Size a DC-DC (alternator) charger from your battery bank’s amp-hours and a target charge rate in C. The result is the charger output in amps and the approximate charge time.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
Charger output
40 A
Charge time
5.0 h
Formula
Charger output (A) = battery Ah × C-rate. Charge time (h) = Ah / charger A. A C-rate of 0.2C charges at one-fifth of the amp-hour capacity, so a full charge takes about five hours before the charge-acceptance taper near the top.
Worked example
A 200 Ah battery charged at 0.2C: charger output = 200 × 0.2 = 40 A; charge time = 200 / 40 = 5.0 hours (before the charge-acceptance taper near full).
A DC-DC charger lets you top up the house battery from the vehicle alternator while driving, safely and at the right profile for the chemistry — which a direct alternator connection cannot do, especially for lithium, whose high acceptance current can overload a stock alternator. This tool sizes the charger and its cabling from your battery bank and how quickly you want to replenish it on the road. Match the charger's output to what the alternator can spare without overheating, size the input and output cable for that current over the long run from engine bay to living space, and fuse both ends; on newer vehicles with smart alternators an ignition or D+ trigger is usually needed to switch it on.
Frequently asked questions
What is a C-rate and what should I use?
The C-rate expresses charge or discharge current as a fraction of the amp-hour capacity, so 0.2C on a 200 Ah battery is 40 A. Lithium (LiFePO4) commonly accepts 0.2C–0.5C for everyday cycling, with many cells rated to 1C; lead-acid and AGM are usually happiest around 0.1C–0.2C. A higher C-rate charges faster but generates more heat and can shorten life, so pick the rate your battery’s datasheet allows and let this tool turn it into amps.
How big a DC-DC charger do I need from my alternator?
Match the charger to the rate your battery can accept and to what the alternator can spare. For a 200 Ah lithium bank, 0.2C gives 40 A, which is a common 40 A or 50 A DC-DC unit. Going larger (say 60 A) charges faster but loads the alternator harder; many builders keep the charger at or below about half the alternator’s rating to avoid overheating it on long climbs. The tool returns the amps for your chosen C-rate so you can pick the nearest standard model.
Why use a DC-DC charger instead of a direct alternator connection?
Lithium batteries can demand huge inrush current when nearly empty, which can overheat or destroy a standard alternator wired directly. A DC-DC charger limits the current to a safe, programmable level and provides the correct multi-stage lithium charge profile that the alternator’s simple voltage output cannot. It also isolates the house bank from the starter battery so the engine always cranks. That current limit is exactly the charger output this calculator sizes.
Why is the real charge time longer than the calculated value?
The simple estimate assumes a constant current right up to full, but real chargers taper the current as the battery approaches 100% (the absorption stage), so the last 10–20% takes disproportionately long. The tool reports the bulk-stage time — a good figure for the fast part of the charge — and you should add roughly an hour for the taper on lithium, more on lead-acid. It also assumes you start from empty; a partly charged battery finishes sooner.
Can I charge faster than the calculated time by using a bigger charger?
Up to a point. A bigger charger raises the C-rate and shortens the bulk stage, but only if the battery, the alternator and the wiring can all handle the extra current. Beyond the battery’s rated charge current you gain nothing and risk damage; beyond the alternator’s safe continuous output you risk cooking it. Use this tool to see the amps a given C-rate implies, then keep that within the limits of every component in the charging path.
How does solar charging combine with a DC-DC charger?
They are complementary and can run at the same time. Solar charges whenever the sun is out and you are parked; the DC-DC charger tops the bank up while you drive, regardless of weather. Many builds use both so the battery is full by the time you reach camp. When sizing, treat them as separate current sources into the same bank — the combined charge current should still stay within the battery’s rated acceptance, which is the figure this tool helps you check for the DC-DC side.
What gauge wire does the DC-DC charger need?
Size the cable for the charger’s rated output current over the run length from the starter battery, through the charger, to the house bank — these runs are often long, so voltage drop matters. A 40 A charger over such a run typically needs 6 AWG or 4 AWG copper. Use the 12V DC Wire Size tool with the charger amps and your actual one-way length, and fuse both ends close to each battery. Undersized charging cable wastes energy as heat and slows the charge.
Source: Charge rate definition C = A / Ah (charger amps = Ah × C; time = Ah / amps) · All sources