December 27, 2008

Charge rate for NiMH batteries

I got a Maha MH-C9000 WizardOne battery charger for Christmas. This charger lets you charge/discharge rates appropriate to your batteries (and your needs, such as how fast you need the batteries). However, the documentation isn’t really clear about just what rates I should use for, say, it’s “Refresh and Analyze” mode; so I went looking.

  • The charger’s “manual” says 0.3C to 1C.
  • Servaas Products says 0.5C charge, 0.25C discharge, although I have no idea how reputable they are.
  • PowerStream kind of says 1C, at least in a “smart charger” like this one that (I think…) senses temperature as well as voltage drop. (But maybe I’m giving this Maha charger too much credit? Will have to look back at the docs.)
  • Panasonic says 0.5C to 1C (PDF). I actually like this document: specifies charging behavior from start to finish, and it agrees with other things I found.
  • Duracell says 1C, or 0.5C to 1C. Starting to see a trend here. Duracell seems to primarily recommend 1C rate, watching temperature, but at the bottom they also list other methods, including less than 1C charge rate with basically the same termination conditions.
  • Isidor Buchmann says 1C more or less. “Nickel-based batteries prefer fast-charge.”

I started thinking I would charge at 0.5C, but now I’m thinking 1C.

What about discharge rates?

  • The charger’s manual says not to discharge above 1C. The charger’s “break-in” mode does discharge at 0.2C, and it seems to indicate that this is an IEC standard.
  • Servaas says 0.25C. I still don’t know who these people are, but I am pre-disposed to liking them since they’re answering the exact questions I’m trying to answer.
  • The NoBS Batteries FAQ says 300mA-500mA. For what capacity? Who knows! For the AA’s I’m looking at right now, 300mA would be less than 0.2C.
  • A thread on suggests 1C but I suspect they’re nuts (and/or dealing with sufficiently different NiMH batteries used in (really big?) RC planes).
  • Mr. Buchmann doesn’t offer concrete answers, but has some nice graphs which seem to suggest that lower discharge current means less capacity loss over time (or, more accurately, over cycles).

I don’t think I found as much concrete information about good discharge rates for NiMH batteries, but I think I’m going to be discharging around 0.2C.