5:33 AM Sun 6 Dec 2009 GMT
|
|
'Probability of cell failure'
.
|
This is Part 3 of a multipart series by
Larry Janke
from Southeast Marine Services.
This week he examines the advantages of having fewer, and bigger batteries, rather then more, and smaller ones.
The first two reasons are obvious:
1. Reduced Maintenance:
Less inspection and watering time; maintaining proper liquid electrolyte level is critical to battery longevity and performance. Inadequate or widely differing cell electrolyte level affects specific gravity causing unequal charging and ultimately affecting the life of the battery.
This is a good place to mention Recombinator + and Hydrocaps. These catalytic vent cap replacements recombine the hydrogen and oxygen released during gassing back to pure water and returns it to the cell. In most cases watering is reduced to once or twice a year, and self discharge is reduced.
Flooded lead antimony batteries self discharge at a rate of about 1% per day. When batteries charge and begin to gas, the bubbles carry electrolyte on their surface which deposits on the top of the battery when the bubbles burst.
This electrolyte increases conductivity across the top of the batteries and accelerates self discharge. Catalytic caps contain the gas and the electrolyte, keeping the top of the case clean and reducing self discharge, not to mention cleanup.
The probability of cell failure goes up dramatically as the number of cells increases (see Figure above left)
Further, the more cells in the system the higher the probability of unequal charging just because of inherent differences in the cells them selves, for example, temperature differences caused by cell position, middle cells will be warmer.
Minor differences in internal resistance cause varying charge rates, end cells exhibit lower resistance and will charge more rapidly. Although these differences may be minor if you multiply them enough times they add up.
2. Potential for corrosion:
Fewer connections reduce the potential for corrosion, voltage drop and improper charging, as well as eliminating the cost of the cabling itself.
3. Plate Mass and Construction:
The third issue, although not so obvious, may be the most important. Positive plate mass and construction are the #1 determining factors in battery life.
Typical Golf cart plate .060 to .080 thickness
Rolls EHG Plate .150 thickness (Rolls golf cart sizes)
Rolls EKG plate .170 thickness
Rolls CS plate .260 thickness 6 OR 8 CS25PS, 12CS11PS AND 4 CS-17PS
Rolls KS plate .280 thickness 4KS-21PS AND 4KS-25PS
The downside of big batteries of course is weight. Rolls modular batteries can be disassembled into individual cells for portability with a ?' wrench and then reassembled in the boat.
Using the 6CS25P for example, the individual cells will weigh less than 105 pounds. This battery will deliver 820 amp hrs at the 20 hr rate. ( The author has a set of these batteries which are 14 years old)
The Rolls 12MD325 and 12 MD 375 and the 12HHG 325 or 12EHG 375 can be disassembled to cells weighing less than 30 pounds, and deliver 325 and 375 amp hrs respectively.
Rolls 4000 series batteries when discharged at the 20 hr rate will cycle to 50% of capacity about 1300 times and the 5000 series dual case modular batteries will cycle at least 3300 times. These figures translate into life spans up to 20 years with appropriate care.
Some comparisons between banks composed of larger and smaller batteries:
Rolls 6cs25p, 820 amp hrs =10,250 watt hrs (20 hr rate) 2 batteries, 6 cells at 12 volts, 1 interconnect cable compared to 8 golf carts, 24 cells ,10 interconnect cables or 6 l-16 size 18 cells 7 interconnect cables
Two rolls 6cs25p will utilize 5.25 cubic feet of space, have a footprint of 3.4 square feet, weigh 636 pounds and have a life of 3300 cycles to 50% of capacity.
8 golf carts will utilize 6.3 cubic feet of space have a foot print of 4 square feet, weigh 496 pounds, 62 pounds each and have a life of 325 cycles to 50% of capacity. (trjoan t-105), the rolls eigh 225 golf cart size will weigh 592 pounds, 74 pounds each and have a life of 1350 cycles to 50% of capacity.
To read the previous Parts of this article click below:
Part 1
Part 2
To make inquiries to South East Marine Services or learn more about them, click
here
.
by Larry Janke
|
Hogan Island, VIC
Other - Random Stuff
Windsurfing - Sails
Stand Up Paddle - Wave & Cruising Boards
Kitesurfing - Kites