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Alternator charging, explained

Your engine already makes electricity while it runs. Here is how that power can charge a van's house battery or a portable power station while you drive, and what to check before you add it.

If you are new to the parts of a van system, start with How a van power system works. This page zooms in on one charging source: the alternator.

What the alternator does

The alternator is a generator driven by the engine. Its job is to run the vehicle's electrical system and recharge the starter battery. When there is power to spare, a charger can send some of it to a second battery. Renogy describes a DC-DC charger (also called a battery-to-battery or B2B charger) as a device that lets you charge an auxiliary battery from the alternator while driving.

Simplified. A DC-DC charger feeds a built-in house battery; an alternator charger made for a power station feeds the power station's charging port. Fuses and wiring are left out for clarity.

Why not just connect the two batteries?

Older setups used an isolator or a relay that simply joined the starter and house batteries while the engine ran. Renogy notes that these do not work as well with modern smart alternators, and that in most cases a DC-DC charger is the more suitable choice. A DC-DC charger:

Controls the current, so it takes a set amount instead of whatever the batteries happen to pull.
Uses a proper charging profile (for example bulk, absorption and float) that matches the house battery's chemistry. Victron describes its Orion-Tr Smart as an adaptive 3-stage charger for dual-battery systems.
Raises the voltage when needed, which matters because a lithium house battery usually wants a higher charge voltage than a smart alternator may supply.

Smart alternators, in plain words

Many newer vehicles have a "smart" (variable-voltage) alternator that changes its output to save fuel. Victron says a conventional alternator produces a fixed voltage (for example about 14V), while a smart alternator's voltage can vary between about 12.5V and 15V on a 12V system, with large swings in vehicles that use regenerative braking.

That matters because many chargers decide "the engine is running" by watching voltage. Two common answers:

Smart-alternator settings. Renogy's dual-input chargers start charging from a smart alternator when the starter battery stays at or above 12.0V for 15 seconds and stop at or below 11.5V. For a traditional alternator the thresholds are 13.2V and 12.7V. Victron's Orion-Tr Smart has a selectable "Smart Alternator" setting (the default) and adjustable start and shutdown voltages.
An ignition (D+) signal. Renogy explains that a D+ or ignition wire tells the charger the vehicle is running, so it does not drain the starter battery when parked. Renogy's manual calls for connecting its ignition signal wire with a smart alternator. Victron notes you can use an external engine-running signal when voltage alone is not reliable.

Renogy's manual suggests a simple check: start the engine with fans, radio and lights off, measure the starter battery voltage, wait 5 to 10 minutes and measure again. Around 14.4V suggests a traditional alternator; around 12.5 to 13.5V suggests a smart one. Your vehicle's manual or dealer is the best source.

Charging a portable power station while driving

A 12V car socket can charge a power station, but slowly. EcoFlow says its 800W Alternator Charger charges up to 8 times faster than a vehicle's auxiliary outlet by using surplus alternator power through a built-in DC-DC converter. EcoFlow also says the charger does not force a fixed 800W: the actual power depends on how much surplus the alternator has.

Two points from EcoFlow's own page apply to any product like this:

Alternator capacity varies by vehicle. Check your vehicle's (or alternator's) manual to confirm it can support the charger's maximum draw.
It connects to the vehicle battery with a fused cable, so it is a wired install, not a plug-in accessory.

Electric vehicles do not have alternators, as EcoFlow notes, so this approach is for gas, diesel and hybrid vehicles.

How to size a DC-DC charger

There is no single right size, but the same few questions apply to every system:

How much can your alternator spare? The vehicle needs its own share first. Your vehicle documentation is the starting point; the charger maker's manual will list its maximum input current.
How much can your house battery accept? Check the battery datasheet for its recommended and maximum charge current. Battle Born, for example, lists a recommended charge rate of 0.5C (50A for its 100Ah battery).
How long do you usually drive? Energy added is roughly charge current × battery voltage × hours driven, minus losses.

Here is that last step as arithmetic:

Charger output	Battery voltage	Hours driving	Energy added before losses
20A	12V nominal	2	About 480 Wh
30A	12V nominal	2	About 720 Wh
50A	12V nominal	2	About 1,200 Wh

Example only. These are round numbers to show the math (amps × volts × hours), not ratings for any product. Real results are lower because of conversion losses, charging stages and a battery that is nearly full.

Remember that a larger charger draws even more current from the alternator side than it delivers to the house battery, because the input voltage can be lower than the output voltage and the conversion is not 100% efficient. Victron's manual notes that input current also causes a voltage drop along the input cable, which its charger accounts for by reducing current.

Safety basics

Fuse both ends. Put a fuse or breaker near the starter battery and near the house battery, sized from the charger manual. Renogy's and Victron's manuals both include recommended fuse and cable sizes.
Use the cable size the manual calls for. Long, thin cable wastes power and gets hot. Victron's free Wiring Unlimited guide explains why.
Set the right battery profile. A lithium battery on a lead-acid profile, or the reverse, can undercharge or overcharge it. See LiFePO4 charging basics.
Protect the starter battery. Use the engine-running detection or ignition wire as the maker describes so the charger cannot drain it while parked.
Get help if unsure. If you are not confident with DC wiring, have a qualified installer do or check it.

Where alternator charging fits

Alternator charging is strongest when you drive often and weakest when you park for days. Solar fills the gap when parked (see How many solar watts do you need?), and shore power covers campground stays. Some chargers combine alternator and solar inputs in one box; Renogy's dual-input models are an example.

Deciding between a power station and a built-in battery first? Read Power station vs. built-in system.

General information, not professional electrical advice. Always follow your vehicle and charger manuals.