What Size DC-DC Charger for a 100Ah LiFePO4? 20A, 30A, 40A or 50A (The Alternator Decides, Not the Battery)

Short answer: for one 100Ah lithium battery, a 25 to 30 amp charger is the right size for most trucks, and 40 amps is the ceiling worth paying for. The battery is not what limits you. LiTime allows 100 amps into its 100Ah and recommends 20. Battle Born allows 50. What limits you is the alternator feeding the charger and the cable between them, and a 40 amp charger pulls nearer 50 to 60 from the alternator, not 40.

The hours are simple arithmetic. From a fifth left, a 25 amp charger refills 100Ah in about three hours of driving, a 40 amp in two, a 50 amp in one and a half. One day of a 12V fridge is about 31 amp-hours, which a 25 amp charger puts back in about an hour and twenty minutes of driving, a 40 amp in under an hour. If you drive between camps most days, the smaller charger keeps up and asks less of the alternator. The bigger one earns its place when the drives are short, the alternator is big, or the bank will grow past 100Ah.

Will a 100Ah LiFePO4 take 50 amps?

Yes, and more. This is the first thing to settle, because it is where most people start and it is not the limit. Three makers’ sheets for a 100Ah battery:

Every charger in this article sits under every one of those ceilings. LiTime’s 20 amp recommendation is the only line that argues for a small charger, and LiTime itself prints the charge time for a bigger one on the same page, “a 40A DC-DC charger (2.5 hours)”, and, for a 50 amp charger of any kind, “50A (0.5C): Up to 100% charge in ~2 hours”. So the battery will take whatever you give it from this range. The question moves to the truck.

Redarc’s advice runs the other way from most buyers’ instinct. Its support article says a bank of 75 to 200Ah has “a recommended charge rate of less than 40 amps, so a BCDC1225D would be the better choice”, and the BCDC1240D’s own page says it “is typically used for lead acid battery banks over 200Ah and lithium batteries over 100Ah”. The 50 amp unit is “used for battery bank sizes over 200AH”. Read plainly: for one 100Ah battery, Redarc sells you the 25.

How long does each size take to charge 100Ah?

Amp-hours divided by amps. The first column is from flat, which you should not do to a lithium battery every day. The second is from a fifth left, which is a normal morning after a fridge night. Neither includes the absorption step at the end, which Battle Born puts at 20 to 30 minutes per 100Ah, or the slowdown as the charger warms up.

All of it is arithmetic from published figures, not a test. The fridge day is the 400Wh our runtime table is built on, which is 31 amp-hours at 12.8 volts, plus the ten percent for losses that our dual-battery guide adds to its own charge-time table. Two published figures land on the same line: Mike Sokol, writing on RV electricity, says “a 25-amp DC-DC charger could fully recharge a 100 amp-hr Lithium trailer battery in about 4 hours of driving time”, and a Go Fast Campers forum member with a 25 amp Redarc on a 100Ah lithium reckons about four hours from drained to full.

What the table says about the choice: if you drive between camps, a 25 amp charger puts a whole fridge day back before the second coffee stop. The 40 amp saves you about half an hour a day. The 50 amp saves you ten minutes more than that. Where the 40 wins outright is a short hop, an hour of driving after two nights parked, where it puts back twice what the 20 does.

What does the alternator actually see?

More than the number on the box. A DC-DC charger steps the voltage up to charge the lithium battery, and the losses come out of the input side, so a 40 amp charger draws more than 40 amps from the alternator. The makers publish output watts and efficiency; divide one by the other, and by the alternator’s voltage, and you have the draw.

Those are best cases at a steady voltage. Owners see more. A Land Cruiser owner on IH8MUD: “my Victron 18A can draw 22-25A on the input.” A Class B Forum member quoting the Renogy manual: “the amperage consumption will be 50% greater on the input side”, so “The 40A might put as much as a 60A load on the system”. That is the number to size the alternator and the cable for, not the 40.

Two rules of thumb circulate for how much of the alternator a charger may take. Spheral Solar’s sizing guide: “your DC to DC charger’s input current should not exceed 50% of your alternator’s rated output current.” The IH8MUD member above, answering whether a 40 amp charger is safe on a 180 amp Land Cruiser alternator: “no more than 50% charger to alternator size, but probably safer to limit to 33%.” Here is what the two rules allow, as input amps, for the alternators common under overland trucks.

