Powering a Big Boat Sound System: Amplifier Draw, an Audio Battery and Charging

Look in each amplifier's manual or spec sheet for a current draw figure or a recommended fuse rating. That's the number to plan around, not the big "peak watts" printed on the box.

If you only have a power rating, you can make a ballpark estimate: total RMS watts for all channels, divided by the voltage (about 12.5 to 14 volts on a boat), then divided again by the amplifier's efficiency (well under 100 percent; Class D amps waste less power as heat than older designs). As a rough example, an amp delivering 600 watts RMS at full power could pull somewhere around 50 to 60 amps.

Two things keep real life below that number. Amplifiers only pull full current at full volume with heavy bass, and most music is quieter on average. But plan for the loud days anyway, because that's when cables get hot and batteries go flat.

Don't forget the head unit. As an example, Garmin's specs for one Fusion Apollo stereo list up to 15 A maximum, less than 700 mA when muted and less than 100 mA when switched off. That last figure is the "off" draw that slowly empties a battery left connected for weeks.

Battery capacity is measured in amp-hours (Ah). Multiply your average audio current by the hours you want to play at anchor. For example, 20 amps average for 4 hours is about 80 Ah.

Then allow for how deep the battery can safely be drained. Flooded and AGM lead-acid batteries last much longer if you don't routinely take them below about half charge (typical; check the battery maker's figures), so that 80 Ah day calls for something like a 160 Ah lead-acid bank. Lithium batteries can usually be drained deeper, but they need compatible charging (see step 5).

Mercury's battery guide sums up the split: starting batteries are built for a short, big burst to crank the engine and aren't meant to run accessories for long, while deep-cycle batteries are built to be drained and recharged over and over. Your audio battery should be deep-cycle.

Every amp-hour the stereo uses has to be put back. Sources are the engine's alternator or charging coil, an onboard shore charger and, on some boats, solar.

Find your engine's charging output in its manual. Small and mid-size outboards often produce much less charging current than a big stereo can use, so a bank drained flat at the sandbar may take hours of running to refill, or may never fully recharge on short trips. If your boat lives on a lift or at a dock with power, a multi-bank onboard charger is the easiest way to top up the audio bank between trips (see the onboard battery charger guide). If you're adding a large bank, test your charging system first (see the charging system test guide).

Three common ways to charge two banks from one engine while keeping them apart:

• Automatic charging relay (ACR). A voltage-sensing relay. Blue Sea Systems explains that its ACRs connect the batteries when either one rises to about 13.0 volts for 2 minutes (a sign that something is charging) and separate them when both fall to about 12.75 volts for 30 seconds. With the engine off and the stereo playing, the audio bank is isolated, so the start battery isn't touched. Thresholds vary by model; follow yours.
• Diode battery isolator. Lets charging current flow to each battery but not back out. Blue Sea notes that ACRs are more efficient because isolators make heat and use some of the charging energy; diode isolators also drop some voltage, which can leave batteries undercharged unless the charging system is set up for it.
• DC-DC charger. Feeds the audio bank from the start side at a controlled voltage. Often the right choice for lithium audio batteries; check the battery maker's charging requirements.

A manual "1-2-Both" switch works too, but only if you remember to switch it every time. Forgetting is how start batteries die at the sandbar.

Use a deep-cycle or dual-purpose battery sized from step 2, or a marine lithium battery if your charging setup supports it (see the lithium battery conversion guide). Several batteries can be wired together as one bank; use identical batteries of the same type and age.

Mount the battery where it's dry, ventilated and accessible, in a box or tray with a hold-down so it can't slide or tip in rough water. Keep it as close to the amplifiers as practical, because shorter heavy cables mean less voltage drop and less copper. Cover every positive terminal with an insulating boot.

The amplifiers need a good home too. BoatUS describes the power amp as a box you rarely touch once installed, so it can go up inside the console or in a dry locker or cabin, but it must stay dry, and it sheds heat when the music is loud. Leave air space around it, keep it out of the bilge and away from spray, and follow the maker's mounting instructions.

