Quickie with rechargable batteries

[Natural Sound]

The company I work for uses an 8.4v 1.2aH battery backup in some of its equipment. These are changed every year but still have plenty of life in them when they go to the recycle bin. Basically they are 7 C-Cell NiMH cells in series. I've been playing around with these to run some DC electronics around the house and in the back yard. I charge these packs at a C/10 rate or 120mA for about 15 hours. This seems to work out pretty well.

I'd like to use these battery packs for my quickie B+ supply. My question is this. would it be better to use four of these for a B+ of 33.6v or five of them for a B+ of 42v?    

[Paul Joppa]

Standard alkaline cells start out a bit over 1.5v, and are considered dead by the time they drop to 1.0 volts. So a "9-volt" battery will run 6 to 9 volts over its working lifetime. NIMH and NiCd cells have a pretty flat discharge voltage of 1.2 volts, so even a 7.2v battery would be enough.

That said, you will get more headroom with more voltage. The circuit will behave reasonably well with up to 67 volts, which is the maximum rated screen voltage for the 3S4.

For what it's worth, I originally chose to use four batteries because they often come in packs of eight at good prices - you can get two complete sets that way. A second advantage is that the battery lifetimes are matched, about 200 hours for the D-cells and for the 9-v batteries.

[Natural Sound]

Excellent, thanks Paul! I'll run it at 42 volts then.