How to measure
Disconnect the charger and the loads and wait. Battery University suggests at least four hours of rest before reading the voltage; Trojan asks for 6–24 hours on lead-acid batteries. Then measure at the battery terminals with a multimeter. On a lithium pack with a balance plug, also measure the cells one by one.
Under load the voltage is lower, and right after charging it is higher, so a reading taken then makes the battery look emptier or fuller than it is. The calculator warns you if you say so under “When you measured”.
Why the result is a range
Resting voltage depends on the cell, the temperature, the age and on whether the battery was last charged or discharged. Makers publish charts that differ by a few hundredths of a volt per cell, and on some chemistries that is a lot of charge. So the calculator shows a typical value and the range covered by all the tables it uses; the tables and their sources are listed below.
LiFePO4: the flat middle
A LiFePO4 cell sits at about 3.2–3.35 V from 20 to 90 % charge. Battery University puts it plainly: on lithium iron phosphate, voltage only indicates full charge and low charge. In a 2025 lab study, a fully rested 26650 cell read 3.295 V at 40 %, and an error of just 15 mV moved the estimate between about 31 and 71 % (Che and others, ACS Energy Letters). That’s why the calculator shows the whole flat zone as a range instead of a single number.
To know the charge of a LiFePO4 battery, count it: a battery monitor with a shunt measures the amp-hours going in and out.
Li-ion and LiPo
A 4.2 V lithium-ion cell rests at about 4.17–4.20 V when full. 18650 and 21700 datasheets rate the cells down to 2.5 V under load, but at rest little is left below about 3.4–3.5 V, where devices usually call it empty. Curves differ with the chemistry: in the middle of the range the four tables we used are up to about 0.2 V apart, so the percentage is approximate.
LiHV cells are charged to 4.35 V instead of 4.2 V. For storage, Grepow suggests about 3.8–3.85 V per cell for both.
Lead-acid: AGM, gel and flooded
A full 12 V lead-acid battery rests at about 12.7 V if flooded and 12.8–12.9 V if AGM or gel. The voltage falls almost in a straight line as it discharges, so a rested reading is a fair guide. Recharge soon after use: a lead-acid battery left discharged sulfates and loses capacity, and Trojan recharges batteries in storage when they fall to 70 % (flooded) or 75 % (AGM and gel).
NiMH
A freshly charged NiMH cell reads about 1.4 V and quickly settles near 1.2 V, where it stays for most of the discharge before dropping towards the 0.9 V end-of-discharge voltage. Energizer’s handbook says voltage can’t be used to determine the state of charge: use a charger with a test mode instead.
Battery chemistries compared
| Chemistry | Cell voltage | Full / empty | Energy | Cycle life | Charging temperature | Self-discharge | Voltage as a gauge |
|---|---|---|---|---|---|---|---|
| LiFePO4 | 3.2 V | 3.65 V charge · 2.5 V cut-off | 163 Wh/kg (EVE LF280K) | 6000 cycles to 80 % (EVE LF280K, 25 °C) | 0–60 °C; full charging power only around 25 °C, 3 % of it at 0 °C (EVE LF280K) | at least 95 % kept after 28 days (EVE) | Poor between 20 and 90 % |
| Li-ion, LiPo | 3.6–3.7 V (LiHV 3.8 V) | 4.20 V charge (LiHV 4.35 V) · 2.5 V cut-off | 220–260 Wh/kg (Molicel P42A, LG MJ1) | 400 cycles to 80 % (LG MJ1) | 0–45 °C (LG MJ1, Molicel P42A) | at least 90 % kept after 30 days (LG MJ1) | Fair |
| Lead-acid: AGM, gel, flooded | 2 V (12 V = 6 cells) | about 2.12–2.14 V full at rest · 1.75 V cut-off | 40–46 Wh/kg (Victron AGM 90 Ah, Trojan T1275 Plus) | AGM 400, gel 500 cycles at 80 % depth; 1500 and 1800 at 30 % (Victron) | below 0 °C only with temperature compensation (Fullriver AGM: down to −15 °C); never when frozen | AGM and gel under 2 % a month at 20 °C (Victron) | Good, after a rest |
| NiMH | 1.2 V | about 1.4 V freshly charged · 0.9 V end | 84 Wh/kg (eneloop AA) | up to 2100 (eneloop); up to 1000 (Energizer) | 0–50 °C, best near room temperature (Energizer) | standard: 50–80 % kept after 6 months (Energizer); eneloop: up to 70 % after 5 years | Unusable |
| Alkaline (single use) | 1.5 V | 0.8 V end in Energizer’s tests | — | single use | not rechargeable; works from −18 to 55 °C (Energizer E91) | shelf life 10 years at 21 °C (Energizer E91) | not covered here |
| CR2032 lithium coin cell (single use) | 3.0 V | 2.0 V end | about 235 Wh/kg (235 mAh, 3 g) | single use | not rechargeable | up to 2 % in 12 months (datasheet) | not covered here |
Figures from one representative datasheet each, as named: other cells of the same chemistry differ. Energy per kilogram is worked out from the rated capacity, voltage and weight.
