Profiling Discharge of LiPo Batteries

Table of contents

RoadTest: Test and Review the Nordic nPM Fuel Gauge Board

Author: ralphjy

Creation date:

Evaluation Type: Evaluation Boards

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?: Maxim MAX17261XEVKIT, TI BQ27545EVM, STM STEVAL-ISB017V1

What were the biggest problems encountered?: The biggest problem encountered was the long time duration required to profile a battery and generate a model. A complete model (which I did not generate) would require 6+ days. I should note that I was aware of the time duration required prior to the roadtest as it is documented.

Detailed Review:

Overview

The Nordic nPM Fuel Gauge Board is a programmable constant current sink device that can be used to characterize power sources.  It is specifically intended to be used with an nPM1300 PMIC Evaluation Kit to develop battery profiles that are used in fuel gauge applications for Nordic Design kits.  There was a prior roadtest for the Nordic nPM1300 Power Management IC Eval Kit last year.  My roadtest will focus on generating and evaluating battery models using the nPM PowerUP application that is included in the nRF Connect for Desktop software.

Unboxing

The parts for the roadtest were very efficiently packed, the typical sturdy oversized box from E14 - this time via Fedex from the UK.  Inside the abundant brown paper packing material were two small product boxes with just the boards in anti-static bags.

 

Here are the front and back views of the two boards connected together.  Nice rubber feet on the back for support.

The nPM1300-EK is rev 1.1.0, PCA10152, 2024.17

The nPM Fuel Gauge is rev 1.0.0, PCA63563, 2024.8

Getting Started

First step is to install nRF Connect for Desktop on your computer, I'm using a Windows 10 PC.  I already had the software installed since I use quite a few Nordic apps, but I did upgrade it to the latest version and installed nPM PowerUP.

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Next the two boards are plugged together. 

The nPM1300-EK requires 2 USB-C connections, a power source to the "USB-PMIC" connector that supplies the VBUS input to the SYSREG circuit that produces VSYS which supplies the battery charger and the external Fuel Gauge load and a data cable from the host PC to the "nPM Controller" connector to power the nRF5340 which is used as the nPM Controller and to connect to the nPM PowerUP app on the host PC.

The final connection is to the battery under test.  There are two 1.0 mm JST battery connectors, a 2 pin for batteries without an NTC thermistor and a 3 pin for those with one.  When using a battery without a thermistor a jumper is required between the NTC and 10k ohm header pins for the onboard thermistor.  Unfortunately that jumper is not provided.  There are also VBAT and GND header pins that can be used to connect the battery without using a JST connector.  I thought that the 1.0mm JST SH connectors were an odd choice since most of the smaller Lithium batteries that I have previously used had a 2.0 mm JST PH connector.

 

Connector diagram:

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Here is my initial setup.  I am using a battery that has a thermistor and a 3 pin JST SH 1.0 connector.

 

On startup the app requires that you select the device to connect (currently only compatible with the nPM1300 Evaluation kit).

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Selecting the EV Kit indicates that the firmware is out of date and requires updating which occurs automatically when you select the Program button.

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After reboot the Fuel Gauge and Battery Status appear on the Dashboard but it is showing a reading based on a default 425 mAh battery model that is preloaded in the PowerUP app.  Since the 600 mAh battery that I am using does not have a model bundled with the app, I'll need to generate one before the Fuel Gauge will have accurate readings.

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Here is a list of the bundled battery models (profiles).  I'm not sure why these specific batteries were selected but the list covers a wide range of LiPo battery capacities.  LIPOL is a Chinese battery manufacturer and RENATA is a Swiss battery manufacturer.  I found that a US distributor, Mouser, sells Renata batteries in single quantities so I'm going to try testing one of their pre-profiled batteries in this roadtest.

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Another thing to note is that the EV Kit showed up as a USB drive on the PC file system.  It had one file on it - a README.txt.

This allows for updating the firmware without using the nRF Programmer application.

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Before moving on to generating a battery model (profiling) I just played around with the PowerUP app a bit.  I used a 165 mAh Renata battery with an existing model.  The app has the capability to have 4 active battery profiles (the default plus 3 available slots that can be loaded).  Later I plan to regenerate a model for this battery and compare it to the existing model.  But for now I'll just use it to get familiar with the tool.

