NIMH Storage

[Ziet]

During winter months I don't fly so need to store my my NIMH batteries. I keep them in my planes because they are not easy to take out. will I need to charge them ever so often, as I have found to my cost leaving them...has ment I have had to replace them. Any help is much appreciated.

[flydiver]

Cycle them occasionally. Unless you use the newer LSD variety they lose most of their charge in 3 months. If they go completely dead you can lose a cell and ruin the pack.
Frequently Asked Questions (FAQ) — The Basics

[fhhuber]

They need to be charged occasionally... about once a month to be paranoid.

NiMh don't need to be cycled except to test for capacity... about once a year.

If the pack fails to give 80% of label capacity you can try another cycle. If it fails the second time you can't trust the pack.

NiCd need to be cycled (or charged, put to use and recharged) at least once a month.

[Rodneh]

Contrary to some of the advice above I have found that as long as the cells do not get a reverse charge it does not hurt them to self discharge to very low levels. In fact, on NiCads, NASA did some extensive tests on them some 30 years ago and found that they stored best being completely discharged with a shorting bar across the terminals of each cell. They were stored at a low temperature just above freezing. I do not know of any similar test being made on NiMH but they have very similar chemistry so should be close in performance. I have had NiMh stored for over a year at low voltage with no problems as long as they were given a forming charge prior to use again. What will quickly damage them is high temperatures (how high? do not know for sure but would assume in excess of 180 degrees F.) or a reverse charge which can happen in you discharge a battery consisting or more than one cell to zero volts as the strongest cell will wind up reverse charging the weaker cell near the end of the discharge cycle. To be safe, just do a forming charge on any Nixx cells you stored for a period of a month or more before use. A storage charge being defined as charging the pack for at least 15 hours at 0.1C.

[kyleservicetech]

Originally Posted by Ziet

During winter months I don't fly so need to store my my NIMH batteries. I keep them in my planes because they are not easy to take out. will I need to charge them ever so often, as I have found to my cost leaving them...has ment I have had to replace them. Any help is much appreciated.

Yeah, it's best to cycle them every month or three. We used over a thousand 24 Volt Sub C type Nicads every month at work, and a number of them would show up in the department where I worked for warranty issues. This was a few years ago.

Now days, many of my club members are going over to the LiFe or A123 battery packs. These batteries far outperform the Nih batteries currently in use. A two cell LiFe or A123 battery is a direct drop in for a five cell "AA" size Nih battery, voltage wise. But the LiFe or A123 units will easily put out 14 Amps in a giant scale model, where as putting a 14 Amp load on a "AA" Nih battery will result in your 2.4 Ghz receiver shutting down. Yup, I've done a lot of testing on this. Just moving the transmitter sticks around on a seven servo giant scale model airplane can and will pull peak currents of 14 Amps.

Nice thing about the 2300 Mah A123 cells. They hold their charge for a LONG time. Mine will lose about 5% of their capacity while in storage for a half year. Then, they are quickly topped off.

If your model uses those digital servos, I'd steer away from the Nih batteries. My 2300 Mah A123 cells are pulling 40 Amps in my electric models. At 40 Amps, they still put out 2.7 Volts DC per cell.

[rtbd]

When Batteries stored in discharged state, it could be spoil. So you can take more advantage from power sonic batteries.

[kyleservicetech]

Originally Posted by rtbd

When Batteries stored in discharged state, it could be spoil. So you can take more advantage from power sonic batteries.

Yeah, its been my experience at work, these batteries have a tendency to develop pin type shorts inside the battery pack. With a fully charged battery, these pin shorts get vaporized, and no worse for the wear.

If the battery is dead, not vaporizing, and those pin type shorts grow to where the cell can not be recovered.

This may no longer be an issue nowdays, the problem was due to minor defects in the cell's separator between the plus and minus plates.