Charts for common batteries
12 V LiFePO4 battery (4S) 24 V LiFePO4 battery (8S) 48 V LiFePO4 battery (16S) LiFePO4 cell (3.2 V) 18650 / 1S LiPo cell (3.7 V) 2S Li-ion / LiPo (7.4 V) 3S Li-ion / LiPo (11.1 V) 4S Li-ion / LiPo (14.8 V) 6S LiPo (22.2 V) 36 V Li-ion battery (10S) 48 V Li-ion battery (13S) LiHV cell (4.35 V) 12 V AGM battery 12 V gel battery 12 V flooded lead-acid battery 6 V flooded lead-acid battery NiMH cell (1.2 V)
Sources
Charts: Renogy, LiTime, Libre Solar BMS, Samsung SDI (Linux kernel), PineNote (Linux kernel), Anbernic RG353P (Linux kernel), Trojan User’s Guide, Battery University BU-903, Fullriver AGM, Deka Gel-Mate.
Limits and comparison: EVE LF280K, Victron Lithium Battery Smart, Samsung INR18650-30Q, LG INR18650-MJ1, Molicel INR-21700-P42A, Grepow, Battery University BU-808a, Trojan battery maintenance, Trojan T1275 Plus, Victron AGM / gel, Energizer NiMH, Panasonic eneloop, Energizer E91, CR2032.
LiFePO4 lab study: Che et al., ACS Energy Letters 2025.
Frequently asked questions
What voltage is a 12 V LiFePO4 battery at 50 %?
About 13.0–13.1 V at rest, but the number tells you little: from about 20 to 90 % a 12 V LiFePO4 battery stays between roughly 12.8 and 13.3 V. Only above about 13.4 V (nearly full) and below about 12.8 V (under 20 %) does the voltage say something reliable.
What voltage is a fully charged 12 V lead-acid battery?
After 6–24 hours of rest, about 12.7 V for a flooded battery (Trojan: 12.73 V) and about 12.8 V for AGM and gel (Trojan: 12.84 V). Right after charging it reads higher because of surface charge.
At what voltage is an 18650 cell empty?
Cell datasheets set the discharge cut-off at 2.5 V under load (Samsung 30Q, LG MJ1, Molicel P42A). At rest, little is left below about 3.4–3.5 V, which is where most devices stop. Fully charged, an 18650 reads 4.20 V on the charger and about 4.17–4.20 V at rest.
How long should a battery rest before I measure it?
At least four hours according to Battery University, and 6–24 hours for lead-acid according to Trojan (Fullriver asks for at least 8 hours after charging). The longer it rests, the closer the reading gets to the charts.
Why does my LiFePO4 battery show the same voltage for days and then drop?
Because its discharge curve is flat: from about 90 % down to 20 % each cell changes by only about a tenth of a volt. Below about 10–20 % the voltage falls quickly, so the drop comes all at once near the end. A shunt battery monitor shows the real charge all along.
Can I tell the charge of a NiMH battery from its voltage?
Not reliably. A NiMH cell reads about 1.4 V when freshly charged, then stays near 1.2 V for most of the discharge; Energizer’s handbook says voltage sensing can’t determine the state of charge. A charger with a test mode, which measures the mAh, can.