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diff --git a/docs/CHARGING_ALGORITHM b/docs/CHARGING_ALGORITHM new file mode 100755 index 0000000000..38a445490a --- /dev/null +++ b/docs/CHARGING_ALGORITHM @@ -0,0 +1,196 @@ +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. |