Constant current, constant voltage 12 volt battery charger questions please? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 03, 2024, 09:47:08 pm
content: I need to have a permanently connected battery charger on a 12V lead acid battery for a 20kW diesel standby, auto start generator. Ones marketed specifically for generators have a high price, can any "battery maintainer" type chargers be used on a battery where the alternator on the generator's engine will start making output and charging when it is fired up? (I.E, back feeding the trickle charger, if I put a hefty diode in one of the leads between charger and battery?) I realise there would the be the voltage drop across the diode to consider.Or on a conventional constant current / constant voltage charger would that diode stop it "seeing" the state of the battery it is charging?I was thinking of building this, 2 Amps might be enough to keep the battery topped up and just running the 100mA or so the control panel draws in standby mode?https://www.engineersgarage.com/designing-12v-lead-acid-battery-constant-voltage-limited-current-charger-for-ups-part-2-17/Thanks.

author: TerminalJack505
author_rank: Super Contributor
timestamp: January 04, 2024, 03:08:51 am
content: Placing a diode in series with an off-the-shelf battery maintenance charger will likely confuse the charger circuit since it will be monitoring the battery's voltage and the diode will add an additional 0.7V or so.I'm guessing most of the off-the-shelf lead-acid battery chargers (even the low amperage maintenance chargers) will use the 4-state method of charging. (Reference.) The charging 'state' is determined by the battery's voltage, so the diode will cause an improper voltage reading.Like you say, you might be better off rolling your own circuit that is always in the float-charge state. (This is what most people call "trickle charge" but the document I reference uses that term differently.) The circuit you reference looks like it could probably work. In the float-charge state, you basically want to apply a low current to maintain a certain battery voltage. This voltage would be set with R2 and R3 in your circuit. The maximum charge current will be determined by R1. The diode should keep any current from flowing back through the circuit.You might stick your circuit in a simulator and see what happens in the case that the battery voltage exceeds the programmed (via R2 and R3) voltage. You would want it to stop charging in that case since either 1) the alternator is running, and/or 2) the battery is at risk of being overcharged. So far as I can tell, it should stop charging in this case.

author: alligatorblues
author_rank: Regular Contributor
timestamp: January 04, 2024, 06:48:56 am
content: You should have a fuse in the charger line too. If you ever get a short, you want that fuse.

author: MarkF
author_rank: Super Contributor
timestamp: January 04, 2024, 01:04:27 pm
content: This Gel Cell Battery Charger circuit may fit the bill:https://www.eleccircuit.com/gel-cell-battery-charger-circuit/The potentiometer (blue) will set the charging voltage and the resistor (red) will set the max charging current.As the battery voltage approaches the charge voltage, the current will reduce to a trickle.