Electronics > Beginners
19V PSU charging 12V lead acid battery
nuclearcat:
--- Quote from: magic on September 16, 2019, 07:19:52 pm ---I have a PFC choke harvested from an ATX power supply which is about 1mH and capable of a few amps. Hopefully that's typical for similar chokes.
19V-12V is 7V across the choke and therefore 7A/ms rise of current during on time, 12A/ms fall during off time. With 100kHz PWM that seems like it ought to be less than 100mA ripple (too lazy to do exact math :P).
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Or, dunno, just get a PSU which current-limits at about 0.5A into 12V (to be determined empirically :)) and you will get the kind of pulsed waveform you initially wanted, averaging to 0.25A. I don't know how it affects battery chemistry, though.
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It seems i will buy some small batteries, lock them in explosive-proof container with ventilation, and will do tests in extreme modes. I think its best way to know.
wraper:
--- Quote from: KL27x on September 16, 2019, 06:18:45 pm ---I say why not?
With the battery on there, you have a buck converter, essentially. Car battery is not picky. As long as the PSU handles it, I don't see any problem.
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:palm: It's as far as you can get from buck converter. You are shorting charged capacitor into battery with very low internal resistance. If this somehow works and pass element for PWM won't fail due to very high peak current, then output capacitor of PSU will certainly fail after some time due to extreme ripple current.
magic:
Another option: just screw PWM altogether.
Low power 12V PSU bricks are common everywhere. Find one with 250mA max current, then it's a matter of changing or adding one resistor to increase its target output voltage to 14V. Then as long as battery voltage is less than the target, the PSU will just keep charging with its maximum current.
nuclearcat:
--- Quote from: magic on September 16, 2019, 08:46:18 pm ---Another option: just screw PWM altogether.
Low power 12V PSU bricks are common everywhere. Find one with 250mA max current, then it's a matter of changing or adding one resistor to increase its target output voltage to 14V. Then as long as battery voltage is less than the target, the PSU will just keep charging with its maximum current.
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Good option as well, thanks for idea.
KL27x:
--- Quote ---Certainly not a buck. As far as a 19V PSU is concerned, the battery across its output is a dead short. It will dump its maximum output current into it. The output capacitors will rapidly discharge when the switched is turned on, possibly shortening their life after many such cycles.
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Ok, geniuses.
You've gone this far. Add the frigging inductor. In this case you don't even need an inductor. Just a power resistor in series. Done.
Battery is your Q. Add resistor. You have a low pass filter and the power resistor drops most all the excess voltage. As I said, as long as the PSU is happy to provide the juice, you're good.
Is there another way to do it? Yeah. Is it better? No. This is a perfectly adequate and perfectly sensible way to charge a lead acid battery, if you desire to have voltage sensing/cutoff and adjustable control over max current and you already have a 19V PSU on your hands. (IMO). If efficiency is important, you could build or buy a proper buck converter with inductor.
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