There seems to be interest in making a really large battery pack for those extended flights to Moscow and Peking. Stringing together the batteries is pretty easy, but how to charge them? And the PSP requires between 5 and 5.5 VDC for its AC adapter port, while lithium batteries are 3.6VDC. You could series two of them to make 7.2 VDC, and then use a linear regulator, but the regulator will get hot and you will lose about 25% of your battery charge to just heat.
Instead, why not make a circuit that will both charge the battery appropriately, and boost the 2.1 to 4.2 VDC native voltage for Li-ion batteries to 5.1 volts? National Semiconductor (
www.national.com) makes two chips which will do this quite efficiently. They are the LM3622 and LM2621. The datasheets for these chips are downloadable in pdf format and each has a reference design which will do just about exactly what we need.
The charging stage will take a DC input of 4.5 to 6.5 volts available from the PSP AC adapter. The datasheet has a very complete desription of how the charging chip works:
http://cache.national.com/ds/LM/LM3622.pdf. Theoretically, you should be able to parallel up as many batteries as you need. R1 and R2 were intended to be soldered-in jumpers that would allow individuals to choose their deep-discharge reconditioning strategy. (Only connect one of these, otherwise you get a short circuit.)
The output stage is based on the LM2621. The datasheet is available here:
http://cache.national.com/ds/LM/LM2621.pdf. The inductor at the input stage is turned on to charge up the inductor’s magnetic field. Then the transistor is turned off and the inductor must discharge through the diode D1 to charge up the capacitor C2. The rest of the circuit is feedback to regulate the voltage to the proper 5.1VDC. ( The reference design uses a 50kOhm resistor at Rf2, but I wanted 5.1VDC for load regulation margin, so I used 47kOhm.) This is sort of an inductive charge pump.
Of course to make this circuit properly with noise immunity, you really need to design a circuit board. This can be done by downloading free (!!) schematic capture and board layout software from
www.pcb123.com. Parts can be purchased from DigiKey or Allied or similar. You really need to have 20 or 50 or 100 boards to begin to make this cost effective though. If you are making just one or two, the setup costs for board manufacture end up killing you. What might be better is to buy a battery pack made by logic3 and dissect the thing for the charging circuit/power supply. They probably have not used this exact circuit, but I bet you it is pretty close. I bought one here:
http://www.nothingbutsoftware.com/
catalog_type_page/ProductCode/34474 for $20. I plan to tear it apart and figure out if I can make it work on a larger battery pack. I’ll let you know how it goes.
One last thing,
www.batterygeek.net is selling a 15 hour Li-ion pack for about $75, which ought to last you on a flight to anywhere in the world.
read more
and
schematics for a LI-ION battery charger/power supply