Batteries Store Power (Part I) article on janice142

Mini-Lesson on Power and Battery Capacity.

I am going to use round numbers for this explanation. It is easier for me to understand that way... Specifically my batteries hold a maximum capacity of 109 amps. For me rounding to 100 is far simpler. Plus over time the capacity decreases thus I believe my rounding down is a more accurate number.

Your standard *Group 27 or *Group 29 battery holds a certain number of amp hours aka power. For math's sake, let's call it 100 amp hours per battery. With the economical batteries (read: cheap) it is suggested that we never use more than half of the amperage in the battery. Thus our 100 amp battery is only going to provide 50 amps of usable power consistently without damaging the battery.

*Group 27 or Group 29 batteries: Batteries are numbered by the external size. Thus a Group 27 battery will be 12 inches long, 7 inches wide about 9 1/2 inches tall. A Group 29 battery is slightly larger. Group 31's are 13" x 7" x 10". The capacity (number of amps/power) does vary between brands.

I opted for the smaller Group 27's. The larger batteries are way too heavy for me to maneuver. The next time I buy more batteries I may chose larger heavier ones as by then I shall be at the "hire testosterone" stage of battery replacement. I will not be able to safely lift them myself.

Each battery can only be recharged a certain number of times before it fails. There are tricks to rejuvenate batts though long term I have not see a lot of success. It is easier to simply replace.

I do not have the funds to continually replace batteries.

Calder's tome [affiliate link→ Calder's Boatowners Mechanical and Electrical Manual 4the Edition] recommends we use a smaller percentage of battery capacity. That will allow more recharging cycles.

Each of my 12-volt batteries holds 100 amps of power. I want to use the smallest percentage of amp hours possible in order to have those batteries last. Here is a chart:

(I use 125 amp hours per day in my life of decadence.)
# of Batteries	100 Amps Each	Total Amps	*Using 150 Amps**	Remaining Amps	Percentage Used
3 batts	100A	300A	150A	150A	50%
4 batts	100A	400A	150A	250A	38%
5 batts	100A	500A	150A	350A	30%
6 batts	100A	600A	150A	450A	25%
Me→ 7 batts	100A	700A	150A	550A	21%

*I rounded up to 150 amp hours per day in my chart to account for the extras. Some days I might watch three movies. Or use the microwave for lots of popcorn while watching said DVDs. It is always better to estimate on the higher side for usage.

Trust me when I say we can always find accoutrements for a more decadent lifestyle. I have never heard a boater say they had too much power available.