Battery Charge Time Calculator
Work out how long a battery bank takes to charge from solar, shore power and an alternator combined, including the slow absorption phase that lead-acid cannot skip.
How to use this calculator
- 1Get the starting state of charge from a shunt battery monitor. Voltage is a rough guide on lead-acid and nearly useless on LiFePO4, whose discharge curve is almost flat.
- 2Enter every charge source that will be running at once — they add, up to whatever limit the battery itself imposes.
- 3Set a realistic peak sun hours figure for the season. Solar cannot run for longer than the sun is up, however large the array.
- 4Watch the absorption column. On lead-acid it usually dominates, and it is the reason a bank that looks adequately charged never actually reaches full.
- 5On LiFePO4, consider setting the target to 90% rather than 100%. It charges faster, stresses the cells less, and costs nothing in usable capacity.
How the calculation works
Solar amps = array watts x system derate ÷ system voltage
Charge amps = solar + shore + alternator, capped at the battery charge limit
Bulk Ah = (bulk-end SoC − start SoC) ÷ 100 x capacity
Bulk hours = bulk Ah ÷ (charge amps x charge efficiency)
Absorption hours = (absorption Ah ÷ (charge amps x efficiency)) x (1 + absorption factor x 8)
Total = bulk + absorption- Bulk
- Constant-current phase. The charger delivers everything it has until the battery reaches absorption voltage — around 80% for lead-acid, 95% for LiFePO4
- Absorption
- Constant-voltage phase. The battery decides how much current to accept, and it accepts progressively less. Adding charge capacity does not speed this up
- Absorption factor
- How much slower absorption is than bulk for a given chemistry. Near 1 for flooded lead-acid, 0.15 for LiFePO4
- Charge efficiency
- Energy accepted divided by energy supplied. Around 80% for flooded lead-acid, where some goes to gassing, and 95% for LiFePO4
The absorption model is a simplification of an exponential taper. Real absorption current falls roughly exponentially as the battery approaches full, and manufacturers specify a fixed absorption *time* rather than a current threshold. The multiplier used here reproduces the well-documented rule of thumb that the last 20% of a lead-acid charge takes about as long as the first 80%.
Absorption is limited by the battery, not the charger. This is the point people find counter-intuitive: doubling the solar array halves the bulk time and does essentially nothing to absorption. A system that cannot finish charging in a winter day is not fixed by more panels.
Charge efficiency and round-trip efficiency are treated as the same figure here. Strictly the charge half of the round trip is slightly better than the discharge half, so this is marginally conservative.
Worked example
A 200 Ah LiFePO4 bank from 40% to 100% on 400 W of solar
- 1.Solar current: 400 W x 80% ÷ 12 V = 26.7 A.
- 2.LiFePO4 bulk runs to 95%, so bulk covers 40% to 95% — that is 55% of 200 Ah, or 110 Ah.
- 3.At 26.7 A and 95% charge efficiency: 110 ÷ (26.7 x 0.95) = 4.34 hours of bulk.
- 4.Absorption covers the last 5%, which is 10 Ah. Its bulk-equivalent time is 0.39 h, multiplied by (1 + 0.15 x 8) = 2.2, giving 0.87 hours.
- 5.Total: 5.21 hours — against only 4.5 peak sun hours in the day, so it just misses finishing.
- 6.The simple answer, 120 Ah ÷ 26.7 A = 4.50 hours, understates it by about three quarters of an hour.
- 7.The same charge on flooded lead-acid would take far longer, almost entirely in absorption.
Result: 5.2 hours
Why charging is not one process
The intuitive model of battery charging — a tank filling at a constant rate — is right for the first three quarters and badly wrong for the rest. Charging happens in phases, and the phases behave completely differently.
In the bulk phase the charger acts as a current source: it delivers everything it can produce, and the battery voltage climbs steadily as it fills. Here the simple arithmetic works. Amp-hours needed divided by amps available gives the time, and doubling the charger halves it.