Put a battery switch or a suitably rated breaker in the audio bank's positive cable so you can kill the amps quickly in an emergency and turn everything off when you leave the boat. Label it clearly ("AUDIO"). Owners who have had an amp start to smoke all say the same thing afterward: they wished they had a switch they could reach without touching a hot battery terminal.

Consider an automatic cutoff too. If the music shares a house battery with the VHF, nav lights or bilge pump, a low-voltage disconnect stops a long sandbar day from draining the battery those need. Blue Sea describes using one of its charging relays this way: the stereo or fridge is wired through the relay, which drops that load when the battery falls to an adjustable set point and reconnects it once the voltage recovers. Ask your installer to set the cutoff where your battery maker says the bank should stop, and never wire the bilge pump or VHF through it. On boats with separate start and house banks, Blue Sea's dual-circuit battery switch is another tidy option: one knob turns both banks on while keeping them isolated, with a "combine" position only for emergency starting.

Federal boat wiring rules (33 CFR 183.455) require a fuse or breaker on each positive wire within 7 inches of the battery or power source, or within 40 inches if the wire runs inside a sheath or enclosure the whole way. That fuse protects the cable: if the cable chafes through and shorts against something, the fuse blows before the cable becomes a heating element.

Blue Sea's guidance is to choose the fuse to protect the wire. If the amplifier maker specifies a fuse that is too big for the wire you planned, use a bigger wire, not a bigger fuse. Use a sealed, marine-rated fuse holder, and put an ignition-protected one in any space where gasoline vapor can collect. Many amp makers also have you leave the fuse out until all connections are finished; then fit it as the last step.

Use the cable size the amplifier maker recommends, and check it against a marine wire chart for your actual run. Blue Sea's wire-size method uses the current and the round-trip length (from the battery to the amp and back) and lets you choose a 3 percent or 10 percent allowable voltage drop. For audio, staying near the lower drop keeps amps from cutting out when the bass hits.

Use tinned, stranded marine cable (Blue Sea notes ABYC-quality marine wire is always stranded and tin-plated), with properly crimped lugs and adhesive-lined heat shrink. Run the amplifier ground back to the battery negative or the main negative bus with cable the same size as the positive. Don't ground amps to a tower, a rail or a random bolt.

The audio battery's negative must connect to the boat's main negative bus (the same ground the start battery and engine use), with a heavy cable. If the negatives aren't joined, the ACR or isolator can't work properly, and audio gear that also connects to the head unit can end up using thin signal wires as a ground path. Follow the wiring diagram that came with your ACR or isolator.

Don't pile more ring terminals onto one battery post. BoatUS points out that stacked terminals may not make full contact and can heat up without ever blowing a fuse. Use a bus bar for multiple connections.

Connect each amplifier's remote turn-on lead to the head unit's remote output, so the amps switch on and off with the stereo. If the head unit's memory wire stays powered, connect it to the audio bank, not the start battery, so its small "off" draw can't flatten the battery that starts the engine. Turn the audio switch off when you leave the boat (see the parasitic battery drain guide if a battery keeps dying).

Reconnect the negatives, then the fuses, and switch on. With the engine off, play music at your usual loud volume and check:

• Voltage at the amplifier's power terminals compared with voltage at the battery. A big difference means a loose connection, a small cable or a poor ground.
• Heat: feel each lug and cable after 10 to 15 minutes. Anything too hot to keep your hand on needs fixing.
• The start battery: its voltage should stay steady while the audio bank does the work.

Then start the engine and confirm the ACR or isolator begins charging the audio bank (its voltage should rise after a short delay).

Check that the battery hold-down is tight and terminals are clean and covered. Look for chafed cables where they pass through bulkheads or up a tower. Keep the audio bank charged during storage, and recheck fuses and lugs whenever you add an amp or speakers.