On 08/07/2012 20:48, Jake wrote:
I have an electronic GPS datalogging device that is designed to be powered
through a 9v battery. It runs the battery down in about 2 hours, and because
it goes through batteries so quickly, I have been looking for solutions to
make it last longer

Sounds like a crap product then unless it records the information by
hacking chunks out of a large tree trunk. Decent GPS logging devices
should last a few days of continuous use.

I have had success in replacing the 9v battery with a 6 AA battery holder
with attached 9v clip and this powers the device for significantly longer
with 2500 mAh rechargeables giving 7.2v.

How much longer?

I guess the small PP3 9v batteries are about 200mAh so you should get
10x longer off a set of AAs (~2Ah). Your best bet would be to use full
capacity rechargeable D cells (~11Ah) assuming weight is not an issue.

I have seen 8 AA battery clips available, that would give me 9.6v but I am
concerned about overvolting and damaging the device. Is the extra 0.6v
likely to cause damage?

Diodes in series with override switches for low battery might work but
it is hard to predict how tetchy the device will be about 9v.

It might extend the lifetime a bit but it really depends on how well the
device tolerates overvoltage and undervoltage. You need to know what
current the thing draws and at what voltage it stops working.

Freshly charged the terminal voltage is something like 1.4v per cell so
6x = 8.4 and 8x 11.2v - and I'd expect *that* might cause trouble.

Some moderately high current devices can be awful on rechargeable
batteries and only work properly on alkaline primary cells. The low
voltage protection circuit triggers early on NiMH - long before the
cells are exhausted. It is very annoying. Typically one weak cell in a
block of 4 or 6 scuppers it and the others will slowly kill it.

--
Regards,
Martin Brown