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InPhase

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Joined: 15/12/2020
Location: United States
Posts: 178

Posted: 01:57am 18 Oct 2021

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A simple circuit that closes a relay at a low voltage and opens it at a high voltage. I will use this to start a generator to keep a battery bank healthy, but it could be used for lots of other stuff too. My generator requires a maintained contact, so this works well, but this wouldn't work for a momentary push button start. If I were to do it over, I would use multi-turn pots for the adjustments. U3 is an LM317HV voltage regulator for 12 volt control power, so it can be used on battery voltages up to 60 V.

I used an adjustable power supply to set the trip points. I already have a couple of improvements to make, but it works very well.

Edited 2021-10-18 11:58 by InPhase

Godoh
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Joined: 26/09/2020
Location: Australia
Posts: 458

Posted: 03:37am 18 Oct 2021

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I wonder what sort of hysteresis or time lag you have built in.
I once saw a commercially installed solar system with a backup generator setup.
It would start the generator run it for a couple of minutes then shut it down.
This was because when a big load was put on the system the voltage dropped below the low voltage setpoint. Then the battery voltage rose very fast with the large current being pumped in by the charger and the high setpoint was reached and the thing shut down again.
This happened many times a day on their system.
Just wondering how you will prevent this happening?
Cheers
Pete

InPhase

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Location: United States
Posts: 178

Posted: 04:25am 18 Oct 2021

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In my use, hysteresis is in the nature of the system. Under the heaviest load I can impose, the voltage doesn't sag below the set point. Only drain can get the voltage that low. Under generator power, the charger doesn't provide enough current to raise the voltage that rapidly. There's also some intelligence on the generator side. Once started, there is a minimum cool down period after the call to start is removed where the genset continues to run until timeout. So there won't be any rapid start-stops with fluctuating load.

Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 05:12am 18 Oct 2021

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It will when the battery starts to get down a bit.

It might all be running fine just above the low voltage trip point, then some massive surge pulls the voltage down to the point where the generator starts up.

So its not really possible to set an accurate fixed known low voltage set point.

One way to fix this might be with a series resistor and big electrolytic with a fairly long time constant. Use the time averaged voltage across the electrolytic for battery voltage sensing. Sudden dips and peaks in voltage will then get smoothed out.

Cheers, �Tony.

pd--
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Joined: 11/12/2020
Location: Australia
Posts: 122

Posted: 12:12pm 18 Oct 2021

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I was away for a couple of weeks and during that time there was a hell of a storm
some how the battery charger for the generator starter battery was disconnected.

With black skies and little solar input eventually the main 48v battery went low voltage and the generator failed to start due to its flat battery so the hole system shut it self down.

Thats when i added the flowing ladder logic to the generators PLC
the generator has its own 12v alternator on it to charge the 12v starter battery.

if the battery voltage drops below 12.3 for 10 seconds
start the generator
when the bat voltage is greater than 13.8 then run for an additional 30minuts
then stop

There is a bit of aux control gear hanging off the starter battery so its only good for about 3 days before needing a topup

InPhase

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Location: United States
Posts: 178

Posted: 12:40pm 18 Oct 2021

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Very good feedback! I also noticed that I have a typo on the board as well. I thought I was clever using a 555 the way I did

An earlier version used an LM358 but had about a dozen extra components. I guess I'll see how glitchy this will get. In my preliminary testing, it has been fine, but I don't (yet) rely heavily on battery power either.

I think I will add the big -ish electrolytic externally to buffer now. Should I use a diode in series with it to stop the cap from draining back towards the battery during momentary surges?

pd--
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Joined: 11/12/2020
Location: Australia
Posts: 122

Posted: 05:16am 19 Oct 2021

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Use a resister
you wont the cap to stay at the battery average voltage.
in my setup the battery voltage needs to be low for 10 seconds before the generator starts

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