Battery Voltage dropping we used MCP73841-420I/UN

Thread Starter

Prasadram@1

Joined May 29, 2021
11
We connect 2 - 3.7V battery Parallely j25 Connector, While it connected with Power suppy ON we getting 4.1V at battery power line. When the suppy is turned off the battery voltage is droping from 4.1V to 2.2V and it is varying like 1.9, 2.1, 2, 2.2V like that can we have any solution on this

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Irving

Joined Jan 30, 2016
2,067
What made you put the reverse protection diode D24 there rather than in the feed from 5v_RAW as shown in the datasheet? When power is off is 5v_RAW open circuit or pulled down to 0v? It seems to me that putting D24 there forces an imbalance between VDD and VBAT which would normally be managed by the body diode of Q2, and so the battery is being discharged through some internal route within IC6. Does IC6 get warm? To pull the voltage down that far implies some serious current draw.
 

Alec_t

Joined Sep 17, 2013
12,200
Are the batteries in good condition? A lithium battery discharged below ~3V will be damaged. A voltage of only 2.2V indicates a dead battery. Protection circuitry inside a charger IC may, for safety, prevent charging of a dead battery. You can't improperly charge/discharge lithium batteries without risking fire or explosion.
 

Irving

Joined Jan 30, 2016
2,067
Protection circuitry inside a charger IC may, for safety, prevent charging of a dead battery.
Good points Alec.

@Prasadram@1 when the power is on, how much current is flowing into the batteries along BATTERY_POWER?

The voltage at this point should never be 4.1v unless batteries are fully charged and charger is just passing a few uA. I think Alec is right, your batteries are not accepting charge, either they are dead or isolated. This charger IC should not be used with batteries with internal protection as it can't deal with the internal BMS.
 

Wolframore

Joined Jan 21, 2019
2,342
How are the batteries connected to J25. Sounds like it’s charging correctly at 4.1V. Possibly have connection issue or batteries are bad.
 

Thread Starter

Prasadram@1

Joined May 29, 2021
11
Good points Alec.

@Prasadram@1 when the power is on, how much current is flowing into the batteries along BATTERY_POWER?

The voltage at this point should never be 4.1v unless batteries are fully charged and charger is just passing a few uA. I think Alec is right, your batteries are not accepting charge, either they are dead or isolated. This charger IC should not be used with batteries with internal protection as it can't deal with the internal BMS.
The current flowing is nearly 1A
 

Thread Starter

Prasadram@1

Joined May 29, 2021
11
How are the batteries connected to J25. Sounds like it’s charging correctly at 4.1V. Possibly have a connection issue or batteries are bad.
Charging happens correctly while discharging it is not constantly discharging it is varying like 2.4V, 2V, 1.8V again increases to 2.2V like this it is fluctuating.
 

Irving

Joined Jan 30, 2016
2,067
Charging happens correctly while discharging it is not constantly discharging it is varying like 2.4V, 2V, 1.8V again increases to 2.2V like this it is fluctuating.
Can you show the discharge path on the schematic.

Does a single battery exhibit the same voltage anomaly?

If the two cells are connected to the discharge circuitry but disconnected from J25 do they exhibit the same behaviour?

Also, can you answer my question regarding the placement of D24.
 
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