Battery – it’s not enough to replace, you need to program – part 2.


Battery – it’s not enough to replace, you need to program – part 2.

As it was in the beginning.

Early battery management system solutions did not measure inrush current. The values they measured were limited to the amount of current the alternator can deliver. An example of a current measurement implementation would be the sensor number 13502261. Using the QST-5 tester, it is very easy to recognize that this is a sensor powered by 5V and with a PWM output of about 126Hz.

Current sensor detected by QST-5 (no current flowing).

The design of this sensor consists of passing a cable leading from one of the clamps through the center of the sensor ring (the sensor itself has a special tab that allows it to be fixed on the cable in a fixed position). Note that this type of sensor Is non-invasive ie. requires running a wire through the sensor without electrically connecting it. Therefore, theoretically, the sensor could be placed on the plus wire. However, for practical reasons, it is basically always the negative terminal side.

With no current flowing through the sensor, the QST-5 indicates 50% PWM signal fill. For the test, a current of 5A was passed through the wire in both directions. This allowed to read signal fill values of 47% and 54%, which means that the sensor detects current flowing in both directions. The fill change is about 9% per 10A (or 0.9%/A). Based on the information that this sensor has a max PWM of 95% and a minimum of 5%, it can be estimated that the 90% range allows to measure current in the range of 100A (taking into account the directionality, it will be +/-50A). As with other sensors, a certain margin of indication is retained which is used for diagnostics. Therefore, the values of 0 – 4 and 96 – 100% can be information about shorting the signal to ground or power supply plus, respectively, or a general failure of the sensor.

Measurement results given by QST-5 when the current is 5A.
When changing the direction of the flowing current (-5A), you can see a change in the filling of the PWM signal read by the QST-5.

Battery condition – how to determine it?

Gradually, BMS systems began to be expanded more and more, but at the same time integrated into more autonomous systems. This led to the introduction in cars so-called. smart glue. This approach gave the opportunity to monitor in addition to the current in addition to the voltage directly at the clamps and the temperature of the battery itself. This design also allows you to measure inrush current which gives you additional information for system diagnostics. However, unlike the first systems, it is already electrically connected to the minus clamp. Due to the amount of information (measuring several parameters), it is already a system with digital communication, usually using the LIN bus.

Resting voltage

The state of charge of a lead battery is well reflected in the no-load resting voltage. That’s why the smart ticker reports as one of the parameters what the resting voltage and current were. However, in order to make such a measurement the car should be out of use for several hours. The controllers at this time will go to sleep and it will be possible to determine the resting voltage. In this state, the current will be drawn in the appropriate range, so that it does not affect the measured voltage (or its effect is known by the manufacturer). Continuous temperature measurement further supports the algorithms that calculate the available energy of the battery. Besides, it allows you to adopt an appropriate charging strategy.

Internal resistance

Analysis of the voltage drop under the influence of the current drawn is another parameter that allows you to calculate the internal resistance of the battery. This in turn is an important parameter for assessing the progressive state of wear and tear or the presence of sulfation.

Of course, newer and newer algorithms are being used, many of which are covered by various types of patents. Since such a sensor is connected all the time and monitors the parameters, it continuously “learns” the battery. No less for such a measuring system to work best it needs some correct parameters given at the beginning. Mostly it is the manufacturer, the type of manufacturing technology and the capacity of the battery, and sometimes it is necessary to enter the serial number. This data allows you to use ready-made algorithms prepared by the manufacturer and this results in the best way to monitor the status and capacity of the battery. Therefore, replacing the battery with a new one requires a procedure called battery adaptation.

Battery Adaptation.

A new battery has different parameters than one that has been in use for several years. As we age, certain measured values change. As mentioned earlier, controlling them allows us to determine two basic variables:

  • The degree of charge (SOC – State Of Charge),
  • state of health (SOH – State Of Health)

These values are crucial for the BMS because it selects the charging parameters so that the battery is always ready to start the engine. At the same time, it ensures that the car battery remains operational for as long as possible. Therefore, replacing the battery with a new one without informing the electronics about it can result in the kind of charging that was “learned” for the old battery. The result will be an underutilization of the full capabilities of the new battery. In addition, it can significantly shorten its lifespan. To inform the car’s electronics about the replacement you need to have the right software. That’s why it’s a good idea to have the battery replaced by a specialist, so that you can enjoy an efficient start-stop system for as long as possible without having to replace the battery prematurely.

You can replace the battery yourself, but …

The purchase of a new battery should be completed with the appropriate service procedure performed by connecting a special tester to the EOBD socket. In most cases, this is a simple and one-time action that informs the car’s electronics that a new battery has been installed. To carry it out, you need the right device and software, which may not be cost-effective for an individual user to purchase. In older vehicles, battery adaptation additionally requires leaving the car closed for several hours. Newer vehicles, due to more intelligent BMS systems, do not require such an expectation and can determine the necessary parameters themselves.


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