Hello, I have a 5Ah 12v SLA that I would like to charge in car. How would I go about doing this?

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Go to a hobby store that sells things like radio-controlled helicopters, boats, planes, cars, and trucks and buy a charger. You could also probably find a charger at the hardware store too. It's probably not worth your time or effort trying to build one since they are so cheap.

I don't think just connecting the lighter plug to the battery terminals would do it properly.

 

If you want to charge it while the engine is running just put a 12V bulb, such as a tailight/turnsignal bulb, in series with the power wire from the car to the SLA battery. The bulb will limit the initial charging current to within the spec of the SLA battery but have a low enough resistance when cold to fully charge it. If you wish to charge with the engine off, you'll need a voltage boosting circuit and dknguyen's suggestion is your best bet.

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No, I meant with the engine running. So using a bulb would work eh? Good idea. Any ideas on what wattage bulb I should use? Lets say I used a 12watt bulb (for easy math), wouldn't that be 1amp going through it, so 1amp charging my battery?

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Wrong math. The car's battery will be about 14V. Your battery to be charged won't be 2V for the entire charging time, will it? The bulb might start the charging by glowing, then as the charging battery's voltage rises the bulb will get dimmer as the current becomes reduced. You need a bulb with a much higher rating than only 12W.

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why do you recommend a higher rated bulb, and what's the math to calculate the rating?

I may try the same thing in the next few months.

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The car battery is charging at avbout 14V. The battery to be charged is about 10V. Then there is 4V across a light bulb and you want the average current to be 1/10th of its A/hr rating. So the current should average at 500mA and be about 1A at the beginning of charging with 4V across the light bulb. 4V at 1A is about 12V at 3A. The bulb should be 12V/3A= 36W.

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Except that the bulb will be pretty cold, so will have a far lower resistance than your calculations would suggest. I would suspect the best method is to stick a bulb in circuit and measure the charging current - starting with the 12W bulb originally suggested.

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It doesn't matter because the battery's voltage will quickly rise, causing the current to quickly be reduced.

I agree, it is best to know.

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Thanks for all the replies guys. But to make the math correct, my car runs at 14.4 volts, and the SLA to be charged is a 12v battery, meaning it should be at 12.75volts @ full charge. I don't have an ammeter to test how much amperage is going through the bulb though. Is there anyway to do the math on paper to figure it out? Would using a resistor inline make more sense than a bulb? Say a 10ohm 1w resistor or something?

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An ammeter is just a low value resistor with a voltmeter across it.
1A through a 10 ohm resistor makes a voltage drop of 10V which is way too high and the measurement will be useless. 1 ohm or less is much better because the voltage drop is 1V or less, but then the voltmeter must be able to measure 0.5V or less, accurately.

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The 12.75V for full charge is measured when the battery has been at rest for a while with no load and no charging current. Typically, to charge a 12V SLA you would apply a constant voltage of around 14.5V without exceeding the maximum allowable charging current. When the current going into the battery drops to a low value (say 100ma or so for your SLA battery) then charging is finished. Then the float voltage should be reduced to around 13.8V or the battery disconnected from the charging source. These numbers I quoted are as examples only. The manufacturer of your battery will have the proper specs.
The bulb helps with the charging because it will reduce it's resistance as the battery voltage climbs towards 14.5V, as the SLA charges, thus yielding a quicker charge time than an ordinary resistor circuit would.
As for the bulb, you can use ohms law to calculate what the absolute max current would be. A 12W 12V bulb would pass a max of 1amp if the battery (heaven help it's life cycle) was totally discharged to 0V.
If you measure the resistance of the bulb (with an ohm meter) you will see how much resistance it will add to the charging circuit when the battery is at full charge and will see that it will introduce very little voltage drop at 100ma or less.

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I know that about charging an SLA, all the voltages etc. The reason I mentioned it is because it had been mentioned about charging an SLA @10v with a 14v source.

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i have a halogen floodlight with a smallish 12v sla in it, the "charging cable" just connects the sla directly to the vech. power outlet, via a fuse rated at a few amps. so far it hasn't blown up or leaked out or anything else foul.

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I was wondering about doing that. Obviously a 1amp fuse couldn't flow more than 1amp, and it can't be more than the vehicles power (14.4volts running). I may try that.

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