Use car to charge solar batteries on cloudy days?

I finally own my own place so I can permanently set up my solar panels. I'm in Michigan so weather caused power outages are a regular occurrence. A decent generator is around $1,000. I can buy a lot of batteries for that.

My thinking is I put up my solar panels and charge a decent bank of 12v batteries. If the power goes out and the panels can't keep up due to short days or clouds, I install a battery isolator in my diesel truck and use the alternator to charge the batteries. This way I don't have to buy a separate generator, buy and rotate gas. I have a 100 gallon transfer tank on the truck to keep it going for a while.

Thought on this concept?

How efficient would this be compared to a generator? I don't think generators are that fuel efficient. Charging batteries for an an hour or two seems better than running a generator all day.

A battery isolator is just a couple of high power diodes, right? Wouldn't this drop the top charge voltage by 0.5-0.7 volts? Would that be an issue with capacity?

Efficiency would be awful. 80-90% of the gasoline is going to be spent just keeping the engine running, the rest will go to electricity.

Add to that - car alternators aren't often much more than 60-70% efficient.
You might get 10% efficiency.

Much better to have a gasoline generator of the appropriate size. A car is not set up to do this at all.

When the power goes out, everyone tries to start their generator. Generators don't start because they have been sitting unused for too long. Cars and trucks start. I got a big inverter that bolts to the car/truck battery. Truck will idle for days on a tank of gas. Inverter runs the refrigerator, computer, some lamps, and the COFFEE POT.

12v batteries suck for this kind of thing, you really want some nice big 2v cells - forklift cells are awesome for off grid use

how many solar panels do you have ? can you not grid connect them, then use then to charge the batteries if the grid goes down ?

get yourself a cheap little suitcase generator... output doesn't really matter... just use it to charge your battered and run everything directly from them


do you want to run everything in your house, or just the essentials ? fridge/router/laptop/couple of lights etc.. ?

knighty said:

12v batteries suck for this kind of thing, you really want some nice big 2v cells - forklift cells are awesome for off grid use

how many solar panels do you have ? can you not grid connect them, then use then to charge the batteries if the grid goes down ?

get yourself a cheap little suitcase generator... output doesn't really matter... just use it to charge your battered and run everything directly from them


do you want to run everything in your house, or just the essentials ? fridge/router/laptop/couple of lights etc.. ?

Click to expand...

When I said 12v I just ment. The voltage, as in not 24v or 48v system. If the budget will allow I might try to go nickel iron.

I currently have about 500watts of panels. Probably add a few more as $ allow. Probably won't grid connect, just charge a battery bank and power loads that I have power for, maybe the fridge and some lights to start.

Any guesses on how many amps I might get from a 150amp alternator at idle?

Whatever voltage of battery you select (battery voltage, not system voltage), make sure to use deep cycle batteries. Any money spent on starter batteries is wasted, as they will die very soon. The batteries that start your car engine have thin plates and were not designed for deep-cycling.

If you want to use a 100-amp truck alternator as a back-up gen, nothing wrong with that, better than nothing. They can be spun by a lawnmower engine to get much better fuel economy instead of running a 5.9L diesel to charge 12V. Only caveat is that you must add a flywheel of some type. A 2-ft diameter steel disc will work.

Also, here's some useful DIY info on home-built generators: http://www.qsl.net/ns8o/genmenu.htm

I built a 4HP Honda edger motor, running a 35A GM 1 wire alternator, for my logging barge. Charged a 8D battery that ran the winch. Used it every day we dove for logs, and it was still going strong 4 years later, when I sold the Barge. Only ran the Honda at less than ½ throttle, to feed 35-40A into the battery.

I may eventually build one for here and run the engine on wood smoke. Got a lot of trees and limbs to burn.

If the cloudy days are typically cold too where you need heat, then using the diesel truck as the charger could make some better sense by making a heat exchanger to get much of that waste heat into the house. You'd just have to make darn sure of no leaks for exhaust fumes to get in. Making use of the heat can change the system from low efficiency to high efficiency.

John

Had a major wind storm yesterday and power was out for most of the day. Had to take the Lifepo4 pack and inverter over to run the heat for a few hours.


Just a quick thought on nickel iron batteries. Are they really that much more expensive If you have to buy 10x the lead so you only use 10% dod. I mean let's say you need 100ah, and to be gentle on the lead to keep the going for years, you only want to use 10%. So you need to buy 1000ah. Where if you have NiFe you can use all 100ah without problems. Is there really thy much cost difference?

500w of generator is not expensive, and would match your panel output. You do have to run em from time to time to keep good gas in em.

But I'd say go get a cheap 3000w, mine was only about 400 bucks. Keep a gallon in it, and every three months or so, use it when it's cloudy. Get fresh gas if you need more, but keep it in a seperate can.

Boy did I look smart two years ago when the ice storm froze all the local electric companies generation plants. Here, they build em to stand heat, and a rare cold had us in rolling blackouts for a few weeks. My house kept heat and microwave to cook the whole time.

The truck idea is fine for a pinch, but not for regular use all the time.

Yeah, I'm thinking I will just find a small gas motor and make my own alternator generator.