| 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. |
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| |
| |
| [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 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. 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. |
| |
| 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 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) |
| - if charging just started (or stopped), ignore the battery voltage for the |
| first 25 minutes |
| - 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 |
| |
| |
| [WHICH CHARGING MODE TO USE] |
| |
| 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. |