USB LiIon/LiPoly charger -- Monitoring Stat1/2 & more ?s

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scurvyrat
 
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USB LiIon/LiPoly charger -- Monitoring Stat1/2 & more ?s

Post by scurvyrat »

I just received my USB LiIon/LiPoly charger and have it connected to one of your 2700mAh 3.7V LiPo cells.

I have hooked up Stat1 and Stat2 to two inputs on an arduino to check the state of the charger. I also setup the Battery Out to the arduino's Analog input. The Arduino is 3.3v and is not connected to the charger. Nothing is connected to the Battery Out terminals.

I can read stat1 and stat2 fine and the voltage via a voltage divider.

My questions are:
1) If I unplug the monitoring arduino, the charger switches to charge "Done." My multimeter says the battery voltage is 4.17v. If I plug the Arduino back in, the "Done" light goes out and the state switches back to "Charging." Multimeter still says 4.17v. I'm confused as to why...

2) I want to use the charger with either the 1A or 2A 6v solar panels that you sell. The battery datasheet says the standard charge is 0.2C with a max of 1C. According to the datasheet for the MCP73833, the max current is 1200mA ( < 833 Ohm resistor), so I'm at about .44C and about 2.25 hours to charge completely (with the 2700mAh battery). Is that right?

3) On the page on making a solar charger, it says "Choose from either a 1W or 2W solar panel. Either will work, if you are using the smaller panel, we suggest removing R4 and replacing it with a 6.8K (or so) resistor which will keep the lipoly charger from trying to draw too much current from the panel." So that would cut the charge rate to 147mA. The smaller panel is rated at 1A...I know that with conditions (clouds, panel temp, etc...) you'll never get the max from a panel, but why is it dangerous to set up the charger to accept a higher current -- like half the rated output or 500mA in the case of the small panel?

4) On a different thread, there was a question of if you have a solar panel/USB connected, a load (like a 3.3v Arduino), when the battery is fully charged does it drain from the battery or does it take juice directly from the USB/panel leaving the battery fully charged? Were you able to discern which is the case?

BTW, the 6v 2A panels are really nice!

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adafruit_support_bill
 
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Re: USB LiIon/LiPoly charger -- Monitoring Stat1/2 & more ?s

Post by adafruit_support_bill »

Some answers:
1) not sure.
2) sounds right.
3) if you draw too much current from the panel, the voltage will drop below the minimum needed to charge.
4) if you draw more than the panel alone can supply, you will start to drain the battery as well. Otherwise, the battery will stay topped-up.

scurvyrat
 
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Re: USB LiIon/LiPoly charger -- Monitoring Stat1/2 & more ?s

Post by scurvyrat »

I checked out the datasheet again and it states:

"If the voltage drops below the recharge threshold, another charge cycle begins and current is once again supplied to the battery or load."

So it seems that the battery will go through a continually charge/discharge cycle, even if power is applied to the charger (albeit going from Vreg down to its programmed recharge level).

My tests show that it took the battery down to 4.09V with the USB plugged and a small load on the Batt Out. @4.09V the charger kicked in again and topped the battery back up to 4.20V.

Some additional logic would need to be put in before the charger to switch over to direct power once the battery is charged.

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ktownsend
 
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Re: USB LiIon/LiPoly charger -- Monitoring Stat1/2 & more ?s

Post by ktownsend »

You could use a FET and a diode for this, but the boards are intentionally very simple and low cost. I seem to recall that there is an app note or an example in the datasheet of one way to do this, though (only drawing power from the source when it's present).

Update: See p.9 here: http://ww1.microchip.com/downloads/en/D ... 51746a.pdf

scurvyrat
 
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Re: USB LiIon/LiPoly charger -- Monitoring Stat1/2 & more ?s

Post by scurvyrat »

Thanks. That appnote recommends the FFDFMA2P857. (http://www.fairchildsemi.com/ds/FD%2FFDFMA2P857.pdf)

Another good note is: AN1149 (http://www.microchip.com/Microchip.WWW. ... 38&lang=en).

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