Hi Olin,

    Are you trying to build a NiMH charger ?

    If so, you should'nt be charging the NiMH battery
    this way. You should use constant current, typically
    at 1C, i.e. if you battery is 300mAH, you should
    charge at a constant current of 300mA. Therefore
    if from fully discharge, it should take around 1 hour
    to charge your battery.
    Your ON/OFF current circuit is not suitable
    for this, you probably need a PWM-controlled
    DC-DC switcher.

    Your charge curve looks more suitable for
    Li-ion batteries. For Li+ you charge using
    constant voltage (typically 4.1~4.2V per cell)
    with current limit (to prevent huge current
    during initial stage of charging).

Regards,
Peter Tiang


----- Original Message -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, May 13, 2001 9:03 PM
Subject: [EE]: NiMH battery packs


> I finally got around to doing some testing with the NiMH battery packs
that
> several people here received recently.
>
> I made a little circuit with a 16F876 that can enable/disable the charge
> current to a pack and also measure the pack voltage.  For an initial test
I
> just used a 36 ohm resistor to +5 thru a PNP as the charge circuit.  I
> wanted to start with low current and with the resistor the charge current
> automatically gets backed off a bit when the pack gets charged up.  I
know,
> this is a long way from the final charge circuit, but I wanted to get some
> idea what we're dealing with before getting more aggressive.
>
> The PIC turns on the charge current for 3 seconds at a time and takes lots
> of voltage readings which get heavily low pass filtered.  It then turns
off
> the current, waits 100mS, then takes lots of voltage readings with the
pack
> open circuited.  These are again filtered.  When this is done (about 200mS
> after the charge current was shut off), it starts the cycle over again by
> turning on the charge current again.
>
> It also send the charging and open circuit voltage over RS-232 where a
> program on the host interprets the value.  This program saves the charging
> and open circuit battery pack voltage, and uses these to compute the
charge
> current and the battery internal resistance.  All four values are written
to
> a CSV (comma separated values) file along with the elapsed charge time.
> Attached is the result of my first test that ran for about 10 1/2 hours.
> The stepped nature of the resistance curve is do to the quantization of
the
> voltages.  Still, I was surprised how low it was.
>
> I plan on trying a differnt virgin battery pack (I've got plenty) and
doing
> this again with higher current.  I'll keep you all posted as I get more
> results.
>

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