7.2v batteries

[nbtsm]

I have two 7.2v nimh batteries powering my starter. They don't seem to last very long anymore. how can I test them. I don't want to shell out for new ones unless I really have to

[Andy Stephenson]

You don't say what size they are or if they are connected in series or parallel. Your best bet is to get a 3s LiPo say 2200 mAh, this is what I use to power a 12V geared starter it has enough power to turn over large 4-stroke engines and it only needs charging a couple of times in a season.

[Martin Harris - Moderator]

I think you’ve almost answered your own question - the capacity appears to have dropped. 
 

You might consider how you’re charging - with a peak detect charger or trickle?  You might be experiencing false peaking on the former - you could try a higher charge rate to improve detection but it’s more likely that if the NiMHs are a few years old, they are past their best. 

[Eric Robson]

I use a 3s 2200 for my starter and it starts up to 35cc petrol engines. If you do go for a lipo 3s battery you will need a dedicated charger, so unless you also fly electric you may be better buying a 12 v small lead acid or some new Nimhs. As Martin advised check the charger first. 

[Cuban8]

15 hours ago, nbtsm said:

I have two 7.2v nimh batteries powering my starter. They don't seem to last very long anymore. how can I test them. I don't want to shell out for new ones unless I really have to

I presume you have a normal 12V starter so you'd be using the the two packs in series - If the battery cells are smaller than C or sub C then they're not really up to powering a starter IMHO. Do yourself a favour and get a 2200 3S lipo - they're not expensive (20 or 30C types will be ample) get a simple charger if you don't already have one, they can be found quite inexpensively as well. I used sealed 12v lead acid batteries of around 7A hr for years but they're heavy to cart around in your field box and won't last long if charged with one of those cheapy Wall Wart chargers  - they always overcharge them and cause the battery to get warm or worse in my experience.

To test your existing nimhs you'll need access to the individual cells and then measure their voltages during an on-load test. One or two are bound to be under performing and collapsing to 1v or less well before the others after a full charge.  Not good.

Good luck, hope this is of some use.

Edited September 14 by Cuban8

[nbtsm]

Appreciate