What the calculator does
A battery does not fill at the charger's rated current from empty to full. It takes the full current only until it reaches the charger's absorption voltage, at roughly 80 % state of charge for lead-acid, and the rest goes in at a tapering current while the voltage is held, which takes a fixed few hours however large the charger. The calculator separates the two: it works out the amp-hours to put back (more than were taken out, because charging is not 100 % efficient), the time at full current to the end of the bulk stage, and the absorption stage that follows, and adds them. It also checks the charger current against what the chemistry tolerates, converts the charge into watt-hours, and reports the charger current that would refill the battery in one eight-hour night.
Formula
C-rate of the charger = Icharger ÷ C
Bulk Ah = C × max(0, SoCbulk end − SoC) ÷ 100 ÷ η Bulk time = Bulk Ah ÷ Icharger
Absorption Ah = C × (100 − max(SoC, SoCbulk end)) ÷ 100 ÷ η
Absorption time = max(Absorption Ah ÷ Icharger, tabsorption)
Total time = Bulk time + Absorption time
Energy into the battery (Wh) ≈ Ah to replace × Vnominal
I8 h = max(Bulk Ah ÷ (8 − tabsorption), Ah to replace ÷ 8), capped at the recommended C-rate
where C is the C20 capacity in Ah, SoC the present state of charge, η the charge (coulombic) efficiency, Icharger the charger current and SoCbulk end the state of charge at which the charger reaches its voltage limit and stops delivering full current: about 80 % for lead-acid and about 95 % for LiFePO4, whose constant-voltage tail is short. The absorption stage lasts the set time, or longer if its amp-hours could not be delivered in that time even at full current. The recommended maximum charge current is 0.1 C for flooded lead-acid (tubular-plate batteries are often rated for a little more), 0.2 C for AGM and gel and 0.5 C for LiFePO4; above it the calculator warns but does not clip your figure. The 8-hour current is the smallest at which bulk plus absorption fit in eight hours: because the absorption stage is the longer of its set time and its amp-hours at that current, the answer is the larger of the two expressions.
Worked example
A 12 V 150 Ah flooded tubular battery of the kind under most Pakistani UPS inverters, run down to 50 % by an outage, on a charger delivering 15 A, with 85 % charge efficiency and a 2.5 h absorption stage.
- Ah to replace = 150 × 0.5 ÷ 0.85 = 88.2 Ah.
- C-rate = 15 ÷ 150 = 0.10 C, at the 0.1 C (15 A) recommended for flooded lead-acid.
- Bulk stage, 50 % to 80 %: 150 × 0.3 ÷ 0.85 = 52.9 Ah at 15 A = 3.53 h.
- Absorption stage, 80 % to 100 %: 150 × 0.2 ÷ 0.85 = 35.3 Ah, which would take 2.35 h at full current, so the 2.5 h set time governs.
- Total: 3.53 + 2.5 ≈ 6.0 h. Energy into the battery ≈ 88.2 × 12 = 1 059 Wh, about 1.06 kWh. From the mains, with the charge voltage above nominal (about × 1.15) and a charger of about 90 % efficiency, expect roughly 1.4 units on the meter.
- To refill in an 8-hour night: the larger of 52.9 ÷ (8 − 2.5) = 9.6 A and 88.2 ÷ 8 = 11.0 A, so 11.0 A (0.07 C). At that current the bulk stage takes 4.8 h and the absorption amp-hours need 3.2 h, longer than the set time, and the two add to 8.0 h.
Chemistry values used by the calculator (per 12 V block)
| Chemistry | Max charge current | Charge efficiency | Absorption stage | Absorption voltage | Float voltage |
|---|---|---|---|---|---|
| Flooded lead-acid, tubular or flat plate | 0.1 C | 80–90 % | 2–4 h | 14.4–14.8 V | 13.5–13.8 V |
| Sealed lead-acid, AGM | 0.2 C | 90–95 % | 2–4 h | 14.2–14.6 V | 13.5–13.8 V |
| Sealed lead-acid, gel | 0.2 C | 90–95 % | 2–4 h | 14.1–14.4 V | 13.5–13.8 V |
| LiFePO4, four cells in series | 0.5 C | 97–99 % | 0–0.5 h | 14.2–14.6 V | 13.5 V, or none |
Assumptions and limitations
- A three-stage charger is assumed: constant current, then constant voltage (absorption), then float. A simple transformer charger with no voltage limit, or a solar controller with an undersized array, behaves differently.
