We're going for 8 X 12 volt 130Ah Deep Cycle batteries which will give us 1040 Ah through connecting them in parallel to increase the capacity (connecting in series would increase the voltage). We had considered buying 6 X 720 Ah 2 volt batteries to make up the 12 volt) but discovered that their life span is not significantly greater, although their cost is double.

All batteries will need replacing within 5 - 8 years (depending on their maintenance / discharge) and the 8 X 130Ah would be cheaper to replace then than the 6 X 2 volt.

Don't forget, if you go the deep cycle battery route, once you have your new batteries, connect them up and allow them to charge on a sunny day for at least 6 hours prior to using them in order to ensure that their charge is at maximum. Using deep cycle batteries which are only charged at say 80 or 90 % will result in those batteries only ever charging to that percentage, as deep cycle batteries have a memory.

I did a posting on our solar installation last year - it can be viewed here : http://ecofootprintsa.blogspot.com/2011/05/our-solar-power-installation-details.html

My (limited) knowledge of dozer batteries is that they are generally a "D8" battery, which is just a scaled up car/truck battery.

D8/8D batteries are mentioned here...

http://www.windsun.com/Batteries/Battery_FAQ.htm

Your research looks pretty accurate to me.

Realize that the charge controller and monitoring hardware is also very important for the health of your battery pack. It should prevent both overcharging and cycling the batteries too deeply. The controller should fit the batteries. If you have an MPP tracker on your solar panel, it should also prevent against overvoltage. If you don't want an MPP tracker, make sure that the maximum efficiency of your solar panels is near the nominal voltage of your battery pack!

If you want to stick to lead/acid batteries, I think deep cycle batteries are the ones to go for. Standard lead-acid batteries don't like being discharged deeply; you'd have to stop discharging when they get below 50% capacity.

But with standard "flooded" lead-acid batteries, you'll have to monitor the level and specific gravity of the electrolyte regularly. That is why I pointed to VRLA batteries, because they don't need much maintenance. Realize that if you skip on that maintenance, it can drastically shorten the lifetime of the expensive batteries, or even detroy them (internal short).

Other types of batteries like Joey's NiCads can fare much better w.r.t. deep discharge. But they need specialized charge controller, and cadmium is an environmental hazard. They are probably also more expensive, unless you can get some second-hand ones.

I'd vote for Nickle-Iron batteries. Majorly Expensive but you never have to buy another one. I can't afford them right now but they are by far the best. Here's a place in Montana that manufactures them... http://www.zappworks.com/

@brian: the battery of a Prius has a quite high voltage (273 V), so it's not really suited for a low-voltage DC solar system. Also its capacity is not very high 273V×6.5A·h×3600s/h = 6.4 MJ. One 8D battery can hold 12V×250A·h×3600s/h = 10.8 MJ

The battery pack from a Tesla is really about 8000 small batteries (individually monitored) put together. And it costs around $30,000!

@anna: even deep cycle batteries should not be discharged below 50% to optimize their lifetime. So the rated capacity of your battery back capacity should be at least twice what you really need.

Daddy --- Looks like Joey chimed in below.

Dani --- Thanks for the reminder to charge the batteries thoroughly before using!

Shannon --- That's what I was thinking, and I appreciate you backing it up with your agreement.

Charity --- That's the exact kind of system we're going to use. We considered the 12 V 35 amp battery that Harbor Freight now sells as an add-on to the kit, but it seemed more expensive per watt than the golf cart batteries we're considering and had no information on longevity. (I'll talk more about that in a later post.)

Joey --- Drat --- that's pretty hard to recreate since we don't know of any electric buses being sold for parts.

Brian --- Interesting idea! I don't think Prius is old enough to be in junkyards yet (at least not around here), but my brother was suggesting reenvisioning a use for those batteries in a few years once they're easily available. No good ideas on a use, though.... (And see Roland's reply to your comment below.)

Roland --- The kit we bought came with the charge controller and (I think...) also monitoring hardware. We're definitely going to have to learn about battery maintenance, but that's something we should have been doing anyway for our golf cart....

Buzz --- Even though they're way out of our price range (over $3,000 for the cheapest battery), I did look at whether batteries like that would be economical. But the truth is that even though they last a long time, they cost so much that it seems like golf cart batteries would be cheaper!

Roland (second comment) --- I was figuring more like only using the top 40%, because I'd read it's best to only use 30% to 40%. See my next post (probably Friday) for more information --- I'll be asking for your feedback there too.

When I was in afdghanistan everyone just used truck batteries. They seemed to last a few years but you do need to add more distiller water once and a while.

While they probably act like car batteries to an extent they also are 3 times the size and so they will do better per charge (more surface area for the electron transfer)

I've done 4 PV installs and live entirely off-grid with a 48V 2kW system. My feeling is that golf cart batteries are the best option for small installs.

