House Battery Storage Capacity

Electrical and electronic topics for small boats
peteinsf
Posts: 64
Joined: Sun Jan 03, 2016 9:04 am

House Battery Storage Capacity

Postby peteinsf » Fri May 26, 2017 7:52 pm

My preference is to have two dedicated engine-starting batteries for my twin 200-HP set-up. My experience is that battery life seems to tied to cycles of discharge, and, by eliminating non-engine [loads], the starting batteries will be topped off almost 100% of the time.

How small a house battery can I get away with? I don't really want a big Group-27 battery living in my Montauk sized console.

All of the electrical items seem to be much lower current than in the past: LED lighting, VHF-radio, Fish finder. I guess bilge pumps are a constant.

Anyone using a tiny console battery? Maybe a "U1" size?

Pete

jimh
Posts: 11673
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: House Battery Storage Capacity

Postby jimh » Sat May 27, 2017 12:29 am

The storage capacity for a HOUSE battery (or HOTEL battery as they are also called) should be sufficient to provide power to loads when the propulsion engine is not running and charging the HOUSE battery for the period of time needed, without draining the HOUSE battery to near complete discharge.

On a small boat the longest period of time an engine might not be running (to create charging current) is during an overnight anchorage away from shore power. You should calculate the time for the overnight period to include any daylight time that the engine is not running. If you anchor at 4 p.m. and shut off the propulsion engine, and don't restart the engine until 10 a.m. the next morning, you need HOUSE battery power without recharging for 18-hours. What loads will be used in that time? Some sort navigation lighting, at the least an anchor light, will be shown for 10-hours. If the lamp uses an incandescent 7-Watt bulb, it will be consuming at 12.0-Volts about 0.6-Ampere. That means 6-Ampere-hours of load.

Will you leave any electronics on overnight? Perhaps the chart plotter so you can verify your position. A chart plotter probably draws about 1-Ampere with a dimmed screen. Leave it on for 18-hours and it will consume 18-Ampere hours.

So one night on the hook with the anchor light and chart plotter running will consume 24-Ampere-hours. If you don't want the HOUSE battery to be chronically discharged below 50-percent charge, it sounds like you need 48-Ampere-hours of storage capacity.

You can figure a different load if you shut off everything as soon as you drop anchor, and you run an LED anchor lamp that draws maybe 0.2-Ampere. That works out to 2-Ampere-hours overnight. Maybe a 5-Ampere-hour battery is all you need for that.

The actual battery case size is not an index of electrical energy storage capacity. Look at the Ampere-hour rating of the battery. Also, if the battery is going to be run down to low charge levels often, look at the design of the battery to be sure it can tolerate a lot of charge-discharge cycles.