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Do you know how long your device will last before the battery runs out?

Remaining Charge and 100% Capacity

How well does your BMS determine current capacity?

Users of devices with batteries find it very important to have a good indication of how long the battery in their device will last.
Without a good capacity indication, the device can shut down and the battery needs to be recharged first.

For vehicles, that can make the difference between getting home or not. Or reaching a charging station, or not.

How good is the State-of-Charge and State-of-Health determination of a BMS? | Engineering Spirit BV
Zie ook de whitepaper: Lithium accu’s and their BMS


There are many factors that affect the residual charge of a battery. Most batteries perform poorly at low temperatures (especially below 0°C). Then the capacity decreases drastically. But also the use of a battery will decrease the 100% capacity.
In addition, there may be a bad or damaged cell in use, whose capacity decreases rapidly.

Manufacturers are therefore reluctant to use State-of-Charge (SoC) indication on their batteries and products. What a customer does not know cannot lead to complaints….
Users, on the other hand, find a good SoC indication extremely important. For them, it saves working time of their tooling. They assume (usually wrongly) that this number is ‘magically’ correct.

Battery suppliers prefer not to touch this and sometimes even let their customers choose which BMS they want, so they can’t be held accountable for the quality or when the SoC value is unreliable.
But who should have the most knowledge about the BMS and the SoC determination? The battery supplier or his customer?

A customer of an autonomous robot for warehouses used a Lithium battery from a large German battery factory.
After a few months, many of the devices were only able to run for a short time and had to be recharged more often. Upon inspection it appeared that the BMS thought the battery was empty, while in reality it was still 50% full.
The opposite also occurred: the BMS thought there was still enough charge, but the device stopped, thus blocking passage for others.
This caused the warehouse work to come to a completely standstill.

Self Learning

The SoC (State-of-Charge) gives a percentage of charge relative to the 100% charge. But to know how far you can go, an absolute number of the available charge is needed. This can be expressed in Ah (Amp-Hour) or Wh (Watt-Hour). To calculate this, the BMS needs to know not only the SoC but also the 100% capacity, and adjust it as it ages.

The Magic

The BMS can determine the remaining charge in several ways.

  • Based on current over time integration
    By keeping track of how much current goes into or out of the battery the Amp-Hour number of the measured.
  • Based on voltage
    Here, voltage is used to look up the remaining charge (State-of-Charge) in a table.
  • Based on a combination of voltage and current
    With a combination, the advantages of the previous methods can be used to compensate for the disadvantages of the other method.

Current Measurement

The most common method of determining the SoC is by measuring the current and integrating the amount of current over time. Because this actually is about charge, and the unit of charge is ‘Coulomb’, this method is called Coulomb-counting. One problem with this method is that changes that cannot be seen in the current measurement (self-discharge, aging) are not accounted for, so that the SoC starts to deviate more and more from the actual remaining charge.
Therefore, the SoC is adjusted under certain conditions, e.g. after (full) charging. When these conditions are never met in the application the deviations still occur.

Voltage measurement

With the voltage-SoC method the voltage of the battery is measured and the SoC value is looked up in a table. The table must be determined per cell type, because variations in cells lead to differences. With a good table, the correct relative remaining charge is always presented, even with self-discharge and aging. But it is relative, as a percentage of 100%, while the user needs the absolute residual value.

This relative value is only correct when the battery is at rest. A load current will cause the voltage to drop, resulting in a false indication.

Current-voltage combination

It is therefore obvious to combine the Coulomb-counting with the voltage-SoC method so that self-discharge, aging or deteriorating cells no longer have any influence. or inferior cells, have no influence anymore and the absolute residual charge is known and accurate.
The algorithm calculates both the SoC and SoH (State of Health), so it can also be seen when a battery is due for replacement.

Fusion algorithm for State of Charge

Engineering Spirit’s BMSs (and its trademark Axtrel.com ) are therefore equipped with a unique algorithm that processes the current and voltage measurement per cell into a SoC and the absolute remaining capacity.
For optimal results, the voltage SoC table and cell model are measured at different temperatures. When for an application, the SoC is not required to be as accurate, we work with standard values.

Would you like to know how to create the best battery for your application?
A telephone conversation with our technologist will quickly tell you.

Functions of a good BMS

  • Protections against damage and fire
  • Display of remaining capacity
  • Balancing of cells
  • Reliable communication
Lithium batteries and their BMS
What and why of the BMS and Lithium (Li-Ion) battery. And is a Battery Management System needed? The backgrounds and advantages.
Request the whitepaper
⇛   Lithium battery, State of Charge, Fuel gauge, State of Health

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