First you need to load the model into an available slot.

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Then the Fuel Gauge and Battery charger are available in the app.

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I was a little surprised to see that the wrong terminal voltage and charging current were loaded in the charger settings.  I suspect that when the model is loaded that the profiling configuration is not loaded.  This appears to be a bug.  The terminal voltage should be 4.2V and the charging current should be 1C (165 mA).

Here is the battery spec

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So the battery started charging slowly at 32 mA and would only have charged to 3.6 V.  There is a nice graph panel in the app that shows the battery charging parameters.  This particular Renata battery does not have a thermistor so those temperature values are from a thermistor onboard the nPM1300 EV board.

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I then updated the terminal voltage and attempted to set the charge current to max (165 mA) but the tool would not allow me to set it to above 82 mA (1/2 C).  I'm not sure where that constraint is coming from unless the app does not allow max rate charging.

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Here is the graph.  In the first third you can see the charging at 32 mA to the terminal voltage of 3.6 V then going to 0 mA until I change the terminal voltage to 4.2 V.  In the middle third I change the charge current to 82 mA and it charges in constant current mode until it gets near the terminal voltage and then it switches to constant voltage mode until the charging current drops to zero.

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Overall it took 3 hours and 5 minutes to charge.  1 hour and 40 minutes were at 82 mA.

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Generating a Battery Model (profiling)

My first attempt at generating a battery model is for a Chinese Liter 600 mAh LiPo battery that I bought on Amazon.   There wasn't a specification sheet available on Amazon so I'm using the spec sheet for a Lipol 600 mAh battery.  These specs are fairly generic.  The terminal voltage is 4.2 V, the max charge current is 300 mA (1/2C).  The only spec that I'm not using directly is the Discharge Cut Off voltage of 2.75 V.  I've read on the Nordic forum that if the internal battery discharge protection shuts the battery off before the profiling process completes then the run will fail and the battery model will not be generated.  I don't relish the prospect of losing ~40 hours of runtime so I'm going to set the cut off to 3 V just to be safe since I don't have the actual spec.  This might result in a slightly lower reported capacity.  Here is the Lipol spec.

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This is the battery that is shown in my initial setup picture.

 

Here is the description of profiling:

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And the name and battery configuration I entered:

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You then create/select a directory to store the battery model, the profile settings, and run logs.

You can choose to profile a single temperature or multiple temperatures in sequence, but each temperature requires about 40 hours plus recharging time between runs.  It is recommended to profile 3 temperatures spaced over the battery operating range.  I chose to start with a single temperature as I don't have a UPS on my setup and I was concerned about what might occur if a run failed.  It turns out that each temperature generates its own individual model file and these are automatically integrated into a composite model at the end if you choose to run multiple temperatures.  If you choose to profile each temperature individually there is a process to manually integrate the model files later.

There is a window that pops to show the profiling status.  The first step is to fully charge the battery.  The battery was partially charged so this step was quick.  The dashboard is grayed out during profiling but provides information in the charging stage.

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Charging current decreases as Vterm is approached and charging mode switches to constant voltage when Vterm is reached.

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Charging terminates when current drops to zero and the USB PMIC connector then needs to be disconnected so that the battery will be the source of the VSYS current to the nPM Fuel Gauge board for profiling.

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Then the battery discharge profiling starts with a resting battery (no load).

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The profile continues with periodic application of load current.  I watched this discharge process for a couple  of hours and it was consuming about 5% of the battery capacity per hour.

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The power consumption slowed over time and the battery had about 25% capacity left after 24 hours.

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Profiling terminated after 32 hours.

Battery consumption by the 3.0 V Discharge Cut Off was about 516 mAh.  Somewhat less than the labeled 600 mAh capacity, but it's possible that the cut off voltage should have been lower.

Battery model completed generating about 2 hours later.  Not sure what computations were being done to generate the model parameters.

