steev

hi,
my dirt bike uses a small battery(lead-acid) of 12v 2.5Ah.
as i haven't used it since 1year,it seems the battery is quite dull.so i want to build a simple charger for it(13.8v charging voltage for 12v battery).
i did some search and found below to circuits.
http://users.otenet.gr/~ahisrod/powe...with_LM338.htm
http://www.vt52.com/diy/myprojects/o...er/charger.htm
one uses LM338 and another L200.can u guys tell me which one of these is better and safe,if iam planning to charge the battery for around 8 to 10Hrs.

thanks in advance

*BATTERY my mistake in subject line.sorry

mneary

Each of those chargers put out way too much current for charging a 2.5AH battery. I would use a "12V" 200mA unregulated wall wart. Just stop charging when the voltage reaches about 14V.

You could just ride the bike to charge it. If you don't plan to ride it, why charge it?

RadioRon

I agree that a simple wall adapter rated at 12V DC unregulated and about 1 amp or so could be made to work. You must put a resistor in the + line between wall wart and battery to limit the charge rate so that the wall wart isn't overloaded. Perhaps a value of 22 ohms might work well, but it would have to be a large resistor with a power rating of one watt or more. The wall wart should be the unregulated type as this type will often put out as much as 17 volts or more when they are delivering small amounts of current, and this is what you want. A wall wart that only puts out 12.0 volts maximum cannot charge your battery to 14.8 volts (which is the peak voltage that you will read while charging the battery).

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RadioRon

steev

actually iam not driving it regularly,so i planned to charge it separately.

so the supply of 14.8 volts with 200ma will enough to charge my 2.5Ah batter for 8-10 Hrs.
i have a 9-0-9 transformer with 500ma rating.with LM317 i can control the voltage to 14.8v.so is this setup gona work?

i don't believe in wall warts as the supply from them is quite unregulated.

RadioRon

Yes, that should work. I recall that the battery manufacturers prefer that you push from a slightly lower voltage than 14.8 volts if you are leaving the charger on the battery for long periods. It might be only a bit less, like 14.2 or maybe 13.8 volts, I can't recall. So if you can have two settings of voltage, that would be helpful. You can push a bit more than 200 mA for a faster charge if you want. It would be quite alright to go with 500 mA (ie. a lower value of resistor), but you might want some sort of method of lowering the current when the battery voltage reaches its peak value. Even cheap off-the-shelf car battery chargers do this.

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RadioRon

stevez

Another approach - configure an LM317 as a current regulator possibly with two resistors in series to set the current limit. The resistors would be selected so that 200 ma (or whatever you determine) current is applied until you switch to something less - maybe 20 ma, 50 ma - as a holding charge.

See the National datasheet for the LM317 - on page 18 is Precision Current Limiter. You would make R1 out of two resistors - R1A and R1B - in series. If you wanted the low current to be 50 ma then the sum of R1A and R1B would be about 24 ohms. If you put a switch across R1A it would effectively take it out of the picture when the switch is closed - so make R1B about 6 ohms for 200 ma.

You 9-0-9 transformer could be used to deliver 18 vac to a bridge rectifier - and you have 20 volts or more available. If you used just a 0-9 or 9 vac you might not have enough voltage to result in the desired current flow.

Caution: using a potentiometer (variable resistor) might not work if the pot is adjusted for current flow that exceeds the limits of the pot.

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stevez