Low Current Li-Ion Battery Capacity Meter

Measuring LiOn Battery Capacity Using

a 100 mA Constant Current Load

I’ve accumulated many lithium batteries of the 14500, 18650, and other small flat package types and needed a way to accurately measure the capacity of these batteries because they did not seem to have the capacity as shown on the case. There have been other instructables providing ways to measure battery capacity but my needs were for an inexpensive way to measure battery capacity at low currents (< 100 mA) that many of my projects need. I wanted to know how long each battery would last in my circuits.

Looking at the discharge profile of Li-Ion batteries it is apparent the optimum operating voltage range is 4.1 to 3.5 Volts. If one stays within the voltage limits of 4.1V full charge to 3.5V discharge, the battery will stay useful over many charge – discharge cycles. Discharging below 3.5 V actually damages the battery by reducing the recharge cycles and capacity. Charging above 4.2 Volts also damages the battery and can be dangerous, exploding or starting fires. Excellent information for Li-Ion battery capacity may be found at: https://www.digikey.com/en/articles/techzone/2016...

What is the useable battery capacity of my batteries between 4.1 V to 3.5V in low current applications? As example, some 14500 batteries were marked 4000mAhr while the larger 18650 were also marked 4000 mAhr. Are either markings accurate? I tried several approaches to measuring the cells including taking dV/dT measurement at ten minutes and extrapolating to a normal Li-Ion curve shown in tables but did not feel comfortable I had an accurate reading of battery capacity because my battery discharge profile did not match the curves. In addition, I did not want to use a clock to keep time and keep monitoring battery discharge voltage manually to determine capacity.

Step 1: acquire the materials needed ~$10 USD depending on shipping:

1 – 18650 Li-Ion Lithium Lead Acid Battery Capacity Meter Discharge Tester Analyzer – Ebay.com. ~ $3.30 USD

1 - 4.8 Ohm 1/8 Watt resistor

1 - 10 Ohm 1/8 Watt resistor

1 - 50K Ohm Potentiometer

1 – LT3092 Constant current source – Digikey.com $3.75 USD

1 – Battery holder appropriate for your battery

Figure 1:

Searching the internet I found an inexpensive battery capacity measuring board on Ebay, shown in Figure 1. But it was advertised for measuring the larger 18650 type. The capacity measuring board even came with a pair of 7.5 Ohm power resistors. A simple Ohms law analysis of a fully charged Lithium Ion battery at 4.1 Volts and a single 7.5 Ohm resistance load means 550 milliAmps of current is drawn. It turns out this high current does not reflect battery capacity for smaller batteries at much lower currents. So I needed a different solution for a current load using this board.

The capacity measuring board has a LED display of capacity and the ability to shut off current when a user selected, lower discharge voltage point is reached. This is important for not damaging batteries by too high discharge current. If a fixed resistor load is used there is quite a current change from 4.1 to 3.5 Volts.

Downside of using a resistor load:

Two difficulties are encountered using a resistor load. First is the accuracy of the capacity measuring board to store the declining current amounts vs time is limited since it only samples discharge voltage once each six seconds. Next, the current draw by a 7.5 Ohm resistor for the smaller 14500 battery or other small cells is high for that size battery.

As most of my home brew circuits that use 100 mA or far less, what I needed to allow more accurate battery capacity measurement, as well as the ability to measure the smaller 14500 battery capacity, was to provide a 100 mA constant current load for the battery under test. First, I tried using a 6V incandescent light as a low current load since it has a non linear resistance curve vs voltage. It was not much better than a resistor. Then I tried a two transistor approach for a constant load but the gain of the transistor pair was not sufficient to hold current any closer than 10% - 20% over the 4.1 to 3.5V range of Li-Ion battery operation.

Figure 2:

A search of inexpensive constant current circuits that would operate lower than 3.5 Volts turned up the Linear Technology LT3902 ($3.75 from Digikey). This simple circuit needs only two resistors, a pot and the chip to provide a 100 mA current load so I could eliminate the 7.5 Ohm resistor load. The data sheet of the LT3092 indicates up to 200 mA constant current is possible using different resistor values but I did not try increased currents. Heatsinking would be more important at higher currents.