Here are the files generated on the host computer:

Directory structure:

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Model files (*.inc, *.json) and profileSettings

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The profileSettings.json file initially only contains the battery configuration that was entered and the path to the run data file.  After profiling this file also includes the model parameters.

{
"name": "Liter602535",
"capacity": 600,
"vLowerCutOff": 3,
"vUpperCutOff": 4.2,
"profiles": [
{
"temperature": 25,
"csvReady": false,
"csvPath": "..\\profile_1\\Liter602535_600mAh_Tp25.csv"
}
],
"appVersion": "1.3.1"
}

 

Voltage and current spreadsheet for the single temperature run

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Debug files

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Model name has been automatically updated to indicate the profiled capacity

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And the available models now in the tool (1 slot still available)

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I then started to recharge the battery but the nPM Controller crashed and disconnected before it completed.  The crash log indicated that it had charged the battery to 4.145 V and there was a kernel panic caused by something in the module_fuel_gauge code.  I later had this happen on another battery near the end of the charging cycle so I believe that there might be a firmware issue in the nPM Controller (nRF5340) code.

Here was the start of the charging cycle (I did not capture the data near the end and it was lost when the controller crashed).

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Model files

The battery model files come in two different text formats a *.inc and *.json.   The JSON format is used with the nPM Controller and the INC format is used with to integrate the model into your final application with a Nordic System on Chip (SoC) using the nRF SDK and the nRF Fuel Gauge library.

 

Liter602535_25C.inc

/*
 * Copyright (c) 2023 Nordic Semiconductor ASA
 * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 */

.param_1 = {0.0, 0.005, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, 0.045, 0.05, 0.055, 0.06, 0.065, 0.07, 0.075, 0.08, 0.085, 0.09, 0.095, 0.1, 0.105, 0.11, 0.115, 0.12, 0.125, 0.13, 0.135, 0.14, 0.145, 0.15, 0.155, 0.16, 0.165, 0.17, 0.17500000000000002, 0.18, 0.185, 0.19, 0.195, 0.2, 0.20500000000000002, 0.21, 0.215, 0.22, 0.225, 0.23, 0.23500000000000001, 0.24, 0.245, 0.25, 0.255, 0.26, 0.265, 0.27, 0.275, 0.28, 0.28500000000000003, 0.29, 0.295, 0.3, 0.305, 0.31, 0.315, 0.32, 0.325, 0.33, 0.335, 0.34, 0.34500000000000003, 0.35000000000000003, 0.355, 0.36, 0.365, 0.37, 0.375, 0.38, 0.385, 0.39, 0.395, 0.4, 0.405, 0.41000000000000003, 0.41500000000000004, 0.42, 0.425, 0.43, 0.435, 0.44, 0.445, 0.45, 0.455, 0.46, 0.465, 0.47000000000000003, 0.47500000000000003, 0.48, 0.485, 0.49, 0.495, 0.5, 0.505, 0.51, 0.515, 0.52, 0.525, 0.53, 0.535, 0.54, 0.545, 0.55, 0.555, 0.56, 0.5650000000000001, 0.5700000000000001, 0.5750000000000001, 0.58, 0.585, 0.59, 0.595, 0.6, 0.605, 0.61, 0.615, 0.62, 0.625, 0.63, 0.635, 0.64, 0.645, 0.65, 0.655, 0.66, 0.665, 0.67, 0.675, 0.68, 0.685, 0.6900000000000001, 0.6950000000000001, 0.7000000000000001, 0.705, 0.71, 0.715, 0.72, 0.725, 0.73, 0.735, 0.74, 0.745, 0.75, 0.755, 0.76, 0.765, 0.77, 0.775, 0.78, 0.785, 0.79, 0.795, 0.8, 0.805, 0.81, 0.8150000000000001, 0.8200000000000001, 0.8250000000000001, 0.8300000000000001, 0.835, 0.84, 0.845, 0.85, 0.855, 0.86, 0.865, 0.87, 0.875, 0.88, 0.885, 0.89, 0.895, 0.9, 0.905, 0.91, 0.915, 0.92, 0.925, 0.93, 0.935, 0.9400000000000001, 0.9450000000000001, 0.9500000000000001, 0.9550000000000001, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99, 0.995, 1.0},
.temps = {22.0},
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.name = {'L', 'i', 't', 'e', 'r', '6', '0', '2', '5', '3', '5'},

