“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.”

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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.

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.

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.

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.

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.