Arduino Forum

Using Arduino => General Electronics => Topic started by: Joes on Apr 09, 2015, 10:28 pm

Title: MCP73871 damaging batteries
Post by: Joes on Apr 09, 2015, 10:28 pm
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 (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 (http://www.hobbyking.com/hobbyking/store/__23320__Turnigy_nano_tech_1200mah_1S_15C_Round_Lipo.html)
Title: Re: MCP73871 damaging batteries
Post by: jremington on Apr 10, 2015, 06:13 am
I would start the hardware debugging with a multimeter or two and check voltages and currents.
Title: Re: MCP73871 damaging batteries
Post by: JohnLincoln on Apr 10, 2015, 08:19 am
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?
Title: Re: MCP73871 damaging batteries
Post by: TomGeorge on Apr 10, 2015, 12:12 pm
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?
Title: Re: MCP73871 damaging batteries
Post by: jremington on Apr 10, 2015, 04:56 pm
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!
Title: Re: MCP73871 damaging batteries
Post by: Joes on Apr 10, 2015, 10:34 pm
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 (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
Title: Re: MCP73871 damaging batteries
Post by: jremington on Apr 10, 2015, 11:23 pm
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.
Title: Re: MCP73871 damaging batteries
Post by: TomGeorge on Apr 11, 2015, 01:12 am
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..... :)
Title: Re: MCP73871 damaging batteries
Post by: Chagrin on Apr 11, 2015, 03:51 am
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.
Title: Re: MCP73871 damaging batteries
Post by: jremington on Apr 11, 2015, 07:19 am
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.
Title: Re: MCP73871 damaging batteries
Post by: Joes on Apr 11, 2015, 10:56 pm
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 (http://uk.farnell.com/vigortronix/vtx-214-005-105/ac-dc-conv-fixed-1-o-p-5w-5v/dp/2401040?mckv=su56pdwB1%7Cpcrid%7C53760420789%7Ckword%7Cvtx-214-005-105%7Cmatch%7Cp%7Cplid%7C&CMP=KNC-GUK-FUK-GEN-SKU-OTH)

so 98% of the time it is being powered by that power supply and the other time on the battery
Title: Re: MCP73871 damaging batteries
Post by: ianroz on Jun 30, 2019, 08:15 am
Beware MCP73871 limitations re excess battery voltage above 4.2.
https://www.youtube.com/watch?v=c60rans82Ug
Title: Re: MCP73871 damaging batteries
Post by: wvmarle on Jun 30, 2019, 01:13 pm
LOL yet another first poster adding a comment to a necrothread. How do they even find this old stuff?!
Title: Re: MCP73871 damaging batteries
Post by: Southpark on Jul 01, 2019, 01:27 am
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.
Title: Re: MCP73871 damaging batteries
Post by: wvmarle on Jul 01, 2019, 04:31 am
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.
Title: Re: MCP73871 damaging batteries
Post by: MarkT on Jul 01, 2019, 04:04 pm
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.
The "nominal" voltage of a PV panel is the open-circuit voltage with 1kW/square metre illumination.

The usable voltage of a PV panel is about 2/3rd of the nominal voltage.

So yes the over-voltage of the chip will be happening since it needs about 5V to operate and 6V max, whereas a PV panel usable to produce 5V at load will be more like 9V unloaded.

This charging chip is designed to work from USB, not a solar cell.  Other chips will perform that function, there are lots of specialist chips for all sorts of configurations of lithium cell charging/protection.
Title: Re: MCP73871 damaging batteries
Post by: wvmarle on Jul 01, 2019, 04:09 pm
...
It always helps reading all comments as you'd have noticed you're replying to a necrothread... from well over 4 years ago.
Title: Re: MCP73871 damaging batteries
Post by: pete_l on Nov 04, 2019, 12:10 pm
LOL yet another first poster adding a comment to a necrothread. How do they even find this old stuff?!
It doesn't matter how old the thread is, the information supplied is still useful and relevant.
As to how do people find this? It is the first search result on Google for MCP73871 and arduino