Marine Battery Sizing

When selecting marine batteries, the rule of thumb is to double what you think you need. In a car it is recommended to use one cranking amp for every cubic inch displacement of the engine. For marine application it is recommended to use two cranking amps. A marine engine typically does not start harder, but this is to provide an extra margin of safety.

For lighting batteries the same rule applies. Double what you think you need. An approximate way of determining battery sizing is to measure the average current draw per hour with all equipment running and lights on. If you don't have an ammeter for doing this then make a list of all the equipment and lights, recording the current draw from the name plate. If the information is supplied in watts then divide the watts by the voltage. A 60 watt light bulb on a 12 volt system draws approximately 5 amps.

Once the current draw is known, certain assumptions will have to be made. An electric refrigerator which draws approximately 5 amps will run approximately one half the time. Therefore, it would draw approximately 60 amp hours in 24 hours.

Once an estimate of the total ampere hours is known over a 24 hour period then determine the number of days the batteries are to last before recharging. Double this figure. A conservative approach to determining battery sizing is to match this figure with the 20 hour rating of the battery.

Remember when batteries are connected in parallel (positive to positive and negative to negative) the ampere hour capacity doubles and the voltage remains the same.

The reverse is true with batteries connected in series. The voltage doubles and ampere hour capacity remains the same.

Current Draw

Estimated Current Draw of Various Lighting/Elec. Items

Group	Device	Amps
Communications	CB Receiver	1.00
Communications	SSB Receiver	1.50
Communications	SSB Transmit	25.00
Communications	VHF Receive	1.50
Communications	VHF Transmit	5.00
Entertainment	Tape receiver	1.00
Refrigeration	Refrigeration(typical)	5.00
General Lighting	Cabin Light (40W)	3.50
General Lighting	Fluorescent Light (26W)	1.80
General Lighting	Fluorescent Light (8W)	.70
General Lighting	Hand Held Spotlight	10.00
General Lighting	Spreader Light	8.00
Heat	Forced Air (11,000 BTU)	3.00
Heat	Forced Air (28,000 BTU)	10.00
Instrument	Knotmeter	0.10
Instrument	Wind Speed Indicator	0.10
Misc..	Anchor Windlass	80.00
Misc..	Auto Pilot	4.00
Misc..	Bilge Pump	2.50
Misc..	Cabin Fan	1.00
Misc..	Fuel Pump	3.00
Misc..	Horn	3.00
Misc..	Inverter (Sm. Microwave)	100.00
Misc...	Inverter(Nicad Charger)	1.50
Misc...	Propane Valve	1.00
Navigation	Depth Sounder	0.10
Navigation	Loran	1.00
Navigation	Radar	4.00
Navigation	Recording Depth Sounder	0.50
Navigation	Satnav(average)	0.20
Navigation	Weather Fax	2.40
Navigation Light	Anchor Light	1.00
Navigation Light	Masthead/Steaming Light	1.00
Navigation Light	Running Lights	3.00
Navigation Light	Strobe Light	0.70
Navigation Light	Tri-Color Light	2.00
Pump	Bilge Pump	5.00
Pump	Fresh Water Pump	5.00
Pump	Head Pump	18.00
Pump	Wash Down Pump	10.00