This past summer I replaced the battery in my 18ft ski boat and went
to a dual battery system with a selector switch. I had probably
shortened the life of my old battey by letting it sit discharged for
too long over the winter.

This year I'm being more carefull. Here are my thoughts on keeping
batteries charged:

Please feel free to add comments or suggestions.

1) Very important: Do not let your batteries get very low or you will
greatly reduce battery life. I have heard that regular batteries will
be affected by letting then sit below 75% charge and deep cycle should
not be left below 50% charged. Keeping them close to fully charged is
best for prolonged life.

2) Be sure wet cell batteries are full using only distilled water. You
can tell the water level is correct when you look into the cell and
the reflection of the water has a "fish eye" look to it and is not a
flat surface. (If you are curious, it is caused by the surface tension
effect when the water level touches to fill level guide)


3) Check the existing charge level

You can do this using a hydrometer. (under $5) The higher the charge
the more dense the liquid and more balls will float. 4 balls floating
= full charge

Question: How reliable are they? I can’t seam to get more that 3
balls to float (75% charge)

You can also measure the charge state using a voltmeter. Let the
battery sit for approx 3 hrs after charging before you take a reading.
You will get an inaccurate, and high, reading if you measure the
voltage at the battery right after charging it.

Question; I've seen different numbers used for full charge: 14V
13.8V 12.9V What is the best voltage to use for fully charged and
where did the number come from?


5) The max rate to charge a battery is 20% of it’s amp-Hour rating.
Example a 100 amp –hour rated battery should be charged at 20 amps
max. Some chargers automatically adjust the current and length of
charge according to the battery's state of charge and then shut off
when the battery is fully charged. If the charger requires manual
adjustment for current or shut-off, check the charger's instructions
to determine the proper current and length of charge based on your
battery's rating.

6) An automatic charger that reduces current as the battery becomes
charged is best and will give the fastest charging without damaging
the battery.

7) Do not leave a battey connected for over 24 hours to a charger that
does not automaticly shutoff even if it is a "trickle" charger. Some
trickle chargers will raise the voltage as the current drops to levels
that can damage a battery if left for extended periods.




Sources:

Charging Batteries

http://www.ibsa.com/www/faqs/tech_ta...e/charging.htm

Battery Tender

http://batterytender.com/

Kip