Project: Battery charger regulator

Thread Starter


Joined Apr 20, 2004
This circuit comes from some years ago as a part of a project we did to make it possible to keep a charger on a battery continuously without it overcharging.

It's mostly a junk box project, at least as far as the components used in the one I held onto. This is like late 1970's technology grafted onto 1960's.

The regulator board is a replacement for whatever was in an old Monkey Ward (that is properly Mongomery Ward) charger, rated at 15 amps. The components labelled T1, D1 & 2, Q3, SW1, the ammeter, and the circuit breaker are all part of the charger.

Other components:

Resistors R1 - 7 - 5% carbon 1/2 watt (1/4 watt should be fine)
Resistors R8 & 10 - 1% 1/4 watt. Oops, R10 should be 2.26K
Trimmer R9 - any multi-turn trimmer will do. Single-turn is not good.
Op amp Z1 - any Op amp should do, as long as it's better than a 741. It's used as a comparator, so good input impedance is better. The CA3140 was the hot op amp back when.
D3 - any 2.5 volt reference diode will do.
D4 - any 1N400X will do.
D5 - this is a 4.7 volt zener. $00 mw is fine - no tolerance necessary.
Q1 - we had cases of 2N1613's. Used them for everything. It's a switch, so 2N2222 is a good sub.
Q2 - had bunches of these, too. Just about any PNP will sub.


The output from the full wave rectifiers is pulsating DC. The diode D4 isolates the op amp from these pulsations, and C1 filters it. Z1 is set up as a comparator with the D3 reference diode presenting 2.5 volts to pin 3. As long as the voltage on pin 2 is less than that, the output will be high.

In operation (with the switch in the regulated position) the voltage on pin 2 will reflect the charge on the battery during the low portion of the rectified waveform. If it is less than 2.5 volts, as set by the R9 trimmer, then Z1 output wil be high. That will turn on Q1. In turn, that will turn on Q2. As the rectified voltage increases, so will the voltage between R6 & 7. This will gate Q3 and allow charge to flow to the battery.

When the battery is up to charge (ideally, 13.6 volts), the voltage on pin 2 of Z2 will be high enough to make the output go low. This will leave Q1 & 2 off, and the SCR will not gate again until the battery charge falls a bit lower.

By using a meter, R9 may be adjusted for a very precise charge on the battery.



Joined Jun 24, 2008
I was wandering if we can replace IC CA3140E with other subs. Any suggestions ?
I am trying to develop a regulator-rectifier circuit for automobile engines but cost was major issue. I really want to bring it as down as possible.
Any feedback on it will be highly appreciated.


Joined Jul 17, 2007
the value of Q3?
Good question, actually.

I'll suggest that the current rating of the SCR you choose should be based upon the current rating of the transformer that you select. Double the current rating for long life. Voltage rating is up for grabs, but just about anything over 50v should do fine.


Joined Jun 13, 2008
However this circuit is ok for NiCd or NiMH batteries.
But best way to charge a Lead Acid battery is with a current limited voltage regulator. This way sets a maximum current of the reg. that will flow to charge the battery. If the current rises over this set current, the regulator will put out a lower voltage. Since voltage drops, so will the current; hence current limited.
While the battery is charging, the current should decrease slowly while voltage starts to increase. In the end the current will be next to zero and the voltage will be equal to the set voltage.
Here is a simple schematics at the bottom of this page.

Or you can go simpler with this however cannot go unatteded:


Joined Jun 24, 2008
What would be the best way to switch ON/OFF a SCR ?

I wanted to use it as a switch in a 12V lead acid charger circuit; i.e when battery voltage goes 14.2 V, STOP charging when it comes down 12V start charging with some 10-15 Apms charging Alternator (after rectification) !

I was thinking to use comparator (IC 741) based switching circuit. I want to make it robust with fewest number of components.

Any ideas will be appreciated.

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Joined Jun 24, 2008
Well, my query was related to beenthere's circuit posted in this thread above; where SCR is working as a switch. I want to modify that trigger circuit with very few components so that rating may improve and it may charge battery fastly with 10-15 Amp single phase alternator !


Joined Jun 24, 2008
Any suggestions on "Specific Voltage comparator" for comparing battery voltage and then triggering SCR ?

I looked all around maxim ICs and other stuffs, it seems op amp based comparator are costly affairs !


Joined Jul 17, 2007
LM393, LM2903, LM339, etc. - would all work. However, you have to look at the datasheet. If it has an open collector output, you'll need to add a pull-up resistor.


Joined Nov 20, 2005
for a car 14.2 is a little hi. TRY 13.68v. An lm317 can do that meaning floating charge. Your way overcharge and let it sit undercharged foruntil charge again.that maybe days before it gets there.Use an lm117 with the output no load set at 13.68v the add a .05 resistor from out to battery to limit the current. WORKS FOR MEshould work for you.


Joined May 12, 2009
Im in the process of building the original regulated battery charger circuit posted.

Im a little confused as to the "Z1" connection. Where does pin 4 go to from my amplifier?? if anyone could offer some assistance it would be greatly appreciated. Im a bit of a noob so instructions in detail if possible, pics or drawings even better. If i get this circuit running id like to fit it into a 6x6x4 box with 120v cord end and + and - leads to the battery. Tryin to impress my teacher but shes gotta work first.

thanks again.
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Joined May 19, 2009
selecting a transformer is a good deal, you need minimum 5 amps transformer to make a battery charger. To include float chargr youneed a good basic knowledge.