Subject: Re: Can bench top power supply act as a charger ?
From: "Rodney Kelp"
[email protected]
Date: 9/30/2004 6:53 PM Central Daylight Time
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Another question: If you put a fully charged 12v battery on a charger in
parallel with a dead battery (as in using jumber cables) is there a danger
of overcharging the dead battery or overcurrenting the full battery?
I guess the answer is, "It depends". Question 1 is, "How dead is dead?"
Usually cars that won't start have a measured no load battery voltage of around
11V, but when you crank the starter, it drops way down. Now at the moment you
connect the second jumper cable (pos to pos, and neg to the car frame) you've
completed an electrical circuit. The amount of current that flows is limited
by the internal resistance of the two batteries, the resistance of the jumper
cables, and the resistance between your chassis ground connection and the -
terminal of the second battery (which by the way is another good reason not to
connect pos to pos and then neg to neg -- the lowered circuit resistance might
contribute to problems described below).
If neither motor is running, your charged battery will send current over to the
other battery until both batteries approach equal voltage. This isn't usually
a good plan, because it's possible then that neither car will start.
Usually, the good battery will be in a running car, which has an
alternator-enhanced 13.8V or so instead of 12V. That greater difference will
result in dramatically faster charging, but it could also result in problems.
First, it increases the potential difference, which increases the chance of
sparking on contact. This sparking can damage electronics. Also, the greatly
increased current drain on the running car will put a strain on the running
car's voltage regulator and rectifier diodes because of increased power
dissipation. Both of these can be a problem, especially when it's hot. I've
toasted my voltage regulator while jumping another car a couple of times. Very
seldom does the charging of one battery off another when using jumper cables
lead to significant damage to the lead-acid battery itself. The total current
being drained from the good battery at the moment that the jump begins is
usually comparable to or less than the current required to start the car.
There's no chance of overcharging the formerly dead battery if the voltage
regulator of the running car is working properly.
Best bet is to monitor battery voltage once or twice after jumping another car
with the motor running and accessories off. If it stays the same, everything's
probably OK.
Chris