[SOLVED] recharging batteries in series

[Zak28]

I have some rechargable NiMH 800mAH AAA batteries nominal voltage is 1.4v which is what the supplied charger charges them at and 250mA per battery as specified on the datasheet. Is it safe to charge them @ 4.2v 750mA in series?

 

[FvM]

Why 750 mA? Current should be the same in series circuit. Another question if it's acceptable to fast charge the batteries at about 1C.

Series charging without balancing means is frequently applied to NiMH battery blocks, but must be never used for LiIon cells. It's only acceptable for batteries made of same cell type and equal state of charge. Overcharging and respective reduction of lifetime may happen though due to differences in cell properties.

 

[Zak28]

Why 750 mA? Current should be the same in series circuit...

Because the total equivalent current for 3 batteries is 750mA thus the buck converter Im going to use would have an appropriate current limiting resistor on its output.

...if it's acceptable to fast charge the batteries at about 1C...

Im sure its fine at least for parallel arrangment since its whats specified on the datasheet for the charger.

 

[BradtheRad]

With batteries present it's difficult to predict their effect on the charging voltage and charger's internal resistance. The 4.2V charger may not push as much as 250 mA through the cells. In that case you are safe.

However 750 mA is about C/1 charge rate. It will make them hot, which can shorten their useful life.

I have a charger which delivers 100 mA. Little AAA cells heat up in it. I ought to be more careful (but I'm not) as to the length of time I leave them charging. Nimh type cannot tolerate abuse as well as Nicad.

 

[KlausST]

Hi,

Because the total equivalent current for 3 batteries is 750mA

This simply is wrong for cells connected in series.

Klaus

 

[Zak28]

Hi,


This simply is wrong for cells connected in series.

Klaus

I absolutely overlooked the simple principle, I will keep it far below 250mA for the series arrangement.