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Multi Battery Charger for Lead Acid

I have a 0 - 15V, 30 amp power supply that I want to use to charge several small SLA batteries at the same time. The problem is the batteries are slightly different capacities and will be at various stages of discharge. What I want to do is have some way of limiting the current going to each battery to about 2 amps. What is the easiest way to do this?

Note, I want to get a minimum of 14V to the batteries, so whatever circuit I use needs to drop no more than 1 volt, preferably less.

I have searched Mouser and Digikey for current regulators but haven't found anything that looks like it will do the job. Obviously I don't know exactly what to look for.

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Chris W
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Reply to
Chris W
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I'm not sure why everybody is being so hard on you. You want a constant current source that has compliance down to .5 volt, and connect one such circuit to each battery. I have no experience with the constant-current circuits using FET's, but perhaps they would have a compliance range that goes as low. I know Win Hill mentioned recently in another thread that the LND150 depletion-mode fet, when connected as a constant current source (requires only one resistor), has a compliance down to .5 volts. Dunno if it would carry 2 amps. Just an idea to get you started, do a little research. I'll get back with a link to Win's post.

Reply to
gearhead

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Well, the jfet idea won't work. That's typically a few milliamps.

If you can deal with losing close to a volt, try this:

15v | ,---------+ | | 1K batt | | | | | _| +-------|_ NTD110N02R | | | | '------\\c | npn |-+ /e | | | | 0.3 | | | | '---+ | gnd

The mosfet has on-resistance of 4 milliohms or something, so at 2 amps the mosfet does not add significantly to the "dropout." You have one base-emitter drop subtracted from your 15 volts, leaving you with 14.3 or 14.4 volts... not bad, actually.

"ground" in the circuit diagram is obviously not battery negative, it's the negative terminal of the power supply, which is not common to the battery. You can get common ground if you want it, though, by flipping the whole circuit upside down and using p-channel mosfet and pnp transistor.

Fiddle with the sense resistor to get the current you want. Resistor power is I squared R and about doubled for safety margin, you want a two or three watt resistor.

The circuit will show some temperature dependence, not enough to notice unless the circuit is exposed to extreme winter/summer outdoor temperatures.

Reply to
gearhead

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