Charging current for home made battery pack

[svdsniper]

Hi

I have just made up a battery pack consisting of 24 AA rechargable batteries configured as below:-

i have made up 8 x three batteries wired in series to give me 4.5v
I have then wired the 8 x 4.5v sets in parrallel to give me a much higher capacity (in this case 22.4Ah as AA cels are 2800mAh)
My question is this , what charging current should i use considering there are three cells in series ?
on one of my other 2700mAh cells it says charge for 7 hours at 540mA but do i stick to this or do i tripple it as there are three cells in series?

Thanks for your valuable advice

svdsniper

 

[Hero999]

What chemistry? NiMH?

Charging NiMH cells isn't easy because you have to watch for a sudden voltage drop which indicates it's fully charged and you need to stop it. To be honest, I won't know where to start with so many cells connected in series and parallel.

 

[svdsniper]

Hi

Yes they are NiMH
it is charging now at 0.5A and all is well , it is showing 4.01v and it has taken in 490Ma in 62 minutes so far. i would like to speed things up to the maximum safe current but i am not sure how high is safe so i am currently charging at just below the recomended charge current for my old 2700mAh cells but i am execting it to take ages to charge. as i have a programmable charger it is set to detect the delta peak (i think that is what its called and isnt that the volt drop point?)

svdsniper

 

[MikeMl]

Paralleling NiCad or NiMh cells will overcharge some and not charge others because the forward voltage under charge is not well matched, cell-to-cell. It is considered a no-no.

Go to this web site. Under "Technical Resourses", read "How to charge a NiMh battery".

 

[svdsniper]

Hi

Yes i had read that article on that web site but i have also seen somewhere on the net companies that make and sell battery acks like i have made for special applications. mine is made as i can not get a high Ah 4.5v battery to fit in my battery compartment.
I would have thought that i treat the 3 cells in series as a single battery and use a suitable charge rate for it and the capacity will just take longer till its filled.

svdsniper

 

[audioguru]

each Ni-MH cell caharges to about 1.5V max so three give 4.5V. But the voltage quickly drops to 1.2V per cell for most of the discharge so your battery is actually only 3.6V, not 4.5V.

 

[svdsniper]

Hi

I have just charged my battery pack and i kept upping the charge current until i hit 2.0 amps and everything was fine. I tested the battery pack in the infra red illuminator and image intensifier that it was built to power as this thing draws 3.5 amps from its original 4.5v wet cell battery. This homew made pack ran continuously for 3 hours before the IR illuminator dimmed to a point where it was very restricted in its ability to illuminate the viewing area and pick out a human. I have now set the battery pack charging at 1.5a and had it running for 10 hours and it is still not fully charged so i will continue and maybe up the current to 2.0a for the next full charge cycle once it has been discharged again as i read that the best charge rate for nimh cells is 1c so as these are 2800mah cells the that would be 2.8a am i right?

svdsniper

 

[audioguru]

A Ni-MH cell gets very hot when it is fully charged and is charging at 1C. It might burst its seal and vent out the good stuff inside it unless you have a circuit to detect the full charge then disconnect the charger.

Yes, your cell is rated at 2800mAh so 2.8A is 1C and it should be fully charged in a little longer than 1 hour.