To accomplish proper charging of flooded cells, the charge rate has to be high enough to overcome the cells’ internal resistance. A PV charge rate of C/20 or better is generally considered the minimum needed. For a 1,000 Ah battery at 24 V, this would be 50 A (plus enough to meet the household loads)—or a PV array rated at more than 1,500 watts. While a C/20 rate is the minimum, the preferred rate is between C/12 and C/6, so for each 100 Ah of flooded battery capacity, the combined DC charge rate should be at least 8 A, or C/12, and not more than 16 A, or C/6. This is the combined amperage of all charging sources, including a PV array, wind or hydro generator, or an engine generator.
In the early years, PV modules were far more expensive than today, and batteries were less expensive. Early practice was to size for 4 to 8 days of autonomy, but that led to small arrays and large battery banks, resulting in chronically inadequate charging. Some old-timers will remember “one module per battery”—modules were 35 W to 50 W, or 6 A per pair of golf-cart batteries. At 220 Ah and 6 A of charge, this resulted in a C/36 rate: too low for good battery care.
Modern systems now call for only 2 to 3 days of autonomy, as long as the system includes a backup generator to make up for extended cloudiness. If the budget allows, array capacity is expanded, rather than increasing battery capacity.
Fewer parallel battery strings in a bank means better performance over the batteries’ life. Slight imbalances between strings within a battery bank can cause increasingly uneven performance, leading to premature failure of part of a bank and early replacement of the entire bank. Larger individual cells allow for fewer strings, as does higher nominal system voltage. A single series string is a wise choice, and two strings in parallel are considered acceptable. Three is the maximum number of parallel strings, but should be avoided if possible.
Battery-based systems are generally wired at 12 V, 24 V, or 48 V. Systems have progressively moved toward inverter-based AC loads, so 12 V system advantages have largely disappeared, and the strong disadvantages of high current and large wire sizes discourage the use of 12 V for all but RV and portable applications and the smallest cabins with minimal loads.
For a 48 V system, a single string of batteries of the proper Ah capacity is recommended. If a single cell fails, it can usually be temporarily bypassed until a replacement cell is installed, and the system can remain in use. Even with the temporary bypass of three cells (an entire battery), as would be the case with a string of eight 6 V batteries, set points can often be adjusted to allow the 48 V system to operate at 42 V.
Twenty-four volt systems are often designed with two strings, as the failure of either a cell or a 6 V battery requires only that one of two strings be temporarily disconnected from the system.
All lead-acid batteries lose their effective capacity as they get cold. The loss varies slightly for different batteries and is almost in direct proportion to their temperature. For example, at 0°F, a battery can supply about 55% of its 77°F capacity. Low-temperature capacity loss isn’t permanent; raise the temperature and the capacity returns. But most off-grid systems are most stressed in winter, when days are shortest and loads are typically greatest. Adding the reduced capacity of frigid batteries only exacerbates this seasonal weakness.
Batteries thrive in a thermally tempered space, with a temperature that seldom drops below 50°F. This can often be achieved by housing them in a space that is well-insulated, has south glazing with overhangs, and adequate thermal mass (the batteries themselves contribute substantially). Batteries don’t need to remain in a heated indoor space, although this too can be done safely and effectively. They do need to be protected from uneven temperatures from radiant heat sources, including exposure to direct sunlight, as identical cells at different temperatures will not perform equally.