12V scuba compressor on board: refilling a mini tank from your boat battery

Yes, a 12V scuba compressor can refill a mini scuba tank on board, running straight off the boat's battery. With the OXSEA 12V/220V mini compressor, allow about 12 minutes for a 0.5 L tank and about 20 minutes for a 1 L tank. The battery handles it well on three conditions: you connect the compressor to the house battery and never to the engine's starting battery, you know how many amp-hours each refill takes, and you charge the battery back up to full right afterward.

Then there is the air. Whatever the compressor draws in on deck, you will be breathing under the hull. Where you set the unit down and whether the engine is running during the fill matter as much as the wiring.

What a 12V dive compressor can and cannot do

A scuba tank is filled to 200 bar (about 2,900 psi). A tire or dinghy inflator works at a few bar, and a shop air compressor tops out at around ten (roughly 150 psi). Neither one comes close to the pressure required, and neither is built to supply air you breathe. You need a high-pressure compressor designed for the job, with its own filters.

The OXSEA mini compressor belongs in that category. It runs on 12 V from a battery or on 220 V from the mains. It fills all three tanks in the range, each with a working pressure of 200 bar: the OX500 and the OX500Plus (0.5 L) in about 12 minutes, the OX1000 (1 L) in about 20. With no full-size scuba tank on board, it is the only way to fill the OX1000, because the manual pump only works with 0.5 L tanks.

A 0.5 L tank gives you up to 8 minutes underwater, a 1 L tank about 15. Those times change with your breathing, your depth and how hard you are working. Clearing a propeller or scraping a hull often takes several dives, which means several refills in the same day. That is where the battery comes in.

House battery or starting battery

Most recreational boats carry at least two batteries: one to crank the engine, and a house battery for everything else. The manufacturer Victron Energy spells it out in its book Energy Unlimited: Electricity on Board: the starting battery should feed the starter motor and nothing else. It is built to deliver a very high current for a few seconds. Its thin plates warp when it is discharged deeply and often, and the book places it among the batteries that are unsuited to repeated charge and discharge cycles.

So the compressor goes on the house battery. Does your boat have only one battery, the one that starts the engine? Then don't run it down at anchor. Fill your tanks in the marina or ashore before you head out, or keep a manual pump on board if you dive with a 0.5 L tank.

Two notes depending on the battery type. Among sealed lead-acid batteries, AGMs cope with high currents better than gel batteries, according to both Victron and INRS, France's occupational health and safety institute. With a lithium battery, look up the maximum discharge current in its manual and make sure it is higher than the compressor's draw.

How many amp-hours one refill takes from the battery

The math fits on two lines. The current, in amps, is the compressor's power in watts divided by 12. The consumption, in amp-hours (Ah), is that current multiplied by the running time in hours: 12 minutes is 0.2 hours, 20 minutes is a third of an hour.

These three rows are worked examples, not measurements. The figure that counts is the current printed on your unit's rating plate or in its manual. If you can't find it for an OXSEA mini compressor, ask us. Better still, if the boat has a battery monitor, note the amp-hours used during one refill: that gives you the real number for your own setup.

Next, compare that with the battery. Its capacity is rated for a slow discharge, often over 20 hours: a 100 Ah battery can deliver 5 amps for 20 hours, as INRS explains (in French). The harder you draw on it, the less it gives back. In Victron's tables, a 200 Ah semi-traction battery returns only about three quarters of its capacity when asked for 40 amps. And on a lead-acid house battery, the share you can really use is no more than half the rated capacity, again according to Victron.

A 100 Ah battery therefore leaves you 50 Ah at best, to share with the fridge, the instruments and the lights. In the 30 amp example, a 0.5 L refill uses 6 of them and a 1 L refill uses 10. Two or three refills over the day are no problem for a battery bank in good shape and fully charged. Eight 0.5 L refills back to back would eat up the entire usable reserve.

A voltmeter gives you a reference point, provided you read the battery voltage after it has rested for a few hours, with nothing charging it or drawing from it. While the compressor runs, the voltage sags and tells you nothing about the state of charge.

These values come from Victron's book, which presents them as a general indication. One last habit, the one that makes a battery last: recharge it fully after the session. A lead-acid battery left discharged sulfates, and in the end the damage can't be undone. Victron recommends never leaving it more than 50% discharged and bringing it back to 100% on a regular basis.

Wiring the compressor with no overheating and no sparks

At 12 V, a few hundred watts already add up to several dozen amps. Choose the cables and the protection to match.

• Keep the original cable. Don't extend it with some wire dug out of a locker. For a cable shorter than 5 meters (16 feet), Victron's wiring guide gives a rule of thumb: the cross-section in mm² equals the current divided by 3, so 10 mm² for 30 amps (close to 7 AWG, so round up to 6 AWG rather than down to 8). A cable that is too thin heats up and drops the voltage reaching the compressor. The guide aims for a voltage drop under 2.5%.
• A fuse on the positive cable. Every device connected to a battery needs its own, Victron writes: in a short circuit, a battery puts out enough current to start a fire. Check that your compressor's cable has one. If you refill often, have a dedicated, fused outlet installed near the cockpit.
• 12 V socket on the electrical panel: read its fuse. Compare its rating with the compressor's current. If it is lower, connect to the battery instead.

