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Sorry if this question is a bit broad, or too part-suggestiony. I've been trying to find a good solution for over a month (and three board revisions) now, and I hope someone has an idea I haven't thought of.

I've got a sort of UPS system, in which I need to charge (at ~1a) a 12v, ~50ah lead acid battery (which has a continuous load of about 0.15a). I have a 15v or 12v supply that I can tap off of, and I have a micro that's monitoring the battery voltage anyway, so I can output a charge termination signal. Pretty basic, right? I first tried an LM317 CC system, but that dissipated way, way too much heat. Also it caught fire. I then spent way too much time trying to find a switchmode battery IC, but I never found one that was:

  1. less than $20 all told
  2. Not huge (pcb footprint wise, I only have about 1 cm^2 of board space).

I then tried to use a standard switchmode regulator with an adjustable current limit (the LM3663), but that didn't work, and I haven't found any other switching ic that met my requirements.

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    \$\begingroup\$ I think we need a diagram of the battery, what supplies are available, what outputs it's providing, switching output between supply and battery. If you've have several goes at getting it right and failed, then I think you need a proper specification. I'm certainly not going to start thinking detail until I've seen a better spec. \$\endgroup\$ – Neil_UK Mar 16 '16 at 21:39
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    \$\begingroup\$ "Also it caught fire." Way to bury the lede! \$\endgroup\$ – user65586 Mar 16 '16 at 21:40
  • \$\begingroup\$ @Neil Ok, I'll try and get a simple schematic up. \$\endgroup\$ – 0xDBFB7 Mar 16 '16 at 22:06
  • \$\begingroup\$ 1 square centimeter for your charger? You can't have tried a TO220 LM317 then, so you went for the SOT-223? With a minimum recommended Vi-Vo of 3 volts and an amp of current? Shudder. \$\endgroup\$ – WhatRoughBeast Mar 16 '16 at 22:24
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I think I've got a dead simple solution; just pwm charge. I've wired a high-side switch up to the battery from a 13.8v transformer, and by pwming the gate at low frequency I can get pretty reasonable average current control. I don't need the charger to be too fancy, so this'll be good enough.

It also injects 8mhz of high-frequency noise into the battery leads, at high current, so FCC testing this is going to be really fun.

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