rockbox/docs/CHARGING_ALGORITHM

CHARGING ALGORITHM

This doc and a part of the charger implementation (especially voltage curves, 
remaining time estimation, trickle charge) is written by Uwe Freese. If you
miss some information here, write to mail@uwe-freese.de.



[INTRODUCTION]

This doc describes how the charging works for the recorder. The algorithm
can be found in firmware/powermgmt.[c|h]. Debug output is done in
apps/debug_menu.c.

Charging for the player and the FM/V2 recorder is done by the hardware and
therefore isn't implemented in rockbox. Only the functions that
calculate the battery level are also used for these models.

All following information is related to the recorder.


[TECHNICAL POSSIBILITIES AJB]

- The AJB can read the voltage of the battery (all four cells in series,
  resulting in about 5V).
- We can switch the charging current (about 350mA, constant) on and off.


[VOLTAGE CURVES]

See http://www.uwe-freese.de/hardware-projekte/rockbox/ladeverfahren.html
for some voltage curves taken while charging and decharging an AJB.

These voltage curves are implemented as arrays in rockbox. We can then 
calculate how full the batteries are (in percent) after taking the actual 
voltage. Both voltage curves (charging and decharging) are used here.


[CHARGE OVERVIEW]

- If voltage drops under a certain value (with "deep discharge" option on the 
  value is a lot lower), charging is started.
- If end of charge is detected, go to top off charge.
- Make the batteries completely full. 90 minutes of top off charge (voltage
  regulation at a higher value).
- After that, trickle charge (voltage regulation at a nominal battery value).
  The trickle charge will continue as long as the charger is plugged in (this
  is a change from the original charge algorithm).


[NORMAL CHARGE]

When charging is started, the charger is turned on. The batteries are charged
with a constant current of about 350mA. The charging is stopped for three
reasons:

- the voltage goes down in a 5 min interval (delta peak, see below)
- the voltage goes up only a little bit in an 30 min interval (is mainly
  constant)
- the charging duration exceeds a maximum duration


[DYNAMIC MAX DURATION CALCULATION]

The max duration is calculated dynamically. The time depends on how full the
battery is when charging is started. For a nearly full battery, the max
duration is low, for an empty one, it is a high value. The exact formula can
be found in the source code. The battery capacity is also considered here.


[LIION BATTERY IN FM RECORDER]

(todo)
http://www.seattlerobotics.org/encoder/200210/LiIon2.pdf


[DELTA PEAK - WHY DOES IT WORK?]

Delta peak means to detect that the battery voltage goes down when the
batteries are full.

Two facts on batteries are the reason why this works:

- If the batteries are full, the charging current cannot charge the battery
  anymore.
  So the energy is absorbed by heating up the battery.
- Each battery has a negative temperature coefficient, that means the voltage
  goes down when the temperature goes up.

NiMH batteries have a smaller delta peak than NiCd, but is is enough for
Rockbox to detect that the batteries are full (in theory :-).

Related documents on the web:

  http://www.nimhbattery.com/nimhbattery-faq.htm questions 3 & 4
  http://www.powerpacks-uk.com/Charging%20NiMh%20Batteries.htm
  http://www.angelfire.com/electronic/hayles/charge1.html (soft start idea)
  http://www.powerstream.com/NiMH.htm (discouraging)
  http://www.panasonic.com/industrial/battery/oem/images/pdf/nimhchar.pdf
  http://www.duracell.com/oem/Pdf/others/nimh_5.pdf (discharging)
  http://www.duracell.com/oem/Pdf/others/nimh_6.pdf (charging)
  Philips TEA1102/1103/1104 PDFs available at www.philips.com.


[TOP OFF CHARGE AND TRICKLE CHARGE]

After a normal charge is completed, trickle charging is started. That means 
charging to keep the batteries full. While trickle charge in other (stand
alone) chargers means charging the amount that the battery loses because of
self decharging, here it's charging the amount the AJB consumes when it's on.
That's because it is not possible to switch off the AJB when charging is done.
It goes on again and then the archos firmware charger code would charge again.
So we have trickle charge in rockbox.

