I have an AC/DC power supply unit (400W) with the controlled output voltage (40V...60V). It would be great to use such a power supply as a current source to implement a dual step current changer. The charger is for four 12V 12Ah lead-acid batteries in series.

The algorithm for dual step current charging is below:

enter image description here


1) How is it possible to implement the dual step current charger with the aforementioned power supply unit?

2) Is there any better algorithm for charging four 12V 12Ah lead-acid batteries in series?

  • \$\begingroup\$ a) Implement a controllable current source (google it). b) Implement a controller (choose your favorite). c) Integrate between the two. \$\endgroup\$
    – Eugene Sh.
    Commented Apr 30, 2015 at 15:36
  • \$\begingroup\$ I need a sound advice on how to build a reliable charger for lead-acid batteries in series. There are very experienced people here and I would appreciate their replies. \$\endgroup\$
    – Konstantin
    Commented Apr 30, 2015 at 15:40
  • \$\begingroup\$ Isn't this the sort of thing that's much easier to buy as a commercial product than build? And if you were to build it, the existing unit doesn't help you very much unless it's (say) digitally controllable. If it's just a box it's no help. \$\endgroup\$
    – pjc50
    Commented Apr 30, 2015 at 15:44
  • \$\begingroup\$ The output of the PSU is digitally controlled (regulated) in the range 40V...60V. I need to develop it as a highly customized device \$\endgroup\$
    – Konstantin
    Commented Apr 30, 2015 at 15:49
  • \$\begingroup\$ Have a look through the excellent www.batteryuniversity.com site at lead acid charging and then ask re anything they do not cover. \$\endgroup\$
    – Russell McMahon
    Commented Apr 30, 2015 at 16:19

1 Answer 1


You have a constant-voltage (CV) power supply, and you need constant-current (CC) power supply. You will want to add an external control loop to command the right voltage so the current is maintained. Luckily, it is not that hard -- your load will not change rapidly, and it can accept short overcurrent intervals.

You will want to add a microcontroller (arduino if you are a beginner, or something more rugged if you can do it) with a way to measure current and voltage going into the battery -- most likely this means you need a shunt and a voltage divider. This microcontroller must control the power supply in whatever way it needs to be controlled.

During bulk charge, the current is only limited by your power supply -- so increase voltage slowly until the current reaches required fraction (80%?) of power supply's capacity. As the battery is charging, change the voltage in small steps to maintain the current. When the battery voltage reaches threshold, you switch to holding mode and decrease the voltage to maintain a much smaller current.

As for charging the series of batteries, the method in the paper will work fine if all batteries are similar, and if they are always discharged simultaneously. If you buy batteries used, and each battery in series is from a different source, OR if you sometimes connect load to only one battery (as opposed to all four in series), then your chain will become unbalanced. If you are worried about that, add more voltage inputs to your MCU to measure the voltage between the batteries. If you detect disbalance, limit the charging current to 0.1C until it disappears.


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