Here is why:

• They have reasonable thick lead plates which means if you keep them charged up they will have a decent life span.
• They are accessible locally, have freshness dates that are usually less than 90 days old and do not require shipping.
• They are robust and designed to do deep discharges.

You can take this from someone who spent a fortune on fancy pants Surrette Rolls Solar industry batteries ($400 each) for a total of $8,000 in batteries on my first PV installs. They are totally not worth the money. They sat for years in a warehouse and had to ship as a hazmat item.

Be sure to never buy batteries until the entire system is completely installed and all the cables are in place. I see so many off-grid installs where someone leaves the batteries sitting in their yard for a year.

I used to own an electric car and know batteries better than I ever thought Roland gave you great battery care advice, but I will chime in to emphasize a couple points.

I would not consider anything other than golf cart batteries or the better lead-acid batteries made for solar installations. Flooded batteries are more forgiving to abuse (higher usage and charging mistakes) but they require maintenance that I did not enjoy doing (checking water levels, cleaning the tops, etc.) On the other hand, sealed batteries do not require maintenance - but if they are not charged carefully, again you'll shorten the life (if overcharged, they will vent and you cannot replace that fluid.) When I set up my very small off-grid system, I plan to use AGM (absorbed glass mat) batteries. They also will not be damaged by freezing weather. My car had Trojan T-125's which are fine but not tops in my opinion.

Make sure your charging system matches the battery type, or you will shorten the life of the batteries. Especially important for sealed batteries. If you use flooded batteries, try to get a charger with an equalization cycle. All batteries are slightly different (like a litter of puppies) and every once in awhile you need to over charge them slightly to even them out. The best charging system of all, but difficult to implement, is to individually charge each battery. One friend had this setup on his car - looked like a rat's nest in there but his batteries lasted a long time.

Try to buy from a golf cart dealer - they have the freshest batteries. A friend bought an Optima yellow-top for his off-grid place - optimas are good batteries, but it had sat at Costco for a year and was DOA. I bought a few from Batteries Plus and regretted it - they were not as good/fresh as from the golf cart place.

I'm so glad I asked my question, because you all keep pointing me toward things I wouldn't have considered! Here are the facts that jumped out at me:

Mikey's excellent point about age of the batteries (well, De's too...) and his firsthand data is pushing me toward what I was already thinking was the best route --- buying golf cart batteries from the closest golf cart store.

De's point about batteries being individualized supports my (odd) plan of having several small solar setups rather than one big one.

Thanks, and keep those comments coming!

These are the ones that do NOT blow up

They have a life expectancy of 10 years. A 12.8V 100AH CALB 4 cell would run $450.

you have to use the right charge and disconnect voltages though (like any battery)

source - http://www.evolveelectrics.com/CALB.html

De's comment that the charger should fit the battery should be printed in big, black letters!

Individual chargers and monitors for each battery sounds like a very good idea. It would prolong the life of the batteries and give you a chance to nip problems in the bud before they become expensive failures.

Instead of several separate systems I would advise building a flexible system where you can disconnect pieces (batteries, circuits) that don't work. Put all the 12 volt battery stacks in parallel to the solar panels (through their individual chargers), with a switch to disconnect a pair if it is misbehaving.

As you've seen with carpenters and woodwork, it might be a good idea to get an electrician on board for this project.

A lot of applicances use DC internally, so they come with a transformer/rectifier. Depending on what you need you should consider wiring your home for 12V/5V DC, so you can supply laptops, dehydrators, camera and phone chargers and perhaps incubators directly.

David --- Hmmm, one of those batteries would hold about 1,280 watt-hours, which is only barely more than a typical golf cart battery (which I estimate at 1,200 watt-hours). But the price-tag is three times as high. If the golf cart battery lasts five years and your battery lasts ten years, you'd still be paying half again as much for your battery's juice. I'm not sure I'm willing to pay that much extra....

Roland --- I'm going to have to mull over your comment when my brain is working more. I suspect it makes sense, but I can't quite remember what all the parts that attach to the battery and panel do. (I clearly need to make a post about it, since that makes the information stick.)

Since we're starting with such a small system, it doesn't make sense to wire the whole house. Instead, we'll just get a car-charger for the laptops, which will let us plug them directly into the DC current.

You still need a charge controller and a inverter. I personally like the Xantrex C40 (24V) for small installs. It can do 12V and there is a 48V option, but I find 24V to be a sweet spot for installs less than 800W. That means you need to buy two more golf cart batteries, but it gives you room for expansion and keeps you cabling cheap. I buy from these guys in Albuquerque because their prices are very competitive.

http://www.affordable-solar.com/store/solar-charge-controller/xantrex-c40-charge-controller

It's silly to run all DC these days we live in a AC world and your cable diameter will be so thick that it is difficult to work with and expensive to run to anything.

