Lead-Acid Battery Runtime Calculator

Amp-hour ratings are measured at the 20-hour rate. Trojan's own data sheet for the T-105 puts the whole ladder on one line: the same battery is rated 250Ah at the 100-hour rate, 225Ah at the 20-hour rate, 207Ah at the 10-hour rate and 185Ah at the 5-hour rate. One battery, four numbers. Pull harder and you get less, because at high current lead sulfate forms faster than it dissolves back into the electrolyte and blocks the plate surface before the plate is spent.

The calculator above does not assume the rated amp-hours arrive at any current. It works out the current your load actually draws, then reads the time off the printed discharge rows: time to full discharge = f / (I / C20), where I is the current, C20 is the 20-hour rating, and f is the share of that rating the maker prints at that current, interpolated on a straight line in log-log space between the two nearest printed rows for the type you picked. Your depth of discharge then takes its share of that time. The rows come from the discharge tables Trojan and Victron print for their own batteries (Trojan for flooded, Victron for AGM and gel), and the curve returns every printed row exactly. Lifeline's printed table for its GPL-31T sits 3 to 8 percent below Victron's between the 8 A and 25 A rates; for AGM and gel this calculator uses Victron's table across its full range.

Two checks against the sheets themselves. Trojan prints 447 minutes at 25A for the 225Ah T-105; the tool computes 7 hours 27 minutes to full discharge, the printed figure, because the curve runs through that row. Victron prints 65% of the rating over 1 hour for its AGM Deep Cycle batteries, a 65A draw on a 100Ah battery; the tool computes 1 hour, again the printed figure. The flat-capacity arithmetic this page used to show gave 9 hours and 1 hour 32 minutes for those same two batteries, and that gap is where the missing hours came from.

What the correction does not cover is temperature or age. Trojan and Victron both print capacity-versus-temperature curves and neither is in the engine, so a cold bank does worse than the figure above. Push the load past the fastest discharge the makers printed and the result panel says so: past that point the tool extends the slope of the last two printed rows rather than reading a curve somebody measured, so the real runtime may be shorter than the figure shown.

Flooded and sealed are the same lead-acid chemistry in different packaging, and the packaging is what you live with day to day. Every figure below is one the maker prints for its own product, with the maker named in the cell.

Flooded wins on cost per amp-hour and is the value play for an off-grid cabin where you can reach the caps once a month. Prices and rates on this page are our own estimates as of the date shown; they change often. Enter your own figures for a current result. Sealed AGM and gel cost more and give up nothing to spillage or hydrogen, which is why they win in an RV, a boat, or a cupboard. Look at where the printed cycle rows land, though: Victron rates its gel line 750 cycles at 50% discharge against 500 at 80%, so the depth you choose moves the replacement date more than the badge on the case does.