Battery Charge Time Calculator

1. Bulk (to about 80% state of charge): the charger pushes its full rated current and the battery voltage climbs steadily. Victron puts the arithmetic plainly: bulk time is the amp-hours you need back below that endpoint divided by the charger amps. At the defaults above, a battery at 50% has 30Ah sitting below the endpoint, which is 3 hours at 10A.
2. Absorption (the rest of the way): the charger holds voltage constant and the current tapers towards nothing. This is where lead-acid loses its afternoon. Victron's charger varies the stage with how flat the battery was, between 30 minutes and 8 hours, and its own worked example gives a fully discharged 100Ah battery 8 hours of absorption on top of 8 hours of bulk. At the defaults the calculator returns 4 hours 17 minutes, longer than the bulk stage that preceded it.
3. Float: a low maintenance voltage holds the battery full without cooking it. Nothing in the charge-time answer depends on it, because the battery is already at 100%. LiFePO4 should be disconnected at full charge rather than floated.

Picture filling a glass. The first three-quarters goes in fast because the glass takes the stream easily. The last inch needs a slow pour, and pouring at full speed just makes a mess. A lead-acid battery behaves the same way: its internal resistance climbs as it fills, so the charger has to drop the current to stay under the absorption voltage East Penn prints for its own batteries, 2.40 volts per cell, which is 14.40V on a 12V AGM or gel monobloc and 14.70V on a flooded one.

LiFePO4 does not have that tail. It takes full current to about 95% state of charge and finishes the last 5% in minutes. A flat 100Ah LiFePO4 on a 20A charger is exactly 5 hours by this calculator, and Amptron prints “approximately 5 hours” for that same pairing on its own 100Ah datasheet. The same job in lead-acid is 8 hours 17 minutes from empty, or 6 hours 26 minutes if you only took it to 80% depth of discharge. If you are weighing the two chemistries on more than charge time, our LiFePO4 against lead-acid comparison puts the running costs beside the charging behaviour.

Victron’s Blue Smart charger manual prints the structure this calculator uses, including two worked examples for a 100Ah battery on a 10A charger. It gives the bulk formula outright and the bounds on the absorption stage, 30 minutes to 8 hours, but it does not print a rule for a battery that is only half flat. So we fitted one to the totals that manufacturers do print: absorption hours = 0.857 x (amp-hours to replace / charger amps), the least-squares line through the six rows below. One thing to know before you lean on it: every printed total behind that coefficient comes from a 10-amp charger, Victron’s worked example and all five rows of NOCO’s chart, so the way the absorption stage scales to bigger or smaller chargers rests on Victron’s printed statement that the time is calculated from the bulk time, not on figures measured at those sizes.

The worst of the six misses by 4.1%. One more caveat belongs with that number: NOCO prints a single chart for a charger with lead, AGM and lithium modes rather than a chart per chemistry, and we read it as lead-acid because the times it prints are about 1.5 times the plain amp-hours-divided-by-amps, which is lead-acid behaviour and not lithium behaviour. Note too that the smallest row, 20Ah on a 10A charger, is a 0.5C charge, above the 0.3C ceiling East Penn prints for lead-acid: the fit uses the row, and the calculator would warn you about that rate.

The LiFePO4 side is not fitted at all. Bulk runs to the 95% endpoint Victron prints, the last 5% follows at the charge current, and that reproduces three printed figures exactly: Victron’s 10 hours for a flat 100Ah battery at 10A, Victron’s 2 hours for the same battery at its recommended 0.5C, and Amptron’s approximately 5 hours at 20A. Battle Born’s printed “30 minutes per 100Ah” absorption time lands on the same half hour. If you want the charge leg and the discharge leg in one view, the charge and discharge calculator runs both.