rockbox/docs/CHARGING_ALGORITHM

CHARGING ALGORITHM

This doc and a part of the charger implementation (especially voltage courves, 
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 is done by the hardware and therefore isn't implemented
in rockbox.

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 COURVES]

See http://www.uwe-freese.de/rockbox for some voltage courves taken while
charging and decharging an AJB.

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


[CHARGE OVERVIEW]

- If voltage drops under a certain value (with "deep discharge" option on the 
  value is 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 high value).
- After that, do trickle charge (max. 12 hours with voltage regulation at a 
  lower value).
- When trickle charge is done and you did not disconnect or shut off your AJB by 
  now, the AJB decharges normally since it reaches a low voltage and everything 
  starts from the beginning.


[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.


[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.

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 300mA. 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 (between 1 and 24 sec). 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 
wanted value, rockbox will charge one second more the next minute. If is is 
greater than this value, is will charge one second less.

Trickle charging runs 12 hours after finishing the normal charging. That should 
be enough for charging the AJB over night and then unplug the charger sometime 
in this 12 hour trickle charge time. It is not recommended to trickle charge 
over days, that's because it is stopped after 12 hours.

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 courves described above and a battery capacity (a 
constant in powermgmt.h), 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.


[BATTERY LAZYNESS]

The battery voltage gives no realistic hint to the actual charging status when 
the charger was just turned on or off.

Assume a 50% full battery and then turn on the charger. The voltage goes up. But 
the voltage is smaller than the voltage of a 50% full battery when the charging 
started at 0%.

To consider this (and make a better remaining time estimation possible), I 
implemented the battery lazyness array. It means (in simple words, let's assume 
the charger is turned on):

- if the charger is just turned on, calculate the battery status (percentage)
  out of the voltage for decharging (even if we actually charge)
- after 20 minutes of charging, calculate the battery status out of the voltage 
  courve for charging
- inside these 20 minutes, mix the voltage courves (another array in
  powermgmt.c tells how)


[WHICH CHARGING MODE TO USE]

(trickle charge on and off not implemented yet!)

If you use your AJB connected to the power supply the whole time, select "deep 
discharge on" and "trickle charge off".

If you want to charge your AJB over night and take it with you the next day, 
select "deep discharge off" (that it starts charging immediately) and "trickle 
charge on" (that the batteries remain full).

A special case: 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 on" 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.