# Can bench top power supply act as a charger ?

Discussion in 'Electronic Basics' started by mowhoong, Sep 30, 2004.

1. ### mowhoongGuest

I build a bench top power supply unit for my hobby 0 to 30 volt 1
amp.I try to
charge my lead acid battery with proper voltage. but the current only
draw less than 50 ma,When I use a battery charger, it draw 400 ma. My
battery was
6 volt 4 amp . Can any person know the reason ? Thanks !

2. ### John FieldsGuest

---
In order to charge a battery you have to make the supply voltage
higher than the battery voltage or else you won't be able to push
electrons into the battery. If your battery doesn't mind being
charged at 400mA when it's discharged, then all you have to do is
400mA into the battery and leave it alone until your battery voltage
rises to something like 7 volts and there's something like 50(?) mA
going into it. It may be more or less voltage and/or more or less
current, and you should either check with the battery manufacturer for
the correct terminal voltage and current or, if you're happy with the
way your charger works, measure its output voltage and current with
the battery connected to it and fully charged and make sure that's

3. ### CFoley1064Guest

Subject: Can bench top power supply act as a charger ?
Assuming everything is working and you have a good battery, there might be a
couple of possible reasons. If your benchtop power supply has current limiting
with shutdown, it might be tripping when connected to the battery.

The second, more likely possibility is that you're setting your power supply by
voltage rather than by current. If you put a meter on the battery charger when
it's charging, you'll see that it is probably well over 6V -- likely somewhere
around 7V or 7.5V. That voltage is necessary to reverse the chemistry inside
the battery.

Here's what to do. To protect the power supply and the lead-acid battery, put
a 3 amp barrel diode such as the 1N540X in series with the positive terminal of
your power supply (anode toward the +), and then connect the cathode of the
diode (the side of the diode with the line) and the - of the power supply to
the battery. Turn on the power supply, then turn up the output voltage until
you either have 7V or 7.5V across the battery terminals, or are getting a
current draw of 400 mA. Then you can see what voltage is across the battery.
Set the voltage of the power supply there the next time. The current will go
down over time as the battery recharges.

Remember to limit the amount of time the lead-acid battery charges -- they're
easy to overcharge. You might want to power your benchtop supply from a timer
of some sort. If not, just watch the time carefully. Also, feel if the
battery is getting overly warm when it's being recharged -- that's a sign that
you're recharging too fast. Also, do your charging in a ventilated place.

Good luck
Chris

4. ### Rodney KelpGuest

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?

5. ### mowhoongGuest

Many thanks all for the info. I have try it worked well.

6. ### CFoley1064Guest

Subject: Re: Can bench top power supply act as a charger ?
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

7. ### Animesh MauryaGuest

Very well described Chris, as is u there were with me, when all above
said happened with me a week ago.

Regards,
AM