For a few quid I can buy a lead to connect my mobile phone to the 12v
> dash socket on my car and recharge its battery. I've got a small torch,
> that can do the same. But the cheapest 12v battery charger for 4x AA ,
> is about £30. I think the phone lead just has a resistance in it to
> bring the 12v down to whatever the phone needs. Could I make up a lead
> like that to charge 4xAA's , and what resistance would I need ?
Re-chargables need a special circuit to charge them properly - something that gives constant current for Ni-Cads. This circuit will be built into your phone and torch, so a simple lead is all that's needed. But to charge batteries on their own needs a charger. However, 30 quid seems a bit steep. Have a look at Maplin for special offers - I think I saw one on a flier recently.