555 Timer-based Charge Controller


Several years ago [Michael Davis] built a charge controller for his wind turbine and published his construction plans online. This build became quite popular, especially among people that live in remote regions. He states that he is flooded with email each day with questions about his charge controller from people trying to troubleshoot its construction or from people who are unable to source the proper parts.

In order to make things easier for people, he decided to revisit his controller design to see what could be improved, and more importantly, what could be removed. The revision was shelved for awhile, but while in the process of working on another project, he realized that most of his original circuit could be easily replaced with a 555 timer. Since the 555 chip is so ubiquitous, he figured it was a fantastic way to simplify his charger, even if he wasn’t using the chip in the manner for which it was originally designed.

He continued revising his charger, sourcing very basic components and simplifying the circuitry enough that even he was able to build it correctly the first time around. Needless to say, this charging circuit will be his entry in the 555 Design Contest.

Be sure to keep reading for a quick video of his charger in action.


13 thoughts on “555 Timer-based Charge Controller

  1. “Before you write to me and tell me that my lower set point is too low and I am over-discharging my batteries, consider that the battery voltage isn’t normally going to get that low except under load. If the load were removed, the voltage would recover over time back up to well over 12V. So the batteries aren’t as deeply discharged as you might think at first glance.”


    Still, if you set the charge point that low, doesn’t it still mean that after a full top-up, the controller allows the battery to drain all the way to empty even when there is power available to charge it?

    It will only resume charging when the battery is almost empty, but then it’s too late and you’ve wasted good wind/solar output by not putting it in the battery and powering your load with it.

  2. And the second issue is that a light load will still deep-discharge the battery if left on for a while. Don’t forget to turn off the radio!

    To maximize the lifetime of a lead acid battery, you should only discharge it to about 25% DoD. 50% is okay if you don’t do it too often. Always keep it full.

    An ordinary car battery will withstand 80% discharge for about a hundred times before it dies. Some cheap models may die after only a couple dozen times of doing that. They’re designed for a very slight, 2-5% DoD per start.

  3. Eikka, he’s actually using a deep-cycle marine style battery if you check the info page. Deep-cycle batteries are meant for longer, less amp draws and are great in solar applications, while car batteries are capable of giving high amperage extremely fast, but only for short periods of time.

    I like the design, but I would say that bumping up the lower limit sounds like a good idea given that you probably want to capture all that energy that could have been lost while it was dumping the load.

  4. To those complaining about the set point: It’s adjustable, adjust it however you like after you build your own.

    Many controllers of this type use a dump load that does useful work. For example a water heater to preheat your shower water. Heating elements make excellent dump loads. Another possibility is a small water pump, perhaps from a shallow well. Then the energy created while the batteries are full is not wasted.

  5. @sandrusz: The regulator is needed since there are voltage set points set using resistive voltage dividers. Therefore the supply voltage must remain constant relative to the battery voltage.

    The output pulse begins when the 555 timer receives a signal at the trigger input that falls below a third of the voltage supply. If the supply voltage droops along with the battery voltage the set points will droop along with it and never cross the threshold.

  6. Very clever circuit. I like that it doesn’t waste power driving the relay coil except when there’s excess energy.

    I also like the low parts count. Following this theme, I’m sure there’s a way the Q1 inverter could be omitted, even if it meant going to a P-channel FET. (Although I respect that you were resourceful and used the surplus parts you had on hand.)

    My biggest concern with the circuit is that the battery could be overcharged, like if you went away on a weekend, especially if you’re float-charging up at 14.9 volts.

    When the charge current falls below something like 0.03C (or about 3 amps for a 100Ah battery), then you know that the battery is almost fully charged and you should reduce the charge voltage to avoid over-charging. This is known as a 3 stage charger.

    Here’s my idea for how to implement this. The battery charge current can be monitored by coiling some number of turns of the +battery wire around a reed-switch. When the charge-current is low, the reed-switch will open. This signal can then be used to adjust the charge voltage in your circuit. This might be as simple as putting a resistance in series with the reed switch and connecting the whole thing across the R2 pot, with one side going to the pot-wiper.

    Suppose we use a sensitive reed-switch rated at 10 amp-turns nominal and 6 amp-turns minimum drop-out. This says if we use 3 turns of wire around the reed-switch, then it will open at around 2 to 3 amps.

  7. Maybe a possible add on I might try to this design will be to also monitor the voltage obtained from the solar panel and then dump the battery to the device on a sliding scale to compensate for any loss due to cloud cover keeping the out put as constant as possible. Maybe even with multiple battery’s and when one is full and no load is applied the next battery is charged.

  8. I agree with Eikka and Angela that the 11.9 volt setpoint is too low. Something closer to 14 volts would be better. The problem with this circuit is that the RS-latch introduces hysteresis, and you don’t really want any hysteresis for this application, but if you use a 555 you have to live with it. I think it would be better to replace the 555 with a dual comparator if possible.

    Also as Eikka said, it would be good to have a second relay (SPST) to disconnect the inverter/load when the battery is low.

  9. There are ready schematics of NE555 automobile battery voltage regulator. They limit charging voltage at 14.4 V. In your case the lower voltage has to be set around 12.9 to 13.2 V. Charging a 12V battery over 13.2 V to 14.4 V is called equalizing i.e. The weak cells charge and the strong cells boil. You don’t need very freqyent equalizing cycles so I set the lower voltage at 12.8 V to give a chance batteries to rest or relax. The comparator has to be a pecission comparator, and low and upper limit has to be set with precission components. Above all the circuit has to be automatic and not to be operated manually with switches.

  10. I did one of this controller, but I have a problem speeding up my wind turbine over 19 volts the regulator 7805 burn up
    my system is to 12volts. I going to try with solar panel maybe work better.

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