Electrical

RV Battery State of Charge Calculator: What Your Voltage Reading Is Worth

By Dale Whitaker Updated October 4, 2026 Specs verified August 23, 2026

Enter your voltage and what is running, and the calculator tells you how much of that reading is charge level and how much is load sag.

On rested lead-acid it will give you a percentage, because that is a case where voltage genuinely works. On lithium it deliberately will not — and the reason is the most useful thing on this page.

Step 1 of 3

What are you measuring?

Results update as you type

Why voltage barely works on lithium

A lead-acid battery’s terminal voltage falls steadily as it discharges, which is what makes a chart possible: 12.7V down to 12.0V across the usable range, a spread of about seven-tenths of a volt.

A LiFePO4 pack does almost the opposite. It holds close to the same voltage from full until it is nearly empty, then falls off a cliff. Across the range you actually use, the whole signal is a few tenths of a volt — and against that you are fighting meter tolerance, temperature, and cable drop between the battery and wherever you clipped your probes.

That flatness is a feature of the chemistry. It is why lithium holds full brightness in your lights right up until it stops. It is also why the number on your multimeter is nearly useless as a fuel gauge.

The bigger problem: you are almost never measuring at rest

The charts assume open-circuit voltage — the battery sitting with nothing connected. In an RV that is a rare condition. The fridge cycles, the propane detector never sleeps, and the moment you switch anything on the terminal voltage drops by current × the battery’s internal resistance.

How far it drops depends on which battery you bought, and almost nobody publishes the figure. Two that do:

BatteryPublished internal resistanceSag at 50A
Victron LFP Smart 12.8/1000.8 mΩ0.04V
Battle Born 100Ah (Smart variant)7 mΩ0.35V

Nearly nine times the difference, from the same chemistry — the Victron uses large prismatic cells and the Battle Born cylindrical ones, and cell format drives internal resistance.

Now put that next to the signal. A LiFePO4 pack spans roughly 0.4V across its usable range. On the Battle Born, a 50A load moves the terminal voltage by 0.35V. The load is very nearly as large an influence on the reading as the state of charge is — so a chart lookup taken with the microwave running is not measuring what you think it is measuring.

The chart nobody can source

Search for a lithium battery voltage chart and you will find plenty. Check who published them: solar retailers, battery-adjacent content sites, an inverter brand’s blog. Then check the manufacturers of the batteries actually in RVs.

We hold records for five lithium batteries and read the datasheet behind each. Between them they publish charge voltages, float voltages, absorption ranges, end-of-discharge cutoffs, maximum charge currents, cycle life at three depths of discharge — a great deal of documentation. None publishes a table mapping resting voltage to state of charge.

That is not an oversight. It is the manufacturers declining to imply a precision the chemistry does not support, and it is worth more than any chart they could have printed.

Where the chart does work: rested lead-acid

None of the above means voltage is useless. On a lead-acid battery that has genuinely rested, terminal voltage maps to state of charge well enough to plan around, and the calculator will give you a figure:

Resting voltageApproximate state of charge
12.7V or above100%
12.5V75%
12.4V50%
12.2V25%
12.0V or belowEffectively empty

Two honest caveats attached to that table. It is a long-standing convention rather than a manufacturer specification — Renogy publishes no resting table for its 100Ah AGM either, so this is received practice, not a datasheet. And “rested” means hours: lead-acid carries a surface charge after charging and a depressed reading after discharge, and both take time to settle. Overnight is the reliable answer.

Read it as accurate to about ±10% and it will serve you well. Read it to the volt and it will mislead you.

The voltages that are published

Published charge and float voltages, by battery

Charging reference for the listed 12V / 12.8V batteries, per battery. These are charger settings, not resting voltages or state-of-charge percentages.

Max = published upper limit, not a target. Recommended = the maker's preferred charge current. Not verified = no confirmed setting here; consult the exact model manual.

Battery Chemistry Charge / absorption Float Charge current
Battle Born BB10012 [1] LiFePO4 14.2V to 14.6V 13.4V to 13.8V Max: 50A Recommended: 0.5C
Battle Born BB10012H [1] LiFePO4 14.2V to 14.6V 13.4V to 13.8V Max: 50A Recommended: 0.5C
Battle Born BBGC3 [2] LiFePO4 14.2V to 14.6V 13.4V to 13.8V Max: 135A Recommended: 0.5C
Dakota Lithium 100Ah Heated [3] LiFePO4 14.4V recommended, 15V maximum Not verified Max: 50A
LiTime 100Ah Group 24 BT [4] LiFePO4 14.4V ± 0.2V Not verified Max: 100A Recommended: 20A
Renogy 100Ah AGM [6] RNG-BATT-AGM12-100-US AGM 14.4V to 14.8V (cycle use, 25°C) 13.5V to 13.8V (25°C) Max: 30A
Victron LFP Smart 12.8/100 [7] LiFePO4 14V to 14.4V, 14.2V recommended 13.5V Max: 200A Recommended: ≤50A
Renogy Core Mini 100Ah [5] RBT12100LFP-M LiFePO4 Not verified Not verified Max: 100A

Match the exact battery model and chemistry. Follow its manual for absorption time, float use, temperature compensation and equalization; this table is not a complete charging profile.

Confirm charge-temperature limits and heater/BMS operation before charging in cold weather. Victron LFP Smart requires an external BMS. Renogy AGM voltages are specified at 25°C.

