How can I disconnect a dead rechargeable battery from a circuit to prevent leakage current from completely draining the battery therefore damaging it? It would be great to find a solution that would have a minimum increase in operating current and a minimum voltage drop from the battery supply.

 

What sort of battery?

 

2 2500 mAh NiMH AA batteries in series

 

how much current do your circuit draw?

 

no more than 150 mA.

 

Well, two AA batteries put out too little voltage to control a MOSFET gate.

BJT's and relays would require too much current.

Probably best to just use a simple switch to disconnect the batteries.

 

The device is enclosed in a case and it can not be easily opened for controling a switch and it is not pratical to have a switch on the outside of the case.

 

I may be wrong here. But the some N fet from International rectifier seams to have a very precise Gate Threshold Voltage fixed at 0.7 volt. As some examples IRF7401, IRF7402,IRL3103,IRL3402,IRL3502. Maybe you can use one battery to control the V gate/source voltage

 

OK, so cut a small piece from a plastic milk jug, and insert it between the terminals of the two batteries. That will prevent any current flow from occurring.

 

Or use a mercury switch, turn box over & batteries are off; used sucessfully in well logging probe 1957.