Batteries, Battery Charging Systems, and Battery Installation
Core concepts
A battery is a chemical storage device; usable capacity depends on chemistry, temperature, age, and depth of discharge. Flooded lead-acid, AGM, gel, and lithium (LiFePO4) have different charge profiles. Flooded batteries can be equalized and need watering; AGM and gel must not be equalized; lithium requires a battery management system and typically no float or equalize stage. Three-stage charging-bulk (constant current), absorption (constant voltage), then float (lower voltage)-restores capacity without overheating. Temperature compensation lowers charge voltage when cold and raises it when hot for lead-acid, roughly −0.03 V/°C for a 12 V battery. Installation requires secure mounting, terminal covers, hydrogen ventilation, and overcurrent protection at the battery source. Cable size is governed by ampacity and 3% voltage drop, though starting circuits may accept 10%. Parallel banks should use the same make, age, and type, wired symmetrically.
Traps
Questions often confuse amp-hours with amps, assume a charger delivers its full rating in every stage, or recommend equalization for AGM or gel. Another trap is charging LiFePO4 below 0 °C or using an automotive charger without a marine ignition-protected rating. Do not assume a battery is fully charged because open-circuit voltage reads 12.7 V; surface charge and load matter.
Worked example
A 12 V fridge draws 5 A for 8 h (40 Ah) and lights draw 2 A for 4 h (8 Ah), totaling 48 Ah/day. At 40% depth of discharge, bank size is 48 ÷ 0.40 = 120 Ah; adding 20% margin for aging gives about 144 Ah, so choose a 150 Ah bank. Rule of thumb: do not plan to use more than 50% of lead-acid capacity routinely.