Building A 2-Stage 12 Volt Lead-Acid Battery Charger

(Click on images for full resolution)

Over the years, I've designed and built many battery chargers.  This one, although not as sophisticated as some, will do the job of keeping your 12 volt lead-acid batteries in shape.

It is designed to be both a slow charger (constant current or "Boost" charger) and a maintainer ("Float" charger).  In other words when connected at all times when the battery isn't being used and when working right (13.7 volt float), it will actually prolong the service life of your battery.  I have had a cheap auto battery on a standby generator that lasted a little over ten years while being float charged for the whole time.  Incidentally, when a flooded cell battery is in good condition, it will lose very little water, only needing a top-up every year or two.

This project is for the reasonably technically inclined.  It is slightly more sophisticated than a simple float type  maintenance charger.  It can be used with lead-acid batteries of all types, both flooded and gel cell.  It should also should work on all batteries from very small 1 Amp/hour to over 100 Amp/hour.  Depending on the capacity of the battery being charged and it's state of charge, charging time to float could be as much as a week.


Boost mode.                                                                        Float mode.

All parts should be readily available through any parts house such as DIGIKEY.COM, MOUSER.COM or others.

For this charger, I simply dug through my parts bins and junk box.  The transformer is out of (I think) a Malibu light power pack.  Any transformer of 2+ Amps rating at about 18 Volts will do fine.  The bridge rectifier also came out of something I junked.  With things like rectifiers, it's always wise to get one rated for at least twice the maximum current it's expected to draw and several times the PIV (Peak Inverse Voltage) you think it will be subjected to. 

The meter is another junk bin retrieval.  I removed the internal resistor that made it into a voltmeter and substituted a shunt of about 0.05 Ohms (also not shown) and scaling resistor to make it into a 2 amp full scale meter.  This is not required but does give some indication of the state of charge of the battery.

NOT SHOWN is a fuse in the 120 volt (220 volt in Europe, etc) line.  The value of this is determined by dividing the primary voltage (in my case 120V) by the secondary voltage (in my case 18V) to get a ratio (6.67).  Use this ratio to determine the primary amp reading by dividing the full load secondary amps (2A) into the ratio (6.67) to get a primary amp figure (0.3A) amps for the full load current draw.  Select a slow blow fuse that has a rating of up to about 50% over that number (for this example a 1/2 amp fuse should do).  You will most likely need a slow fuse versus a fast one because of the inrush current as the filter capacitors are charging.

Schematic of the charger.

There's nothing special about this circuit.  It consists of a dual comparator that is used to switch the MOSFET output transistor.  The upper comparator sets the float voltage of 13.7 volts and the lower comparator sets the boost voltage of 14.2 volts.


In operation (assuming a roughly half-discharged battery of about 12 volts across the terminals) when first connected, both comparators are turned on and, since the boost comparator is set to the higher voltage, it determines the charging voltage until the boost voltage setting is reached.  While the battery is in boost mode, both LEDs will be on until the voltage on the battery passes the float voltage, at which time the green LED will extinguish leaving only the red one on.


When boost voltage is achieved, the boost comparator turns off (both LEDs off) and all charging will cease until the battery voltage slowly falls to the float voltage setting.  (The time this takes is determined by the battery capacity and  state of charge)  When the battery voltage has fallen to the pre-set "Float" voltage, the green float LED will begin flashing.  It will be faster at first then slow to about a two flash per second rate as the battery stabilizes.


Note R8/R9.  These resistors have been selected to provide for about one volt of hysteresis or toggle so once the float voltage is reached, the comparator will remain turned off until the battery voltage returns to 13.2 volts more or less.  When charging a deeply discharged or large battery, the charger may cycle several times from boost to float back to boost, etc.


The light bulb, a TS-1047 may not be available.  I have a bunch of 'em and I think they are some kind of military surplus spotlight bulb with two paralleled filaments, rated at 28 volts and 4 amps.  You can use any lamp or parallel hookup to achieve about 1.5 amps at about 13 volts while in boost.  This is to limit the maximum available charging current and protect the transformer and output resistor from overload.


R3 and R5 are ten turn trimpots.  Refer to the schematic for the adjustment procedure.  If the schematic is unclear, just click on it and the full sized image will be displayed.  You should also be able to copy it from my webpage to your 'puter for printing.


Any components that seem superfluous are used to improve stability.


If  you don't like the slow flash of LED in float, it can be sped-up by lowering the value of C5.  If it is lowered too much, depending on the layout of your wiring, the circuit could become unstable.


In the photos, the heatsink is simply a piece of aluminum bent into a "U" shape with a mounting hole for the transistor.  The transistor is electrically insulated from the heatsink.  If I had one in my junk, I'd use a much larger heatsink because the transistor gets HOT when going full bore in boost mode.


I used a piece of perfboard and an IC socket for the components and used point-to-point wiring.  This is not the best way to construct a circuit of this type but, since I closed my business, I have no means to etch PC boards.  If someone wants to do a layout and test it, I would be happy to add it to this page.  (Please send it as a JPEG image).


12 July 2017:

In response to a numerous request, here are some more float charger schematics:


I used this one to keep a motorhome house battery charged.  It will work fine as an AC powered charger if a suitable transformer, rectifier and filter are used.  D1 is used to prevent the battery from discharging back into the regulator if input voltage is below regulation voltage.

Solar Charger.


This one has a rudimentary overtemp trip circuit so, if the LM7805 regulator gets uncomfortably hot, it will shut be shut down.  Note that the 7805 has an internal overtemperature shut down but it gets pretty hot before it operates.  The 1N4007 diode keeps the battery from discharging through the circuit if the AC power is lost.  The 10K resistor at the meter + lead (1N4007 cathode) keeps the leakage in the diode from causing slight overcharging of very small batteries.

Charger with over-temperature shut-down.


This one has an added current limiting circuit for output transistor and small baqttery protection.

Current limiting charger.


This is about as simple as you can get.  Regulation relies on pushing the bandgap of the LED and small switching diode by changing the current through the diodes.  It works reasonably well.

Simple workable charger.

All of these chargers except for the two-stage version are constant voltage ("float" or "maintenance") chargers and can be left on continuously, prolonging the life of the batteries and minimizing water loss.

Always remember to occasionally check the terminal voltage of the batteries to make sure the charger is working correctly.  

The float current will vary slightly with temperature.  

If you notice that the float current is slowly rising without any load on the battery, it may indicate that the battery is nearing the end of it's useful life.


Good luck!  If  you have any questions or comments, I can be reached at:

[email protected]