Batteries - what type?

[---MIKE---]

I went shopping today. The only type of rechargeable batteries at Wal-mart in the AA size were Lithium and NiMH. I bought the NiMH (Duracell), and will try them.

---MIKE---

[tnom]

There are no rechargeable Lithium AA's

[SteveB]

---MIKE---

What did your manual say?

Steve

[E Z Peaces]

I've seen it with NiCads. You charge one and it appears to be discharged after brief use. Then you put it in a holder with a 2-ohm shunt, and it keeps producing current, but the voltage is low because the internal resistance is high. With the high internal resistance, it may take a long time to discharge completely. Completely discharging gets rid of the high resistance.

It appears that if a NiCad is left charged or partially charged for a long time, a chemical change will cause high resistance in the charged part, but discharging will correct it. Lead-acid seems the opposite; if it is not routinely fully charged, the discharged part will develop high resistance, which can be corrected by fully charging, which can take a long time due to the high resistance.

NASA says NiCads should be drained and shorted if not needed immediately. When you get ready to use them, the first step is to overcharge them, 40 hours at C/20.

Sometimes a NiMH may not charge fully. I think that's from gas bubbles and not memory effect.

In my experience, keeping a NiCad charged eventually results in memory effect, which can be corrected. If you removed one of your batteries, I wonder if it would provide 8.2V across 150 ohms for a couple of hours. If it won't, that could be memory effect.

In my experience, slow or trickle charging NiCads causes electrical leakage and eventually shorting. Internal electrical leakage can be detected by testing capacity after two weeks off the charger.

I think the heating comes from the gases recombining. I wonder if the heat can damage a cell even if there is no venting. (I like to check temperature with an infrared thermometer.)

Are C/10 and trickle charging really recommended for NiMH? In my experience, both kinds of cells do better if I stick to microprocessor-controlled fast chargers.

[AZ Nomad]

My experience with both kinds of batteries is that if I remove a set from service and let it sit idle for more than a few months, they'll be ruined. Every time.