I have thought of making a smart lead acid battery charger using the following schematic. I am not sure if the circuit will work the way I expect it to, or not.

My expectations are:

  1. The battery starts charging at constant current mode through UI, as the relay's NC connects input to U1.
  2. When the closed circuit battery voltage crosses 14.1V (or a little more,) Zener breakdown happens in ZD1 and transistor TIP31 starts conducting. The relay turns on and now U2 starts getting input instead of U1.
  3. Now the battery charges at constant float voltage of approximately 14.2V (considering forward voltage drop at D2, the R4 is set in such a way that the output of the voltage regulating part is little higher than 14.2V, e.g. 14.5V.)
  4. I don't want to keep the charger always connected to the battery. So, when fully charged, I would disconnect the charger manually. This means I don't want the charger to make any extra effort to start or stop charging automatically.

The voltage values might slightly vary, I know. But, will the following circuit fulfill the expectation? Will this circuit fulfill these expectations? I am also in real doubt if I am going in the right direction to charge an SLA battery in 2 modes - first fixed current then fixed voltage. Please let me know if my expectation are right in order to make a working charger.


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Might help if you explain how it works. What causes the relay to turn on and off. My main observation is that the LM317 will probably spend most of its time in thermal shutdown unless the charger current is very small, or the heatsink is very big. \$\endgroup\$
    – user57037
    Apr 20, 2017 at 16:53
  • 1
    \$\begingroup\$ I applaud your attempt to "roll your own". You might however have a look at ICs designed for this purpose. Even if you do not wish to go that route, the datasheets and app notes do a pretty fair job of discussing the stages of battery charging. There are both analog and switch-mode types available. It's been a while, but I recall TI having a number of battery-charger controller ICs. \$\endgroup\$
    – Tut
    Apr 20, 2017 at 17:34

3 Answers 3


Good on you for trying to design your own circuit.

Rather than dissect your circuit, which I think has a few problems, I'd just like to point out a few things with it, analyse what's going on more thoroughly, and suggest you re-think your approach.

First, I'd suggest you try simulating this circuit; perhaps using CircuitLab, if you can stand the nagware, or LTspice or whatever, by simplifying the circuit (substitute a const-current source for the U1 LM317, just put a wire instead of the relay in NC or Open states respectively, a +V source for the battery, and see what your important node voltages & currents are.

If simulating doesn't work out for you, just build this thing, and measure and observe! This will be the best teacher.

One problem that should come up is the TIP31 transistor (BTW it's not a Darlington like the symbol you've used, but that's not important for the analysis) - there will be a ~0.7V drop across base-emitter junction, which I suspect you've not taken into account?

Also, think about the interplay of the amount of current needed to pull in the relay, the beta (amplification factor) of the TP31 transistor at that low current (that's a mighty big transistor for this job!), and therefore how much base current you need - is that not too small a current for this insensitive beast of a transistor? - and where that puts you on your chosen Zener diode's characteristic curve, and what the actual battery voltage will need to be to activate the relay. You will need to dive into the datasheets for all these components to work it out.

More fundamentally, think about what happens when the circuit switches from CC to CV mode, what happens at the D1/D2/R5 node, what effect that will have on maintaining the relay engagement - might you have a problem with oscillation?

Also, what happens if power is applied but without the battery connected?

And if you do actually make this circuit for reals, don't forget to do your resistor power calculations, particularly R1!

Solutions to this kind of application typically don't take your approach, because of the short-comings I think you'll discover, in time. You're going to need a more accurate and stable comparator than a Zener driving the base of a transistor, likely an op-amp comparator chip, and that comparator will likely need to be configured with some hysteresis to avoid oscillations in the system's state switching between CC & CV.

Google "2 stage lead acid battery charger circuit" to get some more ideas.

Then make that circuit, heat and all, and give yourself a pat on the back for getting the job done. Then and only then listen to the other guys here telling you to "just do it with a switch-mode regulator"!


the approach is too expensive in terms of BOM and power dissipation. I would try to plan with only one rung without a relay. You could define the end-of-charge voltage first and then limit the current using PWM.


A simple SMPS with current limit and setpoint voltage is somehow more efficient. For the proposed power rating a SMPS will be the size of your palm, your circuit apart from excessive heating, will be bulky.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.