using a forklift battery with a charge controller

The dilemma is that using the battery costs more than using the grid. So to save money you would want to never use the battery unless you have an outage.

The math works out that if you had 100kwhs of energy coming from your array. if you store the energy and use the inverter later you get to use @65 - 70Kwhs of electric, due to losses in charging the battery and creating a sine wave. If you back feed the grid, you will likely (I don't know how efficient hybrid systems are) get to use 90-92 Kwhs. So for best outcome dollar wise, you don't want to use the battery.

Problem is Batteries like to be used...

Personally I would run it hard once a week when You know you will have a sunny day following, for the first 6 months. While the battery is being 'formed'. Then maybe once a month, and just keep it in float and equalize once a month. Other batteries are better designed for this type of backup application.

With out knowing your situation and costs, it would be hard to hang a dollar figure on it. I don't know of any hybrid solar systems that are cheaper than the grid, though I would think some of the expensive grid areas could be.

If you don't understand my previous post,  I'd guess some where between 30% and 50% more than just back feeding the grid from your system.

If it makes you feel better, pure off grid systems require roughly 3 times the array size for the same delivered kwh, and must be wasteful for a healthy battery.

new2PV said:

Just out of curiousity how much is the cost per kwh using a forklift battery?

I just re-read this and see that you might be thinking that using the battery, since the system exists, is 'free' but it will account for as much as 1.5Kwh that you won't be back feeding the grid.

Maybe that statement will clarify my earlier calculations.

Our sponsor sells Crown batteries and they are a very high quality traction battery.

I purchased mine through GB Battery. They are NOT as reputable a company. It has been discussed here a couple times that they over rate the 20 hour ratings of their batteries. So my 12-85-13 "804 amp hour battery" is more likely closer to 670 amp hour. This make cost comparisons and calculation a battery for your needs more difficult, but they are a bit cheaper and they did deliver in a timely fashion. They did NOT pay all applicable taxes as they failed to pay the Missouri sales tax stating that I was liable for that. Not sure why that's not an applicable tax?

http://gbindustrialbattery.com/Forklift_Battery_Sizes_and_Specifications_Zone15.html

Well, I was hoping someone else would respond, so I don't look like the bad guy here.

I don't think you need to fully discharge, and I wouldn't do it and don't think it's necessary. Solar isn't like a standard battery charger. I don't know the size of your array, but I do know the sun doesn't shine every day. If you discharge to 20% of capacity and the sun doesn't shine, and the battery sits at 20% SOC(State of charge) the battery WILL Sulfate and you will loose capacity. In a warehouse batteries that are taken down to 1.140 or 20% of capacity only last 5-7 years! and that is with prompt recharging! (Please note VTMaps has already stated this)

It is important to properly 'form' your battery. So they need to be used for the first 6 months. I would be sure to draw them down 20% daily (80% SOC), and perhaps 40-50% once a week when you know you will have a sunny day following and can fully recharge them. Please note I say fully recharge them, I don't know the size of your array or orientation so I can't make this calculation for you.

Be sure to equalize at least once a month and some recommend more often. Heavy use and equalizing help to mix the electrolyte.

I'd keep talking with your "forklift guy" only if you have a fork lift or you want to run your battery down daily and recharge with a large battery charger daily from the grid. That is what they know. 

Pretty much every lead acid battery will spend at least a short time in bulk every morning. The array, if oriented south, isn't putting out much current, start with less than the inverter in idle is using. Once the array starts producing meaningful current above what the system is using, the voltage will rise. Once it reaches your set point, the charge controller will maintain that voltage and switch to the absorb mode. It will stay there depending on how your charge controller is setup, for a prescribed time, or until the current reaches your set end amps, then switch to float. Float is a lower voltage that is above the batteries natural resting voltage, but not enough above to force current to flow into the battery.

Equalizing cycle is a higher voltage cycle that comes after the battery is fully charged, with the higher voltage current is flows into the battery, this does several things. A battery is a electrochemical reaction, when equalizing it is forced into a very active state, the current running through the plate tries to convert the sulfates on the plate back into solution, reducing the buildup on the plates. The battery is already fully charged so most of the sulfate is already in solution, so the electrolyte must do something with the energy. The temperature rises and the electrolyte breaks down and releases hydrogen and oxygen. This 'boiling' helps mix the electrolyte. It's important to have a temperature sensor so the battery doesn't get too hot.

Equalizing in short case batteries is primarily to 'equalize' the cells, some cells will have all of the sulfates mixed into solution and have a higher SG (Specific Gravity) than other cells which will still have some sulfate on the plates. You will see some manufacturers recommend only equalizing when the SG is out of balance. This is not true of forklift batteries. They need the equalizing regularly, some recommend every 2 weeks or 10 cycles. I really hate the term cycle when used in solar applications, but understanding why is important. This is why when in an off grid situation, and you have had your battery sitting at a low state of charge for a long time, like 2 weeks of fall storms, that you fully charge and equalize ASAP.

I've never read maintaining a 1.140 SG, I think if that represents the minimal SG at which you won't have sulfating, thinking that this will preserve long battery life. It is just bad thinking and has no bearing in a lift/traction battery world.