The other hazard is less well known. A lead-acid battery gives off hydrogen, especially toward the end of a charge, and some of it lingers around the battery after charging has stopped. That is what the INRS fact sheet on charging lead-acid batteries, mentioned above, describes. In a closed locker, this gas mixed with air ignites from a single spark. A clamp brought close to a terminal is enough to make one.

Hence this sequence, to follow at every refill.

1. Shut down the engine and the generator. If the battery has just been charging (from the engine or a shore charger), open its locker and let it air out before touching anything.
2. Take off rings, watch and bracelet, and never set a tool down on the battery. INRS asks for this to prevent a short circuit across the terminals.
3. Set the compressor up outside, level, wedged in place and in the shade, with the air intake facing into the wind.
4. With the compressor switched off, connect the red clamp to the positive terminal and the black one to the negative terminal. With the unit off, making contact produces less of a spark.
5. Hook up the tank, check the filter, then switch on.
6. Stay next to it, eyes on the pressure gauge. Stop at 200 bar (about 2,900 psi), never beyond.
7. Switch off before disconnecting. Between two tanks, allow the rest time given in the manual.

The whole procedure is filmed in the video “Use the 12 V / 220 V mini compressor” in the OXSEA School. Watch it before your first fill, not on the day a line gets wrapped around the prop.

Engine off while you fill

The temptation makes sense: leave the engine running so the alternator backs up the battery while the compressor works. Don't do it. A running engine means exhaust, and exhaust contains carbon monoxide.

DAN (Divers Alert Network), the reference organization for diver safety, lists engine exhaust among the sources of breathing gas contamination, along with oil from an overheating compressor, cigarette smoke and paint fumes near the air intake. Carbon monoxide has no smell and no taste. Underwater, it impairs judgment and can make you pass out. A study published by INRS (in French), covering fifteen years of compressed air testing at the French Navy laboratory in Toulon, points to the same culprits: exhaust from machines with combustion engines, and lubricants broken down by heat.

The right sequence is simple. You fill with the engine and the generator off. You run the engine afterward, with the tank disconnected, to bring the battery back up. In a marina, look around you too: a neighbor's diesel warming up, a generator sitting on the dock, a car idling near the pier are all exhaust, just like yours.

Moisture counts as well. The same study calls it the least well controlled parameter in compressed air, because very dry air is hard to obtain by compressing the surrounding air directly. Too much moisture encourages corrosion inside a metal tank. On board, the air is damp and salty: keep the air intake out of the spray and keep the filters current. They are consumables, and the “Change the filters” video in the OXSEA School shows how to replace them.

If the air from a tank has an unusual smell or taste, don't use it. The opposite proves nothing, since carbon monoxide can't be smelled. Your protection is where you place the air intake.

In the marina: 220 V shore power

Plugged into the dock pedestal, the same compressor runs on 220 V and asks nothing of the battery. It is the easiest way to leave with full tanks. Fill them the day before, check the gauges, then stow them wedged in, away from heat and knocks.

Do the very first fill and the first trial run in the marina as well. A new tank arrives empty, and you would rather get to know the gear at the dock than at anchor with a line in the prop. The guide on the mini scuba tank for boats covers choosing a model and stowing it on board.

Running the compressor at anchor through a 12 V to 230 V inverter gains you nothing. You put one more device, with its own losses, between the battery and the compressor. Use its 12 V connection.

Still deciding between a compressor, a pump and a transfer from a full-size tank? The guide on how to refill a mini scuba tank compares the three methods, with times and budgets.

Five mistakes that cost you a battery or a day on the water

• Drawing on the starting battery at anchor. If it gives out, the engine won't restart.
• Filling with the engine running. What the battery gains isn't worth carbon monoxide in the tank.
• Extending the cable with thin wire. It heats up, the voltage drops, the compressor struggles.
• Leaving the battery half empty until the next weekend. It sulfates. Recharge it the same day.
• Walking away during the fill. Nobody is watching the gauge, and the 200 bar limit slips by without a sound.

Tank full: the rules before you go down

Refilling is only half the job. Before any work under the boat, four rules are not open to debate.

• Engine off, key out. Give the key to the person staying on board. Neutral is not enough: a propeller put into gear by mistake causes serious injuries.
• Never alone. France's ministry for the sea (in French) reminds divers that in diving, the basic unit is the pair. Under a hull, keep at least one person at the surface who watches you and knows how to raise the alarm.
• Alpha flag up. Under the French rules in that same leaflet, a boat supporting divers must fly this blue and white flag for as long as divers are in the water.
• Keep breathing. Never hold your breath on the way up. According to DAN's Alert Diver magazine, an ascent of about 1.5 meters (5 feet) with your lungs held can be enough to cause an arterial gas embolism.

Stay shallow and head up while there is still air showing on the gauge. The mini scuba tank safety guide goes through these points one by one.