usb lithium charger

[jonsleepy]

Hi, I bought this:
http://adafruit.com/products/259

and connected it to a big lithium battery. The description said there will be an indicator light that will turn on when charging is complete.

My questions are:

a) how does the circuit make this determination? Is there some kind of clock that starts ticking from 0 every time I plug it in? Or does it monitor the battery fill level in real-time and adjust accordingly?

b) Is it bad to plug in the battery when it still has some juice on it? (If I can only charge it for a short while, will the charger cycle through all 3 stages every time it gets plugged in)?

c) http://learn.adafruit.com/li-ion-and-li ... r-charging
refers to an earlier version of the usb charger. What type of resistor should I add, and where, to increase the charge current to 1000 mA?

d) Is 1000 mA sufficiently fast that I need to add the thermistor for safety? My battery is 6,600 mAh.

Thanks so much!



Thanks!
Jonathan

[adafruit_support_mike]

a) how does the circuit make this determination? Is there some kind of clock that starts ticking from 0 every time I plug it in? Or does it monitor the battery fill level in real-time and adjust accordingly?

Both, actually. The control chip has built-in logic that monitors the battery voltage and adjusts the current to do what's best for the battery at each level. It also has a timer that will stop the charging process if the battery goes too long without reaching the proper voltage.

The chip also has STATUS pins that control the LEDs so the user can find out what the chip thinks of its present situation.

b) Is it bad to plug in the battery when it still has some juice on it? (If I can only charge it for a short while, will the charger cycle through all 3 stages every time it gets plugged in)?

That's called a 'partial charge cycle' and opinions vary about how good those are for batteries. The balance of what I've heard says they're probably okay.

The chip may go through all three stages every time (low-current preconditioning, high-current charging, and constant-voltage final charging), but it won't follow the same pattern blindly every time. It constantly measures the battery and adjusts the charging cycle to the immediate conditions.

c) http://learn.adafruit.com/li-ion-and-li ... r-charging
refers to an earlier version of the usb charger. What type of resistor should I add, and where, to increase the charge current to 1000 mA?

You'd solder a through-hole resistor between the pads marked 'PROG' on the board. According to the datasheet for the control chip ( http://ww1.microchip.com/downloads/en/D ... 22005b.pdf ), the charge current is 1000V/Rprog (equation 4-1, page 14), so a total of 1k resistance between the pads will give you 1000mA.

Now, the photo on the product page shows a 1k surface-mount resistor already between those pads, but the text says the default current is 500mA, which would require a 2k resistor. It could be an older photo, or I could be missing something. I'll leave a final call on that to people who know the product better than me.

d) Is 1000 mA sufficiently fast that I need to add the thermistor for safety? My battery is 6,600 mAh.

You actually want to check the amount of current the battery can supply at a given time. That parameter is called 'C', and it's a good idea to use a charging current between .5C and 1C unless your battery is rated for higher charging currents.

FWIW, raising the charge current from 500mA to 1000mA probably won't make the charging time much faster. The battery will charge to 70% faster, but the topping-off stage from 70% to full charge will go slower.

Here's a page with good information about charging LiPos: http://batteryuniversity.com/learn/arti ... _batteries

[jonsleepy]

Thanks, mstone!

I looked at the version of the USB charger I received, and it does indeed have a 2K resistor in PROG. So I think Adafruit's documentation is outdated.

Thanks. In going out to buy a 1.3K, I notice that resistors come with a power rating, and most of them appear to be rated at 0.25 watts. Seems like if I'm plugging in 5v and drawing 1 amp, I'd need a 5 watt rating, so I must be thinking about this wrong?

[jonsleepy]

Looking again at this, my battery is here:

all-battery.com item number 31002

the max discharge rate (C?) is 6A, so based on your suggestion (thanks again), I think aiming for 4.5 amps of charging current would be reasonable, so I'd want a total resistance at RProg of 222 ohms, hence I should solder a 250 ohm resistor over the existing surface mount 2K resistor.

This is a lot different than their suggestion of soldering in a 1.3K resistor, and will presumably lead to much more current going in, so I just want to make sure that sounds reasonable so I don't explode the darn thing in an effort to speed the charge time?

Thanks again!
Jon

[adafruit_support_mike]

The product page for that battery pack lists a maximum charging current of 3A, so trying to charge at 4.5A would probably set the pack on fire.. it might explode if it generates hydrogen fast enough.

The control chip for the charger has a maximum rating of 1A. If you take the current-control resistance down to 220 ohms, the chip will either ignore it and give you 1A anyway (which would be good design) or burn out (which is possible).

It looks like the fastest safe charging time for that battery pack will be somewhere in the 2-1/2 to 3 hour range, and to get that you'll need a charger rated for more current than the one you have. For the hardware you already have, the best charging time you can get will be in the 6-7 hour range.