How to Check a 12V Battery's Capacity

Published October 6, 2026. Built by Jeremy Panasuk.

Direct answer: Trojan’s maintenance page says that for accurate voltage readings, batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs. On that page’s Table 1, a 12 V flooded battery at 12.73 V is 100% state of charge (specific gravity 1.277) and 12.10 V is 50%. The check below is that table. It is not a measured amp-hour capacity, and it is not a lithium resting-voltage chart.

The battery bank calculator answers a different question: how many nameplate amp-hours to buy. For the cabin’s 1,000 Wh/day and 2 days at 12 V and 0.50, that result is (1,000 × 2 × 1.25) ÷ 12 ÷ 0.50 = 416.6667 Ah. A rested reading tells you where a flooded battery sits on Trojan’s table. It does not replace that nameplate.

Rest, then four steps

For accurate voltage readings, Trojan says batteries must remain idle (no charging, no discharging) for at least 6 hrs, preferably 24 hrs. Then:

  1. Disconnect all loads from the batteries.
  2. Measure the voltage using a DC voltmeter.
  3. Check the state of charge with Table 1 below.
  4. Charge the battery if it registers 0% to 70% charged.

If the battery registers below the Table 1 values, Trojan says the battery may have been left in a state of discharge too long, or it has a bad cell. Batteries in these conditions should be taken to a specialist for further evaluation or retired from service. This page does not add a fifth step, and it does not publish a load-test current. The maintenance page’s check used here is the open-circuit test.

Table 1, 12 V open-circuit column

These are the 12 V open-circuit voltages and the specific-gravity column on Trojan’s current maintenance page. The factory specification printed above the hydrometer steps is 1.277 +/- 0.007. This table does not apply that tolerance to the other rows.

State of chargeSpecific gravity12 V open-circuit
100%1.27712.73 V
90%1.25812.62 V
80%1.23812.50 V
70%1.21712.37 V
60%1.19512.27 V
50%1.17212.10 V
40%1.14811.89 V
30%1.12411.81 V
20%1.09811.66 V
10%1.07311.51 V

The form repeats those ten rows. It does not invent a voltage between them.

50% on this table, and 0.50 in the calculator

Trojan says discharges of 50% or less are recommended, and that 80% discharge is the maximum safe discharge. It also says not to fully discharge flooded batteries (80% or more). A separate line says many experts recommend operating only between 50% and 85% of full charge, with a periodic equalization charge. Those lines stay separate. A rested 12.10 V is the table’s 50% row, which is the calculator’s planned floor for lead-acid, not an empty battery. A rested 12.73 V is 100%. This site’s calculator still divides lead-acid and AGM by 0.50. Trojan’s “50% recommended” and Trojan’s “80% maximum safe” are not edited into one divisor, and the 50% to 85% line is not that divisor either.

The 416.6667 Ah cabin result at 0.50 is nameplate amp-hours to install, from the six-step formula. It is not “50% of 12.10 V.” If that bank later rests at 12.10 V, Table 1 is calling the flooded battery 50% charged. The nameplate you bought is still the nameplate.

Do not move this table onto AGM or lithium

Lifeline’s storage instruction says to boost every 90 days or when 12 V open-circuit voltage drops to 12.5 V, which that manual calls about 75% state of charge. Trojan’s flooded table lists 12.50 V as 80%. Same printed voltage, different document, different percent. Do not merge them. The AGM guide is where the GPL-31T count lives. Do not read Table 1 onto a Lifeline AGM and call it the same battery.

Battle Born’s manual includes voltage-versus-capacity charts for its LiFePO4 battery. This page does not read numbers off those charts. Lithium state of charge here is not a resting-voltage table. The manual’s low-voltage note says many chargers must see over 10 V, and that you should jump the battery within 24 hours of entering low-voltage disconnect or you risk damage and voiding the warranty. That “over 10 V” line is a charger-wake threshold. It is not 0% on Table 1, and it is not a percent of a 100 Ah lithium battery. Shelf loss, which is also not this table, is how long a battery holds a charge.

Self-discharge and freeze, from the same Trojan page

Trojan says self-discharge is 5% to 15% per month depending on temperature. It says VRLA/AGM is less than 3% per month at 77°F, and flooded batteries can be up to 15% per month at 77°F. Those rates are not the open-circuit voltages above. Fully charged electrolyte, Trojan says, freezes at −92°F (−69°C). At 40% state of charge it freezes at approximately 16°F (−9°C). A battery you stored at the 40% row is the freeze case in that sentence, which is another reason step 4 says to charge a battery that registers 0% to 70%.

A common mistake

Measuring while the charger is still connected, or an hour after a drive, and treating that surface voltage as Table 1. Trojan’s idle time is at least 6 hrs, preferably 24 hrs. The paired mistake is using 12.50 V as a universal “80%” for every chemistry because it is 80% on this flooded table. Lifeline’s 12.5 V boost trigger is about 75% in Lifeline’s manual. A lithium chart is a third document, and this page leaves it unread.

Limits, and when to call a pro

A voltage lookup does not measure remaining amp-hours. A battery can sit at 12.73 V and still have lost capacity that an open-circuit reading will not show. If the voltage is below 11.51 V, or a cell is the bad-cell case Trojan names, the maintenance page’s instruction is a specialist or retirement, not a tighter formula. Terminal work and the difference between a lithium torque spec and an AGM torque spec are on maintenance, connections, and terminals.

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DC voltmeters on Amazon

Nameplate amp-hours are the battery bank calculator.

Sources

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.