Battery calculator: runtime and charge from voltage

An online tool for battery math: how long a battery will run under a load — directly or through an inverter or UPS — how much charge it holds judging by its resting voltage, and how long it takes to charge. It accounts for the battery type (car, deep-cycle, AGM, gel, LiFePO4, Li-ion 18650, LiPo), the Peukert effect, depth of discharge, cold and wear. The voltage charts come from manufacturers' documents and are printed below.

How to use the battery calculator

Choose the type and voltage

Car, deep-cycle, AGM, gel, LiFePO4, Li-ion or LiPo; 12, 24, 48 V or your own cell count.

Enter the capacity

In Ah, mAh or Wh; if several batteries are in parallel, their number.

Pick what to calculate

Runtime under a load, charge from the terminal voltage, or charge time.

Read the answer and how it was worked out

Below the answer — the working and warnings: surface charge, the LiFePO4 plateau, cold, over-current.

Find out how long a battery lasts and how much charge it holds

Battery type

Starter battery: calcium, hybrid, EFB. Chart: BCI.

Voltage

Other, cells

Capacity of one battery In parallel

How many identical batteries are wired in parallel — capacity adds up, voltage stays the same.

Load

Through an inverter or UPS — an AC load Starting charge, %

Depth, cold, wear

Depth of discharge, % Load is on, % of the time Temperature, °C Capacity left vs. new, % Peukert exponent

The default depth is the long-life one for the type: 50 % for lead-acid, 90 % for LiFePO4, 80 % for Li-ion and LiPo. A fridge cycles on and off — enter the share of time its compressor runs (usually 30–50 %).

Examples

Runs for

—

Resting voltage and charge

Voltages of this battery

How it was calculated

Wiring

12 V battery voltage chart: state of charge

Resting voltage at 25 °C — after 4–24 hours with no load and no charging. Makers disagree: 50 % is 12.24 V on a starter battery, 12.10 V on a Trojan deep-cycle one and 12.35 V on gel. For 24 V, double the figures.

Charge	Car (flooded)	Deep-cycle flooded	AGM	Gel
100 %	12.65	12.73	12.84	12.85
90 %	12.57	12.62	12.72	12.77
80 %	12.49	12.50	12.60	12.69
75 %	12.45	12.44	12.54	12.65
70 %	12.41	12.37	12.48	12.59
60 %	12.32	12.24	12.36	12.47
50 %	12.24	12.10	12.24	12.35
40 %	12.17	11.96	12.12	12.21
30 %	12.10	11.81	12.00	12.07
25 %	12.06	11.73	11.94	12.00
20 %	12.03	11.66	11.88	11.96
10 %	11.96	11.51	11.76	11.88
0 %	11.89	—	11.64	11.80

Sources: BCI (Battery University BU-903, Victron «Energy Unlimited»), Trojan T-1275 and 31-AGM, Discover (gel).

LiFePO4 voltage chart: 12.8, 25.6 and 51.2 V

Between 20 and 90 % the voltage moves less than 0.15 V per cell and hysteresis is 5–25 mV, so a voltmeter can only tell full, middle and nearly empty. An exact percentage needs a shunt monitor or the BMS. Cell makers publish no such table — this is the sellers' consensus (Renogy, UPower).

Charge	1 cell	4S · 12.8 V	8S · 25.6 V	16S · 51.2 V
100 %	3.40	13.60	27.20	54.40
90 %	3.35	13.40	26.80	53.60
80 %	3.32	13.28	26.56	53.12
70 %	3.30	13.20	26.40	52.80
60 %	3.27	13.08	26.16	52.32
50 %	3.26	13.04	26.08	52.16
40 %	3.25	13.00	26.00	52.00
30 %	3.22	12.88	25.76	51.52
20 %	3.20	12.80	25.60	51.20
10 %	3.00	12.00	24.00	48.00
0 %	2.50	10.00	20.00	40.00

Li-ion and LiPo cell voltage by charge

Li-ion is computed from the LG M50 cell model (Chen, 2020); other cells differ by ±0.05 V. LiPo is the table RC pilots use; store a pack at 3.80–3.85 V per cell.

