Battery Charge and Discharge Calculator

Here is one closed 24-hour cycle for a 200Ah 12V LiFePO4 bank on a cabin with solar panels. Every figure below comes from the calculator above at the same settings.

Morning (7 AM - 10 AM): The bank starts at about 29% SOC after the overnight loads. Panels come up with the sun and the charge controller puts a net 15A into the batteries after the daytime loads are covered, so 45Ah by 10 AM lifts the bank to roughly 51.5%.

Midday (10 AM - 1 PM): Peak production, and the controller ramps to 30A. The 97Ah still missing takes 3 hours 14 minutes: 2 hours 54 minutes of bulk at the full 30A, then 20 minutes of absorption for the last 10Ah while the current tapers. The bank is full by about 1:14 PM, and after that any excess solar is wasted unless you have a dump load.

Evening (5 PM - 11 PM): Production is zero. The bank runs lights (60W), the fridge (50W average), a TV (80W) and phone charging (15W), 205W in total. At 80% depth and 90% efficiency the 200Ah pack holds 1,843Wh of usable energy, which is 9.0 hours at this load, or 8 hours 59 minutes on the clock. Six evening hours take 1,367Wh out of the bank, leaving it at about 46.6%.

Overnight (11 PM - 7 AM): Only the fridge runs, averaging 50W. That is 400Wh of work and 444Wh out of the bank over eight hours, and the night ends at about 29%, where the day started. The loop closes, which is the whole test: a system that ends each night lower than it started is losing ground, and no amount of good weather next week fixes it.

Cycle-life numbers are quoted loosely all over the internet, so this table quotes nobody but the manufacturers, and names the model line and the depth each figure was printed against. A "cycle" is a discharge to the stated depth followed by a full recharge.

Two gaps are worth naming rather than filling. There is no flooded row, because the flooded makers we checked publish this relationship as a chart rather than a table of numbers, and reading a value off a chart image is not the same as quoting one. There is no lithium-ion NMC row for the same reason: the cycle counts published for NMC are not tied to a stated depth of discharge, so they cannot be lined up against the rows above.

Read the shape rather than the individual numbers. Halving the depth roughly doubles the count on every row, which is why the depth of discharge box above is the single most expensive setting on the page. The cleanest comparison is Victron against itself at one depth: at 50% the company prints 5,000 cycles for its LiFePO4 Smart range and 750 for its gel range. That is 13 years of daily cycling against two. If you are choosing between chemistries on cost, that ratio matters more than the sticker price, and our deep cycle battery runtime calculator works the runtime side of the same trade.

The discharge half. A battery does not hand back its rated amp-hours at any current. Victron says so plainly on its own datasheet: "The effective capacity decreases with increasing discharge current." So instead of dividing energy by watts, this tool fits a rate correction to the discharge tables the makers actually print, one fit per lead-acid chemistry. Flooded is fitted to the printed discharge table on Trojan's T-105 data sheet, AGM to Lifeline's GPL-31T minutes and Victron's AGM column, gel to Victron's gel column. The rate correction reproduces those printed tables within about 10% for flooded, about 6% for AGM and about 2% for gel. LiFePO4 takes no rate term at all up to 1C, and is run at the 12,8V nominal Victron prints rather than a flat 12V, which is why a 100Ah lithium pack shows 1,280Wh here and not 1,200Wh. If you want the lead-acid side on its own, the lead-acid battery runtime calculator goes further into it.

The charge half. Charging is two stages, not one. Victron's Blue Smart charger manual prints the method in full: bulk time is the amp-hours below the bulk endpoint divided by the charge current, and absorption is a separate stage on top. It puts that endpoint at "approximately 80% state of charge" for lead-acid and "well above 95%" for lithium, and it prints a worked case, a flat 100Ah lead-acid battery on a 10A charger, 8 hours of bulk plus 8 hours of absorption, 16 hours in total. Nothing in that printed method divides by an efficiency figure, and neither does this calculator.

Victron prints the driver of the absorption stage, its bounds (30 minutes to 8 hours) and that one worked value, but no rule for a partial discharge. That gap is filled by a least-squares fit to six printed totals: Victron's 16-hour case plus the five rows of NOCO's GENIUS10 charge-time chart, all at 10A and 50% depth. The fitted coefficient is 0.8571 absorption hours per amp-hour-per-amp, and it lands within 4.1% of every one of the six printed totals. Two printed floors sit under it, Victron's 30-minute absorption minimum, and East Penn's statement that a lead-acid charge returns between 105% and 130% of the amp-hours removed, which stops a shallow top-up finishing faster than the current allows. LiFePO4 is not fitted: bulk to 95%, then the last 5% at the charge current, which reproduces all three printed lithium cases exactly (Victron 10 hours at 10A, Victron 2 hours at 0.5C, Amptron 5 hours at 20A).

What that leaves is a tool whose two answers you can check against a manufacturer's own printed numbers, which is the only kind of checking worth doing. If you only need the charge side, the battery charge time calculator runs the same engine with a smaller set of inputs.