- The bulk stage ends at a fixed state of charge (80 % for lead-acid, 95 % for LiFePO4). In reality it ends when the terminal voltage reaches the limit, which comes later at low currents and earlier at high currents, in cold batteries and in old ones with high internal resistance; the absorption time then covers the difference.
- No temperature compensation. Lead-acid charge voltages should fall by about 3–5 mV per °C per cell above 25 °C; a charger without compensation overcharges in a 40 °C summer and undercharges in winter. The times here assume 25 °C.
- No Peukert effect on charge. Peukert describes capacity lost at high discharge rates. Charging at high rates loses efficiency to gassing instead, which is why the default efficiency for lead-acid is below 90 %.
- Equalisation is not included. A periodic equalising charge at a higher voltage for flooded batteries adds hours beyond the times shown.
- Capacity is assumed healthy. A sulphated or aged battery has less capacity than its label, so it reaches the absorption voltage sooner and holds less; the calculator cannot see that.
- Inverter-chargers limit the current themselves. The hybrid and UPS inverters common in Pakistan have a maximum charge current setting, often left at a default well below the rating, and a separate limit when running from a generator or a weak utility supply; enter the setting, not the datasheet figure. Charging from solar is limited by what the array delivers hour by hour, which the calculator treats as a constant current.
Frequently asked questions
Why can't I charge faster with a bigger charger?
Because above about 0.1 C a flooded lead-acid battery reaches the gassing voltage early and puts the extra current into electrolysing water and heating the plates rather than into charge. A bigger charger shortens the bulk stage, which is only the first 80 %; the absorption stage still takes its two to four hours because the battery can only absorb charge as fast as the acid diffuses into the plates. Sealed batteries take 0.2 C and lithium 0.5 C or more, which is one reason lithium refills so much faster.
What happens at 0.3 C on a tubular battery?
A 150 Ah tubular battery on 45 A heats up, gasses vigorously, loses water fast and sheds active material from the plates. The terminal voltage hits the charger's limit early, so the bulk stage ends well short of 80 % and the battery spends longer in absorption anyway; the calculator's 3.7 h for that case is optimistic, and life falls markedly. Some tubular batteries are rated for a little more than 0.1 C, not for three times it.
Is trickle charging bad?
A float charge at the right voltage (13.5–13.8 V per 12 V for lead-acid) is what keeps a standby battery full for years and does no harm. A "trickle" at a fixed small current with no voltage limit is harmful: it keeps gassing a full battery and dries it out. Lithium batteries need no float at all and are happier resting a little below full.
How do I know the battery is full?
For lead-acid, when the current at the absorption voltage has fallen to about 1–2 % of the C20 capacity (1.5–3 A for a 150 Ah battery) and stops falling; the charger then drops to float. Rested for a few hours off charge, a full flooded battery reads about 12.6–12.8 V per 12 V, and a hydrometer reads the specific gravity the maker specifies for full, about 1.25–1.28. For LiFePO4, full is when the current in the constant-voltage tail has fallen to about 0.05 C, or when the BMS says so.
Why do UPS batteries in Pakistan die early?
Chronic undercharging. The battery on this page needs six hours to refill from 50 %, and the last two and a half of them are the absorption stage. If the utility comes back for four hours between outages, the charger completes the bulk stage and half an hour of absorption, leaving the battery a little over 80 %; the next outage starts from there and ends lower, and a four-hour window puts back at most 4 × 15 × 0.85 = 51 Ah against the 75 Ah each outage takes out. Each cycle starts about 16 percentage points lower than the last, the absorption voltage is never held long enough to convert the sulphate on the plates back into acid, and the sulphate hardens into crystals that will not convert at all. The remedy is a charger current near 0.1 C, an absorption stage allowed to finish, and, where outages are long and frequent, a LiFePO4 bank, which charges at 0.5 C and does not sulphate. The guide has the full arithmetic.
References
- IEC 61427-1:2013, Secondary cells and batteries for renewable energy storage — General requirements and methods of test — Part 1: Photovoltaic off-grid application — cycling and charge regimes for storage batteries in PV systems
- IEEE Std 450-2010, Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications — float and equalising charge, state-of-charge indicators for flooded cells
- IEEE Std 1188-2005, Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications — charging and maintenance of AGM and gel batteries
- IEC 62619:2022, Secondary cells and batteries containing alkaline or other non-acid electrolytes — Safety requirements for secondary lithium cells and batteries, for use in industrial applications — charge limits and battery management requirements for lithium industrial batteries