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.

Charging calculator

DC-DC Charger Sizing Calculator for RV Alternator Charging

Quick answer

A DC-DC charger should draw no more than about half of an alternator's rated output so the alternator can still serve the vehicle and stay within its thermal limit. A 130A alternator suits a 40A to 60A DC-DC charger.

Result

DC-DC charger size 50 A

Current available for charging 52 A

Daily energy added in 4 hours of driving 2720 Wh

The formula

charger amps = alternator rating x available fraction

Lithium accepts current almost without limit, which is exactly why it destroys alternators that are connected to it directly. The DC-DC charger is the current limit.

The figures shown above are computed with these exact inputs: Alternator rating 130 A, Share available for charging 0.4.

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

Renogy

Renogy 40A DC-DC Battery Charger

$186.99

A DC-DC charger is what lets an alternator safely fill a lithium bank, because lithium will otherwise draw more current than the alternator can survive.

Best for: Charging from the tow vehicle or chassis engine.

Check price on Amazon

Intermediate

Litime

LiTime 12V 40A DC-DC Lithium Charger

$155.99

A fixed 14.6V output matched to LiFePO4, which removes the risk of leaving it on a lead-acid setting.

Best for: A lithium-only install.

Check price on Amazon

Expert

Litime

LiTime 12V 40A DC-DC Charger with MPPT

$209.99

Combines alternator charging and a solar MPPT input in one unit, so the two sources share one charge profile.

Best for: A build with both solar and alternator charging.

Check price on Amazon

Expert

Victron Energy

Victron Orion XS 50A DC-DC Charger

Price varies, check the listing

Victron publishes full efficiency curves and the unit is configurable over Bluetooth, which is why it is the reference part in this category.

Best for: A documented build where the alternator load has to be limited precisely.

Check price on Amazon

Frequently asked questions

How does the dc-dc charger size calculation work?: charger amps = alternator rating x available fraction. Lithium accepts current almost without limit, which is exactly why it destroys alternators that are connected to it directly. The DC-DC charger is the current limit.
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.