These values do not configure a 24V/48V bank or combine current limits for multiple batteries. Check the bank arrangement and the combined output of all chargers. 0.5C means half the battery's Ah rating in amps.

Battery / charger: __________________________ Profile checked: ______________ Date: ______________

7 of 8 batteries in our records have a verified charge voltage. Charging references checked 4 October 2026; other product specifications retain their original verification dates.

[1] Battle Born BB10012 / BB10012H manual, charging specifications p. 7

[2] Battle Born BBGC3 product cutsheet (PDF)

[3] Dakota Lithium 12V 100Ah specifications, heating behaviour and warranty

[4] LiTime 12V 100Ah Group 24 Bluetooth specifications and cold-weather guidance

[5] Renogy Core Mini series product page

[6] Renogy AGM100 charging specifications (datasheet)

[7] Victron 12.8 & 25.6 Volt LiFePO4 Smart battery datasheet (PDF)

rvtrove.com/electrical/rv-battery-state-of-charge-calculator/

This is the useful sheet to have on paper, and it is the mirror image of the one everybody prints. Charge and float targets are documented by most makers, because a charger has to be set to something and getting it wrong damages the battery. State of charge is not documented by any of them, because it cannot be inferred reliably from voltage.

Choose Download PDF for a one-page landscape sheet in US Letter or A4, or Print in browser. The chart includes all eight battery models, source links, unverified settings and space to record the charger profile you checked. It stays a charging reference when you change the calculator above.

Recommended charge current and maximum charge current are labelled separately. For example, Victron lists 200A maximum and no more than 50A recommended for the LFP Smart 12.8/100; those are different limits, not interchangeable charger targets. Renogy’s AGM charging values are specified at 25°C. Follow the exact model manual for the rest of the charging profile and temperature limits.

Four ways the reading misleads you

  1. Reading under load. The most common one, and on a cylindrical-cell lithium battery a heavy load moves the number nearly as much as the whole discharge does.
  2. Reading straight after switching off. Surface charge on lead-acid takes hours to disperse. A minute is not resting.
  3. Reading at the panel rather than the battery. Voltage drop in the cable is on top of everything above — the further from the terminals you measure, the lower it reads under load.
  4. Trusting a lithium chart to a tenth of a volt. A tenth of a volt is a large fraction of the entire usable span. Your meter’s own tolerance may be the same size.

What actually answers the question

The four above are compared properly in best RV battery monitors, including the drift problem that decides between them.

A shunt-based monitor sidesteps every problem on this page, because it is not inferring anything. It counts amps in and out at the battery negative, so the figure is arithmetic and it reads the same whether the fridge is running or not. On lithium that is not an upgrade over voltage — it is the only method that works.

For how products on this site are selected and what we do not claim about them, see the editorial policy.

Questions worth answering

What voltage is a 12V battery at 50% charge?

On lead-acid, about 12.4V once it has genuinely rested. On LiFePO4 there is no reliable answer, and that is not a gap in this page — a lithium pack sits between roughly 13.4V and 13.0V across almost its entire usable range, so a tenth of a volt covers a large slice of the battery. Not one of the lithium batteries in our product records publishes a resting-voltage to state-of-charge table.

Why does my battery voltage drop when I turn things on?

Because the battery has internal resistance, and current through resistance produces a voltage drop inside the battery itself. The meter reads what is left at the terminals. Victron publishes 0.8 milliohms for its LFP Smart and Battle Born 7 milliohms for its Smart 100Ah — so the same 50A load costs 0.04V on one and 0.35V on the other. The battery has not lost charge; you are measuring it under load.

Can I use a voltage chart for a lithium battery?

Only as a rough sanity check, and only when rested. The problem is resolution: the whole usable span is a few tenths of a volt, so ordinary measurement error and temperature move the answer as much as real discharge does. Add any load and the sag can exceed the entire span. It is not that the charts are wrong, it is that the signal is too small to read.

How long does a battery need to rest before the voltage is accurate?

Longer than most people wait. Lead-acid holds a surface charge that takes hours to disperse — overnight is the practical answer, and a couple of hours is the minimum worth bothering with. Lithium settles faster but still not instantly. "I switched everything off a minute ago" is not a rested reading on either chemistry.

Is a hydrometer better than a voltmeter?

On flooded lead-acid, yes. Specific gravity measures the electrolyte directly rather than inferring from terminal voltage, and it does not care about surface charge. It is also the only one of the two that can show you a single bad cell. It does not work on AGM or lithium, which are sealed.

What should my charger be set to?

That is the one voltage question with published answers, and the chart on this page has them per battery. Battle Born states 14.2V to 14.6V absorption and 13.4V to 13.8V float; LiTime 14.4V ± 0.2V; Victron 14.0V to 14.4V with 14.2V recommended and 13.5V float. Charge voltage and state of charge are different things — a charger target is not a fuel gauge.

Does temperature change the voltage reading?

Yes, in the direction that makes cold batteries look emptier than they are. It matters most on lead-acid, where the resting table shifts noticeably, and it is one more reason the conventional chart should be read as approximately right rather than precisely.

If voltage cannot tell me, what can?

Counting amps in and out — coulomb counting — which is what a shunt-based monitor does. It reads the same whether a load is running or not, because it is measuring current at the battery negative rather than inferring from terminal voltage. That is the whole reason the instrument exists, and on lithium it is not a refinement but the only method that works.