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Old 05-20-10, 08:51 PM   #1
Reynolds 
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Small SLA battery charging in car - OK?

Excuse me if this is a silly question, but can I charge a 12V 4Ah SLA battery by plugging it to my car's lighter socket while driving?

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Old 05-21-10, 04:17 AM   #2
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You can, but do you have a mechanism to cut off when the battery is fully charged? The battery can become severely damaged if you overcharge it.
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Old 05-21-10, 05:02 PM   #3
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Overcharging should not be a problem. You're hooking one SLA battery in (EDIT)parallel with another lead acid battery (the car battery). If it's good for one, it should be good for the other.

EDIT: NOTE my caveat below; you'll want to have a resistor in the circuit

Lead acid cells are very difficult to overcharge.

In fact, most jumper pack batteries, which are just SLAs in a nice plastic box, come with a charger lead which is just two cigarette lighter plugs on a wire, intended to do exactly this.
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Old 05-21-10, 05:33 PM   #4
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Do not connect the SLA in series in your car. You will make 24V condition which can be damaging.
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Old 05-21-10, 05:47 PM   #5
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Do not connect the SLA in series in your car. You will make 24V condition which can be damaging.
Uh, he's not. he said hooking it up to the cigarette lighter. It's not possible to hook another battery in series with the car battery without disconnecting the car battery and wiring it in series.

Hooking it up in parallel is OK, however, you MUST use a resistor to limit charge current. Standard ohm's law to calculate. If you post how many amp hours your SLA battery is, I or someone else here will tell you what size resistor to put in line.
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Old 05-21-10, 05:54 PM   #6
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however, you MUST use a resistor to limit charge current. Standard ohm's law to calculate. If you post how many amp hours your SLA battery is, I or someone else here will tell you what size resistor to put in line.
No real need. Lead-acid batteries can be safely charged as long as the voltage is properly regulated. The car's electrical system should already be doing that in order not to damage the car battery or the electronics.
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Old 05-21-10, 10:15 PM   #7
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No real need. Lead-acid batteries can be safely charged as long as the voltage is properly regulated. The car's electrical system should already be doing that in order not to damage the car battery or the electronics.
Not entirely correct. The SLA can be over charged severely limiting it's life. A charge controller will be needed. There are cheap shunt regulator types that charge at the standby voltage (otherwise know as trickle charge). This is typically 13.6-13.8 VDC.

But proper charge control can be fairly sophisticated for good battery life. They will go through a pre programmed charging procedure. A Sunguard Charge controller could probably be used but they are really designed for solar panels. You might do better to if you can find one specific to automobiles.


You should never allow an SLA to become fully discharged. And it should be recharged as soon as possible after use.
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Old 05-22-10, 06:25 AM   #8
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No real need. Lead-acid batteries can be safely charged as long as the voltage is properly regulated. The car's electrical system should already be doing that in order not to damage the car battery or the electronics.
I don't think it's a good idea to hook up a discharged SLA to a full 13.8V supply with no current limitation. For one thing, it could blow the fuse in the power outlet.

The car's electrical system IS regulating the charge to the battery. But there's no charge controller going to the cigarette lighter. And you don't want to hook it up to the battery either, that's completely unregulated and provides essentially unlimited current.
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Old 05-23-10, 02:58 PM   #9
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Uh, he's not. he said hooking it up to the cigarette lighter. It's not possible to hook another battery in series with the car battery without disconnecting the car battery and wiring it in series.

Hooking it up in parallel is OK, however, you MUST use a resistor to limit charge current. Standard ohm's law to calculate. If you post how many amp hours your SLA battery is, I or someone else here will tell you what size resistor to put in line.
The suggestion to connect in series was your idea, not the OP's.
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Old 05-24-10, 07:45 AM   #10
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The suggestion to connect in series was your idea, not the OP's.
You're right, I mistyped. I meant parallel. I'm going to go edit it now.
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