Step 2 – Wiring the 4 components needed

The component wiring for this constant current load is so simple I did not draw a schematic but rather show the wiring in Figure 2. The components can be wired in free form, then onto an insulated surface using hot glue to hold them. With 400 mW power dissipation by the LT3092, a small copper surface soldered to the device tab would provide a heatsink. Alternately, a small section of experimenter’s board with 100 mil hole spacing and solder pads the tab can be soldered to would be excellent support and provide heat sinking. In my application I used a small board, drilled holes for the resistors and wired the underside according to Figure 2.

Step 3 – Calibration of the Constant Current Load

Setting the constant current load was easy. Adjust the pot to the maximum resistance of 50K, place a current meter in series with the current sink, with a variable power supply across the circuit, apply 4 Volts and adjust the pot to get 100mA. Verify operation by monitoring the current from 3.5 Volts to 4.1 Volts from the external power supply.

Once satisfied with the circuit operation, wire the current load circuit to the outside end pins on the capacity measuring board. Note the constant load positive pin 1 location and the negative goes to pin 4, in figure 3. The battery under test: positive goes to pin 2 and negative, pin 3.

Figure 3 – Final Assembly

Step 4 system setup:

Connect the battery to be tested and the constant current load to the capacity measuring board. Apply 5V power to the capacity measuring board on the bottom side, or connect a USB cable to supply power and plug it into a computer or charger. The capacity measuring board only requires about 60 mA of current at 5V. Next Press the “+” button on the capacity board to set the 3.5V cutoff point then press “OK” to start the measurement. When the battery under test reaches the 3.5V discharge point the load is automatically removed and the display gives capacity in mAhr units.

RESULTS:

It is good to have an accurate way to measure small Li-Ion batteries under low current conditions. Now I have a better understanding of the active time needed between charges of my home brew circuits using each battery in my collection. I mark each battery with a sharpie pen to record its low current capacity. The smaller 14500 batteries stamped 4000 mAhr exhibited about 100 to 280 mAhr actual capacity which means they take 1 to 2.8 hours to test at 100 mA load. Even though the larger 16850 batteries are also marked 4000 mAhr I find mine exhibited between 1100 to 1680 mAhr - far from the case printed amount. Overnight testing might be necessary on the larger capacities. At least now I know each batteries’ limit and the test automatically ends with no further battery drain below 3.5V. With overnight testing, the capacity displays in the morning.

Step 1:

Measuring LiOn Battery Capacity Using a 100 mA Constant Current Load

I’ve accumulated many lithium batteries of the 14500, 18650, and other small flat package types and needed a way to accurately measure the capacity of these batteries because they did not seem to have the capacity as shown on the case. There have been other instructables providing ways to measure battery capacity but my needs were for an inexpensive way to measure battery capacity at low currents (< 100 mA) that many of my projects need. I wanted to know how long each battery would last in my circuits.

Looking at the discharge profile of Li-Ion batteries it is apparent the optimum operating voltage range is 4.1 to 3.5 Volts. If one stays within the voltage limits of 4.1V full charge to 3.5V discharge, the battery will stay useful over many charge – discharge cycles. Discharging below 3.5 V actually damages the battery by reducing the recharge cycles and capacity. Charging above 4.2 Volts also damages the battery and can be dangerous, exploding or starting fires. Excellent information for Li-Ion battery capacity may be found at: https://www.digikey.com/en/articles/techzone/2016...

What is the useable battery capacity of my batteries between 4.1 V to 3.5V in low current applications? As example, some 14500 batteries were marked 4000mAhr while the larger 18650 were also marked 4000 mAhr. Are either markings accurate? I tried several approaches to measuring the cells including taking dV/dT measurement at ten minutes and extrapolating to a normal Li-Ion curve shown in tables but did not feel comfortable I had an accurate reading of battery capacity because my battery discharge profile did not match the curves. In addition, I did not want to use a clock to keep time and keep monitoring battery discharge voltage manually to determine capacity.

Step 1: acquire the materials needed ~$10 USD depending on shipping:

1 – 18650 Li-Ion Lithium Lead Acid Battery Capacity Meter Discharge Tester Analyzer – Ebay.com. ~ $3.30 USD

1 - 4.8 Ohm 1/8 Watt resistor

1 - 10 Ohm 1/8 Watt resistor

1 - 50K Ohm Potentiometer

1 – LT3092 Constant current source – Digikey.com $3.75 USD

1 – Battery holder appropriate for your battery

Figure 1:

Searching the internet I found an inexpensive battery capacity measuring board on Ebay, shown in Figure 1. But it was advertised for measuring the larger 18650 type. The capacity measuring board even came with a pair of 7.5 Ohm power resistors. A simple Ohms law analysis of a fully charged Lithium Ion battery at 4.1 Volts and a single 7.5 Ohm resistance load means 550 milliAmps of current is drawn. It turns out this high current does not reflect battery capacity for smaller batteries at much lower currents. So I needed a different solution for a current load using this board.

