new charging related doc
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docs/CHARGING_ALGORITHM
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CHARGING ALGORITHM
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This doc and a part of the charger implementation (especially voltage courves,
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remaining time estimation, trickle charge) is written by Uwe Freese. If you miss
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some information here, write to mail@uwe-freese.de.
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[INTRODUCTION]
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This doc describes how the charging works for the recorder. The algorithm can be
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found in firmware/powermgmt.[c|h]. Debug output is done in apps/debug_menu.c.
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Charging for the player is done by the hardware and therefore isn't implemented
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in rockbox.
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All following information is related to the recorder.
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[TECHNICAL POSSIBILITIES AJB]
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- The AJB can read the voltage of the battery (all four cells in series,
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resulting in about 5V).
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- We can switch the charging current (about 350mA, constant) on and off.
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[VOLTAGE COURVES]
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See http://www.uwe-freese.de/rockbox for some voltage courves taken while
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charging and decharging an AJB.
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These voltage courves are implemented as arrays in rockbox. We can then
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calculate how full the batteries are (in percent) after taking the actual
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voltage. Both voltage courves (charging and decharging) are used here.
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[CHARGE OVERVIEW]
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- If voltage drops under a certain value (with "deep discharge" option on the
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value is lower), charging is started.
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- If end of charge is detected, go to top off charge.
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- Make the batteries completely full. 90 minutes of top off charge (voltage
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regulation at a high value).
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- After that, do trickle charge (max. 12 hours with voltage regulation at a
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lower value).
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- When trickle charge is done and you did not disconnect or shut off your AJB by
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now, the AJB decharges normally since it reaches a low voltage and everything
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starts from the beginning.
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[NORMAL CHARGE]
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When charging is started, the charger is turned on. The batteries are charged
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with a constant current of about 350mA. The charging is stopped for three reasons:
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- the voltage goes down in a 5 min interval (delta peak, see below)
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- the voltage goes up only a little bit in an 30 min interval (is mainly constant)
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- the charging duration exceeds a maximum duration
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[DYNAMIC MAX DURATION CALCULATION]
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The max duration is calculated dynamically. The time depends on how full the
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battery is when charging is started. For a nearly full battery, the max duration
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is low, for an empty one, it is a high value. The exact formula can be found in
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the source code.
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[DELTA PEAK - WHY DOES IT WORK?]
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Delta peak means to detect that the battery voltage goes down when the
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batteries are full.
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Two facts on batteries are the reason why this works:
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- If the batteries are full, the charging current cannot charge the battery anymore.
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So the energy is absorbed by heating up the battery.
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- Each battery has a negative temperature coefficient, that means the voltage goes
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down when the temperature goes up.
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NiMH batteries have a smaller delta peak than NiCd, but is is enough for Rockbox
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to detect that the batteries are full.
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Related documents on the web:
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http://www.nimhbattery.com/nimhbattery-faq.htm questions 3 & 4
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http://www.powerpacks-uk.com/Charging%20NiMh%20Batteries.htm
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http://www.angelfire.com/electronic/hayles/charge1.html (soft start idea)
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http://www.powerstream.com/NiMH.htm (discouraging)
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http://www.panasonic.com/industrial/battery/oem/images/pdf/nimhchar.pdf
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http://www.duracell.com/oem/Pdf/others/nimh_5.pdf (discharging)
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http://www.duracell.com/oem/Pdf/others/nimh_6.pdf (charging)
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Philips TEA1102/1103/1104 PDFs available at www.philips.com.
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[TOP OFF CHARGE AND TRICKLE CHARGE]
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After a normal charge is completed, trickle charging is started. That means
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charging to keep the batteries full. While trickle charge in other (stand alone)
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chargers means charging the amount that the battery loses because of self
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decharging, here it's charging the amount the AJB consumes when it's on. That's
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because it is not possible to switch off the AJB when charging is done. It goes
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on again and then the archos firmware charger code would charge again. So we
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have trickle charge in rockbox.