When the battery reaches its absorption voltage — around 14.4 volts for a 12 volt lead-acid bank — the charger switches to acting as a voltage source. It holds that voltage steady, and the current is now determined by the battery rather than the charger. As the battery approaches full, its internal resistance to further charge rises and the current it accepts falls away, roughly exponentially.
This is where the intuition breaks. In absorption, a bigger charger does nothing at all. The battery is the bottleneck, and it will take the time it takes. For flooded lead-acid, that last 20% of capacity commonly takes as long as the first 80% — and no amount of solar, shore power or generator changes it.
The absorption phase is why lithium won
Compare capacity, cycle life and price and lithium looks like an incremental improvement on lead-acid. Compare charge acceptance and it looks like a different category of product, because this is where the practical difference in a solar system actually lives.
Lithium iron phosphate accepts nearly full current right up to about 95% state of charge. Its absorption phase is brief. A 200 amp-hour LFP bank at 40% will take everything a 400 watt array can give for four hours and then finish in under an hour more.
A flooded lead-acid bank in the same system transitions to absorption at 80%, and then spends hours accepting a steadily dwindling trickle. In a winter day with four or five hours of usable sun, it will not finish. It will get to perhaps 90% and the sun will go down.
That would be merely inconvenient if the consequence were not cumulative. It is: lead-acid that does not regularly reach full sulphates, and sulphation is permanent. So a lead-acid bank in a winter solar system is not just slow to charge — it is slowly destroying itself, one incomplete cycle at a time, and the owner discovers this a year later when capacity has halved.
This is the mechanism behind the observation that lead-acid banks in off-grid solar systems die young. It is not the depth of discharge; it is the failure to complete the charge.
What actually limits charge current
Adding charge sources adds current, up to a point, and several different things impose that point.
- The battery’s own limit — lead-acid should not be charged much above C/5 — 40 amps for a 200 amp-hour bank — because faster charging causes excessive gassing, water loss and plate damage. LiFePO4 typically accepts 0.5C and its BMS will simply disconnect if exceeded.
- The controller or charger rating — an MPPT controller has a maximum output current, and a converter has a rated DC output. Neither will exceed it regardless of what is available upstream.
- Available sunshine — a solar array only produces while the sun is on it, and only at full output near the middle of the day. Peak sun hours is a hard ceiling that no amount of array capacity moves.
- Cable and fuse — charge current flows through the same wiring as discharge current, and a system sized for a modest charger will not safely carry a much larger one added later.
- Temperature — LiFePO4 must not be charged below freezing at all — doing so plates lithium metal onto the anode and permanently damages the cell. A BMS with a low-temperature cutoff will refuse charge, which looks like a fault and is a feature.
Three ways to put energy in, and what each is good at
Most off-grid and RV systems have more than one charge source, and they complement each other rather than duplicating.
Solar is free once installed, silent, and requires nothing of you. It is also weather-dependent, seasonal, and limited to a few hours a day. It is excellent at holding a system topped up and poor at recovering a deeply discharged bank in winter.
Shore power is fast, unlimited in duration and completely reliable, which makes it the only source that can comfortably sit through a long lead-acid absorption phase. It requires being plugged in, which is precisely what an off-grid system exists to avoid.
An alternator through a DC-DC charger is the underrated one. Forty amps while the engine runs replaces a typical day of consumption in two to three hours, works at night and in rain, and costs a fraction of the equivalent solar. Its limitation is that it only works while driving, which for a rig that moves regularly is not a limitation at all.
The practical answer for most systems is solar to hold station, alternator to recover, and shore power occasionally to give a lead-acid bank the full absorption cycle it needs. A lithium system needs that last one much less, which is another quiet advantage.
Stopping short, on purpose
The two chemistries want opposite things at the top of the charge, and knowing which you have changes how to set the charger.
