Kindle 3 Battery Usage and Charging

Basics about the Amazon Kindle 3 battery and how to charge it.

Out of  the box, the Kindle is equipped with two  ways of charging – one the supplied charger, the other is to use your computers  USB port as the charger.    The USB port on the Kindle is one of the newer miniature narrower types (micro B),  standard on many cell phones and blue-tooth devices.   Both of these methods work great.  I  don’t recommend trying any other charges unless for some reason, circumstances force you to take a chance on a third party charger.

The Kindle 3 Battery

The Kindle 3 has a 3.7 volt, 1750 mah Lithium Polymer battery for a fairly hefty battery capacity.  The battery is made in China,  which manufactures much of the industrial worlds batteries. The battery is likely to be a fairly generic unit selected for size and capacity.   Charging techniques are well established for LiPo battery chemistries and is basically the same as for Lithium Ion.  The key is control of the current used in charging, and to monitor and control the voltage applied to the battery.   A typical charge curve for a generic 3.6 V LiPoly device employs an initial  850-900 mah constant current charge (CC) for the main charge phase until the voltage climbs to a maximum 4.2V, followed by regulated constant voltage of 4.2V (CV) while  the charge  current drops to approaching zero.  Deviating from this strict regime invites the  destruction of the battery.  All consumer charging circuits are designed to protect the battery while charging it.  By convention, LiPo batteries can be charged at a maximum charge current of 1C (where C is the capacity of the battery) and most commercial chargers provided by the manufacturer  provide less than this 1C charge which, for a 1750 mah battery,  would be 1.7A (amps).    However, if you want the battery to have a long life,  the charge rate should  be less than 1C

The Kindle 3 comes with a  compact 4.9v 850ma charger with a  USB socket for the charge cord that doubles as the USB link to the computer and enables  the Kindle to charge while attached to a  computer USB port.    The computer’s USB 2 port supplies 500 ma,  a lower charge rate than the included charger. Higher charge currents charge faster, so expect the wall charge to charge faster.  850 ma is about 0.5c  rate for the battery so  a full charge is mathematically about 2 hours,  but because of the charge algorithm, charging will take longer for a full charge.   When the battery approaches 4.2V a “smart” charger  switches from a constant current to a constant voltage (CV) for the remainder of the charge while the current drops as the battery approaches a full charge.  For the USB port, the calculated full charge time is more like 3.5 hours, again with a large fudge factor.

I don’t think I’ve ever charged my Kindle as long as 2 hours using any charger.  That’s probably because I don’t let the battery level drop too low.  It’s best to top off the charge whenever you get the chance with Li rechargeables and never let the charge get to the point that the device cuts off the power.

The charge circuit is built into the Kindle.  It is possible the charger is relatively “dumb” (I don’t know and haven’t tried to test it), but the job of even a dumb charger is to  stop the charge as the charging voltage reaches 4.1V.    If the charger did not follow up with a constant voltage charge at 4.1 v, the result  would be a reduction in run-time, but a likely gain in overall battery life, which is fine for me since the run times are already some of the longest you’d see in an e-reader. Otherwise the charge goes on longer and the run time approaches the maximum possible.  Either way works for me but a smart CC/CV charger shows a better engineering effort.  Hopefully I’ll find out what the deal is with the Kindle and get a chance to update this info.

How about using a cell phone (or other device) charger?

While I don’t recommend it,  there are situations where this would be convenient.  I’ve read several posts on the web of people who have used cell phone chargers on the Kindle with no issues.  The problem is that there is a very wide variety of chargers out there, some of which may be incompatible with the Kindle.  Here is some information that I hope reduces the risk to your Kindle if you.  Beware, unless you know the charger and what it is designed for, it is not possible to know what will work and what won’t with 100% certainty.  It’s your call!

Make sure the output voltage is  4.9-5.0v DC  (the voltage “V” will be stamped on the charger).   I don’t know whether the Kindle can handle outputs above 5.0V at this time, so I would avoid higher voltages as unknown and possibly unsafe.  If anyone has tried it let me know.   It is safest to use chargers that are designed for miniature LiPo/Li-Ion batteries in personal electronic devices.  These have the best chance of sharing the same level of technology with the Kindle.  The USB plug must fit.    Most cell phones use with lower capacities than the Kindles so using those chargers should be OK  unless the charger has a higher voltage than 5v or a charge rate exceeding 1750 ma at the extreme end.   Something between 500 ma and 850 ma is optimal.   Most of the time,  the current supplied by the charger to be lower than the Kindles charger,  resulting in a longer charge time.  This is because most cell phone batteries are not as beefy as the Kindle’s as far as total capacity.

More comments on generic, after market chargers, or a charger you have laying around the house. Assume that using any third party charger  not recommended by Amazon will void your warranty.  Common brick chargers  used for cell phones rated at 4.9V – 5.0V and 500 MA or less are probably safe.  Don’ts: If the rating exceeds 1750 ma (or 1.7 amps), don’t use the charger.  Any  charger that puts out AC and not DC is definitely incompatible – this is rare but not impossible with very specialized chargers.  Don’t use chargers designed for printers, dust busters, phones or anything but miniature Li-Ion/Li-Poly devices manufactured recently.   A hobby charger, a specialized charger, or a “smart” charger may be incompatible with the Kindle.  Don’t use these specialized chargers.    If you don’t know what a charger was designed for, don’t use it.

A few charging guidelines.  This would apply to all chargers including the OEM Kindle charger. As always, let common sense and temperatures be the guide in charging electronics.  If the device  ever feels hot while charging,  stop.  Hot is seldom a good thing (unless you race RC cars, in which case safety, economy, and all else is sacrificed for speed).  120F degrees  for anything but RC car batteries is  hot.   Warm… well charging is a warm activity. My Kindle on it’s original factory charger seems to charge without getting warm except possibly at the very end of the charge when the light turns from amber to  green.  The brick itself is warm throughout the charge and cools toward the end.

Run times.   Naturally, the longer the better for convenience.  Some things that affect run time include the following:

• listening to music (reduces run time to a few hours – too bad…)
• wifi/network turned on
• indexing new books (internal processing that occurs soon after downloading new books)
• turning pages

Most rechargeable devices  batteries will survive more charging cycles if they are never fully depleted and never fully charged to their rated capacity.

Some internet research brings up a few factoids regarding the internal components of the Kindle 3.

– 3.7V, 1750mAh (6.5Wh) Li-ion battery . Type number 170-1032-00 model number GP-S10-346392-0100

– AnyDATA DTP-600W HSPA mini PCI-E module
– Freescale ARM 11-based i.MX353 multimedia applications processor. Part number: MCIMX353DJQ5C
– Samsung K4X2G323PB-8GC3 DRAM
– Samsung 4GB moviNand storage chip — KLM4G1DEHM-B101
– EPSON KCRE7000 F10203TYV E-Ink display controller
– Atheros AR6102 ROCm WLAN chip (AR6102G-BM20)
– Wolfson Microelectronics WM8960G stereo codec

Refences:

• Amazons Official Page on Charging the Kindle 3
• Battery Bob’s Kindle Battery Replacement
• IBT Kindle battery information
• Link for a new battery
• You tube video of battery installation
• Kindle 3 Teardown Photos

Disclaimer.

When it comes to  using the information found here and especially trying out third party chargers, please understand that the reader  assumes all risk and liability.