 

Liter602535_25C.json

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0.030276176922283873, 0.030753681142067387, 0.031204263229559024, 0.03163924975664671, 0.032116178451178236, 0.03276684298272673, 0.03348343514508827, 0.034174159592472005, 0.03506276857737792, 0.037535485350660085, 0.04153253542381026, 0.042267773463704264, 0.04272739050458074, 0.043097546015235376, 0.0434125065001195, 0.043697037505212824, 0.0439582150680913, 0.044201053032942875, 0.044429201882559076, 0.044645381336061045, 0.044851657972455335, 0.04504962899494346, 0.045240547175254046, 0.045360824742268054, 0.045360824742268054, 0.045360824742268054, 0.045360824742268054], "param_8": [0.00001781, 0.00001817, 0.00001853, 0.00001889, 0.00001925, 0.00001961, 0.00001996, 0.00002032, 0.00002073, 0.00002135, 0.00002198, 0.00002261, 0.00002323, 0.00002386, 0.00002449, 0.00002511, 0.00002574, 0.00002637, 0.00002700, 0.00002762, 0.00002825, 0.00002888, 0.00002950, 0.00003013, 0.00003076, 0.00003188, 0.00003319, 0.00003450, 0.00003581, 0.00003712, 0.00003843, 0.00003974, 0.00004105, 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"param_11": [0.21092464170622577, 0.0012555727454083248], "param_12": [0.5162281769919267], "name": "Liter602535"}

 

I wasn't able to find a description of the discharge sequence of the battery profiling process or a definition of the parameters in the battery model file, so I submitted a query to devzone.nordicsemi.com.  The response was "This is a part of our IP and is intentionally not described. "  Basically, the model generated is only usable with Nordic hardware and software.  And there is also no option in the PowerUP app to use the nPM Fuel Gauge current sink independently from the profiling process and you cannot modify the load currents or profiling sequence that is used.

I did take a quick look at the API documentation for the Fuel Gauge library and it provided some insight into format of the parameter data in the battery model.  There is a disclaimer at the end about purposefully not documenting the parameters.

The only immediately recognizable field from the data is param_2 which is the battery voltage.  param_1 looks like it may be the state of charge percentage from 0 - 100% in 0.5% steps.  So, the model data is only meaningful to the proprietary algorithm being used.

 

image

 

Discharge Current during Profiling

I was able to gain some insight into the profiling process by examining the the Voltage and Current CSV log file that is generated during the run.  I've attached the final file for the 32 hour profiling run in case anyone is interested.

The CSV file logs the load current, battery voltage and temperature every second.

image

 

The process only uses two load currents ,120 mA initially and 60 mA after the battery voltage drops to 3.5V.  It is modulating the duty cycle that the load current is applied throughout the process.

Initially the load current is applied for 10 minutes and off for 40 minutes, so 20% of the rated current at 20% duty cycle.

 

The first couple of cycles:

image

 

The first 12 hrs:

image

 

At the end of the run the load current was 60 mA, on for 5 minutes and off for 30 minutes, so 10% of the rated current at 14% duty cycle.

 

More battery profiling

As part of the roadtest I wanted to profile one of the batteries that had a bundled model to see how well my results would compare.  I'm profiling the Renata 165 mAh battery that I used to initially try out my test setup.  That battery doesn't have an embedded thermistor so I attached a 10K NTC thermistor to the outside of the case using Kapton tape.

 

The battery with thermistor connected for profiling.  The thermistor connects between the NTC and GND header pins.

 

I wanted to profile this battery at two temperatures (ambient [22C] and 35C) to see the effect of temperature on the battery capacity.   I don't have a temperature chamber so I plan to use the heated bed of my 3D printer for temperature control.  As a result I need to change my PC host to a laptop to be able to operate in proximity of the printer.  It should be straightforward - just need to install the nRF Connect Desktop and nPM PowerUP software.

Installing the programs and setting up the profiling went smoothly.

