Choosing Batteries - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > Contributing Authors > Gear and Maintenance Articles
 Not a Member? 


Closed Thread
 
LinkBack Thread Tools
  #1  
Old 04-07-2003
Contributing Authors
 
Join Date: Jan 2000
Posts: 35
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 0
Kevin Jeffrey is on a distinguished road
Choosing Batteries


It's hard to get to this point if the engine will not start.
Batteries are the heart of any 12-volt electrical system, and choosing the correct size of battery banks is the most important consideration in designing a proper electrical plan. Before calculating the capacity needed in your battery banks, however, you need to make sure your battery configuration is appropriate to your situation. I usually recommend a single main "house" bank and a designated starting battery for each engine on board, whether auxiliary or gen-set. With this arrangement you can track charging performance, load draw, and battery condition with a low-cost system monitor, and there's no need for switching banks to supply your house and engine starting loads. With the addition of a battery link or combiner, even the charge distribution to the house and engine-starting banks will be automatic. 

The only electrical load on a designated starting battery is the engine starter motor. Sizing a starting battery is easy: from the engine manufacturer's literature, determine the cold cranking amperage needed to reliably start the engine. Secondary house battery banks or dedicated batteries are often used for windlasses and other designated heavy loads, and sizing these banks is the same as for the main bank.

The total capacity of a house battery bank relates to two things: 


Making a simple schematic can help visualize how the system works.

 

  1. Your total electrical load drawn from that house bank, including inverter loads (but NOT loads supplied by direct AC power sources such as shore power or a gen-set ).  
  2. The reserve battery capacity you'd like to have, which is the amount of time you can live solely off battery power before needing to recharge.

 
Wet and gel-cell batteries have different charging needs, so size the alternator accordingly.
 
The reserve capacity you choose determines the time between regular engine-charging cycles. If contributions from renewable chargers such as solar cells or a wind generator are small or nonexistent, you'll have to adhere to this regular engine charging schedule once you have chosen the time interval. If renewables make a significant contribution, however, you can extend the length of time between regular engine charging or eliminate the need for it altogether. 

Rules of thumb when sizing house battery banks: 

 Your usable battery capacity is the amount of energy available when the batteries are between 50 and 90 percent of full charge. This means that your usable capacity is about 40 percent of the total capacity. Battery life is extended dramatically if you don't discharge below the 50 percent level on a regular basis. Topping up the last 10 percent of charge usually takes too long with an engine-driven charging source since charging current drops significantly during the latter stages of charging.
 Not all of the charging power that reaches the batteries actually gets stored as electrical energy—some is lost in the process. It's a good idea to make provisions for about 15 percent battery losses. In addition to these rules of thumb, I usually recommend sizing total battery capacity as if no renewable chargers were present. That way the more power renewables produce, the less you'll need to be concerned with a regular engine-charging routine. 

Example: If we assume an electrical load of 110 amp-hours per day and a one-day reserve capacity, sizing total battery capacity would be as follows: 

 110 amp-hours per day load x 1 day of reserve capacity = 110 amp-hours of usable battery capacity needed. 
 Since 110 amp-hours of usable capacity = 40 percent of total battery capacity, therefore: 110 amp-hours divided by 0.4 = 275 amp-hours of total battery capacity before losses.
 275 amp-hours of total capacity x 1.15 (accounts for 15 percent battery losses) = roughly 320 amp-hours of battery capacity required.

Increasing your electrical load or the length of your reserve capacity increases the total battery capacity required. For instance, if you wanted two days between regular engine-charging cycles, you'd need to double your total battery capacity. 

Before running out to buy your batteries, check to see if the total battery capacity you've calculated is suitable for your charging sources. To make the most efficient use of your alternator, total battery capacity of wet batteries should be at least four times the amperage delivered during bulk charging (when your alternator is producing the most current). Gel battery capacity can be two-to-three times the maximum alternator current. With 320 amp-hours of total capacity, you could have a 100-amp high-output alternator producing about 80 amps when it's warm. Larger amperage alternators require more battery capacity to keep charging current levels from dropping prematurely. 

 
These common manual switches are reliable, but battery combiners can automate the system.
 
Next, check if you have enough storage capability for your renewable chargers. You should be able to store all the charging current produced during a full day of maximum output. This is usually not a problem with solar panels, but a wind or water generator could produce upward of 200 amp-hours per day. With a 320 amp-hour battery bank you could only store 200 amp-hours if the bank was discharged to 50 percent at the beginning of the day (with 160 amp-hours to reach full charge) and you used the remaining 40 amp-hours to supply loads. If the batteries were more fully charged to begin with, you used less power during that time, or you had optimal charging conditions for several days, some of that hard won electrical power would go to waste. In this case it might be a good idea to add a bit more battery capacity if you can. 

Although weight and space are always concerns on a sailboat, having sufficient battery capacity ensures good system performance, a reliable supply of power when you need it most, and extended battery life.

Quick reply to this message
Closed Thread

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On



All times are GMT -4. The time now is 09:09 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.