A voltage reading is useful only with its measurement conditions. This tool illustrates the effect of current and resistance on a nominal 12V battery. It gives a rough reference estimate for rested lead-acid, and withholds a percentage for lithium or a battery that is charging, loaded or still settling.
Step 1 of 3
What are you measuring?
The answer
This reading is
—
Estimate charge by tracking current over time
Renogy 500A Battery Monitor with Shunt
Current accuracy: Not stated separately. Renogy publishes one "1% Accuracy Measurement" figure without saying whether it refers to current or voltage
Check price at Renogy(opens in a new tab)
Victron Energy SmartShunt 500A/50mV Battery Monitor
Current accuracy: ±0.4%
Check price at Victron Energy(opens in a new tab)
Bogart Engineering TriMetric TM-2030 Battery Monitor
Current accuracy: Not published
Check price at Bogart Engineering(opens in a new tab)
Victron Energy BMV-712 Smart Battery Monitor with 500A Shunt
Current accuracy: ±0.4%
Check price at Victron Energy(opens in a new tab)
Balmar SG200 Battery Monitor Kit with SmartShunt and Colour Display
Current accuracy: Not published
Check price at Balmar(opens in a new tab)Why the flat lithium curve matters
On a sloping part of a voltage-versus-charge curve, a change in charge produces a visible change in voltage. On a flat part, many charge levels can produce similar readings. That makes small measurement errors much more significant.
The shaded strip represents uncertainty in a voltage reading. Follow where it crosses each curve, then look down to the charge axis. These are conceptual shapes, not calibrated battery curves or lookup charts.
Steeper region · lead-acid example
A suitable rested-voltage reference can give a rough estimate. Battery type and measurement conditions still matter.
Flat region · LiFePO4 example
Many charge levels can fit a similar voltage. Load, charging, temperature and meter error add further uncertainty.
No numeric scales are implied. LiTime explains the flat plateau while also publishing a general voltage chart. A chart’s existence does not establish its accuracy for your particular battery and reading.
Under load, current changes the reading
The simple resistive term is current × resistance. At 50A, an illustrative 7mΩ gives 0.35V; 0.8mΩ gives 0.04V. These are mathematical examples, not predictions for a particular battery or cell shape. A datasheet resistance may use different test conditions from your installation.
Adding that term to a discharge reading, or subtracting it while charging, does not remove polarization, recent-use effects, temperature changes or cable losses. The result is an illustrative adjusted voltage, not a measurement of true open-circuit voltage. A load also consumes charge while it runs; a sudden voltage step is not itself a measure of the amount consumed.
Manufacturer charts exist; applicability is the question
LiTime publishes a general voltage chart and explains that its flat plateau makes accurate charge estimation from voltage alone difficult. Our earlier statement that no battery manufacturer published such a chart was too broad.
A curve needs to match the battery and measurement conditions. This calculator has no validated model-specific lithium calibration, so it does not print a lithium percentage. That is a limit of this tool, not a claim that all voltage-based estimation is impossible.
Rested lead-acid: use the right reference
The calculator uses separate 12V columns from Trojan’s User’s Guide, table 7, printed page 22. Selected points show why one universal table is inappropriate:
| Reference | 100% | 50% | Lowest published point |
|---|---|---|---|
| Trojan AES / AGM | 12.84V | 12.24V | 11.64V at 0% |
| Trojan flooded | 12.73V | 12.10V | 11.51V at 10% |
These values describe Trojan’s listed types, not every AGM or flooded battery. The tool interpolates between points and flags readings outside the reference range. It does not extrapolate a flooded 0% value or promise ±10% accuracy. Low reference points are not recommended discharge targets.
Trojan calls for at least six hours idle, preferably up to 24 hours for its open-circuit test. Follow your exact model’s rest and temperature instructions. A serviceable flooded battery can also be checked using the manufacturer’s specific-gravity procedure; sealed AGM and lithium batteries are not opened for that test.
Charging settings are a separate reference
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.
PDFs use one landscape page. Choose landscape in your browser's print dialog.
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 [5] 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 [6] 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 Rev 038, conflicting temperature limits pp. 7, 16 and 22
[2] Battle Born BBGC3 product cutsheet (PDF)
[4] LiTime 12V 100Ah Group 24 Bluetooth specifications and cold-weather guidance
[5] Renogy AGM100 charging specifications (datasheet)
[6] Renogy Core Mini series product page
[7] Victron 12.8 & 25.6 Volt LiFePO4 Smart battery datasheet (PDF)
rvtrove.com/electrical/rv-battery-state-of-charge-calculator/
The printable chart records charge settings and source gaps under its stated verification date. Those targets are not state-of-charge lookup values. Use the exact battery and charger manuals for the complete profile and temperature limits. Recommended current and maximum current are separate quantities.
Choose Download PDF for US Letter or A4, or Print in browser. The charging reference stays separate from the live estimate above.
Four checks before using a reading
- Confirm the system. This calculator accepts nominal 12V batteries, not individual cells or a 24V/48V bank.
- Record what was running. Charging and discharging change terminal voltage.
- Allow the required rest. Recent charging can leave surface charge; recent discharge also requires settling.
- Check the measurement point and reference. Battery terminals and a remote panel can differ under load. Use the correct battery-type reference.
Track current when the battery is in use
Measures it instead of inferring it
Victron SmartShunt 500A
Estimates charge from current integrated over time, with correct setup and synchronisation.
If it has to be readable without a phone
Victron BMV-712 Smart
Similar published measurement specifications with a local panel readout.
The budget way to stop guessing
Renogy 500A Monitor
Shunt-based for less, with a 20 ft cable — though Renogy publishes less about it than Victron does.
The three picks are compared in best RV battery monitors. A shunt measures current and integrates it over time to estimate charge; it does not directly measure remaining chemical capacity. Correct current paths, capacity and charge settings, and regular synchronisation matter. Victron explains the measurement and synchronisation process.
Related guides
- Do you need a shunt if your battery has Bluetooth? — whether the app you already have makes a monitor redundant. Usually not.
- What size shunt do you need? — the two ratings, once you have decided to measure properly.
- Victron SmartShunt vs BMV-712 — readouts, alarms and control options.
- Best RV lithium batteries — the batteries whose datasheets this page draws on.
- RV battery calculator — sizing the bank in the first place.
- The electrical section collects everything published so far.
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?
It depends on the battery and test conditions. Trojan’s reference gives 12.24V for its AGM types and 12.10V for flooded at 50%. Those are rested references, not universal values. This tool does not assign a lithium percentage.
Why does voltage drop when I turn a load on?
Internal resistance and other electrochemical effects lower terminal voltage under load. Cable drop may lower a remote reading further. A simple current-times-resistance example illustrates one contribution; it does not recover an exact rested voltage. The load also consumes charge over time.
Do lithium battery manufacturers publish voltage charts?
Yes. LiTime publishes a general chart and also explains the flat voltage plateau. The issue is whether a chart supports the precision claimed for your exact battery and conditions, not whether charts exist.
How long should a lead-acid battery rest?
Follow the battery instructions. The Trojan guide used here calls for at least six hours idle, preferably up to 24 hours. Switching off a load a minute ago does not satisfy that procedure.
Can a shunt monitor give an exact percentage?
It estimates charge by integrating current over time. Capacity settings, charge efficiency, correct current paths and synchronisation affect the result. Its display resolution is not a guarantee of state-of-charge accuracy.
Is a charging voltage a state-of-charge measurement?
No. It is affected by the charger’s operating stage and settings. Use the exact battery’s charging instructions; do not convert an absorption or float target into a fuel-gauge percentage.