Off-Grid Batteries: Page 2 of 4

Industrial batteries are available in a wide variety of configurations, up to about 2,500 Ah per cell. Industrial batteries no longer fit a dimensional standard, as they are not adapted from another industry. Rather, the number and size of lead plates determines both the capacity and physical size of each cell. A battery bank is sized to the desired capacity, and cells ordered in cases holding one, two, three, or six cells. Some cells can be removed from the cases for ease of installation, others use welded interconnects between adjacent cells. Installation must be carefully planned to manage a weight that can easily exceed 300 pounds.

Industrial cells are substantially more expensive: $2,000 to $10,000 for a set is not uncommon, depending on capacity. Their substantially greater cycle life—15 to 20 years of good performance is typical—has been shown to be the best bargain on a lifetime basis. This is especially so if replacement labor is amortized into the life-cycle cost, as they are replaced less frequently. Some long-time installers will only sell industrial cells, preferring to maintain a positive relationship with their customers over the long haul.

With newcomers to off-grid living, however, installers may advise a set of L16s or even golf-cart batteries as a training set, rather than suggesting more expensive batteries. Some homeowners are simply better than others at maintenance duties, and a ruined bank of industrial cells is a bitter pill to swallow. An inexpensive first set is a smaller investment, and allows for several years of adjustment to an off-grid lifestyle and load-watching. Plus, because of change in family size and lifestyle, the replacement industrial bank may end up larger or smaller than the load analysis determined in the initial design.

Sealed batteries offer some benefits over flooded batteries. They require no maintenance beyond proper charging. As the electrolyte is either gelled or absorbed, they don’t gas during normal charging. Lacking liquid electrolyte, they are charged to lower voltages and can tolerate small arrays and lower charge rates, as long as they regularly reach and maintain full charge. They don’t require adding water or equalizing, plus they don’t leak and won’t foul battery storage areas or attract corrosion on terminals. Absorbed glass mat (AGM) batteries are non-spillable and non-hazardous, so can be shipped via common ground and air freight with no hazardous material costs. Since access to the cell tops to add water is not necessary, they can be mounted in any orientation without harm. Their stackability means they may occupy less floor space than flooded batteries.

But sealed batteries are not without their drawbacks. AGM batteries may last longer than inexpensive flooded batteries, but not as long as industrial flooded batteries. For example, Concorde estimates seven to 10 years of service in off-grid use. They are substantially more expensive: typically double the cost of industrial cells of similar capacity. They are more susceptible to damage from overcharging.

These batteries are well-suited to homeowners who don’t want to perform their own battery maintenance. This includes many newcomers to off-grid living, who want (and can afford) a professionally designed and installed system, and can live well within its limitations, but prefer not to be involved with battery maintenance. Sealed batteries are also well-suited to cabins and homes with seasonal use and little maintenance, and for weekend cabins in which small arrays and larger banks provide energy for weekend use.

PV-to-Battery Sizing

Originally, deep cycle batteries were meant to be cycled during a work shift and then be immediately and fully charged daily with utility power: a predictable and well-managed charging regimen. In an off-grid home system, both loads and RE resources vary from day to day and season to season. Some days, batteries will be full by mid-morning; at other times, batteries may not be full for days. Opportunity charging is the term given to the treatment of batteries in RE systems: the charging system takes maximum advantage of any RE source when it’s available. In practical terms, this means that charge voltage settings are often set higher, and absorption times are often set to the maximum available—often 4 hours (with the 2% current threshold as an override, to minimize daily gassing when a system is left unused)—see “The Charging Process” sidebar on page 87.