told me the batteries were becoming fully charged. I think 30 seconds was about the shortest cycle time. To my thinking it was just as good as taper charging.

My system was in a basement where temperature was pretty much constant. I did notice the batteries getting warm while charging.
 
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d123

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Hi,

Hi,
Supposing you have a 16Ah 12 cell LiPO battery.

Supposing we charge it up to 47V (3.91V/cell)

We will simply charge it with constant current of 13A up to 47V, then keep it at 47V (Our charger is current limited to 13A).
When it reaches 47V, a comparator simply disables the charger………is it true that the battery voltage will then sink to 44.4v (3.7V/cell)?.....if so, then our end-of-charge comparator will be tripping on and off repeatedly.
Is charge-balancing included in the plan? Voltage-based charge gauging is supposed to be unreliable, quick'n'cheapo, besides cell reaching Vnominal hours before it is actually charged. Sensing end-of-charge current is meant to be more accurate, you could AND the charge rate drop to e.g. 0.05C or better yet 0.01C in your case with another reliable signal, maybe the Vcharged signal to confirm 47Vreached + Iset fallen to 0.01*13A (130mA across a tiny shunt...). I don't know, I'm just starting to learn about it... A bidirectional coulomb counter + MCU would be the best choice.


Also, supposing the charging is done at zero degrees C…and the battery is charged up to 47V….then the temperature rises to 40 degrees c….what will the voltage of the battery then become?
No idea, to be honest, good question. Maybe a better question might be 'What charge rate is appropriate at 0C?'

Sorry can't be of more use.
 
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treez

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Thanks, we are buying a lipo with internal cell balancing included.
Its strangely impossible to find out how lipo battery voltage varies with temperature....eg a fully charged 12 cell lipo....
Sorry can't be of more use.
Thanks, not at all....delighted to hear your take on it.....i think both our unlearnedness about this subject reflects what an area of industrial secrecy batteries are.
 
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d123

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Hi,

:) I think less than secrecy, it's more a case of complicated chemistries. If you think about it, Li-ion is a 'young' technology. There is a lot of info - some more academic and some easier to absorb - available.

Not sure if these may help at all regarding voltage and temperature, and hard to find anything solely covering voltage at different temperatures, I think some of the content is relevant... I can't upload the pdfs as it would take several attempts and all afternoon here...

'the influence of temperature on the operation of batteries and other electrochemical energy storage systems' - pdf is same name with no spaces
'Open Circuit Voltage of a Lithium ion Battery Model Adjusted by Data-fitting' - IRESEC2018_Final.pdf
'A Study on the Open Circuit Voltage and State-of-Charge Characterization of High Capacity Lithium-ion Battery under Different Temperature' - energies-11-02408.pdf
'Thermal Analysis of a LiFePo4 Battery' - David_Balaguer.pdf
'Li-ion battery-charger solutions for JEITA compliance' - slyt365.pdf
 
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Audioguru

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The LiPO batteries for my electric radio controlled model airplanes work poorly when at 0 degrees C. Low maximum current causes the loaded voltage to drop and a low capacity.
The Battery University says to reduce charging current when charging near freezing temperature but they did not show a video that shows the spectacular reason (explosion?).
 
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