I want to build a lead-acid battery charger which can reach an average charge current of 3 to 4.5A, and start with a prototype. The charger needs to be supplied by the mains, up to 250VAC, for example. I was looking for some circuits on the web and did selected the circuit below.


I want to check your considerations and recommendations about this circuit and the components that I have choose (list below). My list of components would be:

  • D1, D2 = B560A, SMC package (60V x 5A schottky)
  • M1 = AmpMeter or short-circuit
  • Rx = 1R or 1.5R or 2.2R, 10W, wirewound
  • R1, R2, R4 = 1K, 1210, 0.5W, 5%
  • R5 = 10K, 0603, 5%
  • R3 = 470R, 1210, 0.5W, 5%
  • P1 = 5K PCB potentiometer, PN CT94EW502‎ (Digikey)
  • D3 = SMD 1N4007, SMA package
  • D4 = BZX84C5V6, SOT23 package
  • C1 = ceramic 10uF , 0805, 16V

  • SCR2 = NYC0102BLT1G (Littelfuse Inc.) https://www.digikey.com/product-detail/en/littelfuse-inc/NYC0102BLT1G/NYC0102BLT1GOSCT-ND/2705214

  • SCR1 = MCR12DSMT4G (Littelfuse Inc.) https://www.digikey.com/product-detail/en/littelfuse-inc/MCR12DSMT4G/MCR12DSMT4GOSCT-ND/964541

  • Both theses SCRs have sensitive gate current (low IGT), lower prices and available in high quantities on Digikey.

  • Is it suitable to use SCR1 as sensitive gate? Or it will be better to use a standard SCR for this part?
  • Also, would it be better to select a higher temperature grade for SCR1?
  • Is it possible to make a low cost protective circuit in case inverted battery connection occurs?
  • What would happen to the output voltage when the battery is not connected to the circuit? If we considerar that the floating voltage of the battery is 13.8V and we set the potentiometer accordingly, the output voltage would be limited to 13.8V when the battery is not present?
  • Also, you can suggest other good circuit for the same purpose (3 to 4.5A average current charge), maybe also a switching circuit.

Next month I will buy again on Digikey, so if you have some recommendations, I can buy any component present there for tests, giving preference for price and quantity available.

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    \$\begingroup\$ I want you to specify Vout vs Iout with tolerances and temp rise and what type of Pb battery and if you considered temperature compensation. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 3 at 18:59
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    \$\begingroup\$ Do you know the overvoltage protection is just the alarm ON (AVISO) with no safety shutdown. Maybe Change Fuse to polyfuse as there is no current control or limiter and with +/-10% tolerance on 240?? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 3 at 19:11
  • \$\begingroup\$ @Tony: I will try to answer your questions. But initially I would use 12V sealed lead acid batteries, of 7Ah and 12Ah, for low power systems. Like these: [ powerkingdom.com.cn/high-rate-12v-12ah-ups-battery ] and [ powerkingdom.com.cn/12v-7ah-ups-battery-with-heat-sealed-design ]. \$\endgroup\$ – abomin3v3l Jan 3 at 20:50
  • \$\begingroup\$ I am prototyping a simple machine. In my system there is the mainboard, and there are some types of slave board, there is a modbus-like protocol over EIA485 between them. The mainboard requires 12V, 5V and 3.3V to fully operate. Currently, I am using a low cost desktop PC 200W PSU to supply it. I want the machine could operate for some time when there is no voltage present on the mains, a battery would be the solution. I maginated this: MAINS -> BATTERY CHARGER -> 12V BATTERY -> 12V INPUT ON THE MAIN BOARD -> TWO BUCK SMPS, ONE TO REDUCE TO 5V AND OTHER FOR 3.3V -> 5V AND 3.3V VOLTAGE INPUTS \$\endgroup\$ – abomin3v3l Jan 3 at 21:04
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    \$\begingroup\$ SMPS are off-the-shelf but you want a charger that has CV and float/cutoff for SLA batteries and this charger does not have cutoff. SCR's are Latches for DC so the SCR's do not meet this critieria \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 3 at 21:11

I want to build a lead-acid battery charger

Stop stop stop.

It turns out battery charging is really hard to do well.

Lead-acid batteries are fragile, actually, and take damage from almost any kind of abuse (and even normal use). A less-than-ideal battery charger will destroy a lead-acid battery in months, and SLAs are even more vulnerable.

The only thing that really works properly is a 3-stage battery charger. These are complex enough that homebrew designing one as a starter project would be madness - you could spend months on it.

So the battery charging problem should be solved with a prefab module from a competent supplier, that performs full and proper 3-stage charging. Save your valuable time for the meat of your project.

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