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Anyone have battery charger monitoring circut plans? Answered

As we all know the biggest problem with cordless power tools is the battery life.  One of the biggest reasons for battery failier is because the companies sell the tools with cheap chargers that cook the batteries. 

The tools I have use 18v nicad batteries.  The chargers are the type that take about 3-4 hours to recharge a drained battery pack.  Does anyone know where I could find plans for an inexpensive circut that I could retrofit into the battery chargers to keep them from cooking the batteries? 

Idealy a circut that would detect when the battery is almost fully charged and drop down from the 'fast charge' to a 'trickle charge' and eventually shut off completely when the battery is 100% charged.

Be a super nice extra bonus if I could put in a circut that monitored the batteries while in storage, maybe topping them off once a week when I go a while without using them.



6 years ago

Thanks for the link.

My battery packs do have a 3rd contact, wasn't sure why so I guess that is where the charger checks the tempurature.

Nicad and NMiH batteries have a property whereby their temperature rises significantly when fully charged.

This is usually detected by a thermistor buried in the battery pack and brought6 out to a third contact.

Intelligent chargers monitor this voltage and shut down or go to reduced charging current when this state is reached.

The RC modelling community is the best source for charging circuits that do this as they want to charge as fast as they can but not over charge.

Google should help there


is one example - I cannot vouch for the cct as I have not built it.