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Date: 7 April 2016. Batteries Store Power (Part I)


My friend says people prefer to have all the information on one page and in one article. I do not particularly care to click through either. In any event, I have covered a lot of material herein. In bite-sized pieces the articles are:

  1. Batteries Store Power (Part I) ←this one

  2. Solar Makes Power (Part II)

  3. Power Used Aboard Seaweed (Part III)

  4. Inverters Make AC Power (Part IV)

The complete version with all the information
contained in the four articles is on this page:

Solar, Batteries and an Inverter, the complete series.


This is Part I, entitled
Batteries Store Power.
A boater wrote asking about solar power. Like many I have embraced the solar movement. The price has come down on the panels. Nothing moves so nothing breaks. It is quiet. What solar panels are not is the complete package. There is more to it.

I started with one 75 watt panel mounted over the cockpit in front of the wind generator.

If you have the room for them, solar is definitely the way to augment your power supply. I love mine. The one complaint I have heard time and again is "I wish I had more space for solar. I envy you folks with room for multiple panels."

A few years back I added two more 100 watt panels atop the pilothouse.

Aboard Seaweed I had a total of 275 watts. At that time I had four 100 amp batteries. The combination was not enough to run my refrigerator 24/7 unless it was windy. Then the wind generator kicked in enough power to put me over the top on power.

I like to make more power than I use thus the batteries stay near full at all times.
According to
Calder's it is best to use the least percentage of battery amperage.


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%
Me7 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.


It is a good idea to have plenty of batteries. It is not just your solar power generation that counts. We have to have a place to store the power. That is the battery bank.


Please Note: This is a multi-part series about power for your boat's goodies. It encompasses solar power, batteries and inverters. Part Two, is next: Solar Makes Power. The complete version with all the information contained in the four articles is on this page:

Solar, Batteries and an Inverter, the complete series.


How many batteries do you have aboard your boat?
Do you prefer easy-to-digest shorter articles or colossal ones that encompass everything?

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