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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I am working on building the 4th revision of my GPS Tracker, and this time i am trying to integrate the batteries down onto the pcb so that there aren't any loose connections to screw me over this time...
Also, to keep things simple, I am having the mess charge itself over usb, which limits my charging voltage to 5v (ie, 1 li-ion cell)
This isn't problem for the main electronics, but I am hoping to also integrate a battery to fire the pyro loads--which really should be at least 2 or possibly 3 cells in series. Now in theory this should be downright easy, since most of the lithium ion chargers are already a buck converter, making one a boost converter one should be trivial... But i can't find any chips that let you do that
So short of having a 5v->13v boost converter followed by a buck converter based charger chip, is there an easier way to do this? I am only using small cells (probably 100ma/hr), but am very tight on pcb space. I also try to avoid leadless parts like the devil.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The first thing that comes to mind is to control the charge yourself and use a standard boost converter PSU with a current limited, constant voltage, output (I'm sure they're all over the Maxim and LinearTech sites). I've had a quick look in the Wiki, and under "battery" there is a link to batteryuniversity which gives Lion charging as constant 4.2v/cell for max charge, 4.1v/cell for max life, and terminate when the charge current has dropped to <3%. The PSU handles stage 1 (constant current) and stage 2 (constant voltage) parts of the charge, just leaving you with either checking the charge current from time to time and turning it off, or having a current measurement feature which latches the supply 'on'. I have a suggestion for the current latch if needed.
You might want to do a bit more research into Lion charging specs than my 2 minutes' worth, especially for your specific manufacturer, before you knacker your batteries with an inappropriate profile.
The Wiki could probably do with a dedicated "rechargable battery" page giving a quick summary of the various types.
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Charging a 3 cell Li-ion batter pack from a USB port
