How to measure voltage of multiple batteries connected in string/array in series or parallel combination with microcontrollers

Measuring an individual battery voltage or a whole bank of battery using any microcontroller(arduino, microship pic, Avr, Atmega, Intel, NXP, stm32)is an easy task. You can find many tutorials on internet on how to do so. But what about if we want to measure an individual battery connected to a cluster of batteries in series or parallel combination. Now its a hard task to accomplish. But still their are some cleaver ways to do so. In this post i am going to enlist some of the ways through which we can measure individual battery voltage which is a part of series or parallel connected string/array of batteries.

Basic and popular battery monitoring technique – Voltage Divider Circuit

Basic and the most popular individual battery monitoring technique using microcontrollers in practice is voltage divider circuit. In voltage divider circuit two resistors are connected in series and source (battery) voltage is applied across its ends. Voltage is divided against the two resistors according to the resistor ohmic values.

Voltage divider voltage divided against two resistors

Why voltage divider?

Microcontrollers work on 5 or 3.3 volts(From here on we will take in to consideration 5 volt, techniques listed below can also be applied to 3.3 volt microcontrollers). So their pins are also working on 5 volt TTL logic. Voltage higher than 5 volts could potentially harm the pin or may fry the microcontroller. Solar panel, car, ups, generator and back up batteries normally are at 12 volts. Microcontroller can not measure 12 volts directly. So voltage divider is used here to divide the voltage in two half while ensuring that the one half voltage can not increase 5 volts in any scenario(charging etc). This half voltage is feed to microcontroller to measure the voltage.

Voltage divider circuit with formula

Now lets calculate the values for Rtop and Rbottom. Here we need some important considerations to be taken seriously.

Low ohm resistors can sunk much current and wires could be heated instantly. Consequently wires can melt down in seconds. So always use sufficient amount of resistors for bigger ampere hour batteries. I selected one resistor Rbottom to be 10k ohm.
During charging battery voltage can increase to 18 volts. For example150 watt solar panel outputs 17 volts at 6 amperes during full sun, output voltage can even reach above 18 volts. Solar charge controller also output voltage approximately equal to 15 volts to charge the batteries.

Calculating resistance values

I am going to measure voltage across Rbottom and i decided its value randomly to be 10k ohm. We know Vout can be up to maximum 5 volts since nodemcu works and accepts maximum 5 volts at its I/O pins. Vin is 18 volts when the battery is charging(worst case scenario). Now we can find Rtop.

Voltage divider resistance value calculation

Now if 18 volts are at battery side it will be divided across resistors, 5 volts drops at 10k resistor and remaining 13 volts drops at 26k resistor. If battery is not charging and say supplying 12 volts what will be the voltage drop across resistors? Lets calculate it

Voltage drop against resistances of voltage divider circuit

Its obvious from the above discussion that the voltage across Rbottom will not exceed 5 volts now. I hope it makes sense to readers about the calculations. The question now is how the 3.33 volts is converted to 12 volt by nodemcu or how from 3.33 volts we can predict that at the battery side the voltage is 12 volts. Well a little more mathematics is involved here. Since the resistor values are fixed we can calculate the voltage ratio across the resistors with respect to the source and use it in code for actual voltage at source. How ratio is calculated is below.

Voltage divider ratio calculation

Two cases are given above when source is at 18 volt and when source is at 12 volt in both the cases the ratio comes out to be constant value. This ratio is utilized in code for predicting the actual source/battery voltage. Ratio is multiplied with voltage at Rbottom for actual voltage value.

In order to take the tutorial on calculating resistance values for a 3.3 volt tolerance microcontroller visit the tutorial below. A demo project with free project code and circuit diagram is presented in the project.

Battery monitoring with 3.3v tolerant microcontroller

Batteries connected in series

Batteries are connected in series to increase the voltage output. For example two 12 volt batteries are connected in series to build up 24 volts. Now how to measure voltage of individual batteries connected in series. See the circuit below. Four 12 volt batteries are connected in series to output 48 volts.

