FWIW, I bought a couple of these some time back and both have exhibited a
symptom of not being able to recharge. Having FINALLY read the LARGE print
on the front, I noticed that you must not let the battery voltage drop below
11.2V. Mine was 10.5V! The problem appears to be that the unit utilises a
sealed battery and for safety reasons, the charging current is limited to a
low value. If the battery volts are low (11.2V), the charger thinks it is
looking at a faulty battery and ceases to charge. The solution to this
problem is to switch the unit on and to recharge via the high current leads
at a low (500mA) rate from another source until the battery volts get above
11.2V and then the provided charger will operate in the manner designed. Son
in law has the other unit, which was last seen being dissected in anger!!

Regards
Capitol