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In simple words, rockbox charges about 15 seconds per minute in trickle mode. An
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AJB consumes 100 mA when it's on and the charging current is about 300mA. So
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charging 15 s and decharge 45 s will keep the batteries full.
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But the number of seconds the charger is on in trickle charge mode is also
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adjusted dynamically (between 1 and 24 sec). Rockbox tries to hold the battery
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level at 5,65 V (top off charge, that means "make the batteries completely
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full") for 90 minutes, then a level of 5,45 V. If the voltage drops below the
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wanted value, rockbox will charge one second more the next minute. If is is
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greater than this value, is will charge one second less.
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Trickle charging runs 12 hours after finishing the normal charging. That should
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be enough for charging the AJB over night and then unplug the charger sometime
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in this 12 hour trickle charge time. It is not recommended to trickle charge
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over days, that's because it is stopped after 12 hours.
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Many chargers do top off and trickle charge by feeding a constant (low) current
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to the batteries. Rockbox, as described, makes a voltage regulation. That's
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because the power consumption of the AJB changes when backlight is on/disk is
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spinning etc. and doing a voltage regulation is the simplest way to charge
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exactly the needed amount.
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There are two charge ICs I want to mention here: The Philips TEA1102 and TEA1103
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do voltage regulation for NiCd and NiMH at 1,325 V per cell. That would be 5,3 V
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for four cells, but I think 5,45 V is best for Rockbox with the maximum time of
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12 hours. Note that the voltage values are taken in the part of a minute where
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the charger is off, so the values are a little bit smaller than the actual
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average of the whole 60 seconds.
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The Philips TEA1102 top-off charge time (with 0,15 C) is one hour.
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My test results with trickle charge (battery capacities measured with an
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external charger):
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- after normal charge and top off time: 1798, 1834, 1819, 1815 mAh
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- after normal + top off + trickle charge (12h): 1784, 1748, 1738, 1752 mAh
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- charged with external charger: 1786, 1819, 1802, 1802 mAh
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Result: Trickle charge works. :)
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[REMAINING TIME ESTIMATION]
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In simple words, it is
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remaining time = remaining battery energy / power consumption of AJB
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With using the battery courves described above and a battery capacity (a
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constant in powermgmt.h), the remaining capacity is calculated. For the power
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consumption, a usual constant value is used. If the LED backlight is set to
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always on, it is also considered.
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[BATTERY LAZYNESS]
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The battery voltage gives no realistic hint to the actual charging status when
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the charger was just turned on or off.
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Assume a 50% full battery and then turn on the charger. The voltage goes up. But
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the voltage is smaller than the voltage of a 50% full battery when the charging
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started at 0%.
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To consider this (and make a better remaining time estimation possible), I
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implemented the battery lazyness array. It means (in simple words, let's assume
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the charger is turned on):
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- if the charger is just turned on, calculate the battery status (percentage)
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out of the voltage for decharging (even if we actually charge)
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- after 20 minutes of charging, calculate the battery status out of the voltage
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courve for charging
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- inside these 20 minutes, mix the voltage courves (another array in
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powermgmt.c tells how)
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[WHICH CHARGING MODE TO USE]
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(trickle charge on and off not implemented yet!)
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If you use your AJB connected to the power supply the whole time, select "deep
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discharge on" and "trickle charge off".
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If you want to charge your AJB over night and take it with you the next day,
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select "deep discharge off" (that it starts charging immediately) and "trickle
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charge on" (that the batteries remain full).
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A special case: If you fill up the batteries that are still nearly full every
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night, it is recommended that you make a complete charge cycle from time to
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time. Select "deep discharge on" and "trickle charge on" and wait till the whole
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cycle is over (you can speed up the discharging a little bit by turning on the
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LED backlight). Even if the battery sellers say NiMH cells don't show a memory
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effect, I recommend making this procedure from time to time (every 10th charging
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cycle). BUT: Don't recharge the batteries completely every time if you don't
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have to.
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