There are many standard abbreviations of lead-acid batteries, and an explanation of what these mean follows:
- Wet/Flooded
- MF
- Calcium-Calcium
- VRLA
- GEL
- AGM (Vlies)
- Hybrid
The wet or flooded battery still dominates. It
consists of all but freely suspended plates that are insulated from
each other usually with the negative plate being sealed in a small
polythene separator bag. The price factor is the main reason why this
design is so prominent. Even though a normal car battery contains
around 130 components, the production method has been refined and the
material can be relatively basic. The result is a low price battery
that offers something that is adequate to meet the needs of many
people. A Maintenance Free battery, MF, is normally a wet battery
where the design keeps gassing to a minimum and includes a battery box
that is sealed to keep the gases in place. An ample amount of acid
means the fluid can last for the entire service life of the battery
without being topped up. Development has been made in three stages.
Twenty years ago, the high antimony battery dominated the market where
the basic element antimony was added to the grid, primarily to
rationalise manufacture. The refined production method saw the
introduction of hybrid batteries with the antimony on the negative
plate being replaced by a calcium alloy which substantially reduced
gassing and thereby water consumption.
The next stage has dominated on a wide front in recent years and is called Calcium-Calcium.
This involves the antimony on both the negative and positive plates
being replaced by calcium alloy. The benefits are obvious. The fluid
loss of the battery is about 80 % lower than that of antimony batteries
and the self-discharge is lower, i.e. they can remain unused for longer
periods without losing a lot of their charge. The disadvantage is that
they are more demanding when charging if they have been
over-discharged. Efforts made in avoiding the gassing had a positive
affect, namely that the bubbles moved about in the acid causing it to
be thoroughly mixed when charging. Without these bubbles the acid can
stratify at different densities, acid weights, and is quite a common
phenomenon. An acid weight of 1.35 or more at the bottom and maybe 1.17
at the top when you are looking for an even weight of 1.28 may cause
the battery to be affected by sulphating and increased grid corrosion
despite the battery being apparently fully charged.
An entirely different way of controlling fluid loss goes under the generic name of VRLA, Valve Regulated Lead Acid.
In this case the battery box is designed as a small pressure vessel
with safety valves. If you confine the oxygen and hydrogen gases, they
can react with each other and be reformed into water. This is called
recombination and is a brilliant way of all but eliminating fluid loss.
It is not possible to confine and recombine all the gas but the loss is
reduced dramatically.
VRLA is available in two main designs, GEL or AGM (Vlies) which are constructed in totally different ways. A Gel battery
has some elements, usually silicon compounds, added to the acid to
ensure it gelatinises and thereby guarantee that no flooded acid can
leak out. The oxygen gas "drills" channels in the gel from the positive
to the negative plate where it meets the hydrogen gas and recombines
into water. They have excellent capacity but the somewhat higher
resistance in the acid means that they have limitations as starter
batteries. They can withstand over-discharging very well, i.e. when the
battery is drained down to 20% State Of Charge. Gel batteries are very robust and are often used in applications such as floor-cleaning machines and golf carts. When gel batteries are mentioned in a motorcycle context, this is often a misconception. It is more likely to be an AGM battery.
AGM, Absorbed Glass Mat, (Vlies) keeps the acid in
place by the separator paper that consists of a fibreglass mat
operating like a sponge. The capillary forces in the separator do the
trick. The batteries can be constructed with extremely thin separators
which keep the internal resistance low. This means that you can
leverage high power from a small volume which makes it ideal as a
starter battery. AGM has one drawback which is that
the amount of acid is limited. All acid must be absorbed by the
separator paper and when the small amount of acid has been converted
into lead sulphate, this signals that the "petrol tank" is empty. To
deal with this problem, AGM batteries often have a somewhat higher acid weight. This means that an AGM battery can, and often should, be charged using a voltage that is a little higher.
VRLA batteries are very sensitive to "unclean current", i.e. high current ripple, see link. For this reason, traditional transformer battery chargers should not be used.
Courtesy of CTEK