[Pb-Acid charger]Solution or Suggesting A better Way - Page 1
Forum section: electronics / projects

author: Mousam Upadhyaya
author_rank: Newbie
timestamp: June 24, 2021, 02:40:45 pm
content: So, I Designed a Simple Lead Acid Battery Charger for a Friend Of mine, I went to Google and Find either over Simplified or extra complicated one, so I designed this on my own (The moderate One ), I used an Adjustable Zener 431 as a Reference that somehow created a Stable 2.5 v which I fed this to inverting input of the Comparator, The Non Inverting one is fed through a Resistor voltage Divider of which one is a trim pot to set the threshold. So by comparing the two voltages and setting the threshold trips the base which then forces the Relay to "N.O" position and the Charging stops (theoretically).The Circuit Works as expected but the time it Fully reaches 14.4v (The threshold voltage) it Behaves like crazy and Switches Relay Back and forth at high speed and it no longer becomes a Battery charger, instead, like a PWM generator. I thought the OP-AMP is doing its business very Aggressively, So I Threw a Big Capacitor C1 to hold the base For a while. It seems that it worked but the Relay still Doing "Tik" and "Tok" (At slow speed). So my question is This an Inherit Problem of such chargers or I went too far or there any Improvement you want to suggest 🧐

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 24, 2021, 10:31:42 pm
content: Use a MOSFET instead of a relay as the switch, that way the rapid switching will not be a problem.

author: Benta
author_rank: Super Contributor
timestamp: June 24, 2021, 10:41:19 pm
content: Place a 470k resistor between output and non-inverting input of your 741. Hysteresis is your friend.And don't use a 741 for single-supply circuits. 1/2 LM358 will do the job better.

author: Mousam Upadhyaya
author_rank: Newbie
timestamp: June 27, 2021, 04:21:04 pm
content: Thanks, I was aware of it, but I have many of such Relays Laying around, and a clicking sound would be satisfying

author: Mousam Upadhyaya
author_rank: Newbie
timestamp: June 27, 2021, 04:27:17 pm
content: AHH, my bad I was using LM358-P, the same you suggested, I mistyped. would be helpful how 470k will help me, I'll try it shortly

author: Ian.M
author_rank: Super Contributor
timestamp: June 27, 2021, 06:35:42 pm
content: As Benta has pointedout, you need to add positive feedback round the OPAMP to provide Hysterisis so it doesn't oscillate. However I think the hystersis range should be greater than Benta's suggested 470K resistor would provide. Ideally it should start charging if the voltage is under 12.6V (fractionally under the resting voltage of a fully charged lead acid battery) and stop when the battery voltage reaches your 14.4V end of charge setpoint. The precise value of the feedback resistor will depend on the equivalent resistance seen at +in from your battery voltage feedback divider, but an order of magnitude greater would get you 'in the ballpark' Adjust the feedback resistor value to get the desired hysteresis range.Also your TL431 circuit as drawn is absolute nonsense. Use the shunt regulator circuit from its datasheet with resistors calculated to give about 6V for your reference voltage (10K lower, 15K upper for the Adj divider should be near enough) Use a 2K7 dropper resistor from your 15V supply to feed it, which will give about 3mA through the TL431, plenty for stable regulation.