How long have you been doing this for (months)? You may have already drastically reduced the life of your battery
Battery & Charging
The Zen’s battery:
The Zen uses a 3.7v rated Lithium-Ion
rechargeable battery. A 3.7v rated Lithium-ion battery actually gets
charged to around 4.2v with a tolerance of +/- .05v. A charge of
3.7v/3.8v is actually about a 50% SoC. At 3.3v, lithium-ion batteries
have typically only utilized 70% of its 100% charge capacity.
Charging Voltage = 4.2v (4.1v)
Voltage = 3.7v (3.6v)
A note on Over-Discharging and Over-Charging:
In general, Lithium-Ion
batteries do not like to be overcharged (usually above 4.2v) or
excessively discharged (under 2.5v-3.0v). If a Lith-ion battery falls
below 1.5v, then typically you shouldn't try to recharge it at all for
"safety" concerns. A fully discharged lithium-ion battery causes the
formation of copper shunt in the cell which begins to get extremely hot
when attempting to recharge. A lithium-ion battery should not be
charged above 4.3 volts. Above 4.3 volts and the cell causes lithium
metal plating on the anode. The cathode material becomes an oxidizing
agent and loses stability and begins releases oxygen. This can cause
the battery to heat up. Lithium-ion cells should never get above 130°C
(265°F). At 150°C (302°F) the cell becomes thermally unstable and can
eventually lead to a thermal runaway. These are the safety reasons why
the low and high voltage cut-off and temperature sensing circuits are
used. If the internal temperature of the battery gets to high, the
temperature sensing circuit can initiate a mechanical pressure switch
that will permanently cut-off the current path and prevent anymore
How do you charge the Zen (Battery)?
It can be charged by
connecting it to a USB port on a computer or by using an AC Wall
Charger within the Zen's charging specs (see below).
Can I use my cell phone's AC Charger to charge my Zen?
I believe the max input
voltage of the Zen is around 5v. Most devices that can charge or run
off the USB host power can handle the max 5v of USB ports. 4.2 volts is
usually the max Lithium ion batteries are charged to. It is usually
around when the high voltage circuit is triggered and stops the
charging of the battery. I would not connect a charger to the Zen that
puts out more volts then 5v with an output current beyond 2400mA (2.4A)
to be safe.
When and how often to charge?
To be safe, I would avoid
frequent full discharges because this puts additional strain on the
battery and usually shortens its life. Partial discharges with
frequent recharges are better than one deep one. Recharging a partially
charged lithium-ion does not cause harm because there is no "memory
effect" like with older type Ni-Cd (nickel-cadmium) batteries.
Do I need to charge the Zen for hours before I use it for the first time?
With Lith-Ion batteries, the
first charge isn't any different then any other charge. There is no
technical need to fully charge it the first time you get it other then
the fact of being able to use it longer since it will have a full
charge. They are not like other types of previously used rechargeable
batteries that needed the full charge at first. Most likely by the time
you get everything all figured out and loaded onto the device, the
battery would be fully charged anyway.
Please note: I do recommend
that you at least connect the Zen to a computer while it has sufficient
charge on the battery in order to properly be detected and have its
devices drivers load properly. In
other words, don't use your new Zen for a period of time to where you
drain enough power from the battery to just come on and turn off
automatically or to the point the Zen doesn't come on at all. If you
do, then you may have detection and charging issues when you go to
connect the Zen to the computer.
How long will the battery in the Zen last?
Lithium-Ion batteries have a life span of about 300-500 discharge/charge cycles or 2-3 years from being manufactured.
Storing or not using the Zen for long periods of time:
Lithium-ion batteries (by
themselves) with no built-in voltage monitoring circuit have a really
low self-discharge rate (1-2% a month) when not being used. Having the
addition of voltage monitoring circuits will slightly increase the
discharge rate, but should not reach the point of excessively
discharging the battery to where it won't turn back on unless sitting
unused for an extended period of time and if its last state before
powering off was a already in a low voltage state. So, as long as you
aren't storing it for long periods of time at a low discharged voltage,
the battery should be ok to recharge.
Accidently left the Zen plugged into the Computer all night or for a long period of time:
I wouldn’t make a habit of
doing so. Long term usage like that could shorten the life of the
battery. Although you may be fine for a while, leaving it plugged in
occasionally all day and night, most likely won't hurt it, but leaving
it plugged in every day and night may not be a good idea. Although once
it is charged to 100%, the battery's internal high voltage cut-off
circuit should keep the battery from over-charging and the devices
charging circuit should keep applying a trickle charge when detected a
drop in charge below full. However, keeping a Lithium-ion battery
plugged in and fully charged keeps the battery's temperature elevated.
Exposure to prolonged elevated temperatures can cause capacity loss
which can then cause the battery to no longer be able to charge or hold
a charge. Plus, it may be a small chance, but there is a chance that
exposure to this condition for long periods could also increase the
chance of a circuit failure (charging, high voltage, temperature
sensing) and allow for a severe over-charge to take place to the point
of thermal runaway which has been known to cause the occasional
"exploding" battery, but in most cases just melt (and take some of the
device with it) or catch fire.
A note regarding USB and charging:
Per the USB specs
USB 1.1 = Minimum supply voltage is 4.4v
USB 2.0 = Minimum supply voltage is 4.75v
Both maximum supply voltages are 5.25v and a current of 500mA.
Before the USB bus interface
can begin charging a device (Zen), it must return a device identifier
to the hub driver (or device driver). Once the hub driver has the
returned information from the device (like power requirements,
supported transfer speeds, etc.), it can then begin charging the device
based on those retuned values. This process is called Enumeration.
Until enumeration has completed, it may not draw more than 100mA from
the bus. Once enumeration has completed, it can then begin drawing up
to the 500mA allowed by the USB bus.