Series batteries combination voltage monitor by microcontroller

In the above circuit four voltage divider circuits are used to measure voltage across each battery. The technique is to measure the voltage across high potential battery first, than against the lower ones and negating the subsequent batteries voltage from the one at higher potential. For example for the above circuit the measured voltage across battery-1 is 48v and battery-2 is 36v. Negating 48v-36v=12v gives us battery-1 voltage. Similarly if battery-3 is at 23v. Than 36v-23v gives 13v. So battery-2 is supplying 13 volts in series string array. Other batteries voltages can be calculated with same method.
In the above scenario for each battery their must be a dedicated analog channel. For higher string of batteries more analog channels are required and microcontrollers usually have 8 analog channels at max. So this method is feasible only when batteries in series combination are not greater than 4.

Note: For the above circuit the resistors values should be selected using the same formula given above.
A demo project using the above technique is made with arduino uno. Project contains free source code and circuit diagram. If you are interested take the tutorial. The link is below.

Measuring string array of batteries voltages using arduino

Opto-isolators or Optocouplers

Using optocouplers is another way to do the same task. A linear optocoupler is one which can perform the job at best. It outputs an equivalent voltage to input but with drop in voltage rating. Optocouplers also isolates the microcontroller from the battery voltage and provides safety from high spikes. The problem with optocouplers is its hard to configure them and they require more effort than voltage divider. Circuit may also be messy. An extra power some time is also needed to power the optocoupler. Amplifier may also be needed at end to amplify the voltage output. Optocouplers also increases the circuit cost. At the end the major drawback is, it still requires a dedicated microcontroller analog channel to measure each individual battery.
You can find many linear optocouplers with different ratings from Texas instruments and other vendors online. At the end the circuit will be hard to design and configure. A typical example is below. I hope no one has time to spend on it 😀

Optoisolated battery monitor with microcontroller

Relays in battery monitoring

Double pole single through relay

Relays can also be used to measure voltage across batteries. A
double pole single through relay is best choice here. A double pole single through relay has a single coil and double channels. When coil is energized two contacts are instantly made. Since two contacts are made. Battery both positive and negative terminals can be connected to this relay for input. A typical foot print of DPST relay is shown on the right hand side. Normally both the terminals are open and upon coil activation both the poles moves and completes the circuit line through which electric power can flow now.

Double pole single through relays with batteries and microcontroller connections are shown below. Take a look I will discuss circuit its pros and cons below the diagram.

Series battery monitoring with microcontrollers

The circuit seems to be pretty simple in diagram but their are some serious pros and cons.
Pros

Only one analog channel of microcontroller is required to measure multiple batteries.

Cons

Digital pins of microcontrollers are required to activate the relay coils and for individual battery an individual pin is required. Digital pins can be reduced by using multiplexers.
Each relay must be properly switched on and off one by one. If the two relays accidentally switched on at the same time their will be a huge blast due to short circuit of batteries.(It happen with me) .

Relay on off increases response time of voltage monitoring.
Relay driving circuit is required.

I used arduino mega to monitor a cluster of 32 batteries with the same relay method. I used ULN2003 relay driver to drive the relay coils. UL2003 input is connected to multiplexer output. 4 to 16 multiplexer is used to drive 2 ULN2003 drivers. First i short circuited the 2 batteries and it cost me much at the end i finally fixed the code and inserted some delays which increased the hardware efficiency.
I made a simple diy project with the same above logic. Arduino relay is used in the project. Click the below button to take the tutorial.

Arduino relay battery voltage monitor connected in series

Analog Multiplexers

Analog multiplexers can also be used instead of relays. Finding a suitable multiplexer and its configuration is as difficult as of optocoupler. I did not have much experience of analog multiplexers i can say any thing more abut them.

Batteries connected in parallel

In parallel combination batteries are connected to increase the shelf life of the source or increase the time of power source to supply suitable voltage to load before needed to be recharged. In parallel combination voltage across each battery remains same. So we can not measure individual battery voltage in this case.

These are some of the ways through which batteries connected in series or parallel can be monitored. If you have any more method in your mind please let me know about it.
If you are a DIY circuit maker or an electronics getting started enthusiastic or if you are a electronic major student the below mentioned project is for you. Its monitoring your car battery voltage, engine temperature and automatically switching head lights on off. Tutorial link is below.

Car automation – Battery Monitor, Engine Temperature, Head lights control

I also made an internet of things project on battery voltage monitoring over WiFi. User can see the battery status now on smart devices like mobile and desktop computers. Nodemcu esp8266 WiFi module is used in the project. Nodemcu Arduino ide is used to write, compile and download the code in nodemcu WiFi module. Project link is given below.

Battery monitor over WiFi using nodemcu WiFi module