Guide
DC-DC charger sizing
Updated October 22, 2026. Prices checked on the makers' own sites that day.
Rule of thumb
20 A for a 100 Ah bank or a small alternator. 40 A for 200 to 400 Ah. Only go past 40 A with a large bank and an alternator you know can take it.
What an hour of driving puts back
| Charger | Wh per hour of driving | Same as this much flat solar all day | Draw on the alternator (about) |
|---|---|---|---|
| 20 A | 243 Wh | 130 W | 24 A |
| 30 A | 365 Wh | 200 W | 36 A |
| 40 A | 486 Wh | 270 W | 48 A |
| 50 A | 608 Wh | 340 W | 60 A |
| 60 A | 730 Wh | 400 W | 72 A |
In winter, one hour behind the wheel with a 40 A charger matches a roof full of panels. That's why the planner counts driving before it sizes solar.
Three things that set the size
- Your battery. Battery makers list a recommended charge current. Redodo, for example, recommends 0.2C (60 A for its 300 Ah battery). For a 200 Ah bank, 40 A is 0.2C: gentle and plenty.
- Your alternator. The charger pulls roughly 20% more than it delivers, so a 40 A charger asks about 48 A of the alternator. Stock van alternators are rated well above that, but they make less at idle and when hot. If you plan to charge while idling, stay at 40 A or below unless you know your alternator's idle output.
- Smart alternators. Many newer vehicles lower alternator voltage to save fuel. A DC-DC charger boosts it back up; some chargers need an ignition or "D+" signal wire to know the engine is running. Check the charger's manual for your vehicle type.
Cable and fuse for each size
The starter-battery side runs through the engine bay, so it uses ABYC's lower engine-room rating. Fuse both ends: at the starter battery and at the house bus bar.
| Charger | Input current | Fuse | 10 ft run | 15 ft run | 20 ft run |
|---|---|---|---|---|---|
| 20 A | 24 A | 30 A | 8 AWG | 6 AWG | 4 AWG |
| 40 A | 48 A | 60 A | 4 AWG | 2 AWG | 2 AWG |
| 60 A | 72 A | 100 A | 2 AWG | 2 AWG | 1/0 AWG |
DC-DC chargers tolerate more voltage drop on the input than a battery does; some manuals allow thinner input cable. Never go thinner than the fuse requires. How we size cable.
Chargers we'd look at
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Alternator and solar in one
Redodo 12V 40A DC-DC Charger with MPPT
Charges the house battery from the alternator while you drive, and has a solar input too.
$175.99$189.99
See at Redodo (opens in a new tab)40 A, alternator only
Renogy 12V 40A DC-DC Charger
Renogy's 40 A alternator charger, without solar input.
Small banks, small alternators
Renogy 12V 20A DC-DC Charger
A gentle 20 A alternator charger for small banks and small or smart alternators.
For a Bluetti power station
Bluetti Charger 1 (alternator charger)
Lets a Bluetti power station charge from the van's alternator while you drive, far faster than a 12 V socket.
$249.00$399.00
See at Bluetti (opens in a new tab)Safety first
Fuse every positive wire as close to the battery or bus bar as you can, and use the wire and fuse sizes from a standard table, not a guess. Boats should follow ABYC E-11; anything that touches 120/240 V AC should follow the NEC. Have a qualified installer check any 120/240 V AC or shore power work.
Run the power planner
Battery, solar, charger, inverter, every wire and fuse, and a priced parts list.
Draw your wiring
Drag parts onto a diagram and get gauge and fuse labels on every run.
Questions people ask
Do I need a DC-DC charger for a lithium battery?
In almost every modern vehicle, yes. Lithium will take as much current as the alternator can push, which can overheat it at idle, and smart alternators drop their voltage too low to fully charge lithium. A DC-DC charger limits the current and charges at the right voltage.
Can I use a battery isolator instead?
An isolator or voltage-sensitive relay just connects the two batteries. That was fine for lead-acid. With lithium it lets the house bank pull too hard on the alternator, and with a smart alternator it won't fully charge. A DC-DC charger costs more and avoids both.
Is a bigger DC-DC charger always better?
No. Keep the charge current at or below what the battery maker recommends (Redodo, for example, recommends about 0.2C, which is 40 A for a 200 Ah bank) and well inside what your alternator can supply at idle. Two 40 A chargers are only worth it with a big bank and an upgraded alternator.
How long do I need to drive to charge my van battery?
With a 40 A charger, each hour of driving puts back about 486 Wh. A van using 1,000 Wh a day gets about half of it back from a one-hour drive; solar or a longer drive does the rest.
Keep going
Wire gauge and fuse chart
12 V and 24 V wire sizes and fuses by amps and run length, from the ABYC tables.
How many solar panels
What 200, 400 and 600 W of flat roof panels make in your state, month by month.
Lithium conversion
Swapping lead-acid for lithium: what it costs and the three things that must change with it.
Power station vs built-in
When a Bluetti box beats a wired system, and when it costs you more.