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Topic: MCP73871 damaging batteries  (Read 5436 times) previous topic - next topic

Joes

hi there i am using MCP73871 in one of my product but after a week or two the batteries had blow apart, i don't no if it is just that my charging current is to high (set to 2k 500mA) or something else, what do you lot think?

here is a link how i have it send up: https://raw.githubusercontent.com/adafruit/USB-DC-Solar-Lithium-Ion-Polymer-charger/master/schem.png

here is a link to the battey i am using: http://www.hobbyking.com/hobbyking/store/__23320__Turnigy_nano_tech_1200mah_1S_15C_Round_Lipo.html

jremington

I would start the hardware debugging with a multimeter or two and check voltages and currents.

JohnLincoln

That circuit seems to have a thermistor to monitor the battery temperature.


Do you have the correct type of thermistor fitted, and is it in good thermal contact with your battery?

TomGeorge

#3
Apr 10, 2015, 12:12 pm Last Edit: Apr 10, 2015, 12:14 pm by TomGeorge
Hi,
Why have you got 150K in parallel with the 10K thermistor, the data sheet for the chip only has the thermistor?
What is the number of the thermistor and is it a negative co-efficient type?

http://ww1.microchip.com/downloads/en/DeviceDoc/20002090C.pdf

Tom..... :)
PS, did you read the feedback on the HobbyKing page, below the product info?
Everything runs on smoke, let the smoke out, it stops running....

jremington

#4
Apr 10, 2015, 04:56 pm Last Edit: Apr 10, 2015, 05:41 pm by jremington
My understanding of the chip datasheet that the thermistor is there to provide fault protection, in case of battery overtemperature, and does not otherwise enter into control of the charge cycle. Charge cycle control is driven primarily by battery voltage and die temperature.

I have the MCP73871 charger from Adafruit, and it is another example of how Adafruit goes so far and then drops the ball. The thermistor in question is a 10K SMD resistor on the board, however they did include plated-through holes for a thermistor to be attached to the battery. The charger works fine for me using solar cells as the input.

So, the question is, why is the OP's charger not going into constant-voltage mode when the batteries reach the selected setpoint of 4.1 or 4.2 V. Time for some actual measurements!

Joes

hi there thanks for all your replies, this is the thermistor i am using:http://uk.rs-online.com/web/p/thermistors/7428427/?searchTerm=742-8427&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D6265724D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5C647B337D5B5C732D2F255C2E2C5D5C647B332C347D2426706F3D313426736E3D592673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D3734322D3834323726

does this look ok? and i put the 150k in as it was in the schematic would you recommend for me to remove it as it is not in the datasheet?

will get the meter out tomoz and see what i have

thanks Joe

jremington

Quote
i put the 150k in as it was in the schematic would you recommend for me to remove it as it is not in the datasheet?
As the datasheet explains, a series and/or a parallel resistor can be used in addition to the thermistor to tune the range over which the battery temperature monitor is active.

TomGeorge

HI,
Fine on the 150K, that will cause the apparent temp range to be lower so cooler batt temp range.
What are you using as a power supply?

Tom..... :)
Everything runs on smoke, let the smoke out, it stops running....

Chagrin

I'm guessing your batteries are popping when you're using a solar power source for the charging. You don't have any sort of over voltage protection when used for solar; what happens when the MCP73871 stops pulling current from the panels, the voltage from the panels increases, and the chip's abs. max of 7V Vin is exceeded?

A typical solar cell will have an open circuit voltage (voltage when no current is being drawn) about 50% higher than its nominal voltage. So if you have a "5V" panel it will be capable of reaching ~7.5V.

jremington

#9
Apr 11, 2015, 07:19 am Last Edit: Apr 11, 2015, 07:21 am by jremington
Quote
what happens when the MCP73871 stops pulling current from the panels, the voltage from the panels increases, and the chip's abs. max of 7V Vin is exceeded?
Good question! Overvoltage protection should definitely be in place, and it is missing from the Adafruit design.

However, I think you are exaggerating the likelihood of the problem.  Small panels are often, but not always, rated by the maximum power point voltage, which is typically 80% of the open circuit voltage. I use the MCP73871 with a "6V" 1W panel from Seeed Studio, with maximum power point voltage of 5.5V, 200 mA and measured open circuit voltage 6.8 V.

Joes

so we are all good with the 150k and the thermistor then

i am not powering this with solar, i am using this VTX-214-005-105 5W

so 98% of the time it is being powered by that power supply and the other time on the battery

ianroz

Beware MCP73871 limitations re excess battery voltage above 4.2.
https://www.youtube.com/watch?v=c60rans82Ug

wvmarle

LOL yet another first poster adding a comment to a necrothread. How do they even find this old stuff?!
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Southpark

#13
Jul 01, 2019, 01:27 am Last Edit: Jul 01, 2019, 01:29 am by Southpark
LOL yet another first poster adding a comment to a necrothread. How do they even find this old stuff?!
Don't know hahaha. I reckon that future forum software could have an upgrade that has a red light or something on the screen that first gets our attention, and says - first post was APR 2015 .... or something. Or have a little coloured thing in the post titles list, somewhere on the side, that has a little indicator that notifies users that the post is at least a year old, or something.

wvmarle

I have often enough run into old threads while searching for stuff.

It does have a nice read warning, latest reply >6 months or so. But well, we can't expect first posters to read that. Or the "how to use this forum" sticky. It'd get in the way of the urge of getting that useless post out.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

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