The capacity measuring board has a LED display of capacity and the ability to shut off current when a user selected, lower discharge voltage point is reached. This is important for not damaging batteries by too high discharge current. If a fixed resistor load is used there is quite a current change from 4.1 to 3.5 Volts.

Downside of using a resistor load:

Two difficulties are encountered using a resistor load. First is the accuracy of the capacity measuring board to store the declining current amounts vs time is limited since it only samples discharge voltage once each six seconds. Next, the current draw by a 7.5 Ohm resistor for the smaller 14500 battery or other small cells is high for that size battery.

As most of my home brew circuits that use 100 mA or far less, what I needed to allow more accurate battery capacity measurement, as well as the ability to measure the smaller 14500 battery capacity, was to provide a 100 mA constant current load for the battery under test. First, I tried using a 6V incandescent light as a low current load since it has a non linear resistance curve vs voltage. It was not much better than a resistor. Then I tried a two transistor approach for a constant load but the gain of the transistor pair was not sufficient to hold current any closer than 10% - 20% over the 4.1 to 3.5V range of Li-Ion battery operation.

Figure 2:

A search of inexpensive constant current circuits that would operate lower than 3.5 Volts turned up the Linear Technology LT3902 ($3.75 from Digikey). This simple circuit needs only two resistors, a pot and the chip to provide a 100 mA current load so I could eliminate the 7.5 Ohm resistor load. The data sheet of the LT3092 indicates up to 200 mA constant current is possible using different resistor values but I did not try increased currents. Heatsinking would be more important at higher currents.

Step 2 – Wiring the 4 components needed

The component wiring for this constant current load is so simple I did not draw a schematic but rather show the wiring in Figure 2. The components can be wired in free form, then onto an insulated surface using hot glue to hold them. With 400 mW power dissipation by the LT3092, a small copper surface soldered to the device tab would provide a heatsink. Alternately, a small section of experimenter’s board with 100 mil hole spacing and solder pads the tab can be soldered to would be excellent support and provide heat sinking. In my application I used a small board, drilled holes for the resistors and wired the underside according to Figure 2.

Step 3 – Calibration of the Constant Current Load

Setting the constant current load was easy. Adjust the pot to the maximum resistance of 50K, place a current meter in series with the current sink, with a variable power supply across the circuit, apply 4 Volts and adjust the pot to get 100mA. Verify operation by monitoring the current from 3.5 Volts to 4.1 Volts from the external power supply.

Once satisfied with the circuit operation, wire the current load circuit to the outside end pins on the capacity measuring board. Note the constant load positive pin 1 location and the negative goes to pin 4, in figure 3. The battery under test: positive goes to pin 2 and negative, pin 3.

Figure 3 – Final Assembly

Step 4 system setup:

Connect the battery to be tested and the constant current load to the capacity measuring board. Apply 5V power to the capacity measuring board on the bottom side, or connect a USB cable to supply power and plug it into a computer or charger. The capacity measuring board only requires about 60 mA of current at 5V. Next Press the “+” button on the capacity board to set the 3.5V cutoff point then press “OK” to start the measurement. When the battery under test reaches the 3.5V discharge point the load is automatically removed and the display gives capacity in mAhr units.

RESULTS:

It is good to have an accurate way to measure small Li-Ion batteries under low current conditions. Now I have a better understanding of the active time needed between charges of my home brew circuits using each battery in my collection. I mark each battery with a sharpie pen to record its low current capacity. The smaller 14500 batteries stamped 4000 mAhr exhibited about 100 to 280 mAhr actual capacity which means they take 1 to 2.8 hours to test at 100 mA load. Even though the larger 16850 batteries are also marked 4000 mAhr I find mine exhibited between 1100 to 1680 mAhr - far from the case printed amount. Overnight testing might be necessary on the larger capacities. At least now I know each batteries’ limit and the test automatically ends with no further battery drain below 3.5V. With overnight testing, the capacity displays in the morning.