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Author Topic: Charging 12v SLA from a vehicle.  (Read 1680 times)
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I know this isn't directly related to the arduino per se, but it's involved in an arduino project of mine. I'm powering this project off of a couple 12v sealed lead acid batteries.  I'm looking to be able to charge these batteries from the vehicle itself.

First of all, I'm not dead set on 12v SLAs, but they seem to make the most sense since I'm running 12v lights from them.  12v lights not negotiable.

I found a commercial product that also charges 12v SLAs from a vehicle (cigarette lighter) and was surprised to find no charging circuit at all!  It has a direct connection from the cigarette lighter to the batteries, with a 20A fuse.  Is this ok?  I like the idea as it would certainly reduce the complexity of my design, but it ok to charge batteries this way?  If not, what would be a preferred method?

I've read alot about battery charging circuits, but they all tend to assume that I'm charging from 110/220AC.
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I found a commercial product that also charges 12v SLAs from a vehicle (cigarette lighter) and was surprised to find no charging circuit at all!  It has a direct connection from the cigarette lighter to the batteries, with a 20A fuse.  Is this ok?  I like the idea as it would certainly reduce the complexity of my design, but it ok to charge batteries this way?  If not, what would be a preferred method?

That's fine. That's essentially how the automotive battery in the car itself gets a charge. The voltage in the car's electrical system will hover right around the correct voltage for keeping a 12v lead acid battery topped up. (typically around 14v).
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Thanks ill.  That's about what I expected.  I suppose I need to make sure that the capacity of the batteries I use stay well under what I expect the vehicle's to have.  I would like to be able to leave these in to charge without leaving me unable to start the vehicle.

What happens if these batteries get plugged into a 12v cigarette plug wall adapter?  Will it have the same effect?  It just 'feels' odd to me to keep supplying a full battery with full voltage with no control over current.  I suppose that's what happens in a running car, though.
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Connecting an sla direct onto a car battery system will almost certainly lead to premature failure of your sla.  These batteries have quite stringent charging requirement and the lack of current or voltage control will lead to failure.  There is also the danger that the battery itself might well explode due to excessive gassing resulting in pressure build-up within the battery.

Google "sla charging requirement" and you'll get a host of info.

Any company that manufactures or sells what they describe as sla charging leads for use within a vehicle that contain nothing but a fuse are potentially criminally negligent

jack
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The only thing is, what's the Ah rating on that SLA battery? Car batteries are pretty good at handling abuses (i.e. high charging current/high temperature/etc.), but if your SLA battery is less than 20Ah, it probably wouldn't hurt choosing one charger that is rated to charge batteries in that class (normally SLAs are not recommended to be charged at over 1/10c rate).
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Although a little pricy, the Triton brand chargers by Electrifly are are great and just about the prefered standard for R/C use. I have used mine for LiPo, NiCd and PB charging specifically through a 12 VDC automotive accessory socket.

http://www.electrifly.com/charger.html
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Charging SLA battery is a good challenge for hobbyist.
I am trying to make one UPS, that requires to build inverter and Charger.(I guess it will publish in forum later smiley )
I done the inverter but programming the charger unit now...

You needed to build one Voltmeter to detect battery voltage which is  too easy to build one.
Grab 4x20K resistor and build voltage divider of 1/4 since resistors are same (that produced in same time) they are act like (or better than) precision resistor.

After you can control a Power Darlington (like TIP145 or BDX33) via PWM. I choose darlington transistor for small base requirements. And you needed to amplify PWM out voltage to charge voltage to feed BDX33. (You can use opamp(LM358) or optocoupler (pc817)).

Since you are sensing voltage of the battery, you can limit charging voltage to 14.40. But if battery is empty, you cant reach that voltage at start of charging...

Problematic part is sensing charge current. You needed to have constant charge current. How to sense current? I can't make it proper yet. But there is much of IC that senses current. You can use shunt resistor. It's possible to use OPAMP to amplify voltage drop/difference of shunt resistor and reference it to ground to read via ADC of arduino. But I have some problems in action, sensing current properly...  Better to use circuits like MAX4172 will give you better results.

Most important part is using FUSE to avoid short circuiting battery. There is too much amperage that will molten any connections and wires in case of short circuit. Quite possible to start fire!.(I am almost  ;D) I placed a 40Amp fuse to Battery connector directly...

Also read here to learn how to charge battery...
Good lucks.
« Last Edit: October 13, 2010, 10:49:58 pm by E.U.A.. » Logged

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