This article is from the Misc Bicycles FAQs, by various authors.
Part 2 - discussion of charging gel cells.
I have spent a lot of time studying the application notes from Gates and from
Yasua. There are several different ways to charge a gel-cell. They are:
I will now attempt to briefly describe each and their pros and cons:
1. This is considered to best method in terms of cycle life of the battery.
If the initial charge is low, the battery will require a large amount of
current initially until the charge is restored. As the charge is restored
the current will lower. In order to constant voltage charge, you need a
good high current regulated supply capable of the initial currents. This
can be expensive. You must also follow the manufacturers spec. sheets for
the voltage range required, and adjust for temperature. Gates and Yasua both
give slightly higher voltages for cyclic (i.e. NOT FLOAT) charging. I don't
have the specs. in front of me, but I do recall that it was about 2.4 volts
per cell plus or minus about 0.1 Volt for the battery. This gives a charging
range of 13.8-15V for a 12V (6 cells) and 6.9-7.5V for a 6V (3 cells). The
numbers given above are good. The thing to realize is that this method assumes
that the charge voltage is removed, or at least reduced to a float charge
voltage when the battery reaches full charge. Leaving it on longer can cause
the out gasing of hydrogen and shorted the cycle life of the battery.
2. This is not prefered, but is useful if you have several cells in series
that have different levels of charge. If too much current is applied for
to long, damage to the cells can occur. Constant current regulated supplies
are not very common and can be expensive. You also need to adjust for temp.
3. Taper current is the least desirable from the cycle life of the battery,
however, it is the easiest, will give good results for the amateur (such as
our applications where we have just one cell, and not trying the maximize
the use of each cell over thousands of units where the $ savings add up).
In this case, a supply, that is not neccesarily regulated, is connected to
the battery. If the initial current is more than the supply can handle, a
current limiting resistor is placed in series. As the initial current is
high, the charging voltage is low, as charge is restored, the current lowers,
and the voltage raises. If the voltage raises too high, the battery can
outgas hydrogen which lessens the life of the battery. If you attempt the
constant voltage charge, and your supply cannot maintain the voltage at the
initial high current, the voltage will lower and the charging behavior will
become a taper current model, which is ok, it just costs more than it needs to.
Most automotive chargers are for the most part taper current models and will
support your needs just fine. Realize that at 15V, you are at the very high
end of the range, and must pay attention not to leave the charger on too long.
The charge current will initially be high and remain high, then fall off
quickly. When it falls off quickly the battery is close to being recharged.
Allow the charger to remain on a little longer, then remove it. In terms of
cycle life, maybe you will get 2000 charge/discharge cycles from your batter
instead of 10000. Clearly acceptible for the needs of a bike light IMHO.
4. Float cycle is for applications where the batteries are present only for
times that the main power source is absent, such as an Uninterrupted Power
Supply (UPS) for a computer system. Float cycle assumes that you have a
very carefully regulated voltage at a low current. The voltage of the battery
is maintained such that a full charge is maintained all the time. If you
have a float charger and the cells are low, if the charger cannot handle the
initial current, once again the model degrades to a taper current, although
if the charger cannot supply much current, the recharge time will be longer,
which is acceptible for these types of applications. Float charge voltages
are lower than cyclic values, therefore, they can be present on the battery
for long periods of time.
5. A step cycle charger is one that starts the charge as a constant voltage
charger until full charge is restored, then swithes to a float cycle. It is
in a sense a specialized charger. It is more expensive than a float charger.
This type of charger is popular in applications such as camcorders where the
cycling of the battery is uneven, and the consumer wants a fast recharge, but
also is apt to leave the battery in the charger for very long periods of time.
A float charger can be thought of as behaving as a step cycle charger for
discharged cells, however, you will not receive the fast recharge
In all of this, I have come to the conclusion that taper current charging is
the easiest, cheapest, and very acceptible for the application of our bike
lighting systems. If you have an automotive charger, USE IT! The only thing
you don't want to do is to leave a charger, that is not well engineered
as a float cycle charger, connected to the battery for long periods of time
as this will shorten the life of battery. If you have access to the
specs of your particular cell, and have access to the types of regulated
supplies to achieve constant voltage, and/or step cycle charging, (and you want
to carefully monitor the charging process to absolutly positively maximize
the life of the battery), go for it, I would rather be out riding! :-)
Otherwise, it would not be worth the expense, or the time, when you can build
a taper current charger for under $10, or use your automotive charger, and
receive good results.
Some more notes:
-Any over voltage charging will cause the out gasing of
hydrogen. The gel-cells are engineered such that most of this hydrogen is
internally recombined. The manufacturers reccomend against prolonged or
severe over voltage conditions. In other words, if you don't have a float
cycle charger, don't over charge for long periods of time.
-The time to recharge is dependant on the initial state of charge, the charge
current, and the size of the battery. The more current you can deliver, the
faster you will charge. An automotive charger can help you here, however, I
have homebrew taper charger that can only supply about 1Amp, and my 12V 5AHour
battery will fully recharge in about 6 hours. I have also used my auto battery
charger with excellent results, however, my charger puts out 15V also, and I
have detected that the battery gets warm and outgases hydrogen, so I won't
leave it on the charger for much longer than it takes the current to fall.
A reasonable indication that you are abusing the battery is if it gets real
warm while you charge. You can add a current limiting resistor in series
to lower the voltage a bit, this will give you a taper current charger and
will lengthen the charge time a bit, but might be easier on your battery.
-If the battery is sufficiently depleted such that the battery voltage is low,
after it is removed from the load, there can be some recombination of the
chemistry at the poles of the cells and the voltage measured will appear back
at the fully charged state. The internal resistance is still high and when a
load is reconnected, the voltage will drop again. If you are not sure if you
battery is charged, connect it to your lights and measure the voltage.
I would highly recommend, for those who are so inclined, to contact the vendor
of your battery and see if they can send you a set of application notes.
I hope this cleared things up for you. I had the same questions and turned
to the application notes for the answers.