Lead acid charger.

[IloveFLIGHT!]

What is actually required to charge a car battery? I have a power supply that will deliver 13.8 volts and 15 amps all day. I would like to make a charger out of it. Thanks for any replies!
Chris

[Bruce Abbott]

You need something to limit current flow into the battery. The simplest method is to wire a power resistor (eg. 0.5 Ohms 10W) in series with one of the charging leads, however this causes the charging current to drop significantly as the battery approaches float voltage.

Using a high-power light bulb (eg. car headlamp) may work a bit better, because as the current drops the bulb filament cools down and reduces its resistance, thus maintaining higher current flow than with a fixed resistor. Other advantages are that you can see the filament getting dimmer as the battery approaches full charge, and it provides better protection against accidental short circuits.

[IloveFLIGHT!]

I had'nt checked this in awhile! That is way simpler than I thought. Can you not overcharge a lead acid battery?

[Alison F]

They're effectively a chemical reaction (like most batteries). Ultimately the plates break down and disintegrate, with the particles falling to the bottom of the battery and shorting out the plates. They'll also release hydrogen (smells like rotton eggs) which in a confined space is explosive. I think it was the Hindenburg airship that they filled with hydrogen and it exploded. Yep; http://en.wikipedia.org/wiki/Hindenburg_disaster

There's bits about it dotted around on the net explaining the chemical reaction/exchange. This one seems good; http://www.bigginhill.co.uk/batteries.htm

[mjsas]

The charge system in a car uses a constant voltage of about 14.4 volts and has no current limiting. Also, all automotive alternators are temperature corrected. Some autos even have a thermistor attached to the battery to control the charging voltage. This is common where the battery is under the back seat or in the trunk.
However the amount of temperature correction is small so a constant voltage of 14.4 volts is good enough.
For trickle charging the voltage should be lower, about 13.8 volts.

[ammlione]

Alison F, The battery whislt charging releases Hidrogen and Oxigen, You cannot smell Hidrogen, (the rotten egg smel is from Hidrogen Sulfide) .
You are right the Hindenberg was filled with hidrogen.

IloveFLIGHT, I would put just the floating voltage to charge it and leave it there, just in case I forget to disconnect/adjust the charger on time and cook the battery (like mjsas said).
It is not a bad idea to limit the inrush current with something, (resistor, light globe or a thermal switch) just in case the battery was damaged in the first place
good luck!!

[vintage1]

The charge system in a car uses a constant voltage of about 14.4 volts and has no current limiting.

Oh yes it does. One way or another. Even if its only inductance and resistance of the alternator.


Also, all automotive alternators are temperature corrected. Some autos even have a thermistor attached to the battery to control the charging voltage. This is common where the battery is under the back seat or in the trunk.
However the amount of temperature correction is small so a constant voltage of 14.4 volts is good enough.
For trickle charging the voltage should be lower, about 13.8 volts.

Agreed, but put some form of resistor in.

[IloveFLIGHT!]

The power supply sits at 13.8 volts. So is the light bulb a good idea? Would'nt I need one heck of a resistor to handle the load?

[Dan Baldwin]

You could use the same current limiting method that some are using to charge A123's at high currents; zip charging, or using a length of lamp cord as a resistor to limit current.

Dan

[Rodney]

If you can set the power supply to 13.7 to 13.8 volts and put a 1 ohm 225 watt resistor in either line from the power supply to the charger, you will be safe. If the battery is shorted the current will be limited to about 15 amps; if the battery reaches full charge no current will flow. If the battery is near normal condition and reading 11 volts (the lowest you should ever let it get to under load before recharging), the inrush current at start of charge will be 13.8-11.0 divided by 1 (the value of the resistor in series) or 2.8 amps. You should not charge a Pb based battery at more than 0.1C.

[Dan Baldwin]

If you can set the power supply to 13.7 to 13.8 volts and put a 1 ohm 225 watt resistor in either line from the power supply to the charger, you will be safe. If the battery is shorted the current will be limited to about 15 amps; if the battery reaches full charge no current will flow. If the battery is near normal condition and reading 11 volts (the lowest you should ever let it get to under load before recharging), the inrush current at start of charge will be 13.8-11.0 divided by 1 (the value of the resistor in series) or 2.8 amps. You should not charge a Pb based battery at more than 0.1C.

Every automotive charging system I have ever seen charges at CONSIDERABLY higher than .1C.

Printed on the side of a Trump 7AH SLA battery;
"Constant voltage charge
Cycle use: 14.4-15.0v
Standby use : 13.5-13.8v
Initial Current: less than 2.1A"

Which is about 3 times higher than .1C

Dan

[Rodney]

Every automotive charging system I have ever seen charges at CONSIDERABLY higher than .1C.

Printed on the side of a Trump 7AH SLA battery;
"Constant voltage charge
Cycle use: 14.4-15.0v
Standby use : 13.5-13.8v
Initial Current: less than 2.1A"

Which is about 3 times higher than .1C

Dan
Dan, you are correct, I should have stated for SLA's as most of them do not recommend a higher charge rate than 0.1C. Non sealed batteries can be charged much faster if temperature is monitored and charge rate regulated accordingly.

[Dan Baldwin]

I have 9 SLA's on my bench right now of different sized, and from different MFGs, and they all have .3C max charge rate printed on them.

Dan

[IloveFLIGHT!]

If you can set the power supply to 13.7 to 13.8 volts and put a 1 ohm 225 watt resistor in either line from the power supply to the charger, you will be safe. If the battery is shorted the current will be limited to about 15 amps; if the battery reaches full charge no current will flow. If the battery is near normal condition and reading 11 volts (the lowest you should ever let it get to under load before recharging), the inrush current at start of charge will be 13.8-11.0 divided by 1 (the value of the resistor in series) or 2.8 amps. You should not charge a Pb based battery at more than 0.1C.
Thanks. I guess I will have to find a resistor.

[IloveFLIGHT!]

After looking around for resistors! I am trying the headlamp idea! I just need to fuse it.

[IloveFLIGHT!]

If you can set the power supply to 13.7 to 13.8 volts and put a 1 ohm 225 watt resistor in either line from the power supply to the charger, you will be safe. If the battery is shorted the current will be limited to about 15 amps; if the battery reaches full charge no current will flow. If the battery is near normal condition and reading 11 volts (the lowest you should ever let it get to under load before recharging), the inrush current at start of charge will be 13.8-11.0 divided by 1 (the value of the resistor in series) or 2.8 amps. You should not charge a Pb based battery at more than 0.1C.
If I read this right. I can charge a 750 ah car battery safely at 7.5 amps, right? This seems completely correct since most car chargers have a 12v 6a selection. I understand OHM's law. I DON'T understand individual battery chemistry. Thanks all!