On Friday, November 13, 2015 at 12:49:07 AM UTC-5, Micky wrote:
I need a DC power supply, bigger than what I've got. Any reason why I
can't connect two automobile battery chargers in parallel to suply the
same vehicle?

YIKES!

A few things about lead-acid batteries:

a) They do not like to be charged beyond about 10% of their AH rating. That is not to suggest that more amps will destroy them - but that some care is needed during the charging process.

b) Charging amps, AH ratings and the actual operation of the battery are, at best, first cousins - related but not directly. For instance, even a high-rated gel-type battery is generally not suitable for motorcycle operation as its chemistry is generally not suitable to provide the very fast discharge (very high amperage) needed to turn the starter motor at sufficient speed to start the engine. Conversely, a much lower *rated* battery may be perfectly fine for the purpose as its chemistry is designed for a very high amperage discharge for the few seconds necessary.

c) That your charger jumped to 10A is not a surprise, most such chargers are 'smart' inasmuch as they recognize the draw of the battery and react accordingly. But 10A is not hardly enough to turn the average small starter motor under load. For instance, a marine-grade Group 24 battery (most common group of that type) is rated at a mere 80 AH, but is capable of 800 CCA (cold-cranking amps) for a few seconds to start a big diesel boat engine, for example. Your little cycle-starter may need 50+CCA or more to operate properly.

d) A motor could care less how many amps are available to it, as long as they are in excess of its draw at a given load. If you want to test the starter, us a regular car battery. If you want to use an outboard device to test, get a Starter/Charger unit. These devices either have a rapid-discharge battery on board or a series of capacitors boosting the output the few seconds needed to start.

e) Ganging up chargers is generally not a good idea unless they are identical in behavior and type. It is generally not a good idea to use these things beyond their design parameters - at least, you would void any warranties. At worst, you would pick the only two chargers that if paralleled would go the way of Chernobyl.

Peter Wieck
Melrose Park, PA