Reducing the LiIon charge termination voltage

[Stereodude]

I have a MightyHat R3 and it charges a Li-Ion cell to just short of 4.3V.  I measured the resting voltage of the 18650 I connected to it a few days ago at 4.28V.  The battery was just under 3.8V when I connected it.  Charging to this level is a bad practice for several reasons.  1) Most Li-Ion / LiPo cells should only be charged to 4.2V.  They're not supposed to breakdown and have catastrophic issues below 4.3V, but that's the maximum voltage, not the working limit.  2) Cell life will be degraded by leaving the voltage of the cell so high.

The practical limit should be 4.2V and frankly 4.15V would be better considering cell life.  I measured the voltage at 4.2V when the charging light went out, but didn't realize it would continue to trickle charge the cell even further rather than stopping.  I will not be using the charging circuit feature any further until I dig into the datasheet and determine how to modify the charging circuit for better behavior.

[Felix]

It's a MCP73831 4.2V charging voltage controller so it should not go above 4.2.
In my own tests, after the charge-LED goes off, the vbat is slightly less than 4.2 (usually 4.15 or so). I use this charger on all my LiPo powered boards.
So this is unusual. I can take a look at it if you want me to, I can replace the MCP with a new one if I find it's defective in some way. If there's no other hardware mistake or fault then swapping the charger chip would be the fastest route to see if there's a difference.
Or you can let me know what you find further and we can go from there.

[Stereodude]

It's a MCP73831 4.2V charging voltage controller so it should not go above 4.2.
In my own tests, after the charge-LED goes off, the vbat is slightly less than 4.2 (usually 4.15 or so). I use this charger on all my LiPo powered boards.
So this is unusual. I can take a look at it if you want me to, I can replace the MCP with a new one if I find it's defective in some way. If there's no other hardware mistake or fault then swapping the charger chip would be the fastest route to see if there's a difference.
Or you can let me know what you find further and we can go from there.

What is the open circuit voltage on the charger if you don't have a battery connected?  The red charge LED flickers on mine with no battery connected and I measure 4.29V.

Edit: I see from the datasheet for the MCP73831 that there is no way to trim the charge voltage.  I will check it again today with another DMM or two.

Edit2: I checked the open circuit voltage from the charging circuit at the leads on the board to the connector with 3 multimeters.  I got readings between 4.285V and 4.295V.

[Felix]

With USB connected:

- With no battery connected, it is 4.3 as you say
- With battery connected, it reads the battery voltage, and slowly charges up to < 4.2

I just put 2 types of batteries (a samsung smartphone battery and a lipo chinese generic battery) to charge, will monitor them for this and come back with results once charged LED turns off.

I also checked all my orders this past year and in all cases it was one of these 2 chargers, both "4.2V" rated:
https://www.mouser.com/ProductDetail/579-MCP73831T-2ACIOT
https://www.digikey.com/product-detail/en/microchip-technology/MCP73831T-2ACI-OT/MCP73831T-2ACI-OTCT-ND/1979802

[Stereodude]

With USB connected:

- With no battery connected, it is 4.3 as you say
- With battery connected, it reads the battery voltage, and slowly charges up to < 4.2

I just put 2 types of batteries (a samsung smartphone battery and a lipo chinese generic battery) to charge, will monitor them for this and come back with results once charged LED turns off.

I'm really interested in what the battery voltage is in the hours / days after the charge light turns off.  After the LED turned off my 18650 cell was 4.202V.  About 36 hours later it was 4.28V.

[Stereodude]

I decided to retest.  I connected up the same Panasonic/Sanyo 18650 cell, now at ~4.15V after discharging it some with a flashlight.  The red charging LED immediately turned off.  However, I noticed the voltage at the battery connector on the MightyHat is 4.178V and the voltage at the battery itself is 4.158V.  I realize it's only a difference of .02V, but that would strongly suggest that current is still flowing into the battery charging it.  I will check the voltage at both points again today after work.

Edit:  After a few minutes the red charge LED turned back on.

[Felix]

I think it may depend on the battery too and how it behaves (quality?).
I have not seen evidence that the charger is charging to a higher voltage than designed.
It is perfectly normal that the voltage is >4.2 during charging, that means charger is in constant current stage and current flows INTO the battery.

