Charging an 12V SLA battery from the car

Joined: 08 Aug 2003 Location: Melbourne, Victoria

HI
I regularly use SLA batteries in the car.
How do I setup the power going to the cig lighter to enable me to recharge the SLA's without just plugging them into the power.
I am looking to regulate the amperage going into the SLA's so I don't fry them.
7A/H SLA's from alarm systems are the batteries I want to charge.
Suzuki Sierra is the vehicle.

later
P

Joined: 11 Jan 2004 Location: Adelaide

It can be done, but how complicated it becomes depends on a number of factors.

I presume you'll be running the engine as the batteries charge? What's the regulator voltage in your car? It could be anywhere from around 13.5V up to 15V - typically it would be 14 to 14.5V. A SLA battery can cope with being charged at this sort of voltage for a while, but long term ("float" charging) above 13.8V is generally not good (some manufacturers will recommend up to around 14.1V for a float charge.)

How flat are the batteries when you put them on charge? For most SLA batteries, the manufacturer's recommendation is for a maximum charge current called C10 (or something like that?) - for your batteries, around 700mA. If you try to charge with a higher current for too long you'll fry them.

So this is where it sucks!

If the batteries are too low when you first put them on, you can end up with too high a charge current, and fry them. The higher your regulator voltage, the worse the problem.

If the batteries aren't too flat, and you leave them on for too long with your regulator too high, you'll fry them. Higher the regulator voltage, the worse the problem.

If the batteries aren't too flat, and you've got a higher regulator voltage, you can charge them real quick - this is good, until you leave them too long - then you'll fry them.

If you have plenty of time to charge the batteries, you could add a resistor (2 Ohm/5W?) to the line - if the battery is too low, the resistor will create a voltage drop to help limit the charge current. As the battery voltage comes up the drop across the resistor will decrease (as the charge current drops) and theoretically you can fully charge the battery. If your regulator is too high, and you leave them too long, you'll still fry them.

So, what can you do?

If you use a multimeter to measure the battery voltage before and during charging you should be able to put together a fairly simple system. If the voltage is reasonable (say above 12V) connect the battery straight to the cigarette lighter. Monitor the voltage, and unplug when it hits 13.8V.

If the voltage is too low, add the resistor to the circuit to limit it. Once the charging voltage hits say 13V, remove the resistor and finish charging without it.

Or you could spend a small fortune on a complex charger to automatically take care of everything for you. What's it worth to you?

Good luck,

Scott

Joined: 08 Aug 2003 Location: Melbourne, Victoria

Yes the engine will be running.
It will be either as I'm driving between jobs or when I'm doing door checks or walk arounds.
Regulator voltage NFI..how do I find out?? I think it's a 60 amp alternator with a 700 CCA battery.

Is there anyway that I can rewire the power to the cig lighter with a limiting device in the line to control the current? If yes should I direct wire from the battery or one of the other primary power leads under the bonnet?

Batteries will be between 30-35% to 60 % flat. is that too flat?
please give me an indication of too long and real quick.

is there a charger or regulator that I could get from jaycar et all and canabilise??

just learning with electrics.

thanks
later
P

Posted: Wed Apr 07, 2004 11:05 pm

There is an SLA charging regulator kit available from tricky dicky (and possibly jaycar) that relies on an external 12 v dc power supply, pretty easy to adapt this to your needs. What sort of current draw will you be pulling from the 7Ahr batts?