# How can we monitor the percentage charging of a battery?

#### mukesh1

Joined Mar 22, 2020
37
I would like to build a system that could show percentage charging of a 12 V DC battery. As we can check the level of battery in our mobile phone. I don't have idea How can we monitor the percentage charging of a battery?

#### jpanhalt

Joined Jan 18, 2008
10,480
State of charge while charging may be difficult, if you try to base it on voltage. It will depend on battery chemistry and charger. Take a single cell charger like the BQ21040 from TI.

Here is its profile:
If the battery voltage is below the LOWV threshold, the battery is considered discharged and a preconditioning
cycle begins. The amount of the current goes into the battery during this phase is called pre-charge current. It is
fixed to 20% of the fast charge current.
Once the battery voltage has charged to the VLOWV threshold, fast charge is initiated and the fast charge
current is applied. The fast charge constant current is programmed using the ISET terminal. The constant current
provides the bulk of the charge. Power dissipation in the IC is greatest in fast charge with a lower battery voltage.
If the IC reaches 125°C the IC enters thermal regulation, slows the timer clock by half and reduce the charge
current as needed to keep the temperature from rising any further. Figure 5 shows the charging profile with
thermal regulation. Typically under normal operating conditions, the IC’s junction temperature is less than 125°C
and thermal regulation is not entered.
Once the cell has charged to the regulation voltage the voltage loop takes control and holds the battery at the
regulation voltage until the current tapers to the termination threshold.
The termination current is set to 10% of
the fast charge current. The CHG terminal is low (LED on) during the first charge cycle only and turns off once
the termination threshold is reached, regardless if termination, for charge current, is enabled or disabled.
Thus, voltage during charging will not give the answer. You might try to integrate the current.

#### Irving

Joined Jan 30, 2016
1,064
I would like to build a system that could show percentage charging of a 12 V DC battery. As we can check the level of battery in our mobile phone. I don't have idea How can we monitor the percentage charging of a battery?
What battery chemistry?

The basic approach, as outlined, is a voltage divider to bring battery volts in range of an analog-digital converter (ADC), a microcontroller to encode the data, a bluetooth transmitter to communicate to phone, a small app on phone to display info.

The ADC, microcontroller and bluetooth transmitter can be separate modules or combined in one device. There are several types from different suppliers, eg ESP-32WROOM, Adafruit BlueFruit, STM BlueNRG-2, etc.

What phone are you looking to use, Apple or Android?

#### ScottWang

Joined Aug 23, 2012
6,960
You could divide the 12 Volts as 120 Vpoints for each 0.1V, and full charge to 100% as 100 Cpoints, so 1P%=100Cp/120Vp=0.83V, and each 0.83V = 1 %, 12V = 99.6% or 100%.

You could use the uC that you know, Arduino or pic or ADC card with computer to measuring the charging voltage, etc.

If the uC that you chose there is no ADC chip inside then you have to add one.

#### Irving

Joined Jan 30, 2016
1,064

How is it being used? what is it powering, how do you charge it.

Measuring voltage may or may not be useful depending on these answers.

#### andrewmm

Joined Feb 25, 2011
643
A few things here.

Any battery has a discharge characteristic, at a given load, at a given state of discharge you get a given voltage out.
so if you can measure at a known current, and measure the voltage, you get a discharge curve thats "repeatable". If you measure a few times that curve from full to empty, then you can estimate how far down the curve you are at any time, and hence how much charge you have left.

Over time , ay given battery will vary, but give a fairly stable environment, like in your laptop , this provides a good estimate.

A relatively simple solution are coulomb counter,

https://www.analog.com/en/parametricsearch/11384#/

If you want more resolution, then you can step up to a battery management system chips. Look at people like analog devices ,

https://www.analog.com/en/applicati...ification/battery-management-systems-bms.html

#### Irving

Joined Jan 30, 2016
1,064
A few things here.

Any battery has a discharge characteristic, at a given load, at a given state of discharge you get a given voltage out.
so if you can measure at a known current, and measure the voltage, you get a discharge curve thats "repeatable". If you measure a few times that curve from full to empty, then you can estimate how far down the curve you are at any time, and hence how much charge you have left.

Over time , ay given battery will vary, but give a fairly stable environment, like in your laptop , this provides a good estimate.

A relatively simple solution are coulomb counter,

https://www.analog.com/en/parametricsearch/11384#/

If you want more resolution, then you can step up to a battery management system chips. Look at people like analog devices ,

https://www.analog.com/en/applicati...ification/battery-management-systems-bms.html
True to some extent,but only in respect of open circuit volts (OCV). On load, its anybody's guess what SoC is - in the lab maybe you can get close but in the field, not a chance. On a 12v lead-acid for example, the OCV fully charged is ~13.8v but will drop to 13v at 50% SoC when measured a few minutes after coming off load. Leave it 24h and SoC can apparently climb 20% or more. Until you put it back on load and the volts just die. And the older and more damaged the battery is by being mis-charged, over-discharged, wrong volts, not being left long enough, etc etc the more the relationship of volts to true SoC goes out the window.

For Lithium cells the change in voltage from 90% to 10% can be 'in the noise' - a few tens of mV for a 24v pack, and that's only valid if the load is truly constant. Also internal resistance varies with SoC. Basically determining SoC for Li by voltage is pretty useless.

The only real way to determine SoC is columb counting, and then only if battery is correctly fully charged and balanced each time. The more 'partial charges', the less accurate it becomes, because the Ah to go from, say, 10 -> 20% is different from 70 -> 80%, by as much as 10% in my experience.