How and what to do with excess solar...

[phazaar]

Okay, so here's an area I realise I've got no prior knowledge on, yet can't really find good resources for, so at the risk of universal condemnation, thought I should ask.

I've got a LiFePO4 battery setup, and my understanding of this chemistry is that float charging is a 'no no'. I'll be adding 400W of solar to this, which most of the time will only be adding supplementary charge amperage, whilst a generator/alternator provides the bulk of the input. Sometimes, if we've not used as much electricity as we normally would, I imagine the battery will be sitting comfortably at its 'high' point, and whilst I can stop the generator/alternator attempting to push voltage to it until it drops to a certain point, I can't do this with solar.

By can't, I mean 'can't, efficiently' - I assume my MPPT controller will allow me to select a voltage to stop trying to charge the batteries at*... But if this is happening during peak sun I'm wasting resources, and don't like that idea at all.

So, what I'd like is to be able to use that excess charge without passing it through the battery system. I'm intending to have a small 10L immersion heater which would be a prime candidate for using excess current - a bit like an extra battery but for heat energy. That way I can come in and either choose to top off the temperature of the tank, or temper it with cooler water depending on how much excess charge it's picked up through the day. Other thoughts include putting it through an alternator to trickle charge some of the stuff I would regularly charge when we've got a generator/shore power hook-up.

Now, is there a glaring problem with this? Is there a product I'd need to make it work? Is this really basic circuitry that I'm being an idiot in missing?

Any help, humiliation, or advice welcome

Cheers!

*(bonus question: if the batteries don't require any charge for a week or two, will this damage the panels? I'm used to grid-tie and have always been taught that panels shouldn't collect sun if they're not using it)...

[PNPmacnab]

I heat water with my excess solar and it is a lot more than 10L. Attaching a heating element to a PV panel is inefficient except for about 2 hours a day. This market hasn't developed yet and available products are scarce and expensive. I built my own and this is likely beyond your capabilities. Anything exposed to the sun ages, no more than if it isn't used.

[Mike90250]

The midnight Classic charge controller has a Waste Not feature that can activate a load. ( icemaker, water heater.....

[phazaar]

I heat water with my excess solar and it is a lot more than 10L. Attaching a heating element to a PV panel is inefficient except for about 2 hours a day. This market hasn't developed yet and available products are scarce and expensive. I built my own and this is likely beyond your capabilities. Anything exposed to the sun ages, no more than if it isn't used.

Thanks for your response. I'm aware it's inefficient, hence I'm not sure exactly what to do with the excess yet, but it's certainly less inefficient than simply wasting the amps.

I'd be interested to hear more about your setup, whether it turns out to be beyond my capabilities or not

The midnight Classic charge controller has a Waste Not feature that can activate a load. ( icemaker, water heater.....

That's exactly the sort of thing I'm looking for, but unfortunately I need it in a 12V package...

[azdave]

How much money are you looking to spend in order to use the excess? Sometimes the most economic decision is to do nothing if it means you will have to purchase and maintain more components.

[phazaar]

How much money are you looking to spend in order to use the excess? Sometimes the most economic decision is to do nothing if it means you will have to purchase and maintain more components.

Always a challenging question to answer, since it largely depends what I can achieve with the excess.

Case in point, water heating is something that's got a couple thousand ear marked for it in this build, so if this is effective I wouldn't mind a fairly large chunk of that.

On the other hand, if it's just some trickle charging of batteries I'd otherwise need to charge off the generator, maybe 100GBP just for the neatness of the system.

[PNPmacnab]

Cheers! Obviously the land of 240V? 120V heaters are probably hard to find. Just what is your panel array voltage? Some have used a voltage switch to turn on when a certain voltage has been reached and power an MSW inverter off that. A 110V inverter will give you 1/4 of the 240V heater elements rating. This could be a very light load. In addition, adding a diode would even get you half that much or 1/8 of the elements rating.

[PNjunction]

On the other hand, if it's just some trickle charging of batteries I'd otherwise need to charge off the generator, maybe 100GBP just for the neatness of the system.

