Absorbed glass mat (AGM) batteries are the star when it comes to remote systems that receive little or no maintenance, but their benefits come at a price—they are two to three times the cost of flooded cells for a battery of the same capacity. AGMs use lead-calcium chemistry and contain integral catalytic recombiners to prevent gassing under normal charging conditions. They are sealed so they can’t spill, and are almost immune to freeze damage no matter what their SOC. But because they are sealed, there is no way to top off the electrolyte if too much gassing occurs, so following the battery manufacturer’s charging instructions precisely is essential.
Nickel-iron (NiFe) battery technology goes back more than 100 years, and has recently been experiencing a resurgence in popularity for off-grid systems. NiFe cells have an extremely long lifespan—at least 25 years compared to the four to 10 years expected from lead-acid chemistry—but are considerably more expensive. They can be left idle in storage, not charging or discharging, for long periods of time over a range of temperatures (-40°F to 140°F) without damage, and are not harmed by a low SOC. However, during daily cycle use, they consume a lot of distilled water. NiFe cells would be best-suited for a remote vacation cabin in which the entire system—including PV input, all controllers, and inverters—is shut down for long periods of time.
Lithium batteries (there are many different specific chemistries available) are the relative new kid on the block, and certainly show promise. (See “Gear” in this issue for a few Li-ion options.) They are expensive (but coming down in price), and require a sophisticated battery management system to keep them healthy during cycling. Advantages include long cycle lifespan; efficient charging; very low self-discharge rates when left idle; and small size and weight. Be sure to check their specified operating temperature range, both high and low—charging at low temperatures can permanently damage them.
Silicon salt batteries are a promising newcomer to the world of remote off-grid system design. They claim to handle a high rate of charge and discharge, are nontoxic and maintenance-free, and are rated to perform in a very wide temperature range (-40°F to 158°F). They are more expensive than AGMs, and have not been around long enough to know if the claims of a 15-year lifespan are accurate.
Saltwater batteries are another very new addition to energy storage technology for off-grid systems. They are expensive and can’t provide much surge current (for example, to start an off-grid well pump) without paralleling additional stacks, which can create a battery bank capacity that is larger than necessary. While they can survive temperatures as low as 15°F, their capacity will be permanently lowered. On the positive side, they are maintenance-free, nontoxic, nonflammable, and can be left at a partial state of charge and even discharged to 0% SOC without damage.
Automatic Generator Start (AGS) Systems
For larger remote cabins in which owners don’t want to shut off all the loads and drain the pipes during extended absences, most off-grid inverter/chargers can be programmed to automatically start a backup battery-charging generator if battery voltage (or even battery SOC) gets low (i.e. reaches a certain threshold setpoint). Gasoline, propane, and even diesel generators are possible for AGS with the addition of some simple circuitry from the inverter/charger manufacturer or third-party companies. Complicated startup routines for diesel generators are no problem—AGS circuitry can preheat the glow plugs, attempt to start the generator, sense if it actually starts or not, and, if unsuccessful, retry the whole procedure after a waiting period.