 

profileSettings.json
{
"name": "ICP641620PA",
"capacity": 165,
"vLowerCutOff": 3,
"vUpperCutOff": 4.2,
"profiles": [
{
"temperature": 22,
"csvReady": false
}
],
"appVersion": "1.3.1"
}

 

Unfortunately, about 10 hrs into the profile run Windows decided to do an update and rebooted the laptop so I lost the run (there is no provision for recovery in the Nordic software and a model cannot be generated from a partial run).  I had been careful to change all my power settings so the the laptop would always be active and I had turned off automatic updating, but I guess security updates will still occur unless you suspend (pause) all updating.  Nordic actually recommends that the PC be put in airplane mode to prevent updating during profiling.

The partial run data is available in the csv file, so I took a look at it to see if the profile process appeared to be the same as for the previous battery.  The profiling started out with a 20% load current (33 mA) and the same 20% (10min on/40min off) duty cycle as it did with the Liter 600 mAh battery.

image

 

I recharged the battery and restarted the profiling process.

After 24 hrs the battery had about 23% capacity remaining.  Again very similar to the previous battery.

image

 

Profiling terminated after 33 1/2 hours.

Battery consumption by the 3.0 V Discharge Cut Off was about 166 mAh.  Slightly more than the spec'ed capacity of 165 mAh but less than the 172 mAh of the bundled model of the same battery. 

Battery model completed generating about 1 hour later.

Current model sets loaded in the PowerUP app.  All 4 available slots are used.

image

  

The only planned task left to complete is to profile this battery at another temperature and merge the two sets of temperature data into a single model.   Unfortunately, I have some personal constraints that require me to conclude the roadtest at this point.  I don't believe that addition testing will give me further insight into the profiling process and I'm confident that the merge would work.

nPM Fuel Gauge Circuits

The subject of this roadtest is the nPM Fuel Gauge Board.  The key to simplified profiling of battery capacity is a consistent programmable constant current load.  I thought that it would be good to examine the circuit schematics of the key functional blocks of the Fuel Gauge Board.  It is interesting to note that I did not find a specification sheet per se of the Fuel Gauge Board itself.  Because it is only used in conjunction with the nPM1300 EK some specs can be inferred and some limits are stated in the Fuel Gauge Board User Guide.

Features/Specs

  • Battery types supported (nPM1300)
    • Lithium-ion (Li-ion)
    • Lithium-polymer (Li-poly)
    • Lithium iron phosphate (LiFePO4)
  • 800 mA battery charger (nPM1300)
  • 100 mAh - 3000 mAh battery capacities supported (nPM PowerUP limitation)
  • VSYS 2.3 V to 5.5 V (nPM1300 EK system voltage, used for battery characterization)
  • VBUS_nPMC 4.35 V to 5.5 V (nPM Controller VBUS, powers the nPM Fuel Gauge Board)
  • VDDIO 1.7 V to 3.3 V (TWI voltage reference level)
  • 600 mA maximum profiling load current (this is inferred from the run data and the maximum supported battery capacity)

 

Top view (no components on back)

image

 

Block diagram

image

 

Schematics

Current monitor / current sink

image

 

DAC / voltage reference /temperature sensor

image

 

Key components

Q1

PSMN9R5-30YLC

30 V N-Channel 44 A 9.8 mOhm 33 W LFPAK56, Power-SO8

U1

MAX4238AUT+T

IC OPAMP GP 1MHZ RRO

U2

INA236AIDDFR

48-V, 16-Bit, Current, Voltage, and Power Monitor With an I2C Interface

U3

MCP4726A0T-E/CH

Single, 12-BIT NV DAC, Ext Vref, I2C

U4

N34TS04MT3ETG

Digital Output Temperature Sensor with On-board SPD EEPROM

U5

ADR360AUJZ

2.048V 0.29%, 25ppm/C

 

This is a clean simple design that serves its designated purpose well.  I like that the current setting, current monitoring, and load temperature monitoring are using I2C for data communication.  I think that this board could be a standalone product as a programmable current sink that could be used with other MCUs but that would require additional specification and documentation.  I understand that it was not Nordic's design intent.

