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Understanding the House Batteries

Large "house" batteries provide 12-volt electric power for lights, furnace, and electronics when not connected to shore power at a dock and when the engine is not running.  

A separate, smaller battery is on board to start the engine.  The engine battery is not used for lighting, pumps, etc. and only serves the engine starter so that there is no risk of running it down and not being able to start.

Cassiopeia's house battery will power the boat's lights, pumps, stereo, and small items like phone chargers for at least a day -- and can last two or more days after being fully charged if energy is conserved by turning off unnecessary lights and instruments, and the furnace when not needed.

Always be aware of electric consumption on board when not connected to shore or running the engine.  Watch the kids and look into heads and cabins for lights left burning when no one is there.  Don't leave computers, inverters and phones plugged in if they don't need to be.

The house batteries become fully charged when connected at a dock for more than a few hours.  They never become fully charged by running the engine except, possibly, on a long motoring trip.  The batteries, however, can take a partial charge quite quickly when running the engine while cruising or when idling the engine.

Batteries are quite robust and can provide lots of energy if treated with understanding, but can be run down and permanently damaged by careless use. 

Trying to use battery power after the battery has become discharged will damage the house battery permanently, so care must be taken to conserve power and to monitor the "battery state" frequently when using power. 

Fortunately, monitoring the battery state is simple.  Cassiopeia has a meter at the nav station which shows the voltage of each battery.  It also can show tank levels, but that is another subject.

Basically, all that has to be done is to toggle the switch below the gauge to the left and right.

  • One way will give a high reading and that can be ignored because that is the engine battery which has nothing to do with instruments, lights, or anything the boater uses -- except the starter.  It is always fully charged and cannot be used for anything else.

  • The other will give a lower reading and that is the reading that must be watched.  Comparing that reading to the reading of the engine battery shows how close the house battery is to being fully charged.  If it gets down near "12", the battery is getting low.  At "12", it needs charging.

The battery bank consists of four 6-volt Trojan 175 ampere hour golf cart batteries in a series-parallel configuration.  The safe usage capacity is 175 ampere-hours which means that -- in theory -- one typical 10-watt cabin fluorescent light would take roughly 175 hours to run a fully charged battery bank down to the lowest allowable state of charge -- 12 volts or 50%.  

The small, bright ceiling lights throughout Cassiopeia are LED lights and use very little power -- about one watt each. Eight of them use the same power as one standard halogen, but each gives give the same light as 10 watt halogens.  Each has an individual switch on it to assist in energy conservation.

Or, two lights would run it down in 87 hours and four lights in 44 hours, eight in 22 hours...

Add the draw from several phone chargers, tablets, a laptop running on an inverter, the water pump, the furnace and its fans, plus accidental discharges from leaving the nav instruments or the running lights on after anchoring... and the propane solenoid switch, and in real life if you get two days off the grid, without using the engine before the voltage drops to 12 volts, you are doing very well.

Never let the voltage drop below 12 volts.  There is a danger zone shown as striped on the meter.  Don't go there.  Start the engine or plug in to shore power to charge.

That is the simple rule, for those who need simple rules, but for those who wish to push the limits, here are some details...

The best and most accurate time to read voltage is when all the DC switches have been off an hour.

  • If lights and appliances are on at the time of the reading or recently, the reading will be lower by a hair than after a rest period and underestimate the remaining charge

  • If the engine has been running or the shore power connected, the battery will appear to have a higher charge than it really does.

The bottom line is, however, if you are so close to the 12 volt cut-off that it matters, it is very near the time to recharge the batteries.

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While the information provided here is believed to be correct at time of publication, errors are possible
and things may change, so readers should verify details before making important decisions.

- Home | Greeting | Features | Specs | Video | Marina | Cruising | Adventures -
- Forward Cabin | Saloon | Aft Cabins | Sails | Cockpit | Enclosure | On deck -
 - Plotter and Radar | Marine Head | Refue
lling | Seacocks| Engine | Batteries | Dinghy | Anchoring -
- Inventory | Manuals | US/Canada Border | Winter | Thoughts | Troubleshooting -