Charging 2x 10 Eneloop (12V) 4 000 mAh

Discussion in 'General Electronics Chat' started by alan1975, Jul 14, 2012.

  1. alan1975

    Thread Starter Member

    Jan 1, 2012
    Hello again,

    I building battery pack based on eneloop 2000 mAh batteries.
    I have 10 batteries in series (10x1.2V) = 12V and connected in parallel with same pack to extend capacity.

    So it should have 4000mAh of capacity.

    I need to build charger, with current limiting.

    My question is:

    1) As fare i know, i should load them with 1/10c what should be as fare i know, (correct me if im wrong) 400mA (from 4 000 mAh).

    2) Second question, is about voltage... what charge voltage should be fine?

    I would use constant voltage, using some kind of LM stabilizer (variable).
    I know it can't be just 12V because it's to low for fully charge, so what's the threshold ? Is 13V will be safe and fine ?

    If i set for example that 13V (if You correct me for the value, i will use other) and just use Ohm law to calculate power resistor for current limiting ?

    From my calculation for 13V and 400mA output i will need 32.5Ohm resistor with about 5W power.
    I found the closest will be R33Ohm, i could use 10W power to be safe and not burn anything.

    I would like have LED what will show me "charge finished" or charge is in progress.

    I think by myself that i should use 1R resistor in series with output, and somehow measure voltage across... If there will be no current flow or current flow will be low then set green led if not then red.

    I'm not sure how do this... maybe using voltage comparator, (there will be low voltage across resistor proportional to current flow) but when i will connect one leg of the resistor i will read 13V between opamp ground... just get it... sorry...

    Should i turn charging off ?
    How react that kind of battery ? NiMH eneloop.
    Is battery will stop drawing current when it will be fully charger, and it can be connected all time ?
    Or it should be disconnected after getting fully charged ?

    Please help
  2. crutschow


    Mar 14, 2008
    You need a constant current charger, not a constant voltage. An LM317 can be configured as a constant current source. You would need about 15V to power the LM317 for charging a 12V NiMH battery.

    Because of slight differences in battery voltages between similar batteries, charging NiMH batteries in parallel is problematic. Better to charge each series group separately or use two separate 200mA constant current sources.

    You can not determine when a NiMH is charged by simply measuring the voltage or current. The only way is to use a specialized circuit that monitors the change in voltage with time (Google "NiMH charger IC").

    A simple charger will continue to charge the battery even after it is fully charged. Charging at 1/10C indefinitely may not harm them but it's better to terminate the charge after 15 hours or so.
  3. alan1975

    Thread Starter Member

    Jan 1, 2012
    Thank You for reply but it's looks to complicated!

    How about something like this ->

    Is parameters are ok ?

    It says it can charge 10 cells.
    It use 500mA current.

  4. crutschow


    Mar 14, 2008
    That should work fine, but I would still charge each series connection of cells separately to avoid any possible problems with current hogging.
  5. alan1975

    Thread Starter Member

    Jan 1, 2012
    Thank You,

    I will order and maybe post result how it's working after few charges.

    If anyone, want...
  6. kevin.cheung19

    New Member

    Nov 22, 2011
    For best long life of the battery:

    1) Make sure that you are not mixing old and new batteries.
    2) don't overcharge your batteries, if so, generally, the battery with the higher output voltage will breakdown first.