Off-grid alternative energy homes typically rely on battery banks to store surplus power for use when the various generation devices aren't working at their strongest (i.e. solar panels in cloudy weather or at night, windmills when it isn't windy, micro-hydro when the water flow has dried up, etc). The battery most often used in these installations is called a deep-cycle battery, and is substantially different than the battery you would expect to find in an automobile.
Automobile batteries are designed to discharge a lot of power very quickly, and then recharge for their next use. The lead plates in them tend to be very porous to help with this rapid discharge (a porous lead plate has a much larger surface area that can interact with the acid solution and provide a fast chemical reaction). Over time the lead plates can deteriorate more rapidly than a more solid plate, so start batteries are inappropriate for use in off-grid systems.
Deep-cycle batteries, by comparison, tend to be larger and heavier than starter batteries in order to accommodate more storage capacity. Their plates tend to be more solid in construction and can last a long time when well cared for.
A third common battery frequently encountered in improvised off-grid systems are "marine" batteries which are designed for use in RVs and boats. These batteries tend to fall somewhere between starter batteries and deep-cycle batteries in their capacity, size, and cost. Good off-grid scroungers will also sometimes get their hands on surplussed telephone company batteries (high capacity, low voltage batteries) or "fork-lift" batteries (very large industrial batteries often times used as part of emergency power systems for large warehouse or other commercial installations).
Battery capacity is measured in amp hours (multiples or fractions of), often measured over a specific period of time. In the case of deep-cycle batteries the time period is typically XXX amp hours over a 20-hour period (as in "This battery can discharge 300 amp hours over a 20 hour period"). Batteries can actually discharge power faster than that rate, but when they do their efficiency goes down. Additionally battery efficiency is affected by temperature (they like to be at ~72°F), and it will quickly do harm to a battery to discharge it past the 50% point in its capacity.
So enough theory!
The system that I inherited was, evidently, on it's second or third set of batteries. I'm not real certain what was used previously other than a vague comment made about "all those car batteries we had at first." The bank I inherited is what I describe above as "forklift batteries." There seems to be some debate about why they're called this, with the two most popular theories being that these are the kinds of batteries used in forklifts (this is, I think, wrong); and the other theory being that they're so called because a forklift is needed to move them.
These two batteries are estimated by the prior owner to be ~1500 pounds each. The are about three feet tall, three feet long, and a foot wide. They consist of twelve cells wired in series to create a 24 volt battery, and the two batteries are wired in parallel to double their storage. The solar installer that was paid to evaluate the system documented as 1500 amp hour capacity, but I haven't found anything on the actual batteries that backs this assessment up.
These batteries were supposedly sourced from "a friend that worked at a warehouse that was getting rid of the old batteries in their power backup system." That means these are of unknown age and unknown health. Well maintained these batteries SHOULD last for up to 20 years though, so I haven't rushed into replacing them. I've been using a small battery desulphator on them for a few years now, and the addition of graphing of historical data to my system will be able to give me a decent idea about whether their health is deteriorating over time.
I have a love/hate relationship with these beasts. I want to replace them with a new battery bank of L16 battery with somewhere around 2000 amp hours of capacity. I can't bear to get rid of these batteries, though, if I think they're still good. So here I am nursing them along...
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