MCP73871 IC charging profile on USB / DC / Solar Lithium Ion

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Nicola1337
 
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MCP73871 IC charging profile on USB / DC / Solar Lithium Ion

Post by Nicola1337 »

Hi,
I'm charging this li-ion battery with the breakout board in the subject and a photovoltaic panel and at some point I wanted to measure the current flowing to the battery. So I cut the positive cable to the battery and I connected in serial my tester. (Battery voltage: 3.56V)

Result:
380mA

The solar panel is a 6V a 6W photovoltaic panel. In the same sun conditions it outputs: 960mA ISC

So I checked the MCP73871 datasheet and I found a graph showing the typical charging profile.
Screenshot.png
Screenshot.png (64.87 KiB) Viewed 729 times
This shows clearly that the IC stays in the "Fast Charge Mode" for a short period. It outputs 1A for a few minutes then the current decreases over time. The voltage, instead, stays constant over time.

I found that in standard linear lipo chargers there are two stages:
- Constant current charge
- Constant voltage charge

Following this profile:
lipo battery current rating.png
lipo battery current rating.png (16.78 KiB) Viewed 729 times
So the question is: how can I charge my battery at a constant 1A with this IC? Is it actually possible? If the IC actually behave like this, charging times increase exponentially! :( Is there something I am missing?

Thank you in advance,
Nicola

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adafruit_support_bill
 
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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by adafruit_support_bill »

The charge cycle implemented by this charge controller is designed for safe (for the user) and healthy (for the LiPo cell) charging. Just pumping a constant 1A into the cell will damage the cell and could result in fire or explosion.

Constant current "fast charge" mode lasts until the cell voltage reaches Vreg (4.2v). It then transitions into constant voltage mode to 'top off' the battery. It will stay in this mode until the charge current drops to the threshold level.

See page 20 for an explanation of the charge cycle.
https://cdn-shop.adafruit.com/datasheets/MCP73871.pdf

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Nicola1337
 
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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

Ok, so this IC doesn't use "Constant current mode" + "Constant voltage mode", right? Because, in the test I made today the battery voltage was only 3.56V, so far away from VREG. And charge current was only 380mA.

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

I just discovered that plugging the board input into a 1A wall adapter and replacing my tester with charger doctor, now I get 730mA. I don't know why my tester shows another result.

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

Bill please, just to understand: I connect the battery to the charger board with a JST 2-pin cable.
To measure the current that flows into the battery is it correct to cut the positive wire and connect the multimeter in series to it?

Thank you.

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

It really looks like the current flowing to the battery decreases just because there is the multimeter connected to the circuit. Is it a normal behavior? Is my tester gone? I set the tester range to 10A

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by adafruit_support_bill »

The charger does not necessarily jump right into fast-charge mode. The charge current depends on the phase of the charge cycle and the detected battery voltage. See page 19 of the data sheet.
https://learn.adafruit.com/adafruit-neo ... e/overview

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Nicola1337
 
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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

Yes, thank you Bill, I read the operational flow algorithm and the pin description. The strange thing that I noticed is that with the multimeter placed in serial to the battery, for example, I read ~250mA. While if I disconnect the multimeter and I place the charge doctor between the wall adapter and board DCIN, the current displayed is ~500mA. If I connect both, even the current displayed on the charge doctor decreases at the same value as multimeter's.
This is an example, the values change according to the charging stage.

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by adafruit_support_bill »

There will be a small voltage drop through the multimeter. To measure current, the meter inserts a shunt resistor in series with the load. That voltage drop will affect the battery voltage perceived by the charge controller.

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

Wow! That explains everything. Thank you! What is the best way in your opinion to monitor the charge cycle speeds?

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by adafruit_support_bill »

Testing upstream of the battery is your best bet. The Charger Doctor uses a 0.05 ohm shunt resistor for current measurement, so there will be a slight voltage drop from that too. But since it is on the input side of the charge controller, it won't affect the feedback from the battery.

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by Nicola1337 »

Do you suggest to measure current upstream or downstream the Schottky diode?

Thank you

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Re: MCP73871 IC charging profile on USB / DC / Solar Lithium

Post by adafruit_support_bill »

The current should be the same on either side of the diode.

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