Jaycar used to sell an electronic battery rejuvenator, the design was in silicon chip magazine some time ago. From memory it it pulsed a higher voltage very short duration spike across the battery, no idea what it was supposed to do or if it worked.

I've got a battery charger/conditioner, about $40 i think it was.

It's got 3 main modes-
Motorcycle (1.2-14AH bat) 14.4V at 0.8A max.
Car (20-120AH bat) 14.4V at 3.6A max.
Cold weather (14-60AH bat) 14.7V at 3.6A max.

It has 5 steps to it's charging function.
1 is for extremely discharged (possibly dead) batteries.
It will charge a battery with as little as 7.5V using pulses. It will do that till the voltgae reaches 10.5V

2 is for a heavily discharged/flat battery.
From 10.5V it will charge at 3.6A till the voltage rises to 12.8V and from there it will then charge at 3.0A.

3 is the final stage.
From 14.1V it will charge at 0.8A untill the voltage reaches 14.4V (fully charged).

4 Once fully charged it continues to trickle charge at 100mA (0.1A) to maintain the charge.

5 If at that charge rate the voltage drops to below 12.8V it will then charge at 0.8A again till the battery is fully charged (14.4V), then go back to the 100mA trickle charge mode.