In-circuit 4 x Ni-MH charger

I have a device that runs on 4 x "AAA" Ni-MH cells, being 4.8VDC.

I would like to be able to plug in an external power supply that will
charge the cells while in-circuit. The device does not need to run
_while_ being charged, although this would be a plus.

The charging duration will be operator-timed, so it does not need to
be electronically monitored.

Can anyone suggest the simplest schematic for this based on input from
an ordinary 6 or 9VDC regulated wall pack? Is a constant current
source strictly necessary?

All schematics I have found are for complete chargers that require the
cells to be removed and charged individually.

Thank you very much.

James

 

You don't say what time period you want to charge these cells in.
Without this basic info we can't make meaningful suggestions.

Some things you might consider in the meantime...

It is unwise to design a charger, particularly one meant to charge the
cells in the shortest recommended time, without automatic shutdown or
protection. While you may think it is unneccessary and you can do this
function manually, what happens on the odd occasion you forget to
stick around or you get involved with some other problem and forget to
switch it off after the recommmended time? Or, somebody other than
yourself is using the charger and they ae not aware of the requirement
for manual control.

There are a number of fully auto chargers available to meet your
requirements at far less cost than it would take to design and build
your own. Have a look at this website
http://www1.jaycar.com.au/index.asp

Under Category Search choose Batteries and Chargers and in the sub
category choose Chargers and then hit GO. You can then see the range
of products available ranging in price from AUD25.95 - AUD88.95.

 

Further to Ross H's reply, one of the failure modes for NiXX cells is heat
damage. Unterminated charge is the main cause of this. Charge termination
needs to be deterministic not probabilistic i.e. their needs to be certainty and
repeatability. Manual termination does not provide this reliably.

Timer-based systems depend critically on the state of charge (SOC) of the cells
when charging is commenced. If this is variable, so is the outcome.

"Smart" chargers look for some end-of-charge (EOC) indication from the cells.
This can be a voltage dip or a temperature rise, or rate-of-rise. The voltage
inflection is less pronounced in NiMH than NiCd. "Smarter" chargers use a
combination of these EOC techniques and usually fast charge (0.5C to 1.0C).
Temperature rate of rise is direclty related to charge current, and can be
fairly reliable. High temp termination at 40-45C is reasonably reliable,
subject to ambient temp and cell temp at the start of charging. But any
temperature-related termination requires a sensor in intimate contact with the
cells, and that means getting them out of the appliance or modifying it to
incorporate the sensor.

I'd suggest buying a decent charger and a spare pair of cells, rotating them as
required.

 

Use a series incandescent light bulb. Gives a bad approximation to
constant current over a wide range of differential voltage. Fault
tolerant. Gives some
visual indication of current. Cheap. Just keep the current below the
maximum continuous charge rating of the cells. Operators WILL forget
eventually.

Another thing that works is a charge dump. Q=C*deltaV
mike
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