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.

Comments (4)

  1. December 27, 2008

    1C is a safe charge rate for any NiMH battery, though it is quite fast. My philosophy is, if you have the time to charge at 0.5C, then your batteries will likely live for more cycles, and if you are in a hurry, then 1C is a good rate to get the job done quickly.

    (Servaas Products are the Australian Distributor for Maha Powerex)

  2. November 26, 2009
    Bob Roberts said...

    I have a question about the charging rate for the new 2300 mAh NiMH batteries I just purchased to replace the 1300 mAh battery pack that originally came in my portable radio. The new batteries are flat tops and came with solder tabs that I need for a battery pack I’m going to put together and install in the radio. The radio came with a wall charger that plugs into the radio and delivers a charge of 100 mA at 6 volts.

    In other words, I’m stuck with this charger and need to know if I’ll be damaging, or shortening the life of the batteries, charging them at such a slow rate. I ask this question because I read somewhere that NiMH batteries don’t like long charges. By the way, I recently purchased the Maha MH-C9000 — which I have not used yet — in order to condition my new batteries.

    B. Roberts

  3. June 21, 2011

    This section examines discharges under different C-rates and evaluates the depth to which a battery can safely be depleted. But first lets look at charge and discharge rates also known as C-rate..

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