The alternator figures are the thinnest part of this page, and I say so. The Jeep ones are Mopar part listings (180, 220 or 240 amps for the 3.6 litre JL and JT). The Land Cruiser’s 180 is an owner’s statement on IH8MUD. The Toyota body-on-frame figure is one forum member’s recollection, “usually around 130-140”, and the aftermarket page I found for the Tacoma contradicts itself. Read the rating off your own alternator or the dealer’s parts list before you size against it.

Idle is the other half. A charger that is fine at highway revs can drag the system down in a car park. The Class B member with the 40 amp Renogy on a 160 amp Chevy alternator: it “charges at a full 40 amps”, but, idling in traffic with the air conditioning running, “I ocassionally noticed a voltage drop to low 12’s or even high 11’s voltage unless I add gas.” On the other side, a 2018 Land Cruiser owner with a 50 amp charger and 200Ah of lithium: “I get an honest 50 amps out of our 2018 LC @ idle.” Another member’s estimate is that “modern alternators can easily put out over 50% of maximum amps at idle”, and one aftermarket maker’s figures for its own 270 amp Tacoma unit show the shape: 200 amps at idle cold, 180 at idle hot. No maker I found publishes an idle curve for a stock alternator.

What cable and fuse does each size need?

This is where the 40 and 50 amp units cost more than their price tag. Redarc’s wiring-kit manual has one cable size for the 25 amp charger and a heavier one for the 40 and 50, by run length:

An etrailer expert gives the same line for the BCDC1240D in feet: under 16 feet, 6 gauge; “between 18 and 20 feet you’d want to use 4 gauge wire.” Victron’s dealer page for the Orion XS 50 amp says “6 B&S is recommended, with 4 B&S recommended for longer runs”, with a 70 amp fuse at the starter battery and 60 at the house battery, and Victron’s own sheet caps the terminal at 4 AWG. The target behind all of it, from the IH8MUD thread: “Keep the voltage drop under 3% at full load.” The gauge table by amps and feet, and why a long thin run quietly turns a 40 amp charger into a 30, is in our dual-battery guide, and the same arithmetic for the fridge’s own cable is in what gauge wire a 12V fridge needs.

Heat is the quieter cost. The older Victron Orion-Tr Smart 30 amp runs at 87 percent, so at full output it sheds about 54 watts into wherever it is mounted, and Victron derates it 3 percent per degree above 40C. The Orion XS at 98.5 percent sheds about 11 and derates 1.5 percent per degree. Redarc’s BCDC1240D is rated to 80C and potted for the engine bay. Renogy’s advice for its 40 amp unit is to mount it flat for venting and near the house battery. Put the charger where air moves, and remember that a lithium battery in the same box has its own limits: a JL owner’s rule of thumb is that “LiFePO4 (Lithium Phosphate) batteries don’t like it hotter than about 112F.”

Which size, by truck and by trip?

I would draw the line at the alternator, not at the battery. Two owners who bought big say the same. A Go Fast Campers member who installed a 40 amp unit for a single lithium battery says in hindsight 20 amps would have done, and “The key is making sure you leave some headroom on the alternator.” On MotorhomeFun, a van owner asking whether a 40 amp charger suits a 110 amp alternator got this: “A lithium will gobble up as many amps as you can throw at it” and “110amp is a pretty small alternator these days.” He kept the 20.

One Ram 1500 owner on Overland Bound, with 150Ah of lithium and a fridge, weighed the 40 against the 50 and landed on the 40: “I don’t think that what I gain from the 1250 is worth it.” Another member’s reading of Redarc’s range matches the table above: the 25 is the usual pick for batteries up to 100Ah, and the 40 “packs more value in terms of price/output” once the bank is bigger.

Why does a smart alternator change the install?

Older alternators hold a steady voltage, around 14 volts, whenever the engine runs. A smart alternator does not. Victron’s engine-detection note says they “generate a variable output voltage which can range from 12.5V to 15V”, and can sit at under 12.5 for a while, especially with regenerative braking. Redarc puts it bluntly: “This lower voltage (often as low as 12.3V) was too low to charge deep cycle batteries at all.” A charger that decides the engine is running by watching the voltage will switch itself off.

