Strange battery pack (3x AAA) behavior

Hi,

I'm succesfull running an Arduino on a breadboard, powered with 3 AAA recharchable batteries. After a couple of weeks, the Arduino stopped running (earlier than I expected since the sketch is sleeping in low power mode most of the time).

When I measured the voltage of the 3 batteries I noticed something strange: - 1st battery: 1.2v - 2nd battery: 0.9v - 3rd battery: 0.2v

Anybody has an idea what could cause 1 battery depleting much earlier than te rest? They were connected in series and all of them were completely charched in the beginning.

Thanks, Jan

Were they of the same manufacture model and age? Were all three equally fully charged before starting your test run?

Mixing random cells rarely work out well for any series/parallel arrangement, especially rechargeable types.

Lefty

Thanks for the swift reply! They were charged, but one of them was of a different brand then the other two. I'll try again with 3 identical batteries.

Thanks again, Jan

jtlns: Thanks for the swift reply! They were charged, but one of them was of a different brand then the other two. I'll try again with 3 identical batteries.

Thanks again, Jan

Different brand/models are likely to have different mAH ratings and therefore discharge at different rates. So best to use identical cells of identical usage history, as mAH capacity can also decrease with total number of charge/discharge cycles a specific cell has undergone. Rechargeable cells don't have an infinite lifespan nor a consistent Mah rating over time.

Lefty

As lefty indicates, this is a very common phenomenon with rechargeable batteries, if the cells are not well-matched in regards energy storage, plus similar health and age. In fact, the better cells can actually overwhelm the poorer cells, and drive them to "reverse " polarity. I've seen it. R/C car people use matched cells when assembling series battery packs.

You can see results like this even with batteries of the same type and model from the same manufacturer. NiCd and NiMH batteries have such steep roll off curves that this will happen in all but the most expensive battery packs. One reason good, expensive packs are good and expensive is because the manufacturer takes time to makes sure the batteries are well matched so that this sort of thing is minimized. They are not all created equal. Using different batteries though will almost certainly guarantee that this will happen.

Thanks for all the input. The battery pack I'm talking about is just a set of 3 AAA batteries in a plastic case. Besides using the same batteries, or an expensive real battery pack, would there be anything I can do about this? Putting components between them? Working with a voltage regulator? J

jtlns: Thanks for all the input. The battery pack I'm talking about is just a set of 3 AAA batteries in a plastic case.

This happens a lot with rechargeable batteries. They rarely charge to the exact same capacity. I once built an Arduino-powered battery discharger so I could rate my batteries and put them into sets with similar capacity.

jtlns: Besides using the same batteries, or an expensive real battery pack, would there be anything I can do about this? Putting components between them? Working with a voltage regulator?

A DC voltage booster can make sure things keep running even if one battery dies early:

eg. http://www.pololu.com/catalog/product/2115