Lithium Ion battery indication by led through voltage measurement

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1. Have to make a led level indicator of lithium battery of 7.4V (two cells in series of 3.7V each).

Led level indications are 0-20,40,60,80 & 100% i.e by 5 leds.

 

2. Need to make it simple by voltage measurement only. Voltage range of battery pack is 5.7V to 8.4V. Need to break that into 5 different voltage ranges & then indicate by led.  

 

3. I know voltage measurement is not accurate way, but in most application its good enough & I ave to keep simple in this application. 

 

4. Currently doing it by MSP430G2553 MCU & its working fine, but I was thinking is there any dedicated IC available for this, need to be real low power?

Problem with MCU is everytime I have to burn program in it.

 

Check lots of chinese power banks, they have some dedicated IC, but there parts number are not traceable or available on net.

Any low power ic solution for this?

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Vindhyachal Takniki wrote:
in most application its good enough

Is it?

 

EDIT

 

Silicon Lightworks wrote:

The nature of Li-ion cells is such that the relationship between state of charge ("SOC") and voltage is fairly flat throughout much of the cell’s discharge range.

 

A typical discharge voltage curve is shown below:

 

Image result for lithium ion battery discharge curve

 

https://siliconlightworks.com/li-ion-voltage

 

Note that there is virtually no voltage difference at all between 80% and ~ 50% ...

 

 

EDIT 2

 

Link

 

#LithiumDischargeVoltage

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Last Edited: Tue. Jun 12, 2018 - 06:30 AM
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What you need is a coulomb counter, not a voltage measurement.

 

ps. A quick google leads to this. https://github.com/sparkfun/LTC4...

 

Ross McKenzie ValuSoft Melbourne Australia

Last Edited: Tue. Jun 12, 2018 - 10:43 AM
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Vindhyachal Takniki wrote:
Need to break that into 5 different voltage ranges
If it were a linear discharge then obviously (8.4 - 5.7) / 5 = 0.54 so you would need to use steps 5.7 .. 6.24, 6.24 .. 6.78, 6.78 .. 7.32, etc. but Andy's picture in #2 shows that this is decidely non linear so you would spend almost the entire life of the pack with 3 LEDs lit with just a very short amount of 4/5 and 2/1 LED activity at either end.

Last Edited: Tue. Jun 12, 2018 - 10:52 AM
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valusoft wrote:
What you need is a coulomb counter

Absolutely!

 

aka "gas gauge"

 

https://github.com/sparkfun/LTC4150_Coulomb_Counter_BOB

Note that Linear Tech is now Analog Devices: 

 

http://www.analog.com/en/products/power-management/battery-management/coulomb-counter.html

 

Other manufacturers are available.

 

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1. Th gas gauge are expensive for this application & used where accurate capacity measurement is requried in percentages.

2. The 5 leds already have low accuracy, each led represent +-20% of capacity accuracy so expensive gas gauge ic not required

3. almost all power bank chinese ones , have voltage one & it works ok as it gives rough idea & inexpensive too. 

 

4. Also I need a switch too. When siwtch is pressed, led go on for 2-3 seconds, then turn off & IC go into low power mode for battery saving. 

With MCU i am able to do these things, but i want non programmable solution

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If you insist on this approach you are wasting our time in my opinion. You have been given good advice... don't throw it back at us.

 

Ross McKenzie ValuSoft Melbourne Australia

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I don't recommend building your own custom circuitry when working with Lithium Ion 18650 batteries.  These are such a world-standard device that many circuit modules are cheaply available on eBay. 

 

The cheapest and most common is the TP4056 module board that sells for about $0.50 USD each in quantity:10.  This is a four-stage charging board specifically for Lithium-Ion batteries.  It takes +5V regulated and charges the 18650 at 1 Amp constant current when the battery has a voltage between 3 and 4V.  When the battery reaches +4V, it switches to constant voltage charging and monitors the input current.  When that current falls to @90mA, it shuts off all the charging and goes to high-Z outputs, essentially removing itself from the battery circuit.   There should be one TP4056 module board for each 18650 battery.

 

Order a few of these INA3221 IC module boards:  https://www.ebay.com/itm/1-pcs-N...

Each one has three independent channels. Each channel can read voltage to 25v and current to 3 Amps.  It connects to an AVR using TWI (I2C).  There are several good Arduino libraries.  With a single INA3321 you can read the charging current for each 18650 and the total load current for the +7.3V application circuit.  If you know the total milliAmp-Hour capacity of the +7.4V battery pair when charged, then you can determine the amount of battery energy left by multiplying the number of minutes by the milliAmp discharge rate per minute.

 

This should give you a "gas gauge" for the 18650 batteries for only a few US dollars in part's cost.

Last Edited: Thu. Jun 14, 2018 - 10:30 PM
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Simonetta wrote:
This should give you a "gas gauge" for the 18650 batteries for only a few US dollars in part's cost.

but an actual gas gauge chip only costs a "few US dollars" ...

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Although the LTC family of "gas gauge" ICs may cost only a few dollars and appear to be nearly identical in cost when compared to the INA3221 and TP4056 boards, they are a different class of components.   The module boards from eBay are physically large enough to work with without specialized soldering and magnification equipment that can only be afforded by well-financed corporate electronic development labs and repair workshops.

 

   The "gas gauge" ICs are very small (2mm by 3mm is normal).  They are designed to be manufactured by robotic 'pick-and-place' machines that are only used when thousands of units of an individual design are being made.  They are not for prototypes and the precision rework that is used to troubleshoot prototypes.  It is difficult to buy small quantities of these (or any type of specialized) ICs (for example: 2-3 units).  One must request samples or enter into a special program offered by the IC maker for tiny-quantity purchases.  

 

The INA3221/TP4056 boards are easy to order and obtain from the eBay sources.  They are mounted on boards (along with all the needed resistors, capacitors, and clock crystals/resonators) that have standard 0.1" spacing for connecting and mounting holes.  The INA family of voltage/current sensors are cheap, configurable, versatile to the application, and well-documented.  At @ $0.50 per module board, the TP4056 is a fantastic deal given the complexity of Lithium-Ion charging requirements.  These modules are much easier people who are not professional engineers and embedded-system designers to obtain and work with.

 

In this day and age, I recommend designers to always go with an inexpensive module board holding a 'solution' to a specialized application instead of a single manufacturer's specialized IC.  This is contrary to normal electronic industry wisdom, but the approaches that made sense in the 1980s are sometimes not the best way to approach a solution in the 2010s.