Thought - it's ending summer now. Does your power budget take into account winter hours in GB? Will you be relying mostly on the generator during those months?

The easiest thing to do is, yup - add more storage as a "plan b", or just in case scenarios - or just to save even more in petrol.

My first thought is to get hold of a high-end agm, like and Optima or an Odyssey, that can gulp down nearly it's own capacity in current without harm, especially important if your sun periods may become short-lived during winter in Britain.

Back of napkin calc - 400w / 18v = 22.2A. Heh, the small Optimas can handle that easily. Perhaps opt for a larger 75ah yellow or blue top. Of course that will cost since they are all usually "pure lead" types, and not conventional agm's. No "black case" starter versions - stick with the gray-cased ones...

So there's your miscellaneous portable battery you can use and maintain without wasting idle panel energy....

[phazaar]

Cheers! Obviously the land of 240V? 120V heaters are probably hard to find. Just what is your panel array voltage? Some have used a voltage switch to turn on when a certain voltage has been reached and power an MSW inverter off that. A 110V inverter will give you 1/4 of the 240V heater elements rating. This could be a very light load. In addition, adding a diode would even get you half that much or 1/8 of the elements rating.

Indeed, 240V here. We're running a 12V array. Perhaps this is obvious to you, but what's the advantage of stepping up the voltage that high and moving to AC? Given the low current involved, surely the inverter losses eat up any advantage from AC compared to just using a native 12V DC element?

I suppose my other question beyond that in what you've described is what to do about the panel->charger->battery connection. If we use some kind of voltage sensing relay to switch on an element running off the charge controller load or battery terminals (perhaps via a capacitor?) do we just sever the connection to the battery whilst the element is running, and have a second voltage sensing relay across the battery itself to tell when to switch back over to charging the battery?

We're quite heavily relying on a coulometer to monitor battery usage given how well this works for LiFePO4 compared to voltage monitoring - would it be worth trying to create a system that could incorporate this to decide when to charge versus when to use the element, or better to just read use this read out to determine good switching voltages?

Thought - it's ending summer now. Does your power budget take into account winter hours in GB? Will you be relying mostly on the generator during those months?

The easiest thing to do is, yup - add more storage as a "plan b", or just in case scenarios - or just to save even more in petrol.

My first thought is to get hold of a high-end agm, like and Optima or an Odyssey, that can gulp down nearly it's own capacity in current without harm, especially important if your sun periods may become short-lived during winter in Britain.

Back of napkin calc - 400w / 18v = 22.2A. Heh, the small Optimas can handle that easily. Perhaps opt for a larger 75ah yellow or blue top. Of course that will cost since they are all usually "pure lead" types, and not conventional agm's. No "black case" starter versions - stick with the gray-cased ones...

So there's your miscellaneous portable battery you can use and maintain without wasting idle panel energy....

Hopefully we won't be stuck in the eternal cloud isle for much longer; winter down in portugal has a bit more sun going on, but yes we've plenty of head room both from our generator, hookups and alternator, and from having far more capacity than we're intending to use through the summer (hence looking for something to do with the excess).

I think the problem with extra batteries is just space+weight, and the fact that it doesn't really utilise the excess so much as reduce the frequency with which we would have excess. No matter our capacity, there will always be times when we have had 'shore power' for a few days and had more than enough time to charge any amount of batteries we fit. It would just be cool if we could then use our solar for something (and protect the batteries from float charging too).

[PNPmacnab]

In the absence of complete information, I had to assume the van had a standard 240V heating element in the tank. In that case some boosting has to occur to get any wattage. I don't condone this or battery voltage elements although a lot of people do this. I like to run directly off the panels and that generally requires at least 50V. My decisions on diversion are strictly based on PV voltage and this doesn't necessarily require MPPT. Energy can be harvested in the times the controller's duty cycle reduces. Most good ways require some electronic nouse. It is not easy to come up with an effective solution the general public can implement. Counting schemes are interesting, but worthless if the charge controller doesn't use them. You just want to take the energy that the controller doesn't use and that gets back to panel voltage.