Project files: SLA battery charger

What is it?
For one of my work-in-progress builds I needed a simple charging circuit for an SLA (sealed lead-acid) 12V battery. So, good excuse for another project 🙂

Compared to the Li-Ion or LIPo-batteries used in most applications today, charging an SLA is extremely simple. You use a fixed voltage and then set a limited charge current of around 1/10C or a trickle-charge current of around 1/20C and off you go. (“C” is the nominal capacity of the battery, so 1/10C for a 7Ah battery is 700mA).

A bit of searching online turned up a few suggestions but one appealed more to me than the others, mainly because of its simplicity. The circuit is built around the L200 voltage regulator which features programmable voltage- and current limits, which is exactly what this application calls for. I believe the original circuit is from Elektor, but I am unable to find it so I relied on the web versions here and here instead.

How big are the boards?
The board measures 1.95″ x 1.6″ (app. 50 x 41 mm) without the heat sink.

What is the status of the boards?
The published version of the board is 1.1. Changes from the prototype v1.0 that I built are fixes for some misprinted LED-labels (doh!) and some minor touch-ups to the silkscreen – nothing major.

Does it use any special/expensive/hard-to-find parts?
Not really. Just be a bit careful checking component dimensions against board footprints because there isn’t much space left over.

Anything else I need to know?

  • The PCB is meant for currents smaller than 1A due to the limitations of the rectifier and protection diodes. With a bit of searching it is possible to get 2A components that fit, however whether this is a good idea from a thermal POV remains to be seen.
  • The component values are normally meant for 12V batteries (and a 13.8V charging voltage). A few of the articles have component values for 6V battery use as well, but if anyone can easily recalculate for 24V (2 batteries in series) please let me know.
  • The L200 regulator must be heat sinked, but how much depends on charging current and input/output voltages. The one on my picture was the only one I had with pre-drilled holes, so don’t take that as a recommendation/indication in any way 🙂
  • Although not shown, fuses should always be used in series with the charging battery. An SLA battery is capable of some truly terrifying short-circuit currents, so there should always be a fuse to protect the circuit from damage.

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.