Sorry for the short title, it was just to get around the title bug i read you're having

Im making a simple battery charger where the goal is to feed a constant voltage of 4.25 volts to the battery until the charging current reaches 1 mA. That is the battery resistance increases until this current is reached. But how do I measure the charging current? The voltage across the battery is of course kept constant, so I can't just measure that. I'm using a LM317 as a voltage source configured as in the picture. My R2 is fixed to 560 ohms and C2 wasn't needed.

 

Why 1mA? A constant voltage charger eventually reaches (essentially) zero current when the battery emf equals the set point voltage. Set it and forget it.

If you must, you could use a current sense shunt resistor in series with the battery, and use a comparator to watch the ∆V across that resistor.

 

Connect your milliamp meter in series from the output of the regulator circuit to the positive battery terminal.

 

Thanks! I don't exactly know why I would stop at 1 mA. It's stated in a charging procedure that I read. I'll try to just leave it for a while and see what happens.

 

If you pick the right voltage, you can leave the charger on for years. It's called, "float" charging. My lawn mower has a sealed lead acid battery and an LM317 regulator and it stays plugged in all the time, but it never gets below about 4 ma. Probably because it's a lot bigger than your battery. I once stored a car for a year and that battery floated at about 20 ma. It's about internal leakage and self-discharge. As far as I know, all batteries bigger than coin cells have a little bit of that self-leakage.

 

I'll try to just leave it for a while and see what happens.

That works great for lead acid. You have to pay attention to the temperature since this affects the target voltage setting, but otherwise it's fine.

I'm pretty sure constant voltage is not as reliable for other chemistries, though.

 

The MAX712/713 is a nice chip for various charging apps.
http://www.rason.org/Projects/gelcell/gelcell.htm
Max.

 

the goal is to feed a constant voltage of 4.25 volts to the battery

That sounds like Li-Ion or some LI chemistry. You need a very accurate chanrge voltage to make sure and not overcharge the.

Sanyo and Sony wanted 4.200V +/- 0.5% for the chips we were going to make to terminate charge.

An LM317 is pretty crappy for voltage accuracy and temp drift.

If it is Li, be careful not to overcharge it.