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Charging

What Size DC-DC Charger for a 150 Amp Alternator?

Researched from published standards and manufacturer specifications. Updated .

Quick answer

A 150 amp alternator can typically support a 60 amp DC-DC charger. Taking roughly 40 percent of rated output for charging leaves about 60 amps available, with the rest kept for the vehicle's own electrical loads and for the alternator's thermal headroom.

An alternator is rated at its maximum output, not at what it can produce continuously without overheating. Most are air cooled and spend their lives well below that rating, so drawing the full nameplate figure for battery charging is a good way to shorten an alternator's life or fail it outright on a long climb.

This matters far more with lithium than it ever did with lead-acid. A depleted LiFePO4 bank will accept nearly unlimited current, so connecting it to an alternator through a simple relay lets it pull everything the alternator has, continuously. The DC-DC charger is the current limit, and that is its main job. Size one for your vehicle on the DC-DC charger calculator.

How much charging current can a 150 amp alternator supply?

Available charging current from a 150 amp alternator
Share taken for chargingWhen this appliesCurrent availableCharger to fit
30 percentOlder alternator, hot climate, heavy vehicle loads45 A40 A
40 percentTypical modern vehicle60 A60 A
50 percentUpgraded cooling or a known continuous rating75 A60 A

Convention Source: Planning guideline. Manufacturers rarely publish a continuous rating for automotive alternators.. If your alternator has a published continuous output figure, use that instead of a share of the peak rating.

How much does this actually put back in a day of driving?

A 60 amp charger at about 13.6 volts delivers roughly 816 watt-hours per hour of driving. Two hours of travel puts back about 1,632Wh, and four hours about 3,264Wh. In amp-hours that is about 240Ah over a four hour drive, which for many rigs covers a full day of use.

Alternator charging has one large advantage over solar: it works at night, in rain and under trees. It has one large disadvantage: it only works while you are driving. Most well-built rigs use both. Compare them on solar vs alternator charging.

What cable does this run need?

A 60 amp charger over a 20 foot run from the engine bay to the house bank needs 1/0 AWG copper to hold 3 percent voltage drop. That is a long, heavy cable, and it is the main cost and labour of a DC-DC install. Fuse both ends of the run, because a cable connecting two batteries can be fed from either direction.

Do I need a DC-DC charger at all?

If your house bank is lithium, yes. A voltage sensitive relay or a simple isolator connects the banks directly, which lets a depleted lithium pack draw far more current than the alternator can sustain. If your house bank is lead-acid, the battery's own internal resistance limits the current, and a relay is workable, though a DC-DC charger still gives a proper multi-stage profile and will fill the bank fully rather than partially.

DC-DC chargers for this alternator

Expert
Victron Orion XS 50A DC-DC Charger
Victron Energy

Victron Orion XS 50A DC-DC Charger

Price varies, check the listing

Victron publishes full efficiency curves and the unit is configurable over Bluetooth, which is why it is the reference part in this category.

Best for: A documented build where the alternator load has to be limited precisely.

Check price on Amazon

Frequently asked questions

What size DC-DC charger for a 150 amp alternator?
About 60 amps. Taking roughly 40 percent of the alternator's 150 amp rating leaves around 60 amps for charging, with the remainder kept for the vehicle's own loads and for thermal headroom. Going larger risks overheating the alternator on long grades or in hot weather, when it is least able to shed heat.
Can I connect a lithium battery straight to my alternator?
No. LiFePO4 has very low internal resistance and will accept nearly all the current an alternator can produce, continuously, until it is full. Automotive alternators are not built for sustained full output and commonly overheat and fail when treated this way. A DC-DC charger limits the current to a safe figure and supplies the correct lithium charge profile at the same time.
Will a DC-DC charger drain my starting battery?
A properly installed one will not. DC-DC chargers include an ignition or voltage sense input so they only run while the engine is running and the alternator is producing. When the engine stops, the charger stops and the two banks are isolated, which is exactly the behaviour you want overnight.
How long does it take to charge a bank from the alternator?
At 60 amps you add roughly 60 amp-hours per hour of driving. Filling a 100Ah lithium bank from 20 percent takes about 1.3 hours of driving. Real time is a little longer because the charger tapers near the top of the charge, as any multi-stage charger does.

Lithium safety. A LiFePO4 bank needs a charger with a genuine lithium profile, and that means the converter, the solar controller and any DC-DC charger all have to be set correctly. A stock lead-acid converter will never bring a lithium pack to full. Charging a lithium battery below freezing damages the cells permanently, so the pack needs either a low temperature charge cutoff in its BMS or a self-heating design if you camp in the cold.