Lead-acid must reach 100% regularly. Every cycle that ends short leaves some lead sulphate un-reconverted, and over time those crystals harden into a form that no longer participates. A monthly full charge — from shore power or a generator if solar cannot manage it — is genuine maintenance, not a nicety.
Lithium iron phosphate is the reverse. It has no sulphation mechanism, and holding it at full charge is mildly stressful to the cells. Cycling between roughly 20% and 90% measurably extends life compared with cycling to 100%, and because LFP’s usable capacity is so much larger to begin with, giving up the top 10% costs less than it sounds.
This has a practical consequence for charge time that the calculator shows directly: setting an LFP target of 90% eliminates the absorption phase entirely, because 90% is still inside bulk. The bank charges at full current the whole way and then stops. On a short winter day that difference — an hour of absorption avoided — can be what lets the system finish before sunset.
What this assumes, and where it stops
Assumptions
- Bulk charging runs at the full available current until the chemistry’s absorption threshold — 80% for flooded and gel, 85% for AGM, 95% for LiFePO4.
- Absorption is modelled as a multiple of the equivalent bulk time, calibrated so that a flooded lead-acid last 20% takes roughly as long as the first 80%.
- Charge sources add together, capped by any battery charge current limit entered.
- Solar output is the array nameplate reduced by the system derate, available for the peak sun hours entered.
- Charge efficiency uses the chemistry’s round-trip figure unless overridden.
Limitations
- Real absorption is an exponential taper and chargers implement it as a fixed time at voltage rather than a current threshold. The multiplier here reproduces the observed rule of thumb rather than modelling the curve.
- Solar output is treated as a flat rate across the peak sun hours. In reality it ramps up and down, so the bulk phase in the middle of the day is faster and the shoulders slower.
- Battery temperature affects charge acceptance substantially, and LiFePO4 must not be charged below freezing at all. Neither is modelled.
- Battery age reduces charge acceptance as well as capacity. An old lead-acid bank spends even longer in absorption than this predicts.
- Charger behaviour varies. Some enter absorption early, some hold it for a fixed period regardless, and some never complete it if the battery is large relative to the charger.
Common questions
How long does it take to charge a 200Ah battery with 400W of solar?
For LiFePO4 from 40% to full, about 5.2 hours — 4.3 in bulk at 26.7 amps and under an hour in absorption. For flooded lead-acid the bulk phase is shorter, because it ends at 80%, but the absorption phase that follows takes several hours, and in a short winter day the bank often will not finish at all.
Why does the last 20% take so long?
Because charging switches from constant current to constant voltage. Once the battery reaches its absorption voltage the charger holds that voltage and the battery decides how much current to accept — which falls away as it fills. In absorption the battery is the bottleneck, so a bigger charger or more panels makes no difference at all.
Does more solar make my battery charge faster?
It shortens the bulk phase in direct proportion, and does nothing for absorption. If your bank spends most of its charge time in absorption — which is normal for lead-acid — doubling the array will barely change when it finishes. That is one of the strongest practical arguments for lithium in a solar system.
Should I charge lithium to 100%?
Not usually. LiFePO4 has no sulphation mechanism, so there is no penalty for stopping short, and holding it at full is mildly stressful to the cells. Cycling between about 20% and 90% extends life, and it also eliminates the absorption phase entirely — because 90% is still within bulk, the bank charges at full current the whole way and stops.
Why does my lead-acid battery never seem to reach full on solar?
Because the absorption phase needs hours of steady charging and a winter day does not provide them. The bank reaches 80% quickly, then sits accepting a declining trickle until the sun goes down. This is how sulphation starts, and it is permanent — which is why a periodic full charge from shore power or a generator is real maintenance for a lead-acid bank.
Sources
- Batteries, charging and electric vehicles — US Department of Energy
- Home energy storage — US Department of Energy
- Energy storage research — National Renewable Energy Laboratory
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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