Another straightforward question with incredibly complicated and various responses. There are now some major cases:

Warranty replacement of a defective cell battery.

It is not frequent, but it does occur in a small percent of cases where the original battery achieves the degradation level normally set at 70% during the general warranty term, i.e. the battery capacity has dropped by 30% compared to its new state. If the car’s warranty history is in order (it was taken in on time for obligatory servicing, if required, and these were correctly recorded), the owner will receive a new, 100% battery.

There is a complicated prelude to this, during which the after-sales service tries to repair the battery, and if the standard measurement cycle also shows that the capacity is below the specified percentage, then the replacement is only time-related, not material. In the case of Teslas, battery replacement follows slightly different rules, it is also a little easier, but let’s not complicate my answer with this.

Replacing an old battery with a broken but better one.

Not all electric cars receive the same amount of usage; there are batteries that have been roughly used, charged many times, and are thus in worse condition, as well as spare batteries that have been used far less. It is particularly true with older battery chemistries that they age quite differently depending on usage.

When a car crashes the battery is very rarely damage, so these cars with a total loss of economy often have batteries in surprisingly good condition. In such cases, you simply have to remove the old battery, put on the new one, and some configuration may be required due to the different production numbers, but the transition is practically seamless. This is almost a routine task today. The range of the car increases already on the way out of the service, and with the new battery, longer distances can be covered.

Replacing an old battery with a larger capacity one.

Some car models were produced with several battery sizes. One of the best-known such cars is the Nissan LEAF, the earlier version of which was made with 24 and 30 kWh batteries, and the later version with 40 and 62 kWh batteries. But even more “hitting” is the case of the BMW I3, which is made with a 60 Ah, 94 Ah and 120 Ah battery, i.e. the original can even be replaced with a double-sized battery.

While several service centres undertake to replace the battery with a larger one in the case of the LEAF, which has much more limited knowledge, only 1-2 highly qualified service centres are able to transform the car’s electronics in such a way that the older construction can easily handle the larger battery of the newer version. If the deconstructed, smaller-capacity battery can be sold, part of it will return, but the hybrid functioning of regenerative solar inverters is not yet supported everywhere, so installing battery energy storage alongside the own solar systems is not worth it.

Replacing old batteries with aftermarket batteries.

Thoroughly familiar with the more hidden corners of the Internet, I have already found many aftermarket battery solutions. Technically, such a thing is not difficult, but legally it is rather tricky, the electric car is a dangerous operation battery fires have already happened with the largest manufacturers. Thus, those who wish to officially distribute aftermarket batteries do better for their own sake if they do not leave the mandatory certification circles, i.e. obtain the CE, TÜV, UL, EMI/EMC permits for their construction.

That was the easier part of his question. Now comes the digital one, how can a buyer know the condition of the used battery he wants to buy? The question is especially important when you want to buy a used car. Well, my answer is from nowhere! Lithium battery cells are bagged, cylindrical or rectangular, hermetically sealed units that do not contain any sensors or indicators inside, such as e.g. an acid level control window on top of an old lead acid battery.

All battery-related data is determined by the BMS, the battery monitoring unit, based on the usage data of the previous 1-2 days – or even 2-3 months (!). It measures the temperature and voltage of the cells and knows when, for how long, how much current was charged, or how much energy was taken out of it.

Based on this data, it estimates a battery condition, which is called SoH (State of Healt) and is measured as a percentage. The calculated SoH value – like all related data – is stored in non-volatile memory, and when we query the state of the battery with a smartphone, tablet or factory diagnostics, this stored data is shown to us. A hacker like myself can fake this in a dozen ways, but an average car-nepper has no difficulty in manipulating it in half a dozen ways.

How can cheats with battery information?

-With factory diagnostic software, you can issue a BMS-RESET command, which restores the BMS data to a factory-new state – at the end of production, or in the case of a complete cell replacement, this informs the BMS that the battery is new.

-Cloning software is used to read the good data of the BMS of a good battery and transfer it to the BMS of an old battery, which from then on sees the old battery as a new one. There isn’t much of a clue, because the data is believable, it’s not very clear what the fraud is about.

-You replace the BMS module with one bought from the Internet, which carries in its brain the data of a barely used battery of a crashed car – even from several years earlier, if it has spent the last years on a shelf. It’s also hard to spot because the data is believable.

-The non-volatile memory of embedded computers, called EEPROM, is unfortunately sometimes misleading. In such a case, the BMS resets itself rather than writing nonsense with incorrect data, e.g. 10,000 km range. Such a self-repairing failure can also be caused by various cheap tricks, e.g. with sparking, the 12V battery is put on (that way easy to damage the electric system and generate more fault) taken off until it catches the moment when the car is updating the EEPROM data because of this, the data will be damaged, and the battery will magically be flawless, as the BMS restores the factory data. Thus, a dealer can perform a BMS RESET even without the car’s factory diagnostic software.

-He moves the BMS from the faulty battery to the car with the better battery, drives it for 1-2 weeks, and the BMS realises that the battery is in much better condition than he previously thought, adjusting the SoH estimate higher. When the buyer applies for the automobile or the battery, he replaces the upgraded BMS, therefore the upgrade costs him nothing.

In the case of the stupid BMS, a somewhat more advanced method. Nissan LEAF is to heat up the battery, e.g. with multiple fast charging or car tendering, and the stupid BMS software measures a lower internal resistance (Rx) on the hotter battery, based on which it guesses the battery is better than it really is.

-Arbitrary data may be input directly into the BMS memory using computer hacking methods, much as a kilometre clock can be simply changed forward and backward. A cell defect, for example, can be removed such that the BMS does not detect it for weeks. When driving in the city, it may not be obvious, but on the first vacation, the automobile comes to a halt on the highway as if it has been nailed.

-You just take and place the BMS of a larger battery into the box of a smaller battery, taking advantage of the fact that various types of automobiles are constructed with varying battery sizes. Because there is no overhead reduction in other countries and the hybrid battery solar plant is also encouraged, many batteries are removed, and their worthless factory BMS is sold for a shockingly low price.

This is a trick for the fraudster, but what can the unsuspecting the buyer? Once again, you have to look at the in a reliable place. You can ask for the drive battery information data list that shows the KM you have traveled and time (day-month) also some model navigation unit hold the traveled distance.


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