Panel must remain on when MCU is dead, circuit question

[russpatterson]

Hi There,

I've got a problem for a solar project which is: I need to turn the panel off if the battery is nearing overcharge, however I need the panel to be on if the battery, and the MCU (PIC microcontroller in this case), are dead. I've been through a few circuits (and burned up a PIC or two) but I think I have a solution.

Does this schematic look legit?

Low cost backyard solar energy: N-Channel Mosfet Panel Solution

I'd be interested in any other designs for this kind of thing if you know about one.

Thanks!

 

[mneary]

So the default in the event of any failure is to overcharge the battery?

The circuit? Looks problematic; don't ever recall seeing a MOSFET operated with a gate-drain potential. And any static charge, however small, would be applied directly to the gate.

 

[Mr RB]

You could use something that is a simple analog voltage limiter and a "shunt regulator". You can use a voltage detector to turn a FET or power transistor to waste power and keep the battery voltage from getting too high.

This would be independent from the micro and should not use up any significant power unless the battery is full.

 

[bgudgel]

Does this schematic look legit?

Low cost backyard solar energy: N-Channel Mosfet Panel Solution


Thanks!

I would at least add a zener diode to keep the FET gate from going above 16V or something less than the max Vg-s rating. A diversion load may be better to keep the battery from overcharging. Usually, a PWM controller will work
to switch on and off a load resistor when the battery voltage goes above a preset threshold, such as 14.3 volts or so.
Then, the PV is always on.

boB

 

[russpatterson]

Thanks for the reply. If I just switch the PV off (by disconnecting ground) will that cause a problem? Is it better to divert the load into a big resistor? Sounds like extra parts but maybe there's a reason why that's better?

 

[Mr RB]

If you use a load dump it gets the heat away from the panel so it's nicer for the panel and that's how they do it on larger systems where the panels are expensive.

But you can just disconnect the panel if you like, the panel will run a fraction hotter and your electronics will run a bit cooler. It probably doesn't matter that much. With a small setup you can just put a 5 watt zener diode across the battery and that will be good enough to prevent overcharging in a lot of cases.