Posted to DIR_UK

After a number of Q's and for some other reasons I put the following
info together regarding the use of Sealed Lead Acid batteries in dive
lights.

Storage Temperature
The rated output capacity of a battery is based on an ambient
temperature of 25C (77F). It is important to realize that any variation
from this operating temperature
can alter the performance of the battery, and shorten its expected
life. High temperature (above 30 C) reduces the battery life greatly.

DEEP Discharge
Never run a battery pack below about 10 volts as this will cause
excessive long term damage which severly affects the capacity of the
cells.  It is important that you know how long your battery pack will
run for, as using the old "turn it off when it goes yellow" is way TOO
late as this means you batteries are < 10 volts and heading into deep
discharge.  To know how long you can run your light do a controlled
burn test (see later)

Improper charging
To properly charge a SLA battery, a DC voltage higher that the open
circuit of 2.15 volts per cell (there are 6 cells ina 12v battery) is
applied to the terminals of the battery.

During constant voltage charging, the battery's current acceptance
decrease as voltage and state of charge increase. The battery is fully
charged once the current stabilizes at a low level for a few hours.

For our light use, we consider it a Cycle Applications and we neeed to
limit the initial current to 0.20 * C (where C is the nominal A.H.
capacity of the battery). Charge it until the battery voltage (under
charge) reaches 2.45 per cell at 68 degrees F (20 deg.C). Then to
ensure we have 100% cahrge we hold at 2.45 volts per cell until the
current drops to approximately 0.01 * C ampere. At this point the
battery is fully charged and needs to be either removed from the
charger or "float" charged

For "Float" charging, the charger needs to hold the voltage at 2.25 to
2.30 volts per cell continuously. When held at this voltage, the
battery will seek its own current level and maintain itself in a fully
charged condition.

Continuous over or undercharging is probably the single worst enemy of
a lead acid battery. Caution should be exercised to insure that the
charger is disconnected after cycle charging, or that the CORRECT float
voltage is applied

Because there is a chance of off-gassing hydrogen and oxygen if the
battery is overcharged, it is important to provide adequate air
circulation. ( Never charge or
discharge a battery in a hermetically sealed enclosure.)

Batteries should not be stored in a discharged state (or in a hot
place). If a battery has been discharged for some time it may not
readily take a charge. To overcome
this, leave the charger connected and the battery may eventually begin
to accept charge.

Due to the self-discharge characteristics of this type of battery, it
is imperative that they be charged after 3-6 months of storage,
otherwise permanent loss of
capacity might occur as a result of sulfation. To prolong shelf life
without charging, store batteries at 50 degrees F (10 deg. C) or less.

Burn testing
You should burn test your battery pack at least yearly in order to
establish the actual burn time of your light. Frequent divers or those
who rely on longer burn times may choose to test their light more
frequently (I do my lights every 3 months).

Ensure you have a fully charged the battery before you start.

To load the battery use your light head and immerse it in a bucket of water

Turn on the light and take a voltage reading of the battery

Record the time and voltage every ten minutes until the pack reaches 10
volts NO LOWER !!!!!!!  Notice how white the bulb still is, its not
gone yellow !!  (If you have a dead /old battery try leaving it until
it goes yellow and note the voltage - its around 7 Volts ie deep
discharge)

Note As the battery nears 10 volts, the voltage will drop rapidly, so
stay with it and watch it continously. Record the time when the battery
reaches 10 volts - this is your burn time

IMMEDIATLY Charge the battery

I found this web page http://www.batterytester.co.uk/
which claim to have a tester which tests the real cpacity of your cells
- I have never tried this unit so can't comment - if any one has the
£££ to spend let me know if they work