[TomWS]

Given that the voltage drifts slightly above 4.2V without the battery installed, it may be that the protection circuit on the battery is simply 'opening' up to prevent overcharge of the battery, allowing the voltage at the battery connector to rise above spec.  This could happen if the battery is fully charged.

If you have a LiPo battery where you can get to the actual battery terminals, this would be one way to check to see what is actually happening in this case. 

I'd be surprised if these battery managers overcharge.  The manufacturer (Microchip) is a good one and the Vreg spec is really tight (4.232 volts max).

Tom

[Stereodude]

I think it may depend on the battery too and how it behaves (quality?).

I'm using a very high quality 18650 cell.  It's a Sanyo/Panasonic NCR18650GA 3500mAh cell.

It is perfectly normal that the voltage is >4.2 during charging, that means charger is in constant current stage and current flows INTO the battery.

No it's not.  A proper Li-Ion charger should switch from CC to CV before the voltage exceeds 4.2V.

If you have a LiPo battery where you can get to the actual battery terminals, this would be one way to check to see what is actually happening in this case.

I'm charging a bare unprotected 18650 cell in a 18650 battery holder.  There is no protection circuit.  I have access to the actual battery terminals.  The voltage at the battery's terminals is over 4.2V.

I'd be surprised if these battery managers overcharge.  The manufacturer (Microchip) is a good one and the Vreg spec is really tight (4.232 volts max).

So would I, but the measurements seem to speak for themselves.

[Felix]

I'm using a very high quality 18650 cell.  It's a Sanyo/Panasonic NCR18650GA 3500mAh cell.
So would I, but the measurements seem to speak for themselves.

I dont know what else to do on my end, everything looks good. I had several lipos charge since yesterday. They settle around 4.15 after led shuts off, USB plugged or not.

If you want me to investigate more, you can send me the board you have, along with the LiPos you're testing. I will try my batteries with the board, then try your lipos with that board and other boards with the same charger.

Otherwise, you may want to bring this up with Microchip.

[Stereodude]

I dont know what else to do on my end, everything looks good. I had several lipos charge since yesterday. They settle around 4.15 after led shuts off, USB plugged or not.

If you want me to investigate more, you can send me the board you have, along with the LiPos you're testing. I will try my
batteries with the board, then try your lipos with that board and other boards with the same charger.

Well, I'm repeating the test today.  I will let you know the outcome.  Depending on the results we can look at next steps.

Note, I am not using a Lithium Polymer battery.  I am using a cylindrical Lithium-Ion cell.  However, they charge the same way and are interchangeable.

Edit: The battery was at 4.281V 12 hours after completing.  I'm in the process of running it back down to a lower voltage.  I guess I'm interested in getting you the board and the battery to test since I won't use this as is.

[Felix]

Check out this interesting video of some cells with/without protection circuitry.
Although the datasheet specifies a typical 4.2V regulated voltage for the MCP7383X-2 with a MAX of 4.232V , maybe the protection circuitry (or lack of it) has to do with this.

[TomWS]

Check out this interesting video of some cells with/without protection circuitry.

I'm not sure what this video adds to the discussion.   I think it's worthwhile checking the charger circuit with a discrete protection circuit connected to a bare battery so that voltage at the battery terminals and charger terminals as well as current flow can be simultaneously monitored.

Tom

[Felix]

I'm not sure what this video adds to the discussion.

My thought was maybe the MCP73831 assumes there is a battery protection circuit that drops some voltage. (since I cant find anything in the DS). But that seems to contradict the regulated voltage spec. Otherwise I have no clue how to explain the 4.3v vs 4.2 claimed.

[Stereodude]

My thought was maybe the MCP73831 assumes there is a battery protection circuit that drops some voltage. (since I cant find anything in the DS). But that seems to contradict the regulated voltage spec. Otherwise I have no clue how to explain the 4.3v vs 4.2 claimed.

That doesn't really make sense to try to account for that because the voltage drop would be variable.  The less current you're pushing or pulling into the battery, the less voltage drop it would have.  By the time you're in the CV stage and at 10% or less of the charging current about to switch off the charger, the voltage drop would be quite small.

Do you want me to send you a PM to get your address or how do you want to handle my sending the mightyhat + battery to you.