Simple circuit for Battery Charging

Discussion in 'General Electronics Chat' started by vonsworld, Sep 29, 2014.

  1. vonsworld

    Thread Starter Member

    Apr 27, 2011
    I'd like to build a simple circuit for trickle charging a NiMH battery, and so please could I ask for some advice :)

    I understand that if the battery is to be charged continuously it should only be fed a small current like 300ma, otherwise it will warm up and get damaged.

    The power supply I need to use is 12v and 4 amps, therefore how can I ensure that no more than 300ma reaches the battery?

    I've noticed that with some buck and boost converter circuits you can regulate the current output and well as the voltage output, would one of these be suitable?

  2. ronv

    AAC Fanatic!

    Nov 12, 2008
  3. adam555

    Active Member

    Aug 17, 2013
    I designed last year a very simple circuit for charging 2 x 1.5v NiMH batteries from a USB port, which I have been using since regularly without problems. Maybe you can adapted it to 12v.

    It only does one thing: it supplies 50mA to the batteries, or to the LED when they are charged. I chose 50mA because this is what old commercial chargers used; but you can increase the current if you like.


    If you need something better, you can find another good design from Bountyhounter in this thread:

  4. ian field

    Distinguished Member

    Oct 27, 2012
    Some manufacturers claim that their cells can be float charged continuously if you keep below 1/20th of the Ah capacity (1/20C).

    The automatic chargers sense the small reduction in terminal voltage that occurs when end of charge is marked by the start of temperature rise - I just use a thermistor and sense the temperature rise directly.

    Its not a huge task to hook the thermistor up to a window comparator followed by a bistable latch that switches the current limited feed to the battery.

    Years ago I hooked up what was intended to be a Ni-Cd fast charger - but it didn't work out quite like that.

    Using the thermistor/window comparator/bistable, the current limit bit was the element in a tabletop oven used as a dropper resistor. The charge current was so high that the cells started to warm up long before fully charged, if left on longer; the time it took to alternate between the window comparator temperature trip points, gradually got less, when it was switching on and off in less than 1 second - the battery was fully charged.

    The charger that was intended to be fast, took just as long as a regular bought one, but batteries I'd got by skip raiding were outperforming new ones that were being charged in a bought charger by a substantial margin.