555-timer Charges Lead Acid Batteries


[Kenneth Finnegan] took the focus of a great design and redirected it to solve his own problem. What results is this lead acid battery charger based on the 555 timer. It’s not a top-of-the-line, all the bells and whistles type of charger. But it gets the job done with a readily available IC and no need to code for a microcontroller.

The original idea came from a solar battery charger entered in the 555 timer contest. The main difference in application between that and [Kenneth’s] application is the source. A solar array or wind turbine is limited on how much juice it can produce. But mains power can push a shocking (har-har) amount of current if you’re not paying attention. Herein lies the alterations to the circuit design. To control this he’s using a Laptop power supply as an intermediary and only implementing the constant current portion of the tradition 3-stage lead acid charging profile (those stages are explained in his write up).

He did a talk on the charger at his local radio club. You can see the 90-minute video after the break.

16 thoughts on “555-timer Charges Lead Acid Batteries

  1. Attention 555 haters.

    We are now open for business, please form an orderly queue, thank you for your patience and we look forward to your insightful (inciteful?) commentary. As always.

  2. In TFA it says he only implements the Constant Current stage but yet he says he floats the voltage @ 13.4~13.8v. Every charger I have including an SLA type all claim that float-charge is also a separate stage so this is really a 2 stage charger and no worse than cheap-ish solutions from retailers. Nice!

  3. Nice hack, but please consider installing a proper sized Grommet for the sharp edged hole in the case that the charging wires penetrate, it will eventually wear through the wire insulation and short out.

  4. Lead-acid is probably the easiest kind to charge. Just put 2.3V per cell (13.8V for a 12V battery), and when it’s low on charge, it will take a lot of current, which you can limit if you want to; but when it’s fully charged, it will quit taking current. You don’t need to time it and turn it off. My own experience says you can actually go quite a bit higher than that, as I had a car with an old mechanical regulator that kept it above 16V. I was always blowing out lights, but the battery lasted ten years.

    1. I think the practise of not taking batt off charge may be ok for bigger auto batteries but definitely not for smaller sla types. I have a set of bike lights powered off a 6v 7ah sla, you can leave it on a single stage CC charger for 8 hours more than the recommended 8 hours but it will be quite warm! the older 4.5ah battery it used to take would handle far less abuse than this too

      1. Before we had the efficient LED bike headlights, I made my own with a 12V halogen spot light of the kind you use in track lighting, but the narrowest beam I could find, and used a 12V 7AH sealed lead-acid battery (yeah it was heavy!) which I kept on the charger all the time when not using it. It lasted a long time too. A lead-acid battery should only get warm at really high charge rates though. Once this battery was fully charged, the charge current at 13.8V dropped to a few microAmps, even though the charger had no intelligence in it. Constant-current is the way to charge NiCd batteries, but lead-acid should have constant voltage.

  5. Hi Kenneth,
    while using ICM7555 from Intersil with 18V Vcc limit, why not use either internal discharge FET (Pin 7) or even the output in lieu of 2N3904, and feed the timer from the 12V battery voltage? Any suggestions?

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