The fix is a wire. Redarc’s blue wire “only needs to be connected to ignition when the BCDC* is installed in a "smart" alternator system”, draws about a microamp, and must go to a point that shows battery voltage with the ignition on. Renogy ships a three metre ignition wire with its 40 amp unit and lists a separate input range for “Smart Alternator (Euro 6)”. Victron’s Orion-Tr Smart has a remote on/off connector for the same job, and the Orion XS detects the engine on its own. On a JL, an owner’s advice is unambiguous: “the JL most definitely has a smart alternator” and “You should definitely have a wire to the DCC30S/50S telling it that your engine is on.” Renogy’s phone support told the same thread’s starter the opposite. Wire it.

What to skip

• Sizing the charger to the battery’s maximum. LiTime’s 100 amps is a ceiling, not a target. No DC-DC charger in this class can feed it, and it would take half of even the biggest alternator here.
• A 50 amp charger on an alternator under 150 amps. It draws about 55 from the alternator at full output, which is a third of a 165 amp unit and more than that of anything smaller.
• Buying the 40 and keeping the 25’s cable. The run past the engine bay goes from 6 AWG to 4, and the fuses from 40 to 60. Thin cable gives you a slower charger that runs hot.
• Charging at idle in a car park and expecting the label. One 40 amp unit pulled a 160 amp alternator into the low 12 volts at idle. Drive, or set the charger lower.
• Leaving the ignition wire off on a smart alternator. The charger will stop when the alternator rests, which is often. Connect it.
• Sharing a fuse with the winch. The Go Fast Campers member’s warning: the winch “draws alot”. Its own circuit, its own fuse.
• Trusting a phone rep over the forum. Renogy’s support told one JL owner the Jeep has no smart alternator. It does.

Build the rest of the rig: the dual-battery setup, sized and wired · how long a power station runs a 12V fridge · what gauge wire a 12V fridge needs · charging a power station while driving · running a fridge off the starter battery overnight · how many amps a 12V fridge really draws · best 12V fridge for overlanding · which fridge battery-protection setting to use

I have not installed any of these chargers or metered one on an alternator. The charge currents, efficiencies, cable sizes and fuse ratings are the makers’ published figures, the hours and the input amps are arithmetic from them, and the owner reports are quoted from the forum threads listed below. Where two published numbers are set side by side, the conclusion is mine.

FAQ

Is a 20 amp DC-DC charger enough for a 100Ah lithium battery?

For one battery and a fridge, yes, if you drive. It is LiTime’s recommended charge current for its 100Ah, and it puts back a 400Wh fridge day in about 1.7 hours of driving, or 100Ah from flat in 5. It is the right size for an alternator of 110 amps or so. Where it falls short is a short hop after two nights parked.

Can I run a 50 amp DC-DC charger on a stock alternator?

On a 180 amp alternator, owners do. A 2018 Land Cruiser with a 50 amp charger reports an honest 50 amps at idle. The charger draws about 55 amps from the alternator at full output, which is under a third of 180. On a 130 or 140 amp alternator it is over a third and near the half rule, and I would not.

How long does a 40 amp DC-DC charger take to charge a 100Ah battery?

About 2.5 hours from flat and 2 from a fifth left, before absorption. LiTime prints the same 2.5 hours for its 100Ah on a 40 amp charger. One fridge day is back in about 50 minutes.

Does a bigger charger need thicker cable?

Yes, and heavier fuses. Redarc’s kits go from 6 AWG and 40 amp fuses for the 25 amp charger to 4 AWG and 60 amp fuses for the 40 and 50, on any run past the engine bay. The input draw of a 40 amp charger is nearer 50 to 60 amps, which is what the cable has to carry.

Why does my DC-DC charger keep switching off while I drive?

Most likely a smart alternator. It lets the voltage fall when the battery is full or the car is coasting, and a charger that watches the voltage to decide whether the engine is running turns off. Connect the ignition wire (Redarc’s blue wire, Renogy’s IGN wire, Victron’s remote connector) to a point that shows battery voltage with the ignition on.

Are these numbers from a test?

No. The charge currents, efficiencies, cable sizes and fuses are copied from the makers’ pages and manuals. The hours and the input amps are sums from those figures. The owner reports are quotes from forum threads. Nothing on this page was measured by me.