Can bench top power supply act as a charger ?

--- 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 adjust the output voltage of your power supply until it's putting

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 what your supply does.

-- John Fields

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

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?

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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

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

Regards, AM