Question about the 390 Lipo charger

[ShirleyD]

Hi,

When i was using the 390 Lipo battery charger (with the MCP73871 chip) to charge a Lipoly battery (3.7V, 840mAh) with a solar panel (5V, 7W), something weird happened:
When i plugged the solar panel into the USB port of the charger, which was connected to the battery, the "PWR GOOD" and "CHRG" LEDs started blinking, and when i unplugged the solar panel, the "CHG" LED remained stably on for about 20sec instead of going off immediately.

Can someone tell me why this happened?

Thanks
Shirley

[john444]

Shirley,

Did you install the 4,700uF cap?
If you did, then re-check the solder connections.
The power & charge status LEDs will do that without the cap.

John

[ShirleyD]

Shirley,

Did you install the 4,700uF cap?
If you did, then re-check the solder connections.
The power & charge status LEDs will do that without the cap.

John

Hi John, i did solder the capacitor onto the board. Tried it again today and somehow the LEDs are just fine, not blinking anymore. However, i had a new problem with the "CHRG" LED. I measured the voltage at the battery terminals while it was being charged, and when the voltage hit 3.7V i expected "CHRG" to turn off and "DONE" to turn on, however it didnt happen and the yellow "CHRG" LED just remains on and the battery voltage doesnt go any higher. Does that mean it is still trying to charge the battery even though it is fully charged? How can i solve this problem?

Thanks
Shirley

[john444]

Hi Shirley,

A couple of things here...
You might post a clear photo and we can check your soldering & connections.
I only say that because the vast majority of issues are due to poor soldering.
Also,

Tried it again today and somehow the LEDs are just fine, not blinking anymore.

the LEDs will blink when the input current is low.
If I remember correctly, the blink rate was about 1/sec with 1mA into the charger.

when the voltage hit 3.7V i expected "CHRG" to turn off and "DONE" to turn on

The "DONE" LED will not come on until 4.1~4.2V.
It sounds as if the cell is still charging.
Can you use your meter to measure the current into the 390?
It will take a long time to charge when the input current is low.
It is quite possible that you are not getting much current from the panel - especially if it is inside out of direct sunlight. Also, the 390 was designed to operate from 6V panels. It is even possible that your 5V panel does not have enough headroom to completely charge the LiPo to 4.2V, especially if it is starved for sunlight.
Measuring the input current will tell us a lot.

John

[ShirleyD]

Sure, i will upload a pic tomorrow. But i just did another experiment, i the blinking stopped whenever i held the solar panel closer to the light. But the voltage at DCIN remained unchanged. So i am guess is it just an increase in the current but no change in voltage? ( I dont understand why though...)

The "DONE" LED will not come on until 4.1~4.2V.
It sounds as if the cell is still charging.
Can you use your meter to measure the current into the 390?
John

The battery i have is a polymer lithium ion (3.7V, 850mAh).
What i dont understand here is that the battery voltage will never be 4.2V, so does that mean the charger i have is not suitable for my battery? (But it says in the datasheet of 390 that it can be used for LiPoly/Li-ion 3.7/4.2V batteries)

Also i did a small experiment with the pass-through on the 390 board. I connected a LED to the load terminals on the board. But it surprised me that the LED didnt light up. Can you tell me why?

Thanks
Shirley

[ShirleyD]

O and also... Can i put a dc/dc converter between the charger and the solar panel to boost up the voltage to 6V? Do you think that would work?
And how do i measure the current that my solar panel puts out? (I would need to put my meter in series between the solar panel and one terminal of DCIN, to let current flow through the meter to measure. But its a USB port, how do i do that?

[adafruit_support_bill]

The battery i have is a polymer lithium ion (3.7V, 850mAh).

That is the 'nominal' battery voltage. When fully charged it will be about 4.2v. When fully discharged it will be about 3.0v. For most of its useful life, it will put out about 3.7v.

the LED didnt light up. Can you tell me why?

How did you have it connected?

Can i put a dc/dc converter between the charger and the solar panel to boost up the voltage to 6V? Do you think that would work?

That won't help. A voltage booster will increase the voltage, but decrease the current. The problem is not just a lack of voltage, it is a lack of power (voltage times current). If your solar panel is not producing enough power, it can't provide sufficient current at a high enough voltage to charge the battery.

And how do i measure the current that my solar panel puts out? (I would need to put my meter in series between the solar panel and one terminal of DCIN, to let current flow through the meter to measure. But its a USB port, how do i do that?

You would need to cut one of the wires coming from the solar panel to the USB plug and insert your meter there.

[john444]

Shirley,

the blinking stopped whenever i held the solar panel closer to the light. But the voltage at DCIN remained unchanged. So i am guess is it just an increase in the current but no change in voltage?

That is exactly what is happening. The 390 is taking as much current as the solar-panel will put out while not letting the voltage collapse below the charging voltage. the 390 is a pretty smart charger.

What i dont understand here is that the battery voltage will never be 4.2V

No. You are mistaken here. The battery voltage will reach 4.2V when it is fully charged then drop back to 3.7V when the charger shuts off. As Adafruit said, your battery will spend most of it's life at about 3.7V.

I connected a LED to the load terminals on the board. But it surprised me that the LED didnt light up.

Not sure about this one. Some white LEDs need over 3V to work. If you use a red LED (~1.5Vf) then you should use a resistor of about 200-ohm to prevent blowing it. Otherwise, LEDs only work with one polarity. The longer lead goes to +.

Can i put a dc/dc converter between the charger and the solar panel to boost up the voltage to 6V? Do you think that would work?

No, if you are not getting enough power to run the 390 with a solid charging light, then you don't have enough power to run a boost-ckt.

how do i measure the current that my solar panel puts out?

I would do what Adafruit Support said, cut one of the panel wires and connect your mA meter across them.

Sounds as if this thing is coming along.
Good Luck, John