Universal Charger project

[Dennis Thornton]

Universal Charger project

I have several battery powered tools. Some are without chargers. I'd
like a universal charger that would handle any battery type from 7.2v
to 24v in any battery pack from any manufacture. Too many physical
configurations to expect a plug in setup so I'd use wires and clips to
connect.

I have a Hitachi UC 24YFA 7.2 thru v24v for both Ni-CD or Ni-MH. It
has 4 pins plus, minus, T, and S. I do not have a charger manual or a
Hitachi battery to disassemble, but I assume the T is for temperature
control by an internal thermistor and the S for sensor and I think
this my biggest unknown{different internal resistor for each
voltage?}.

How could I use this as a start for my project?

 

[Martin Riddle]

Use an external temperature sensor, pot it into a magnet and slap it onto the outside of the battery when charging.
I use a constant current switching supply, delta T charging method. And never had any problems.
Include some sort of backup termination timer.
Include a thermostatic switch in the supply.

Cheers

 

[Dennis Thornton]

My thoughts about the external sensor too, but I don't know what to
use.

Nor do I know what to do about the S terminal.

I'm also reading that it's a good idea to plug it into a timer. I
just finished reading some older model Hitachi charger info that says
not to leave them plugged in.

 

[Martin Riddle]

I just used a plain old RS thermistor fed into a comparator. My timer is a 4040 driven by a ~16sec or so astable 555.
For battery packs with the 'S' terminal you can leave them unconnected. Most packs have a thermo switch in them. So with an external
ntc and the internal switch your pretty safe.

 

[Dennis Thornton]

I stayed with you through the RS thermistor and got lost with the
comparator and timer. I got back on track with the thermo switch.

Comparator?

 

[Martin Riddle]

Ok,
My Thermister (ntc) is fed into a comparator.
The comparator is set to trip at 35c (max cell temp, from a panasonic data sheet).
This terminates the constant current charge cycle.
A backup timer also can terminate the charge cycle ( either one can terminate charge anytime ).

If you want I can email you a schematic.

Cheers

 

[Dennis Thornton]

Martin, I'd appreciate that! My email is

DennisT

@

agencyins.net

with no returns of course.