Adventures in batteries!

Well, the time for new batteries has finally come.  Our original batteries have been dropping off one-by-one and although they've been more than adequate throughout the summer (when the days are long and the clouds are few and far between), they have little chance of making it through the winter.  Our current batteries have lasted up to 6 years. I only say "up to" because some have lasted longer than others and some were purchased earlier than others.  I don't know all the details because I didn't do the purchasing. Had the batteries been taken good care of, we may have gotten 10 years out of them...but they weren't.  The previous owners did a number of no-no's associated with the batteries.

• No-no #1: They didn't really have a good idea how much energy they were going to use when they purchased the batteries. It turns out running grow lights uses a LOT of energy and their batteries weren't really sized to handle the demand. 
• No-no #2: When they discovered the battery bank wasn't big enough, they added more batteries in series instead of starting from scratch. Mixing old batteries with new batteries is always a bad idea.  Even if the batteries are of the same type, as batteries age the lead plates get thinner and thinner.  Even when the batteries are resting current flow from the new batteries to the old ones. The old batteries work harder than they should even when they should be resting and the new batteries get dragged down to the level of the old ones. 
• No-no #3: When they added the new batteries, they created 5 parallel strings.  The fewer string the better.  Five is a lot!  With five strings, an equalize cycle is required frequently to keep the batteries at similar state of charge (SOC).  
• No-no #4: They didn't know how to run an equalize cycle!  I asked the previous owners how often they equalized the batteries. I don't remember what they said, but I know the take home message was, "we don't".  

Given all of the no-no's, I guess 6 years isn't so bad.  At last not until you considered they spent about $7000 on batteries.  Ouch.  Lucky for us, we were able to learn from some of their mistakes.

So how did we go about deciding what to get for a new battery bank?  There are a ton of choices out there.  First, we needed to know how much power we use. This varies a bit, but on big days we are at about 3500wh/day.  The general rule of thumb these days is to get a battery that has about 4 times the amount of your daily requirement. This gives you about 2 days of autonomy if you keep your batteries above 50% SOC.  Traditionally, people planned for much more that 2 days of autonomy.  There has been a shift to smaller batteries as of late.  The logic for this has a lot to do with the price of renewable energy supplies.  The price of batteries has remained pretty stable over the years.  The price of solar, however has been going down everyday.  Since batteries don't last very long if they are undercharged, it just doesn't make sense to get a battery that is oversized for a charging system.  Batteries are rated in amp-hours (ah) -- at 24V (our system voltage), 3500wh/24V = 145AH. We use about 145AH per day from a 24V battery bank.  145AH x 4 = 590AH.  So, we were looking for a capacity of about 600AH.

I looked at a number of different options with the following in mind:

• 600 - 800AH capacity
• Lead Acid - I like being able to equalize and monitor the specific gravity of individual cells, so I decided against sealed batteries. 
• No more than 2 strings - preferably one.
• Long life
• Affordability

I finally decided on a forklift from Giant Battery I ended up getting a model 12-85-13.  Fork lift batteries are big and heavy, but can last up to 20 years with proper care.  The model I got weighed about 1100lbs.  Since it weighed so much, putting it in the crawl space was not an option.  Instead, we will built a small (2'x4') extension off the back of the house so we could move it into place with my neighbors backhoe.  The additional bonus of having it up and out of the crawl space is temperature.  Batteries perform there best at around 70F.  We were losing a fair amount of capacity by storing our batteries in the 45F crawl space.

The forklift battery at home in our new mini-extension. 

The new battery has been in place few a few days now. The installation went off without a hitch.  We'll see how it does this winter.  Time will tell!