Limit the current to perhaps 0.5A via series resistor or current limited PS and there is little danger--of course wear your safety glasses if you have your face in it.Won't applying high voltage is dangerous.
An interesting tip. Lead acid batteries may in some deap discharge cases get something that is it is referred to as "cell reversal. it is possible for one or more cells to reverse polarity. One cure for this as I have heard is charging the battery with overvoltage. But I do not know if this is true or not. Doing some quick Google serch. I found out that the most common tip to fix "cell reversal". Is to compleatly discharge the battery and then Then recharge at VERY LOW charge rates. But I have no personal experince with this in any way.What I have observed with dead gel cells is that if you apply a high voltage (e.g. 25V) and monitor the current, you can predict what will happen. If the current decreases, the battery is likely history--if the current gradually increases (possibly unstable at times), it will continue to increase and will eventually start taking a reasonable charge, at which time you start reducing the voltage.
If you have a scope. You can use the method outlined here http://et.nmsu.edu/~etti/fall96/electronics/induct/induct.html. A sound card may act as sinwave source.I have problem with building the de-sulphator. I got no way of measuring the inductance of the inductors I can get.
I know this happens with NiCads--causes cells to short--may be cleared via momentary high current charge.An interesting tip. Lead acid batteries may in some deap discharge cases get something that is it is referred to as "cell reversal. it is possible for one or more cells to reverse polarity. One cure for this as I have heard is charging the battery with overvoltage. But I do not know if this is true or not. Doing some quick Google serch. I found out that the most common tip to fix "cell reversal". Is to compleatly discharge the battery and then Then recharge at VERY LOW charge rates. But I have no personal experince with this in any way.
by Jake Hertz
by Jake Hertz
by Don Wilcher