Quote:
Originally Posted by MotorBikeLes
A caution for anybody repairing a very old charger.They were originally equipped with metal rectifiers which have a fairly high voltage drop.
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That would be correct for the typical ones built into a tin plate box or similar, probably the same type as the one mentioned by Rog in a previous post. I suspect that it was the old metal rectifier developing a very high resistance that caused his charger not to be able to overcome the battery terminal voltage that he related about in his post.
This Lucas 'wall-wart' type charger is completely different in that there would never have been room for a metal rectifier. The small solid state bridge rectifier block with its flat protruding terminal connections looks to be an original fit with the way they're formed into the confined space within the charger.
I'm now charging the forth battery with this charger and it's another car battery of unknown age that I was given a few years ago. It's quite old and has the six removable plugs for topping up the cells. Bulb light up was normal when first plugged in and charging current was a good 1.25 amps. I've just left this one charging and when last checked, the terminal voltage of the battery was 14.9 volts and the current was 0.8 amps. It's not been an age since it was last charged, but was obviously due for one, so we'll see how it goes - no Variac in line with this one this time.
As regards the bright flash of the lamp when first plugging in that the OP related, I think that this may be down to the charger seeing a high impedance from the combined series resistor and the possibly slightly sulphated cell he's charging - perhaps something to do with the transformer inductance and seeing little load on first plug in, a bit like those gel batteries that sometimes need to wake up a bit before they'll pass full current, but just guessing really.