Charge	Li-ion 18650/21700	LiPo
100 %	4.18	4.20
90 %	4.10	4.11
80 %	4.03	4.02
70 %	3.94	3.95
60 %	3.84	3.87
50 %	3.75	3.84
40 %	3.66	3.80
30 %	3.58	3.77
20 %	3.48	3.73
10 %	3.29	3.69
0 %	2.50	3.27

Charging and float voltage at 25 °C

Type	Absorption, V	Float, V
Car (flooded) · 12 V	14.4–14.8	13.5–13.8
Deep-cycle flooded · 12 V	14.82	13.50
AGM · 12 V	14.2–14.6	13.5–13.8
Gel · 12 V	14.1–14.4	13.5–13.8
LiFePO4 · 4S · 12.8 V	14.2–14.4	13.50

Trojan, Victron, Yuasa, Battle Born. Lead-acid charge voltage is temperature-compensated: plus 24 mV for every degree below 25 °C on a 12 V battery, minus above (Victron). LiFePO4 needs no compensation.

Features

The chart of your battery type

50 % charge is 12.24 V on a starter battery, 12.10 V on a Trojan deep-cycle and 12.35 V on gel: the calculator uses the chart of the chosen type, not one for all.

An honest answer for LiFePO4

Between 20 and 90 % LiFePO4 voltage barely moves, so the charge is shown as a range with the reason, not as a made-up number.

Runtime through an inverter or UPS

Inverter efficiency and idle draw, the share of time the load runs (a fridge), the starting charge and a safe depth of discharge.

Peukert effect, cold and wear

At high current a lead-acid battery gives less than its rated capacity, and less again in the cold: both effects are checked against Victron data.

Charging: time and voltages

Time to the charge you want, with the slow absorption phase, plus charge and float voltages with −24 mV/°C compensation for 12 V.

18650 packs and LiPo

S×P wiring, the current limit from the C-rating or the cell current, and an over-current warning.

Frequently asked questions

Why is 12 volts not «half charged»?

On a car battery at rest 12.0 V is about 15 %, on AGM about 30 %: half charge is 12.24 V. The whole scale runs from about 11.9 to 12.7 V, so every tenth of a volt matters.

Why must the voltage be measured at rest?

Right after charging or driving the plates carry a surface charge and the meter reads high; under load it reads low. The charts describe a battery that has rested 4–24 hours. If you cannot wait, switch the headlights on for 2–3 minutes, switch them off and measure again.

Why is LiFePO4 charge shown as a range?

Between 20 and 90 % a LiFePO4 cell stays around 3.25–3.32 V, and hysteresis reaches 25 mV. A voltmeter can tell only full, middle and nearly empty; an exact percentage needs a shunt monitor or a BMS that counts amp-hours.

Why does a UPS run for less than the battery capacity suggests?

A UPS draws a high current, and by the Peukert effect a lead-acid battery then gives fewer amp-hours: AGM over one hour gives about 65 % of its rating. Inverter efficiency and idle draw take another 10–15 %. Tick «Through an inverter or UPS» and the calculator accounts for both.

How deep can I discharge the battery?

Lead-acid — to 50 % for long life: Victron rates AGM at about 400 cycles at 80 % depth, 600 at 50 % and 1500 at 30 %. LiFePO4 cell makers recommend staying within 10–90 %. Change the limit under «Depth, cold, wear».

Can LiFePO4 be charged in freezing weather?

No: lithium batteries are charged at 0 °C and above (EVE cells: 0–55 °C), otherwise lithium plates on the anode and capacity is lost for good. Discharging in the cold is fine, but at −20 °C only about 70 % of the capacity is available.

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Published 18 May 2026 Author: The Supreme