Battery Charger Design Reviews - anyone? - Page 1
Forum section: electronics / projects

author: xibalban
author_rank: Regular Contributor
timestamp: May 31, 2015, 02:16:05 pm
content: The above is a schematic of my design for a 12 V lead-acid battery charger using a 18 V (peak) solar panel. I've employed Op-amp LM324P as the heart of the system. The first U1A acts as a multivibrator to flash a red LED1 when battery voltage drops below 11.8 V, U1B acts as a comparator to light a green LED2 once the battery reaches a voltage of about 13 V and to switch the load on (indicated by LED4) while U1C turns the charging MOSFET off at the same instant. During the charging period, the yellow LED3 (in parallel with the MOSFET) acts as charging indicator.I've simulated it on the computer but I'm yet to test this on the breadboard and wanted to review it first. My concerns are:1. Has my placement of yellow LED (which indicates "charging" status), been proper?2. If I were to use the remaining pins of the op-amp (U1D), and construct a fixed frequency AM receiver around it, would the multivibrator (low charge indicator) interfere with radio reception?Cheers!!

author: HalfSpace
author_rank: Regular Contributor
timestamp: June 02, 2015, 09:13:09 pm
content: Hi Xibalban,Looking at your circuit I don’t see how Q1 will be turned on by U1C. Q1 gate needs to be a few volts higher than its source to turn on, but Q1 source pin is connected to the 12V battery and so is U1C positive supply pin so how does U1C put out a higher voltage than its supply rail to turn on Q1?LED3 would be off if Q1 was turned on charging the battery which is the reverse to what you want.The circuit below may be better a way of connecting the solar panel. You could use a PNP transistor instead of the Pch Mosfet and D2.HalfSpace

author: nctnico
author_rank: Super Contributor
timestamp: June 02, 2015, 10:31:20 pm
content: Beware the LM324 doesn't swing to ground so it might be your charger FET never turns off. I'd simulate this circuit first before committing to a prototype.

author: xibalban
author_rank: Regular Contributor
timestamp: June 03, 2015, 02:21:37 pm
content: Quote from: HalfSpace on June 02, 2015, 09:13:09 pmIf you observe carefully, the MOSFET's source, though appears to be connected to the positive supply, is in fact connected correctly (just observe closely). The reason for connecting the charging indicator LED in parallel to the MOSFET is to ensure that it draws minimum current, and the bulk of the charging current flows through the MOSFET to sufficiently charge the battery.I ran a simulation on "Multisim" and it works as expected. Thanks for your advice though.

author: xibalban
author_rank: Regular Contributor
timestamp: June 03, 2015, 02:23:08 pm
content: Quote from: nctnico on June 02, 2015, 10:31:20 pmDoesn't swing to ground? Could you explain please! Are you saying that LM324 doesn't in fact behave like an Op-amp?

author: metacollin
author_rank: Contributor
timestamp: June 03, 2015, 02:59:58 pm
content: Swinging to ground has nothing to do with op amp behavior. Most op amps powered from a single supply will not swing to ground. In fact, none of them do, but certain ones (like rail-to-rail op amps) can get 100mv or even some millivolts above ground, but they'll never actually reach it. The only way to get an op amp to swing to ground is with a negative voltage rail.That said, whoever said the lm324 doesn't swing to ground is off their rocker. I just double checked the datasheet, and in single supply mode, it swings to within 5mV of ground, 20mV maximum. For driving NFETs, 20mV is much too low to cause any problems, there is no issue with the op amps output voltage range in this circuit. They should drive the mosfets just fine.

author: metacollin
author_rank: Contributor
timestamp: June 03, 2015, 04:12:25 pm
content: You say this circuit works in multisim? I just built rebuilt it in LTspice and it doesn't seem to work at all. The solar panel is able to trickle charge the battery through the LED in parallel with the charging FET, but enough voltage difference between the MOSFET's source and the gate ever develops to turn it on. And yes, I used the same components as you, and even a proper SLA subcircuit model for the battery.Unless I made an error transcribing the circuit, but I've checked it several times...and I don't actually see how that FET could ever turn on in your schematic ether. But it works in multisim you say? Hmmm. I have no idea.

author: xibalban
author_rank: Regular Contributor
timestamp: June 04, 2015, 01:42:38 am
content: Quote from: metacollin on June 03, 2015, 04:12:25 pmWell, I've re-done the schematic and attached with this post. I couldn't figure-out which component to use as "rechargeable" battery so I've used a variable voltage DC source in the simulation. The Solar Panel (18V) may be plugged or unplugged (denoted by the switch S1). You say that you used SLA Sub-circuit model for the battery, and I seem lost here.Thanks for your observation.

author: HalfSpace
author_rank: Regular Contributor
timestamp: June 05, 2015, 10:39:03 pm
content: Hi Xibalban,Looking at your modified circuit I don’t see how Q1 will now turn off. If Q1 was in its off state its source pin would be at around -4V but its gate can never be at -4V to turn it off because U1C output cannot go below its negative supply rail pin which is at 0V.Adding a resistor and zener to the gate circuit of the Nch FET should resolve it. (See below)HalfSpace

author: nctnico
author_rank: Super Contributor
timestamp: June 06, 2015, 11:23:48 am
content: Quote from: metacollin on June 03, 2015, 02:59:58 pmBut if you look at the specs they say with a load of 10K Ohm. I ran into trouble with the LM324 not really swinging to ground with some load attached to it.