Installing a DC-to-DC Charger: Charging a Lithium or Second Battery Bank from Your Engine

Find your engine's alternator rating in the owner's manual or on the engine maker's spec sheet. Small outboards may have only a modest charging output, while inboards and big outboards have much more. A DC-to-DC charger is sized by its output current; choose one that the alternator can feed with room left over for the engine's own needs and your running electronics. Many units also let you limit the input current so a smaller alternator isn't overworked; Victron's Orion XS manual lists an input current limit setting for exactly that reason.

Two numbers matter, and Mercury explains the difference in its own article on outboard alternators: the rated output printed on spec sheets is the most the alternator can make, while the net output is what's left for the batteries after the engine's own loads (ignition, fuel pumps, trim, steering). Output also rises with rpm, so an engine that spends the day at idle or trolling speed delivers far less than its rating. Many smaller outboards don't have a belt-driven alternator at all but a flywheel charging coil (stator) with a modest output; on those, ask the engine dealer before adding any DC-to-DC charger, and pick a small unit or set a low input limit.

Match:

• Input voltage to your starting system (12 volts on most boats).
• Output voltage and chemistry to the house bank. Boating Magazine lists models in 12-to-12, 12-to-24 and 12-to-48-volt versions, and units that can charge flooded, AGM, gel and lithium batteries.
• Output current to your alternator (Step 1) and the battery maker's recommended charge rate.
• Marine suitability: look for a unit sold for marine use, with the enclosure rating and mounting instructions you need. If it must go near the engine on a gas boat, it has to be ignition protected (Step 3).

This is where many installs go wrong. Boating Magazine's install used a charger that the maker states is not ignition protected, so they kept it out of the engine compartment, and also kept it from being mounted above a lead-acid battery that could release fumes. Federal rules require electrical parts in a gasoline engine space to be ignition protected; see ignition-protected parts. Diesel boats don't have the same vapor rule, but a hot, wet engine room is still a bad home for electronics.

Good spots are dry, ventilated, close to the house bank and easy to reach. Victron's Orion XS manual asks for a non-flammable surface, about 10 cm (4 inches) of free space around the unit for cooling, and never mounting directly above a battery. A DC-to-DC charger makes heat; in a sealed locker it will reduce its output to protect itself.

A typical layout has three heavy cables:

• Input positive: from the starting battery (or the starting side of your battery switch, per the manual) to the charger's input.
• Output positive: from the charger's output to the house battery positive.
• Negative: a common negative, run as the manual shows.

Add any small signal wire your setup needs (Step 9). Sketch it, mark where each fuse goes, and decide how the new bank will connect to your battery switch and battery monitor shunt. If the house bank has a battery monitor shunt, all house negatives, including the charger's, must land on the shunt's system side so the monitor counts the charge.

Cable size depends on the current and the round-trip length: from the battery to the charger and back. Too thin a cable wastes voltage as heat, and a DC-to-DC charger may then sense a low input and cut out. Victron's wiring guide says the correct size is in each product's manual and suggests aiming for a voltage drop below about 2.5 percent; ABYC sets its own limits for marine circuits, and Blue Sea and Ancor publish marine wire-size charts based on them. Use tinned, stranded marine cable, and if you can't find one thick enough, Victron says two cables can be doubled up as long as their combined cross-section matches the recommendation.

We don't list cable sizes here on purpose: they change with the model, the current and the run length. Use the manual's table or a marine wire chart and, if in doubt, go one size larger.

Both positives are connected to a battery at one end, so each needs its own fuse or circuit breaker near the battery it comes from. Boating Magazine's project put a breaker near each bank, which also gives you a handy way to switch the charger off. Use the fuse or breaker rating the charger's manual specifies for that cable. ABYC sets how close to the battery that fuse must be, with a few exceptions; the battery cable replacement guide explains the distance rules. Don't skip the fuse on the input side because "it comes from the start battery": that cable can short just as easily.

Crimp tinned ring terminals with a proper crimping tool, seal them with adhesive-lined heat-shrink, and label both ends. Route the cables with chafe protection, support them every so often, and keep them away from hot exhaust parts, sharp edges and moving steering gear. Victron's manual stresses strain relief: the weight of the cable should not hang on the charger's terminals.

With every battery switched off and the fuses or breakers open:

1. Mount the charger.
2. Connect the cables at the charger first: input, output, negative, as the manual shows (Boating Magazine connected the start bank to "in" and the house bank to "out," then the common negative).
3. Connect the negatives at the batteries or shunt.
4. Connect the positives at the batteries, through their fuses.
5. Recheck every terminal is tight and booted before you close any fuse or breaker.

The charger should only draw from the start battery while the engine is charging it. Units handle this differently:

• Voltage sensing: many chargers watch input voltage and start when it rises to charging level. Boating Magazine's charger only charges the house bank when the starting battery is at or above charging voltage.
• Engine-shutdown detection and input lock-out: Victron's Orion XS uses the alternator voltage to decide whether the engine is running, plus an input-voltage lock-out that stops charging if the start battery falls too far.
• A wired on/off or engine-run signal: some installs use a small remote wire. Victron warns against simply using the ignition switch, because the charger can then drain the start battery whenever the key is on but the engine isn't running.

Choose the method your manual recommends for your engine, and test it in Step 12.

Before connecting or enabling the output, set the profile. Victron's manual says to set the charging algorithm before a battery is connected to the output. Use the settings your battery maker publishes (absorption voltage, float voltage or no float for some lithium batteries, and maximum charge current), or the built-in preset that matches them. On units with DIP switches, Boating Magazine notes there is a flow chart in the manual; follow it carefully. If the lithium battery has a BMS charge-disconnect contact or a network link, some chargers can be controlled by it; set that up as both manuals describe.

Close the fuses or breakers, switch the batteries on, and look at the charger's status light or app. With the engine off it should sit in standby and draw nothing meaningful from the start battery. Measure both batteries with your meter and write the readings down.

Start the engine (outboards in the water or on external flush muffs, never dry; inboards in the water) and run at a normal cruising rpm. Within the delay your charger uses, it should switch to charging: check the status light or app, and watch the house voltage climb while the start battery stays at the engine's normal charging voltage. Then shut the engine down and confirm the charger stops and doesn't keep drawing from the start battery. Boating Magazine says on-water testing from idle to cruise is the best way to judge the system.

After a long run, carefully touch-test the cable ends and fuse holders with the back of your hand. Warm is normal; hot means a loose or undersized connection that needs fixing before the next trip. Record the charger model, settings, fuse sizes and the voltages you saw in your BoatMend boat profile.