Batteries in serie for powering microcontroller ...

[voidptr]

hi

It started has an experimentation ...
I used 4 NiMH batteries in serie to directly power a small PIC.

my little circuit run fine for 20 days(kindof) from around 5.2v when all full charged
to 3.x volts (this is fine too)....

my problem are :

only the top ( the one near the +) battery got completely drained, all 3 others got a bit discharged, but the top one got really drain ....
noob question why ? i always tought that all batteries will be drained equally ...

is there some circuits with no regulator (780x) etc . available ( the simpliest the better )...

 

[ChrisP58]

The volts vs time discharge curve for NiMH cells is very flat, only dropping near the end of the cycle. This is particularly true at low discharge rates.

As such, it only takes a few percentage points of difference in capacity between cells, for the behavior you describe to show up.

 

[MrAl]

Hi,

I agree with Chris.

Do this experiment:
Replace the top cell with a brand new fresh cell. See how long it takes for another cell to 'drain' as low as the top one did.

 

[Nigel Goodwin]

Yes, batteries used in this way must be identical, including the amount of charge in each one.

Try charging them all individually, to ensure they are fully charged and then try again - if still faulty, you have a duff battery.

If it is duff, then replace all four with new ones!.

 

[voidptr]

thanks , I guess i 'll have to do more experiments...
right now im using old batteries so in fact they might be used and old ...
I got a set of near new batteries too, I'll try with them and see ...
good idea too fine the next one to fail ,I might even try to see discharged current curve of left current inside them ...

 

[MrAl]

Hi,

I had similar questions long time ago so i started testing my batteries (the rechargeable types) by charging them up and they draining them down while measuring the current and voltage, then calculating the AHr capacities. I found a wide variation in capacities even of new cells.
I also was able to find that many of them had a high rate of self discharge. After charging and waiting as little as 24 hours before the test showed a huge loss of capacity in some cases.

 

[voidptr]

i started testing my batteries (the rechargeable types) by charging them up and they draining them down while measuring the current and voltage, then calculating the AHr capacities. I found a wide variation in capacities even of new cells.
I

i did that last year but more for caracterisation of type, NiMH NiCd Alkaline etc etc .... need to go and try li-ion now too )

 

[4pyros]

is there some circuits with no regulator (780x) etc . available ( the simpliest the better )...

Yes but most of them will wast power.