25A vs 40A DC-DC charger: which should you buy?

Short answer: a 40 A charger only helps if your battery is happy to take around 40 A and your alternator and cabling can supply it. For a typical 100 Ah battery, 25 A is often the better fit. For 150–200 Ah of lithium, or if you drive short distances and use a lot of power, 40 A starts to make sense.

	25 A	40 A
Rule-of-thumb battery size: lithium (~30% of capacity)^[1]	around 80–100 Ah	around 130 Ah and up
Rule-of-thumb battery size: AGM (~20% of capacity)^[1]	around 125 Ah	around 200 Ah
Ah put back in a 2 h drive (no loads, bulk stage)	up to ~50 Ah	up to ~80 Ah
Cable and fusing	lighter	heavier; check the manual's table^[3]
Alternator demand	lower	higher

Battery sizes are calculated from Enerdrive's 20% and 30% rules of thumb. Your battery's recommended charge current is what really decides it.

When 25 A is the right call

A single battery of about 100 Ah. Enerdrive's B-TEC G2 100 Ah, for example, has a recommended charge current of 33 A or less.^[2]
You drive a few hours most days, so there's time to recharge.
Long cable runs to a canopy or tub, where heavier cable is a hassle.

When 40 A is worth it

150–200 Ah or more of lithium.
Short drives between camps, or a big daily draw (fridge plus freezer, inverter use).
You plan to add battery capacity later. Some chargers let you turn the output down; Enerdrive, for example, notes that its chargers are adjustable, so you can buy bigger now and set it to suit a smaller battery.^[1]

Put your own numbers into the DC-DC charger size calculator and check the wiring with the voltage drop calculator.

Sources

This is general information, not a substitute for the manuals for your specific charger, battery and vehicle. Fixed wiring and anything near the starter battery should be installed by a qualified auto electrician.

Last updated: 15 October 2026