A fresh alkaline AA reads about 1.5 V under load and is empty at 0.8 V. A 12 V flooded deep-cycle battery reads 12.73 V full at rest and 12.10 V half charged.
Battery voltages: full, partly used and empty
Battery voltage chart
| Battery | Nominal | Full or fresh | Partly used | Empty or dead | How it’s read |
|---|---|---|---|---|---|
| AA or AAA alkaline | 1.5 V | About 1.5 V under load, about 1.6 V at rest | 1.1 V under load: about 20% left | 0.8 V under load | Under load, 1 to 2 seconds |
| AA or AAA lithium | 1.5 V | 1.79 to 1.83 V at rest | Over 1.74 V at rest: still good | Under 1.70 V at rest | At rest, after recovering |
| AA or AAA NiMH rechargeable | 1.2 V | About 1.4 V at rest, just charged | Stays near 1.2 V, so voltage can’t tell | 0.9 to 1.0 V | Under load |
| 9V alkaline | 9 V | Not given | Not given | 4.8 V | Under load |
| 9V lithium | 9 V | Not given | Not given | 5.4 V | Under load |
| 9V NiMH rechargeable | 8.4 V | Not given | Not given | Not given | Not given |
Lithium: Energizer Ultimate Lithium (lithium iron disulfide).
| Battery | Nominal | Full or fresh | Partly used | Empty or dead | How it’s read |
|---|---|---|---|---|---|
| CR2032 lithium coin | 3 V | Not given | Not given | 2.0 V | Under a 15k ohm load |
| LR44 alkaline button | 1.5 V | 1.55 V under load | 1.33 V at 40% used, 1.20 V at 80% | 0.9 V | Under a 7.5k ohm load |
| 357 or SR44 silver oxide | 1.55 V | About 1.6 V at rest, 1.55 V under load | 1.40 V at 40% used, 1.34 V at 80% | 1.3 V under a 6.5k ohm load | Under 100 ohms, 5 seconds |
Silver oxide’s 1.6 V at rest is Energizer’s figure for silver oxide cells in general.
| Battery | Nominal | Full or fresh | Partly used | Empty or dead | How it’s read |
|---|---|---|---|---|---|
| 18650 lithium-ion (Molicel P28A) | 3.6 V | 4.2 V, fully charged | Not given | 2.5 V, the discharge cutoff | Maker’s charge and cutoff limits |
| 12 V flooded deep-cycle lead-acid | 12 V | 12.73 V (100%) | 12.10 V (50%) | 11.66 V (20%, Trojan’s limit) | At rest 6 to 24 hours |
| 12 V AGM deep-cycle | 12 V | 12.84 V (100%) | 12.24 V (50%) | 11.94 V (25%); 11.64 V is 0% | At rest 6 to 24 hours |
| 12 V LiFePO4 lithium (12.8 V) | 12.8 V | 13.6 V at rest (Renogy) | 13.3 to 12.9 V from 90% down to 20% (Renogy) | 11.2 V, end of discharge (Victron); 10.0 V is 0% (Renogy) | Full at rest; others not stated |
Lead-acid and AGM: Trojan’s open-circuit table, 12 V column. Its AGM table has no 20% row, so 25% is the last row above Trojan’s limit.
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At rest or under load
- Alkaline: a reading with nothing connected is misleading, at best a rough estimate. Test a 1.5 V cell under about 10 ohms for one to two seconds, as a battery tester does with its load set for each battery type.
- Lithium AA and AAA: read at rest, after the battery has recovered from use. A lithium cell at 1.6 V is spent, though an alkaline cell at 1.6 V reads as good.
- 12 V lead-acid and AGM: Trojan’s figures are open-circuit readings. Leave the battery idle, with no charger or load, for at least 6 hours and up to 24 before you measure.
- Button cells: Energizer’s middle figures for the LR44 and 357 are readings under a set load, at 40% and 80% of the cell’s rated capacity used.
Why the middle is hard to read
An alkaline cell’s voltage slopes down as it’s used, so a mid reading means something: 1.1 V under load is about a fifth left. A charged NiMH rechargeable drops quickly from about 1.4 V to a 1.2 V plateau once it’s in use. That plateau is too flat for voltage to tell how much charge is left.
LiFePO4 is flat too. On Renogy’s chart a 12 V LiFePO4 battery falls only from 13.3 V to 12.9 V between 90% and 20%. Victron’s LiFePO4 batteries have no state of charge reading built in, and Victron’s manual says to add a battery monitor to show it. Their battery management system warns when a cell falls under 3.1 V, 12.4 V across the four cells of a 12.8 V battery, and cuts the load when a cell falls under 2.8 V.
Makers’ empty points also differ. For a NiMH cell, Energizer’s handbook uses 0.9 V and its AAA sheet rates capacity to 1.0 V. For LiFePO4, Victron ends discharge at 11.2 V, while Renogy’s chart puts 0% at 10.0 V.
12 V deep-cycle batteries
The 12 V rows are for deep-cycle batteries in RVs, campers, boats and backup systems. Trojan says not to discharge past 80% depth of discharge, 20% charge left, to avoid damage. A reading between two rows sits between those charge levels: 12.4 V at rest is between 70% (12.37 V) and 80% (12.50 V) on a flooded battery. For sizing one, see what size battery for an RV. This site doesn’t cover car starting batteries.
Safety
- Don’t charge alkaline, lithium AA and AAA or lithium coin cells. A battery not marked rechargeable can rupture or leak if you try to charge it.
- Don’t use or charge a lithium-ion cell or battery that is damaged or out of shape.
- Lead-acid batteries give off gas, most of all while charging. Charge them where air moves freely, and keep sparks, flames and cigarettes away.
- Recycle spent batteries: see storing and throwing away batteries, and how to clean a leaking battery if one has leaked.