My opinions on a suitable battery system for an off grid power system:
I consider overall (long term) costs to be the single most important factor. Getting the best value for the battery is paramount. I write "value", not price. What kills a battery is excessive discharge and temperature extremes. Overcharge is also a killer, but it's not so much a problem as the other two. I say go with a new forklift battery. Size the battery such that a full charge on the battery will provide you with all the electricity you need over a 24 hour period without dropping the state of charge below 60%. You never want to drop below 50% state of charge. The bigger the battery, the longer it will last all else equal. The charging efficiency of a battery is very low while it's at a high state of charge. However, with the price of solar panels down so much I consider it preferable to choose a larger battery that stays at a high state of charge and buy extra panels. The overall efficiency can be improved dramatically by using most of your electricity while the panels are producing as this effectively bypasses the battery charging. One example would be placing a thermal mass in freezers and refrigerators and putting them on timers to operate during the day while the panels are producing. Another example is using any high power electrical appliance only when the panels are producing.
Forklift batteries are hands down the best value I've seen in an off grid battery system. You're looking at about $130-$150 per kilowatt hour of rated storage capacity (80% discharge at 20 hour rate). So, a 1050 pound 24 volt forklift battery will cost you about $2500 delivered in the 48 States (see
www.giantbatteryco.com), and will provide 19 KWh electricity at rated capacity. In practice, if you limit discharge to no lower than 60% state of charge and considering inverter losses, then a fully charged battery will provide about 8 KWh of AC electricity, and this is more than most off grid home need each day. Keep a wood gas engine system around for backup charging, and keep a large solar array to minimize wood fuel consumption for this purpose.
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