Off-Grid Batteries

An Eccentric Anomaly: Ed Davies's Blog

The other day somebody asked on the Navitron forum about batteries for off-grid use specifically mentioning the use of lithium-ion batteries. My reply encapsulates the beginnings of my thoughts on the subject well enough and, unlike most forum posts, stands alone well enough that I thought it was worth re-posting here with some light editing.

At the moment almost everybody off-grid uses some form of lead acid battery: typically either deep cycle, forklift or the in-between golf cart/off-grid (e.g., Trojan) types.

From what I've seen most people who try deep cycle kill off one set learning that they're not really as suitable for deep cycle as some would have you believe, though they're much better than car batteries and the like. I'm hoping to skip that episode.

There are two fundamental problems with lead acid: they don't like being deeply discharged and all the time that they're left standing in a somewhat discharged state they "sulphate" (the lead sulphate on one of the plates forms into bigger crystals which are difficult to remove and reduce the cell's capacity). Forklift types are less vulnerable to deep discharge than most but I've yet to see any convincing evidence that they don't sulphate. The people I most respect to give a straight opinion on them seem to charge them up fairly quickly after use.

Another problem with lead acid is that they need to be charged quickly at first (fine for a generator) but then need a long absorption phase at low current which is a bit more awkward (or, at least, wasteful) to provide from a generator.

In general, the way you'd like to use a battery for off-grid living: charging it up during the day then using some energy during the evening, leaving it sitting overnight a bit discharged, then maybe partially recharging it the next day and so on until there's a windy/sunny day to fill it right up again is almost designed to destroy lead-acid batteries. The solution generally adopted is to oversize the bank so that it doesn't spend much time too discharged.

For these reasons the use of lithium-ion batteries is, indeed, intriguing. They are in many ways a much better fit for off-grid. For example, it's actually better for them to be left in a partially discharged state.

The big problem, of course, is cost. Some figures I came up with a while ago were 48 p/W·h (pence per watt·hour) for LiFePO4 (lithium iron phosphate) vs 8 p/W·h for forklifts. (Actually, that 48 p/W·h is a bit out of date; appear to be doing them for about 33 p/W·h now.) When comparing LiFePO4 against sealed deep-cycle batteries the contrast is not so great as you really need to only count a fairly small proportion of the "sticker" capacity of those batteries (maybe 20%) to get the practical capacity you can use without reducing their life significantly.

An argument against lithium-ion batteries is that they need a battery management system to keep the charge on the cells even. In an ideal world, though, you'd have a BMS on lead-acid batteries, too. People get away with not having them by a) using a closely matched set of cells and b) slightly overcharging ("equalizing") once in a while to make sure all the cells are the same level of charge (there are other reasons for this overcharging as well - it's also something you can't really do with sealed batteries).

The conservative answer is to use forklift batteries or similar but smaller "wet" (not sealed) cells. If you're willing to take a bit more care then sealed batteries can work fine.

Lithium ion are a possibility but the technology is less well understood as yet and therefore all a bit more risky.