Zach said:
>This is not exactly a pic question, but it is about the battery pack I am
>using to power a pic.
>
>I recently bought four new C size ni-cad batteries (1.2V 2200Ah).  I wired
>them up in series (making 4.8V). I read on the side of the battery "Charge
>for 15 hours at 220mA."  Now here is my question- what is an easy way to
>keep the current at 220mA.  Sure, you could put a 21ohm resistor in series
>with the pack, that is if the battery pack had no internal resistance.  But
>the amount of current drawn by the pack changes depending on how charged it
>is.  Am I worrying to much about details?
>
>Well, here is why I ask.  I charged this pack up over 15 hours, and then
>connected my pic.  The pic ran for about 5 minutes and then stopped.
>Voltage on the batteries was 4.1V and falling.   I measured the current
>drawn by the pic circuit.  It was only pulling about 60mA.  Why didn't the
>batteries last MUCH longer?!?
>
>I am either not charging the pack correctly or I have a bad cell.  If I
>have a bad cell, is there any easy way to test this? They were supposed to
>be new batteries, they were expensive ($7 bucks each) .
>

Unless all four cells are at the same state of charge initially, charging
them in series can lead to problems.

Measure the voltage of each cell under load. If there are dramatic
differences, (20%) you should charge them individually before using them in
series. Simplest way is to break the series, use a 12VDC supply and
independent dropping resistors of appropriate wattage for each cell.

Reg Neale