Are these figures published standards or rules of thumb?

Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.

Why does this calculator sometimes say there is no answer?

Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.

Do I still need an RV technician if I use this?

For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Batteries calculator

Battery State of Charge by Voltage Calculator

Quick answer

A rested 12V lead-acid battery reads about 12.73V at full and 12.10V at 50 percent. LiFePO4 sits near 13.1V across most of its range, which is why voltage cannot tell you lithium state of charge and a shunt monitor can.

Result

State of charge 72 %

Reading Within the table

Valid only on a battery rested several hours off charge and off load.

Know your numbers before you leave the driveway

The RV Power System Planner

That state of charge is exactly what the Monitoring Log tracks every time you take a resting voltage reading.

11 fill-in worksheets, 33 print pages
2 filled-in examples to copy the format from
Companion Excel / Google Sheets tracker with the battery, solar and wire gauge formulas built in
Online buyer guide with budget ranges and Amazon links, by ladder and budget tier

Replaces the guessed wire gauge, the converter left on the wrong charge profile after a lithium swap, the solar array sized off a rule of thumb, and the pre-trip check done from memory.

Get the kit $29

30-day money-back guarantee. Use it all; if it doesn't help, every dollar back.

Look inside all 11 worksheets

Instant PDF download · Apple Pay / Google Pay · 30-day money-back guarantee

The formula

state of charge is interpolated from a published resting voltage table

Valid only on a battery rested several hours with no charge and no load on it. Under load the reading is depressed and under charge it is inflated.

The figures shown above are computed with these exact inputs: Resting voltage 12.4 V, Chemistry lead.

Components that match this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Expert

Victron Energy

Victron SmartShunt 500A Battery Monitor

Price varies, check the listing

A shunt counts amp-hours in and out, which is the only accurate way to read lithium state of charge. Voltage alone cannot do it on a flat discharge curve.

Best for: Any lithium bank you plan to boondock on.

Check price on Amazon

Intermediate

Renogy

Renogy 500A Battery Monitor with Shunt

Under $100

Includes a wired display head, so you get a reading without needing a phone in your hand.

Best for: A panel-mounted readout.

Check price on Amazon

Intermediate

Litime

LiTime 500A Bluetooth Battery Monitor

Under $150

Covers 8V to 120V, so the same shunt works if you later move the bank to 24V or 48V.

Best for: A build that may change system voltage later.

Check price on Amazon

Beginner

HENGSHAN

AILI TR16 Shunt Battery Monitor

Under $50

The cheapest true shunt-based monitor here, and a cheap shunt still beats an expensive voltmeter for reading lithium.

Best for: A first real state of charge readout.

Check price on Amazon

Frequently asked questions

How does the state of charge by voltage calculation work?: state of charge is interpolated from a published resting voltage table. Valid only on a battery rested several hours with no charge and no load on it. Under load the reading is depressed and under charge it is inflated.
Are these figures published standards or rules of thumb?: Both, and the page labels which is which. Conductor ampacity, resistance, voltage drop limits, overcurrent sizing factors and device ratings come from NEC, ABYC and NEMA and are labeled as published standards. Depth of discharge, solar derate factors and inverter efficiency are planning conventions with no standards body behind them, and are labeled as conventions.
Why does this calculator sometimes say there is no answer?: Because there genuinely is not one within the published tables. Rather than extrapolating past the end of an ampacity table or inventing a figure to fill a gap, the calculator returns nothing and explains why. A run that is too long for the current at 12 volts needs a shorter run or a higher system voltage, not a made up wire size.
Do I still need an RV technician if I use this?: For DC work many owners do their own installation, and these figures are the ones that work is sized from. Permanent AC wiring, anything connected to the shore power inlet and anything behind the distribution panel is work for a qualified RV technician or a licensed electrician. Sizing a circuit correctly and installing it safely are two different jobs.

Researched guidance, not professional advice. These results are planning figures from published standards and manufacturer specifications. Never exceed a conductor's rated ampacity, fuse every positive conductor close to the battery, never run a generator in an enclosed space because carbon monoxide is lethal and odorless, and have permanent AC wiring done by a qualified RV technician or licensed electrician.