Do I need a DC-to-DC charger to charge a lithium battery from my alternator?
In most modern vehicles, yes. A DC-to-DC charger sits between the vehicle's starting battery and your lithium house battery. It isolates the two, boosts the alternator's output to a proper LiFePO4 charge voltage of about 14.6V, and limits current so the alternator isn't overworked. Most DC-to-DC chargers deliver 25 to 50 amps, which suits Expion360 batteries from 100Ah up.
Which DC-to-DC charger size fits my battery?
Expion360 recommends charging at about 0.2C — 20A per 100Ah of battery — with up to 0.5C allowed.
| Battery capacity | Recommended charge (0.2C) | Max charge (1P) | DC-to-DC charger that fits |
|---|---|---|---|
| 80Ah (Group 24) | 16A | 40A | 12A or 25A |
| 100–120Ah (Group 27) | 20–24A | 50–60A | 25A |
| 162Ah (GC2) | 32A | 81A | 25A or 40A |
| 200–240Ah (two Group 27s, EX2 Edge 240Ah) | 40–48A | 100–120A | 40A or 50A |
| 450Ah (EX1) | 90A | 225A | 50A, or two units |
Why not wire a lithium battery straight to the alternator?
Three reasons. LiFePO4 has very low internal resistance, so a discharged lithium battery will pull as much current as the alternator can make, for as long as it takes — heat the alternator wasn't designed for. Many vehicle alternators also regulate below the 14.4–14.6V a LiFePO4 battery needs, especially smart alternators that drop voltage once the starting battery is full. And a direct connection ties the house battery to the starting battery.
A DC-to-DC charger fixes all three: it draws a set current from the alternator, delivers a regulated LiFePO4 charge profile whatever the vehicle's voltage, and keeps the starting battery separate so you can't drain it running the fridge overnight.
What size DC-to-DC charger do I need?
Match the charger to the battery's recommended 0.2C rate, then check the alternator can supply it. A 25A charger suits a single 100–120Ah battery and comfortably charges a 162Ah GC2. For 200Ah and up, step to 40A or 50A, or run two units in parallel. A DC-to-DC charger draws somewhat more input current than it delivers, so leave alternator headroom for the vehicle's own loads.
Two 120Ah Group 27s (240Ah) × 0.2C = 48A. A 50A charger, or a 40A charger with a little more drive time.
Roughly capacity ÷ charge current. 100Ah from empty at 25A is about four hours of driving.
Which DC-to-DC chargers does Expion360 sell?
Expion360 carries its own E360 charger and REDARC in-vehicle units.
| Output | Inputs | Best for | |
|---|---|---|---|
| E360 12V DC-DC Charger 25A | 25A, 14.6V two-stage CC/CV | Alternator, with ignition control | Single Group 27 or GC2 battery; compact, IP68-potted |
| REDARC BCDC 25A Dual Input | 25A | Alternator + solar (MPPT) | One battery with roof solar |
| REDARC BCDC 40A Dual Input | 40A | Alternator + solar (MPPT) | 160–240Ah with solar |
| REDARC BCDC 50A Dual Input | 50A | Alternator + solar (MPPT) | 240Ah and up |
| REDARC BCDC 12A In-Trailer | 12A | Tow-vehicle supply at the trailer | Trailers charged from the tow vehicle |
Set any charger to its LiFePO4 profile before first use. Dual-input units combine alternator and solar charging in one box.
What charger settings should I use for an Expion360 battery?
Use the LiFePO4 profile, or set it manually: bulk and absorption 14.6V, float 13.6V on 12V models. Expion360's charging guidance gives an absorption time of about two hours. The E360 25A DC-DC charger ships with a two-stage constant-current, constant-voltage profile at 14.6V, so it needs no setup for Expion360 batteries.
Where should a DC-to-DC charger be installed?
As close to the house battery as practical, in a dry spot with moderate temperatures — inside the rig, on a frame rail or in the engine bay, depending on the unit's rating. Size the cable to the charger's current; Expion360's guidance runs from 8 AWG to 4 AWG depending on amperage and length. Fuse both the input and output, and use the ignition or voltage-sense wire so the charger only runs with the engine on.
Will a DC-to-DC charger charge my lithium battery in winter?
It will try, but an unheated LiFePO4 battery won't accept charge below 32°F (0°C) — the BMS blocks it. On a VHC™ heated Expion360 battery, the cells warm first and then accept charge from the DC-to-DC charger down to -4°F (-20°C). If you drive to cold trailheads or ski areas, the battery is what decides whether the drive recharges it.