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Choosing a battery for your renewable energy system can be a bit of a minefield. There are several different technical variables to think about which will affect the battery performance - and the price of course. Battery capacity The first thing to look out for is the amp hour rating of the battery. Generally speaking, the higher the amp hour rating, the more energy the battery will hold. Bog-standard car batteries will have an amp hour rating of about 40-50 Ah, more specialist batteries can be anything from 100 Ah upwards. However, that's not the whole story, because most batteries are designed to be discharged only to a certain 'depth' before they need to be recharged. If you go below this depth of discharge (DOD), the battery can be permanently damaged, or have a shorter life. Why normal car batteries aren't a good idea Using standard 12V car batteries doesn’t work well with renewable energy kit because they are designed to deliver only a short burst of energy (to start your car). Typically they only have a DOD of about 5% of their total charge before needing to be recharged again. If you drain them right down, they will deteriorate inside and will be good for nothing, even if you try and recharge them. Battery cycling Deep cycle batteries are specially designed to be “deeply” discharged – in other words, to empty themselves of a lot of their charge without getting permanently damaged. You might discharge them by 80% of their full charge without hurting them. Unfortunately, they tend to be more expensive. The best deep cycle batteries are designed for industrial applications like fork lift trucks and have solid lead plates inside them (making them extremely heavy). Generally, the thicker the plates in a battery, the longer it is likely to last. Fairly decent semi-deep cycle batteries are commonly used in golf cart buggies and some marine applications, and this makes them a fairly economical choice for a small renewable energy system. A golf cart battery might be good for 500-600 cycles of discharging and recharging to a 50% DOD. Testing of the battery charge level You can quite reliably measure the charge level of a lead acid battery from the voltage at the terminals, as long as the battery is resting at room temperature and hasn't been charged or discharged for 5 or 6 hours. Fully charged lead acid batteries should read at around 12.7 volts. If you want to avoid discharging your battery bank below a certain level, you can monitor the voltage, either manually, or using controlling equipment, and cut off the load when it dips below a certain point.
% Charge = Volts
100% = 12.70
90% = 12.64
80% = 12.46
70% = 12.36
60% = 12.28
50% = 12.20
40% = 12.12
30% = 12.04
20% = 11.98
10% = 11.94
Some example batteries Trojan's T105 / T106 225 Ah / 260 Ah is a favourite deep cycle battery amongst renewable energy enthusiasts. It has a low initial cost and a 3 - 5 year life and is suitable for small remote renewable energy applications. The T105 will deep cycle 300 - 500 times. The Trojan L16H 420 Ah battery is a more expensive cousin of the T105, which has a better cycling capacity and lifespan. However, at a cost of around £60 each, building a battery bank of these can work out pretty expensive. A compromise is to buy second hand batteries - you may stumble across a good deal on ebay.
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