Dennis --- That was my second bullet point: "RV or marine batteries are the next cheapest, but still aren't recommended by experts. They're usually easy to find locally and they do last a bit longer than car batteries, but probably will die within a year. They also have a tendency to explode if not cared for well."

I had originally thought they would be fine, but all of the solar experts warning against them on the internet makes me think we'd best avoid them.

I don't have anything to add and just about a zero knowledge base, but we are looking at putting an rv on our unimproved land and have been looking into solar more.

This blogsite had alot to say about batteries and what had and hadn't worked in the past, it is n't mine, I don't know them but the info looked great.

http://hardworkhomestead.blogspot.com/

The intended usage of your batteries (backup for power outages) might not be a good thing for their longevity, nor an effective use of the hardware.

For long periods, the only thing that the solar panels will be doing is topping up the batteries self-discharge. This topping-up should be stopped by the charge controller once the cells reach their maximum capacity (and voltage); without this cut-off, a lead-acid cell would start to electrolize the water in it's electrolyte once it reaches a certain voltage. This might be an excellent source of hydrogen explosions, which might be interesting if they weren't happening in your home!

Depending on the relative size of your solar panels and batteries, your panels will be effectively doing nothing for long periods of time.

I think it would be better to put the energy from the solar panels to other uses once the batteries are charged. Like e.g. pump water from the creek up to the holding tank. This would be simple to control, separately from the battery charge controller.

Once the charge controller cuts off the current to the batteries, the voltage from the panels goes up. This should open a relay (an electrically operated switch) that powers a small pump. If the battery charger or something else on your "grid" starts pulling power, the voltage will drop which should make the pump relay disengage.

It might be even better to use the batteries regularly.

Alison --- Thanks for the interesting link!

Roland --- We don't plan to use them for backup alone. That's the primary goal, but we plan to simply convert the things we want to keep running when the power goes off to DC so that we can use the system all the time for those gadgets.

He there, I am totally new in this area of solar energy, but my first experiment with a 10 W, 12 V cell, gave me that taste of continue to find out, and decision to turn my hole property into 12 V SWG (Solar Water Ground)... Questions: 1. Must the charging source be stronger than the container (battery)?. or can it even be less, and still charge?? 2. I have wired up a pile of diff car and mower batteries, aal of them 12 V, of course, in serie, and that multiply currents....does that effect LED lights that I have several of for lightening, and also ventilation fans; all of them 12 V?? 3. I read somewhere that shunting several solar should never exceed 70 amps, that could destroy the cells: is that the same case with a battery bank?? Thank you!

You can find used Prius traction batteries on eBay. People crash the cars, but the battery is well protected. Shipping can get ugly, but they typically sell for around $300, because that's what Toyota dealers pay to retrieve and recycle them (so try there, too). Using it for storing energy is even better than recycling. I don't recall exact numbers, but the pack is just under 300 volts, NiMh (nickel metal-hydride), around 220 rectangular batteries in series at 1.25V each. But there's a quirk -- Toyota sets their software to maintain the State of Charge from above 60% to below 90% of full capacity to make them last longer. If you charge them to 100% and discharge to 20% SOC, they likely won't last "as long" as in the car. There are YouTube videos that show you how to disassemble the pack safely; if one cell fails, it can be replaced, so it should be easy to assemble packs of any voltage you wish. Technically, you should be wearing those giant rubber 2,000-volt gloves. Or follow directions. In fact, any hybrid junk car would have a giant battery pack in it.

I'm thinking about the Harbor Freight Kit as a great way to provide power for a mobile HAM radio. Found a great youtube on it. Found another spot recommending LiFePo4 batteries.

Is the Harbor Freight Kit charger controller compatible with Lifepo4 batteries or would I also need to get a lifepo4 charger to go in between the charge controller and the battery?

I need to be able to run my propane furnace and propane fridge in addition to some 12Vdc LED lights, a 110VAC TV for 2 hrs a day (for some entertainment), run the 12Vdc hydraulic pump for slide-outs & landing gear (only some times), and still have enough left over for braking power (4 x 12Vdc brakes while in transit), The propane furnace and fridge both require at least 5AMP hrs at 12Vdc for the electric boards and fans. I only need power to last 3-4 days when traveling on the road for now. maybe for longer periods later but that can be accomplished by adding more batteries. its the battery selection thats my issue now.

My first thought was to go with 2 huge 12Vdc Lifeline batteries that would cost approx $1500.00 but Im not sure if this is the right way to go. They are supposed to last 10 years but Golf cart batteries are cheaper and probably the same or less cost if I have to replace them more frequently. Plus the lead cell deep cyle won't freeze which is a major concern for me being in Ottawa Canada. Now Im reading abut all these other types of batteries so I am really confused as to what kind of batteries would make sense.

The Solar panels, MPPT Regulator, and inverter now appear to be the easier part of the setup.

Any help will be greatly appreciated.