Problems
The BMS, the dark side of the force (1)
Lithium technology batteries have a special chemistry that needs to be closely controlled. This is the role of an electronic element, the BMS (for "Battery Management System") which will recover physical measurements from sensors placed in the battery, and will first ensure its safety, by turning it off and therefore stop the load or the vehicle if necessary, so that out-of-specification currents or temperatures do not damage it. Badly managed, a lithium battery can catch fire...
As a battery is made up of several theoretically identical cells but in reality with different behavior, the BMS will also ensure the correct distribution of the currents between them, in order to balance them.
What interests us here is the other role of the BMS, which is to provide useful information to the driver of the vehicle, primarily of course the current level of charge (SOC for "State of Charge"), because it is better for him to know if he will soon run out of fuel or not... It is also the BMS which will provide the SOH, here we are.
The SOH is a calculation of the BMS (2)
Being the result of the comparison between the current capacity of the battery with its original capacity, the SOH is expressed in %. So a 75% SOH indicates that the battery has lost a quarter of its original capacity.
The problem of its determination seems simple: if we compare the battery to a tank which shrinks with wear, the SOH is its current volume, while the SOC is its gauge. To know its current volume, simply empty it completely, then fill it to the brim and measure how much you have put in. Bad luck, that's where it gets complicated.
Indeed, technically, the SOH, no more than the SOC, is not a direct physical measurement as one can measure the number of liters that one has put in a tank. It is an estimate made by a BMS algorithm, resulting from complex calculations made from direct measurements that it compares to theoretical models.
The precise calculation of the SOH requires calibrations (3)
Builders are judges and parties (4)
A user who notices a sharp reduction in his autonomy will have to go to his dealer for a diagnosis. In the case of rental batteries, therefore potentially exchanged free of charge, we have unfortunately seen testimonies of aberrant diagnoses, without written report, with so-called 100% SOH on vehicles several years old. Let's kindly chalk it up to incompetence, but at the very least it underscores the user's brand dependency situation, until now.
Manufacturers manipulate the SOH (5)
Many car manufacturers limit the performance of their new batteries by software at the BMS level. Their goal is to give a good image of the durability of their equipment by artificially delaying the moment when it is time to change the battery.
For example, a manufacturer can decide, via the BMS, to limit the performance of its new battery to 90% of its real values. When the customer sees the SOH drop below the fateful 75% of the warranty, he goes to his dealer, thinking he has to change the battery. The dealer is then content to carry out a simple reprogramming of the BMS, paying most of the time, so that it uses all of what is available, which effectively increases the autonomy, and reinflates the SOH.
Of course this trick has its limits since in this example we can only do it once, but we understand that the SOH provided by the BMS is decidedly far from being an absolute truth and that the manufacturers do not want their customers access it too easily.
Solutions
Access the SOH yourself
Thus the SOH is, to date, never indicated on the dashboard of an electric vehicle (except on the Nissan Leaf, but with great inaccuracy), for the reasons given above. To access it, it is necessary to use an external electronic box which will be able to read a lot of information, including the SOH, via a diagnostic socket, the famous OBD socket (for "On Board Diagnosis") which is now fitted to all vehicles. Many solutions exist on the market, from the simple dongle that can be read with a smartphone application via Wi-Fi to the sophisticated device with touch screen. (6)
However accessing the SOH information does not make it more reliable, but given what we have seen of the technical operation of the BMS, we can at least try to improve its accuracy.
Make the SOH more accurate
To properly calculate the SOH, we now know that the BMS needs measurements made at the ends of the recharge range, in other words calibrations.
In practice, a calibration will consist of:
- Take a preliminary reading of the SOH via an external box.
- Drive enough so that the remaining charge approaches 0%, without of course running out of energy.
- Leave the vehicle at rest "for a while" so that the battery cools down, the ideal being an internal temperature of 20°C. An external box is usually able to provide it.
- Start charging up to 100%. You shouldn't settle for 98 or 99%, and if the load freezes before 100%, the BMS is a bit lost in its calculations. In this case, you have to unplug and reconnect until you reach them. For those who have a choice between slow or fast charging, slow charging should be used so that the BMS can recalibrate finely.
- Take a new reading of the SOH and note the difference with the first reading. The second should be closer to reality.
The opacity that surrounds the BMS algorithms makes it impossible to know if a single calibration is sufficient to obtain an immediately more precise SOH. It is more than likely that the BMS performs smoothing to eliminate occasional errors. On the other hand, we know very well that putting the battery in a state of discharge that is too low (less than 10%) harms its longevity. It is therefore necessary to avoid repeating the calibrations too often.
Evolution of practices and legislation
Regarding the sometimes questionable actions of brands and their dealers, things are moving a little. For example, Renault, via its Mobilize subsidiary, is beginning to offer on-line diagnostics (7), which can circumvent the incompetence of certain dealers, and we have also seen the emergence in Europe of independent companies, such as Moba in France, for achieve more objectively (8).
On the other hand, European legislation, through the implementation of the future "digital battery passport", should force manufacturers to provide the actual capacity of their batteries and their state of health, among other things (9 and 10).
Conclusions
An external box therefore makes it possible to know the value of the SOH, bearing in mind that it is only an estimate of the BMS which must be calibrated beforehand to work better, and that it can be biased by the will of the manufacturer.
We can hope for positive developments, which are essential for consumer information and confidence, through the development of healthier practices on the part of manufacturers, through competition from independent companies and through the legal obligation to provide information via the future "digital battery passport".
Sources
(1) Electropaedia - "Battery Management Systems (BMS)" : https://www.mpoweruk.com/bms.htm
(2) Example of algorithm development for the calculation of the SOH: Thesis of Aurélien Lièvre (in French) - "Développement d'un système de gestion de batterie lithium-ion à destination de véhicules "mild hybrid" : détermination des indicateurs d'état (SoC, SoH et SoF)" : https://theses.hal.science/tel-01178049
(3) MDPI - "State-of-Charge Estimation with State-of-Health Calibration for Lithium-Ion Batteries" : https://www.mdpi.com/1996-1073/10/7/987
(4) Evidence of dubious diagnoses (in French)
Véhicules électriques : http://www.vehiculeselectriques.fr/viewtopic.php?f=72&t=17191
Facebook : https://www.facebook.com/groups/twizyfans/posts/1724882684341726/
(5) Challenges - "How automakers are dodging electric car warranties" (in French) : https://www.challenges.fr/automobile/dossiers/comment-les-constructeurs-esquivent-la-garantie-des-voitures-electriques_785612
(6) Example of Twiz O'Meter : https://lifeintwizy.blogspot.com/2023/01/twiz-ometer-screen-that-says-lot-about.html
(7) L'Argus - "Electric car. How to know the health status of the battery?" (in French) : https://www.largus.fr/actualite-automobile/voiture-electrique-comment-connaitre-letat-de-sante-de-la-batterie-10945393.html
(8) Moba - "SOH battery and capacity: what you need to understand" (in French): https://get-moba.com/soh-batterie-et-capacite-ce-quil-faut-comprendre/
(9) European Parliament - "Agreement on new EU battery rules" (in French): https://www.europarl.europa.eu/news/fr/press-room/20221205IPR60614/accord-sur-de-nouvelles-regles-europeennes-relatives-aux-batteries
(10) L'argus - "Electric cars. Highly polluting batteries banned in 2027" (in French): https://www.largus.fr/actualite-automobile/voitures-electriques-les-batteries-trop-polluantes-interdites-en-2027-10677395.htmlhttps://get-moba.com/soh-batterie-et-capacite-ce-quil-faut-comprendre/
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