Author Topic: Reducing the LiIon charge termination voltage  (Read 20080 times)

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #15 on: December 12, 2018, 03:13:18 PM »
Before we do a return, do you happen to have other LiPos to test this with?
(Just for comparison sake)

Stereodude

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Re: Reducing the LiIon charge termination voltage
« Reply #16 on: December 12, 2018, 05:51:33 PM »
Before we do a return, do you happen to have other LiPos to test this with?
(Just for comparison sake)
I have a number of cylindrical Li-Ion cells of different sizes from several makers.  I'll have to see if I have any bare single cell LiPo batteries.  I have a number of 3S and 4S packs, but that doesn't help me here.

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #17 on: December 13, 2018, 09:22:21 AM »
I meant lipos with protection circuitry, the ones commonly found online.
Even batteries from cell/smart phones would work (not fakes like in the video I linked) and you'd know they have protection.
Let me know what you want to do.

Stereodude

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Re: Reducing the LiIon charge termination voltage
« Reply #18 on: December 13, 2018, 11:39:23 AM »
I meant lipos with protection circuitry, the ones commonly found online.
Even batteries from cell/smart phones would work (not fakes like in the video I linked) and you'd know they have protection.
Let me know what you want to do.
I have cylindrical Li-Ion cells with integrated protection circuits also.  However, I won't be able to measure the true battery voltage as I don't have access to the cell side.

What exactly do you want me to test and measure?

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #19 on: December 13, 2018, 03:01:06 PM »
The VBAT of the MCP73831, during and after it's charged, of a LiPo that has its own protection circuitry.
You can bet the internal voltage is lower if it has such a circuit.

Stereodude

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Re: Reducing the LiIon charge termination voltage
« Reply #20 on: December 16, 2018, 10:05:51 PM »
The VBAT of the MCP73831, during and after it's charged, of a LiPo that has its own protection circuitry.
You can bet the internal voltage is lower if it has such a circuit.
So a protected battery was 4.205V at the terminals after charging.  As soon as you flip the switch the voltage drops to 4.19V.  The protection circuit is stopping the battery from being overcharged.  The charger circuit is still trying to charge the battery.

Also, the red LED never came on while charging the protected cell and the LCD got all wonky and scrambled.

Uncle Buzz

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Re: Reducing the LiIon charge termination voltage
« Reply #21 on: December 17, 2018, 03:32:25 AM »
Since the diode of the P mosfet doesn't block current from VUSB_OR_VBAT to VBAT if it's higher than VBAT + the mosfet's Diode forward voltage , (which should not be the case with common values), maybe some current are flowing to the battery through the mosfet without be controlled by the charger MCP73831 ?

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #22 on: December 17, 2018, 07:35:27 PM »
Uncle Buzz,
You mean PMOS1, when USB is active and PMOS1 supposed to be off?
The PMOS1 body diode voltage is -0.8 with a max of -1.2. Add to that the schottky and we're above 5v already, so I don't think there is a reasonable case that 5VUSB is charging an unprotected battery unintentionally.
The other thread about inverted PMOS1 suggests this could cause a bit of extra voltage in the battery if its unprotected.
But I dont see that with any unprotected batteries, which is only what I have used.
« Last Edit: December 17, 2018, 07:39:04 PM by Felix »

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #23 on: December 17, 2018, 08:04:56 PM »
Stereodude,
Given the comments by UncleBuzz, if VUSB was much higher than 5V maybe that can explain the overcharging. Can you check with the 4.3V on the Lipo, what the VUSB is?
In any case with a protected LiPo this should never happen. Plus the protection circuit offers other protections not just overcharging, which is why I would recommend anyone to use a protected cell, more is better when it comes to protection circuits and Lithium cells.

As a "fix" for this edge case (unprotected cell, VUSB>>5V causing VUSB drainage into VBAT):
Inverting the PMOS1 should avoid this problem, or maybe using a schottky instead of the PMOS1 source->drain.
I can do either for you, if you really want to use the unprotected cell, and are willing to do the return and wait a few days.

Uncle Buzz

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Re: Reducing the LiIon charge termination voltage
« Reply #24 on: December 18, 2018, 03:38:48 AM »
The PMOS1 body diode voltage is -0.8 with a max of -1.2. Add to that the schottky and we're above 5v already
It's what I meant when I said it wouldn't be the case with common values, but is a lower body diode voltage possible on this board in particular ? higher VUSB ? It's just a possible scenario you didn't check even if it's unlikely.

0.8V (minimum body diode voltage) + 0.3V (forward shotky diode voltage, don't know the real value of yours) + 4.2V = 5.3V, so any VUSB > 5.3V will flow current into a full charged battery through PMOS1 (with those values)

But it could be less because of the forward shotky diode voltage which is not a constant value but depends of the current flowing through it, and with some mA or even µA, this value could be lower (but needs lot of time to overcharge the battery)

EDIT : for the low current through the shotky, it's possible only if you can shutdown or sleep anything powered by the mightyHat, but maybe on the same way, the body diode voltage of the PMOS could be lower with very low current ?
« Last Edit: December 18, 2018, 04:01:19 AM by Uncle Buzz »

TomWS

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Re: Reducing the LiIon charge termination voltage
« Reply #25 on: December 18, 2018, 08:05:35 AM »
Any leakage through PMOS1 will charge a battery in CV mode and, since PMOS1 body diode is forward biased, there will be some leakage, no matter what the current.  In this case, the MCP73831 is completely off and is not responsible for the increased charge.

A protected battery will prevent overcharge, but you'll still get a higher voltage at VBat (during Charging).  I would not use an unprotected battery with this circuit.

Tom

Stereodude

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Re: Reducing the LiIon charge termination voltage
« Reply #26 on: December 18, 2018, 01:12:33 PM »
Plus the protection circuit offers other protections not just overcharging, which is why I would recommend anyone to use a protected cell, more is better when it comes to protection circuits and Lithium cells.
Yes, over-discharging and overcurrent protection.  Given that you've designed the circuitry and the base sketch I would hope that you've already taken those into account.  Neither will offer thermal protection on the battery.

Also, I didn't see any mention anywhere on the MightyHat pages that a protected LiPo/Li-Ion battery was required.  Perhaps you should add that.

I can check what VUSB is on my board later when I get home, but your board has the same 4.3V open circuit voltage on the battery terminals too.  So it's not just something specific to my setup.

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #27 on: December 18, 2018, 07:43:13 PM »
Also, I didn't see any mention anywhere on the MightyHat pages that a protected LiPo/Li-Ion battery was required.  Perhaps you should add that.
Good point, I should add something. It's not really required, except until the next revision when this is fixed in the circuit. I would say it's recommended. And yes the MCP73831 is just a nice basic charger, does not include thermal protection.
5.2V+ on USB is not normal either :)

TomWS

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Re: Reducing the LiIon charge termination voltage
« Reply #28 on: December 18, 2018, 08:28:51 PM »
5.2V+ on USB is not normal either
But within USB standard spec...

Felix

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Re: Reducing the LiIon charge termination voltage
« Reply #29 on: December 18, 2018, 08:47:02 PM »
But within USB standard spec...
Quite barely. It's actually:
Limits: 5.00V+/-0.25. But this is upstream from a device. Cables drop some voltage (ball park it 50mV). So you end up with ~5.2 (or less with china cables?). Measure 5.2+ at the device and I call it out of spec.