Dumb battery charging question

Discussion in 'The Projects Forum' started by N0ctrnl, Jul 21, 2010.

  1. N0ctrnl

    Thread Starter Member

    Aug 14, 2009
    14
    0
    I searched and searched but couldn't find anything that answered what I need, so please forgive me if it's there and I just missed it.

    I have 3 batteries (1.2v 2300mA) wired in series. If I am using a constant voltage to charge them, how would I go about calculating how long they would need to be receiving voltage before they're fully charged?

    I was sure I'd seen the math to figure this before, but I can't seem to find it again.

    Thanks!
     
  2. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    What type of batteries are they? For example Nicads need a different charge method to NiMH.
     
  3. N0ctrnl

    Thread Starter Member

    Aug 14, 2009
    14
    0
    I'm sorry. I forgot to list that. They're NiMH.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You don't charge them using a constant voltage.

    You charge them at a constant current until they hit the Delta V "bump". The bump is usually reflected by a change in temperature. It's very difficult to sense the bump via voltage or current; you'll usually not detect it unless you are monitoring the temperature.

    Read this page: http://www.powerstream.com/NiMH.htm

    If you try to charge using a fixed voltage and your batteries are very depleted, you will likely cause damage to them.
     
  5. Bosparra

    Member

    Feb 17, 2010
    79
    3
    I would assume the best place to measure the temperature would be on the positive pole of the battery? Does it matter which battery is measured i.e. is it better to measure the middle battery or the last one, for instance?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    In the case of automotive or marine or deep-cycle lead-acid top-post batteries, the positive terminal is generally larger; so it's the preferred place to get the battery internal temperature.

    The old carbon-zinc batteries were completely enclosed by a zinc casing, except for the positive end. The casing was filled with a paste, a metal-capped carbon rod inserted in the middle, and the end sealed with some type of epoxy or plastic.

    I've not disassembled a NiMH battery (nor do I recommend attempting to do so) but I'd assume that the negative (flat) end of AAA through D cells would be the better terminal to measure the internal temperature.

    You don't charge them in series.
    If you attempt that, you will very likely overcharge one and undercharge another.
    Each cell should be charged/monitored separately.
     
    Bosparra likes this.
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