Battery Voltage Chart: How to Tell If a Deep-Cycle Battery Is Bad

How to take an honest reading

A battery's resting voltage, called open-circuit voltage, is the cheapest way to check how full a lead-acid battery is. All you need is a digital multimeter set to DC volts. The catch is that the battery has to be resting.

1. Disconnect the charger and every load. Even a small draw, like a fish finder left on standby or a bilge pump float switch, pulls the voltage down.
2. Wait. Rolls Battery says to let it stand for at least 4 hours so the surface charge from charging goes away. Lifeline also says at least 4 hours. Battery University notes that some makers recommend 24 hours for lead-acid. Trojan says at least an hour for its flooded batteries.
3. Measure right at the battery posts, not at the end of a long wire.
4. Compare to the chart for your battery type. AGM, flooded and lithium read differently.

If you can't wait, a reading taken under a steady, light load is better than nothing, but it reads lower than resting voltage. The more current you're pulling, the lower it reads. Rolls points out that a battery at 50% reads about 11.6 V under load, well under its resting voltage.

AGM voltage chart

This is Lifeline's own table for its 12 V AGM batteries, measured at rest at 77 °F. Lifeline says new batteries read a little higher, about 13.0 V or more when full.

From the Lifeline AGM Technical Manual, Appendix C, at rest for at least 4 hours at 77 °F. Other AGM brands are close but not identical. Check your battery maker's chart if they publish one.

The 50% line matters because lead-acid batteries last far longer if you don't go past it. That's why our Trolling Motor Calculator and other run-time tools count only half of an AGM's rated amp-hours as usable.

Flooded battery chart and the hydrometer

Flooded batteries, the kind with removable caps, read a bit lower than AGMs at the same charge. Battery University's table, from the Battery Council International, shows a 12 V flooded battery after a 24-hour rest at about 79 °F:

Battery University (BU-903), citing the Battery Council International, for a starter battery. Deep-cycle batteries can run a little higher. Trojan says a reading of 1.277 or more means full charge for most of its deep-cycle batteries.

With a flooded battery you can do better than a voltmeter. A hydrometer measures the specific gravity of the acid in each cell, which is how heavy it is compared to water. Trojan calls it the most accurate way to check charge. It also shows you a bad cell, which voltage can't. Renogy says a fully charged cell reads 1.265 to 1.299, and the cells should be within 50 points of each other after charging. Trojan uses a tighter 30 points as the sign the battery needs an equalizing charge.

Lithium (LiFePO4) voltage chart

A 12 V lithium iron phosphate battery has a very flat voltage curve. LiTime says its batteries change very little between 20% and 90%. Battery University says voltage on these batteries "only indicates full charge and low charge," and the middle can't be read accurately. Here's roughly what you'll see at rest:

Ranges from LiTime's and Renogy's published LiFePO4 voltage charts, which differ by about 0.1 V. Readings shift with temperature, brand and how recently it was charged or used.

So a lithium battery reading 13.2 V could be at 40% or 70%. Don't guess. A shunt battery monitor counts every amp in and out and shows the real percentage. LiTime calls a dedicated monitor the most accurate way to track a battery. Many lithium batteries have a Bluetooth app that does the same. See our battery monitors page for options.

One upside: when a lithium battery's voltage finally does drop, it drops fast. If you see 12.8 V or lower at rest, it's nearly empty.

How to tell if a deep-cycle battery is bad

Batteries wear out slowly, so the first sign is usually shorter run times on the water or at camp. Here are the signs Renogy and Trojan list, and what they usually mean:

Signs from Renogy's guide to bad deep-cycle batteries and Trojan's FAQ.

Before you blame the battery, check the simple stuff: corroded or loose terminals, a charger set to the wrong battery type, a small load draining it between trips, or two batteries of different ages in the same bank.

Three tests, from easy to sure

1. The overnight voltage test

Fully charge the battery, disconnect everything, and check the resting voltage the next day. If an AGM has fallen under about 12.5 V, or a flooded battery under about 12.4 V, it's not holding charge well. For a bank, Trojan suggests comparing each battery after charging and an hour's rest: if the 12 V batteries differ by more than 0.22 V, something's wrong with one of them.

2. The load test

This is what an auto parts store does with a carbon-pile or electronic tester. Renogy's version: fully charge the battery, disconnect the cables, and pull half the battery's cold cranking amp (CCA) rating for 15 seconds. A healthy 12 V battery stays at 9.6 V or higher. For sealed AGM and gel batteries, a conductance tester gives a quick read without the heavy load.

3. The capacity test

This is the sure one, and it's how you test a lithium battery too. Lifeline's method: fully charge the battery, then run it at a steady 25 A until it reaches 10.5 V and time it. Compare the minutes to the battery's reserve capacity rating. Lifeline suggests a conditioning charge if it's under 80% of the rating, and replacing it if it's under 50%.

You can do a home version with a known load, like a trolling motor on a steady speed or an inverter running a light, and a shunt monitor counting the amp-hours until the battery hits its cutoff. Compare the result to what our Trolling Motor Calculator or Inverter Run Time Calculator says a healthy battery should give you.

Keeping a good battery good

• Recharge after every trip. Lead-acid that sits partly charged sulfates. Trojan says storing discharged leads to sulfation and early failure.
• Top up in storage. Lifeline's AGMs lose 1% to 3% a month at room temperature. Lifeline says to recharge every 90 days, or sooner if it drops to 12.5 V. A maintainer does this for you. See our chargers page.
• Keep it charged in the cold. A charged battery is very hard to freeze, but Trojan says a flooded battery at 40% can freeze near 16 °F, which can crack the case.
• Keep it cool. Lifeline says battery life roughly halves for every 18 °F (10 °C) hotter it lives.
• Check water in flooded batteries. Trojan says if the acid drops below the top of the plates, the damage can't be undone.
• Use the right charger setting. Our charging time guide covers charger sizes and settings.

Questions people ask

What voltage is a dead 12 volt battery?

For AGM, about 11.6 V at rest is empty by Lifeline's chart, though the battery isn't damaged yet if you recharge it right away. Under about 10 V at rest, it's deeply discharged and may be ruined. A lithium battery usually shuts itself off around 10 V.

Is 12.4 volts good for a deep-cycle battery?

It's about 70% on both an AGM (by Lifeline's chart) and a flooded battery (by the Battery Council table). It's fine to use, but charge it before it goes into storage.

Why does my battery read 12.6 V but won't run anything?

Either you're reading surface charge right after charging, or the battery has a bad cell or high internal resistance. Let it rest 4 hours and read again, then do a load test. Renogy says a battery that reads fine at rest but collapses under load usually has a defective cell.

Can I tell my lithium battery's charge from its voltage?

Only roughly. Above about 13.4 V at rest it's full, and at 12.8 V or below it's nearly empty. Everything in between looks almost the same, so use a shunt monitor or the battery's app.

Can a sulfated battery be saved?

Sometimes, if you catch it early. Flooded batteries can get an equalizing charge, which Trojan recommends when cells read 30 points apart. Some smart chargers have a recondition mode for lead-acid. Victron's runs a 12 V battery up to 16.2 V for up to an hour. Heavy sulfation from months sitting dead usually can't be reversed.

How do I check a battery in my boat without taking it out?

Turn the battery switch off, or unhook the negative cable, so nothing is drawing on it. Let it sit a few hours, then read the voltage at the posts. A shunt monitor wired in permanently saves you the trouble.

Should I replace both batteries in a bank if one is bad?

Yes, for a series bank like a 24 V or 36 V trolling motor. The weak battery drags the others down, and a new battery mixed with old ones wears out early.