In simple words, rockbox charges about 15 seconds per minute in trickle mode.
An AJB consumes 100 mA when it's on and the charging current is about 350mA.
So charging 15 s and decharge 45 s will keep the batteries full.

But the number of seconds the charger is on in trickle charge mode is
also adjusted dynamically. Rockbox tries to hold the battery level at
5,65 V (top off charge, that means "make the batteries completely full")
for 90 minutes, then a level of 5,45 V. If the voltage drops below the
desired value, rockbox will charge one second more the next minute. If
is is greater than this value, is will charge one second less.

The number of seconds the charger is on in top off and trickle charge
modes is also dependant on the charger's output voltage: if the charger
supplies less than about 10v, the current into the batteries is less and
thus the percentage on is increased to maintain the proper current into
the batteries.

The original recharging algorithm stopped trickle charging after 12 hours,
at which time the battery would be discharged until the the batteries
fell below the "start charging" level.  At that time the charge cycle
would be repeated.

The time limit was removed by Jerry Van Baren (along with other changes)
in the February, 2005 timeframe.  The rationale for this is that the
trickle charge level is very low.  In addition, it is disconcerting to
have a AJR plugged in and "recharged" only to find out that the battery
is only 86% full.  This was giving the Rockbox recharging algorithm a
bad name and frustrating our users.

Many chargers do top off and trickle charge by feeding a constant (low)
current to the batteries. Rockbox, as described, makes a voltage regulation.
That's because the power consumption of the AJB changes when backlight is
on/disk is spinning etc. and doing a voltage regulation is the simplest way
to charge exactly the needed amount.

There are two charge ICs I want to mention here: The Philips TEA1102 and
TEA1103 do voltage regulation for NiCd and NiMH at 1,325 V per cell. That
would be 5,3 V for four cells, but I think 5,45 V is best for Rockbox with the
maximum time of 12 hours.
Note that the voltage values are taken in the part of a minute where 
the charger is off, so the values are a little bit smaller than the actual 
average of the whole 60 seconds.
The Philips TEA1102 top-off charge time (with 0,15 C) is one hour.

My test results with trickle charge (battery capacities measured with an 
external charger):

- after normal charge and top off time: 1798, 1834, 1819, 1815 mAh
- after normal + top off + trickle charge (12h): 1784, 1748, 1738, 1752 mAh
- charged with external charger: 1786, 1819, 1802, 1802 mAh

Result: Trickle charge works. :)


[REMAINING TIME ESTIMATION]

In simple words, it is

remaining time = remaining battery energy / power consumption of AJB

With using the battery curves described above and the battery capacity you 
selected in the settings menu, the remaining capacity is calculated. For the 
power consumption, a usual constant value is used. If the LED backlight is set 
to always on, it is also considered. Having a modified Jukebox with 8 MB of
RAM leads to about 22 percent longer estimated running time.


[BATTERY DISPLAY HOW THE USER EXPECTS IT]

To not confuse the user with the shown battery level, some tricks are used in 
the battery level calculation (this does not affect the charging algorithm, 
because it uses the raw voltages):

- if charging is completed, top-off charge or trickle charge is running,
  always set the battery level to 100%
- the battery level is only allowed to change 1% per minute (exception: when
  usb is connected, it is allowed to go 3% down/min)
- after turning on the device, add another 5% to the battery level, because
  the drive is used heavily when booting and the voltage usually gets a
  little higher after that (rebounds)


[WHICH CHARGING MODE TO USE]

Jerry Van Baren's revised recommendation: Select "deep discharge OFF"
and "trickle charge ON".  This will keep your batteries charged up and
IMHO will not damage them.

Original recommendation:

A special case: If you use your AJR connected to the power supply all
the time or if you fill up the batteries that are still nearly full every
night, it is recommended that you make a complete charge cycle from time to 
time. Select "deep discharge ON" and "trickle charge OFF" and wait till the
whole cycle is over (you can speed up the discharging a little bit by turning
on the LED backlight). Even if the battery sellers say NiMH cells don't show a
memory effect, I recommend making this procedure from time to time (every 10th
charging cycle). BUT: Don't recharge the batteries completely every time if
you don't have to.