 

Conclusions

I've used PMICs on many remote sensor projects to monitor battery voltage and warn of low voltage conditions but haven't really used fuel gauge applications to make accurate state of charge predictions.  It was interesting to go through the process of generating a state of charge battery model for use with Nordic fuel gauge hardware/software.  Nordic generally does a good job of integrating and documenting their hardware designs and software applications.  In this particular instance because the battery model and fuel gauging software are only intended to be used within their hardware/software ecosystem they have "purposefully" not documented the battery profiling process or the model parameters.  Understandable in the context of IP protection, but disappointing nonetheless.

The battery profiling process is very time consuming; ~30+ hours plus charging time per temperature.  Nordic uses an estimate of 48 hours per temperature and recommends profiling at 3 temperatures within the battery operational temperature range for best accuracy.  3 temperatures are the maximum allowed.  A complete model would take 6+ days to generate and use of a temperature controlled chamber is required for good results.  An interrupted run is unrecoverable and partial runs are not usable so this process is somewhat fragile.  I don't have any applications that would warrant this amount of time investment and would tradeoff some loss of accuracy for a shorter time to generate a model.  It would be interesting to see where others are using these models.

The use of the nPM PowerUP app in nRF Connect Desktop makes the profiling process and model generation straightforward.  Nordic has also integrated the models in the nRF SDK for its development boards using the nRF Fuel Gauge library so adding fuel gauging to supported boards is also easy.

The nPM Fuel Gauge Board is a nicely designed constant current sink that integrates well with the nPM1300 EK and the nPM PowerUP app to provide the battery discharge current required during profiling.  I had hoped that the app would allow manually setting the current value and that the graph would show the sink current in addition to the other battery parameters but those features are not available.  I suggested on the Nordic DevZone that it would be nice to add those features since that would allow one to characterize battery voltage vs different discharge currents, but I got no response.  I also inquired as to whether I could change the load currents used during profiling, but that is also not possible.

In summary, the nPM Fuel Gauge Board performs well in its intended application.  It would have been nice to have the ability to manually control the current sink.  I think there is an opportunity to use this board as a standalone programmable current sink. 

Thanks to Nordic and E14 for the opportunity to participate in this roadtest.

Anonymous
  • I'm glad to hear you got it running.

    This is something that might be easy to miss, so I'll see if we can add a pop-up when illegal name is used.

  • Cleaning up the name worked. Thanks  .

    I had been using the battery part number, which had a non alphanumeric character in it.

    Now it is going to take a few days to profile several batteries.

  • Hi Doug,

    This is most likely because you have not entered a name or used special characters in the name in the "Name your battery" field at the top in the Profile wizard.

    Please let me know if this is not the case, and you are still not able to start the profiling session.

  • Hi   I am trying to profile a Li-Po battery using the Profile Battery wizard in the nPM PowerUP utility, with the nPM1300 EK and Fuel Gauge module. I can charge the battery fine, but the "Select folder" button in the Profile Battery wizard is unhighlighted and non-responsive. Do you have any ideas what I am missing?

  • Hi Michael,

    Thanks for your feedback. 

    We support running our fuel gauge algorithm on non-Nordic SoCs. It's correct that we don't share detailed information about the battery model and the fuel gauge algorithm, as this is to protect this valuable IP. However this information isn't needed to implement fuel gauge on non-Nordic SoCs.

    A good starting point is our fuel gauge sample: nPM1300: Fuel gauge. So far we have support for any M4 and M33 based devices, but more will be added based on customer feedback.

    We also have a user guide describing the essentials for the nPM1300 fuel gauge: Using the nPM1300 Fuel Gauge

    I hope this helps.

    Best Regards,
    Eirik Slettahjell
    Technical Product Manager - PMICs Nordic Semiconductor

  • Thanks very much for the detailed review.

    I was very interested to compare what you get from some low cost eval boards compared with buying a full blown bench type instrument and your review answers that question very well.

    I was disappointed that Nordic seem to have a very definite closed IP policy which seems to be designed much more to lock you in to their chips  than help the customers' development investment.

    MK