RC Tank Warfare community hobby forum

This is a easy way to charge a battery with auto shut off. First you need a dud rechargeable battery off a power tool, cut the battery pack open and remove the thermal trip, they are nearly all similar and consist of a metal can with two wires coming out or one wire and a tag, they all break contact around 40 c, because the temperature rises rapidly towards the end when charging a ni-cad battery, and the thermal trip switches off the charge at the optimal moment.

The setup requires a few other bits to make it work, if you just used the thermal trip as soon as it cooled down it would close and start the charge again, so you need a relay to hold the power on until the thermal trip opens, this then breaks the supply to the relay and shut’s the system down until the start button is pressed to re-start a charge.

Any 6-12v relay with 2 poles will do also a push button switch to start the charging cycle, plus a small 2 pin connector for the trip switch.

All you need to do to charge a battery is plug in the main charge plug and the connection to the trip switch, then press the start button. It will switch it’s self off when the battery is fully charged.

The power requirements are basically what ever you have, any 12v supply with an out put of 1 to 10 amp DC will do, I use a 2 amp supply which charges a 7.2v 1700 m/a in about 45 minutes, the bigger the out put in amps the faster it will charge with 10 amp being max but I think this rate would de-crease the life of a battery, so I would  probably keep it below 6 amps.

One other thing if when you connect up and press the start button the relay buzzes then you have the thermal switch case on the wrong battery case.

Have fun

Al

That's a really good tip Al. I think I've got an old cordless drill battery around somewhere (unless my better half has already disposed of it!). Its a pity battery pack suppliers don't fit one of these in each battery pack as standard. I wonder if it is possible to get hold of the trip-switches separately.

Thanks for posting
Cheers
Chris   

You can buy them Chris but they cost more than a battery pack.

Cheers Al

PS: Just a thought you can often find duff battery packs at car boot sales in the junk boxes.

Having another think about the charge control relay etc, the same control unit can be made just using a SCR thyristor and a 270 ohm resistor, the thyristor I used is a C160D which is a common 4 amp switching IC.

For anyone not familiar with thyristor’s they have three legs which are Anode, Cathode, and Gate, a Negative supply is connected to the Cathode and by applying a Positive supply to the Gate, the negative supply is allowed to flow through the Anode and this will continue to flow after the Gate supply is removed, it will only be switched off when current ceases to flow through the Anode, such as when the circuit is broken.

This is a lot easier to make than the relay system the only other thing is that you need a 15v dc supply. Also you will need to bolt the Thyristor to an aluminium plate or heat sink to take the heat away.
Al