Checking and correcting voltage drift in a battery pack
Tags: Battery Life, Energy, 18650 Battery, Battery, Lipo

In a multi-series battery pack, it’s critically important that all cells within the pack is balanced — that is, its voltage be identical across all cells within the pack.

We will go through a couple methods of checking voltage drift, and how to correct variances if they’re detected.

Before getting started

A few assumptions will be made.

Your battery pack has balance leads installed already, to be connected to a BMS

You have a multi-meter

A way to charge or discharge batteries (more on this later)

What series and parallel count means in a battery pack

You understand what voltage drift is and why it needs to be corrected

The battery

In all examples, we will use a hypothetical 10s pack, at 37v volts. This means each cell’s voltage will be 3.7 volts (37v divided by 10 cells).

A battery pack cell will denote individual batteries connected in parallel, or a parallel group.

Method 1: Cell voltage relative to overall battery pack

This first method requires some light math, but ensures balance leads are connected correctly.

First, set the multimeter to DC, and connect the positive probe to pack positive (alligator clips are useful here).

From here, we will begin measuring each cell’s voltage starting from cell 10, down to cell 1.

Assuming the balance leads have been spliced into a harness, likely a JST connector, you can measure each cell’s voltage reading by touching the area in red with the multimeter negative probe:

Voltages should follow:

This multiplying in cell voltage should be in sequence, and is an indication that the balance leads are installed correctly.

Take these voltage readings and subtract each cell’s voltage from the previous one:

Variance in voltage readings in the “Cell Voltage” column should be no more than 0.075v, as the BMS may fail to enter into balance charge if the voltage variance is greater than this amount.

Method 2: Individual cell voltage

This method is a bit easier, and can be used if you’re certain the balance leads are installed correctly.

Simply measure the higher cell with the positive probe, and the cell immediate below with the negative:

When you reach Cell 1, measure Cell 1 with positive, and pack negative with the negative probe, that’ll be Cell 1’s voltage.

Correcting voltage drift

Let’s suppose a cell’s voltage is lower than 0.075v compared to rest of the pack.

It’s possible to charge just the out of balance cell by connecting a DC source to it.

WARNING: This technique is completely manual and has ZERO fail safe. It will require constant monitoring and will lead to battery damage, leading to a fire, if not done correctly. Do not leave the battery unattended.

ANOTHER WARNING: Connecting the power supply wire incorrectly will lead to a violent short and will / may cause permanent damage.

Locate a 5 volt DC power supply that’s no longer used.

Something like this:

Cut the connectors off and measure the wires to identify the positive and negative wires.

So, let’s say Cell 9 voltage is too low, connect the power supply to the balance leads:

Then carefully monitor the cell’s voltage until it reaches the desired charge.

AGAIN: Doing this incorrectly will definitely be a disaster for you, please be careful!

If a cell’s voltage is too high, it can be drained down to the desired charge using some type of electrical load.

Attaching the cell’s balance leads to a 5 volt bulb would be simple way to create load:

Be sure to isolate the bulb as it can burn through whatever surface it touches.

In addition, a resistor could act as a load as well, something like this:

If using a resistor, be sure to use the correct type by using Ohm’s law calculator. Resistors convert energy into heat, and drawing too many amps will yield very high temperatures, hot enough to ignite combustible materials.

I built this little appliance, which connects four resistors to impart a load of 5amp @ 50volts:

Conclusion

Checking your battery pack balance should be a routine maintenance check. Catching issues early on will lead to longer battery life, avoid failures, and most importantly, offer insight into the health of your battery.