12 V Lead Acid Charger

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ljbeng
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12 V Lead Acid Charger

Post by ljbeng » Thu Feb 13, 2003 7:53 am

I am making a hand held logger with a 12v lead acid battery inside. I want the user to be able to recharge by plugging unit into the car cigarette lighter. What is a simple way to limit the amount of charge the hand held battery receives from the car? Do I need to limit the charge? The hand held battery is similar to a camcorder battery (12V 2.0 Ah) Thanks.

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Chris Smith
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Re: 12 V Lead Acid Charger

Post by Chris Smith » Thu Feb 13, 2003 9:08 am

You need to limit the current rush only. The battery voltages and charge voltages are the same, but a large resistor of 100 watts or so, 5 or ten ohms in value, will stop the large battery and wires from attempting to rush in and charge the small battery too quickly. Wires tend to melt when over loaded. <p>Other than that, they are a matched pair as far as their both lead acid, and 12 volts, and require the same conditions.<p>You could also place a large light bulb between the two batteries charging the battery through the bulb, as a regulator, of around the size of the amp hour rate of charge of the small battery.<p>You will need at least a twenty or more watt bulb to do the job unless you want to trickle charge the battery.<p>[ February 13, 2003: Message edited by: Chris Smith ]</p>

Dimbulb
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Re: 12 V Lead Acid Charger

Post by Dimbulb » Thu Feb 13, 2003 9:32 am

A battery charging regulator can also shut off when it compares voltage of the battery being charged to a reference and also the temperature of the battery in the event the battery no longer is cooperating. I do not recommend using only a resistor, the light bulb idea is inexpensive 40 watt 110VAC type measure 26 ohms the temp curve 3/2 for tungsten.<p>[ February 13, 2003: Message edited by: dim bulb ]</p>

natcsparky
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Re: 12 V Lead Acid Charger

Post by natcsparky » Thu Feb 13, 2003 11:29 am

I agree with Chris. If the power outlet on the car is used for charging only, then you could use a smaller (less wattage) higher resistance (like the 10 Chris suggests, albeit at a lesser wattage). The data logger draw in this state is the unknown which must be accounted for. The following is based on a 0 mA draw at the data logger circuitry in the charge (quiescent) mode. Please advise if this is not the case.<p>The rule of thumb is to never charge a battery continuously at over 10% of its' amp hour rating (trickle charge principle). Working at 50% voltage as a design point (the battery voltage when EXTREMELY exhausted) would yield a potential of 7 volts accross the "R" isolator. Using 200 mA (trickle current limit) that works out to be 14 Ohms, multiplying the short circuit (i.e. 0 volts on the output or shorted battery) current of 1 Amp times 14 volts would yield 14 Watts power disipation on the resistor (worst case). A 25 Watt 10 Ohm or 15 Ohm resistor should work pretty well.
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Ron H
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Re: 12 V Lead Acid Charger

Post by Ron H » Thu Feb 13, 2003 1:23 pm

Rick, you boofed the resistor calculation. It comes out to 35 ohms, 5.6 watts. You could use 33 ohms, 10 watts.<p>Ron

ljbeng
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Re: 12 V Lead Acid Charger

Post by ljbeng » Thu Feb 13, 2003 1:48 pm

Thanks for all the advice. It sounds fairly simple.

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