What if you run out of battery in a vehicle with Start/Stop?

The protagonist of these lines is a SEAT Leon (5F) in FR finish, with 1.4 litre TSI mechanics, without cylinder disconnection or ACT, to which my father and I have done more than 44,000 kilometers in 2 years and 8 months, which comes out to about 1,375 kilometers on average per month.

Our previous car, a SEAT Córdoba (6K) bought in 1994, was much simpler, so after the acquisition of the new member of the family we defined rules of use for elements that we didn't have before, such as air conditioning and Start/Stop. With regard to the latter, we decided to use it only for city journeys, and to switch it off for motorway and road trips, thus facilitating rapid engine warm-up. For many months, the Leon has been making short city trips of about 2.3 kilometers, from home to my father's work, accumulating less than 10km per day, and not consuming a whole tank per month. This, in theory, represents a perfect scenario for the use of Start/Stop, but in the end, I'm the only one who respects this rule when using the car in the city; you know, because turbocharged direct injection engines produce cancer-related emissions.

Given this picture, the question is inevitable: "why the high mileage of the car?" And the cause is none other than the server himself; until 5 months ago I lived and worked in Germany, so when I returned from vacation I accumulated between 3,500 and 6,000 kilometers per trip, filling the tank between 2 and 3 times a week. Since my return to Spain, I've been more relaxed (about 10,000 kilometers in total), but the aging rate of the Leon has accelerated noticeably.

What if you run out of battery in a vehicle with Start/Stop?

About 2,000 kilometres ago (mid-June), a childhood friend of mine living in London, M., decided to take a short holiday and visit Tenerife for a few days. We usually meet in the evenings at a petrol station, located on the way to Playa de las Teresitas in Santa Cruz, so on the way to our meeting point I kept the Start/Stop running. The first night, the system worked without problems, but on the second night, the S/S didn't come into action. I put this down to the fact that the car had hardly driven any kilometres in the last 24 hours, and that perhaps the battery was in a low state of charge, so the car had decided not to perform the Start/Stop function. Two more days went by, and when I took the car again, the S/S still didn't kick in.

"Very good", I said to myself.

On the fifth day I had to drive about 50 miles on the motorway, which was more than enough for the alternator to fully charge the battery. The car was stopped for about an hour while I was doing a mid-drive errand, and on the way back I had to start it twice because it wouldn't start the first time; I thought it was strange, but I put it down to the fact that maybe I had rushed it by turning the key, not giving the starter motor time to start before returning to the ignition position. There was no warning light on the battery and no sign of a problem on the instrument panel, and after returning to town, the Start/Stop still didn't work.

Alerted by the situation and with obvious evidence of an impending battery failure, I decided to tell my father when I went to pick him up from work. After not believing me, and after convincing himself by playing with the Start/Stop deactivation button that something was wrong, he decided to take the car to the garage where we used to go with the Cordoba. At the garage he was advised to change the battery immediately at a dealer, and after parking there, he was asked to move the Leon to another parking space: he went back to the car, sat down, turned the key, and it wouldn't start. In fact, from that point on, the factory battery was unable to restart the car on its own.

The dealer didn't have a suitable battery for our car at the time, so they gave us a conventional replacement until it arrived, and obviously advised us to disable the Start/Stop. The next day, we called the salesman with whom we negotiated the purchase of the Leon, who is a family friend, and he reminded us that we had taken out a SEAT warranty extension for two more years, i.e. four years from the date of purchase of the vehicle, and recommended that we go to the official technical service. We returned the replacement battery to the dealer, recovered the factory battery from the dealer, and after starting the car we went to the official technical service where we left the car. In total, since the story began, six days have passed, and 24 hours after the original battery failed, the car was in the hands of the official SEAT workshop.

On the fourth calendar day after dropping the car off, with a weekend in between, I called. After going through several people working at the dealership, I was finally able to speak to a mechanic in charge of our case, and his response was as follows:

We have proceeded to run a diagnostic on your car, whereby we connect a device to it, get the data recorded by the vehicle and send it to SEAT. SEAT has just answered us recently about your case, and the diagnosis is as follows: at mileage 41.981 the car has registered an excessive use of the lights which has caused the battery to lose the minimum voltage of 12V. Therefore, we ruled out battery failure. On the other hand, we performed a slow charge procedure on your battery, but after performing it, the battery is not able to start the car. This is because if a battery loses the minimum voltage of 12V, it is no longer able to recover it no matter how hard you try to recharge it.

And just like that, just like that.

He wasn't even able to tell me directly that SEAT refused to cover the battery failure, he just kept on talking and just told me the prices of a replacement. But what I would like to know is how one can make "excessive use of the lights" in a model whose full LED headlights and automatic light management were a major selling point during its conception and launch, because let's not forget that the Leon was the first compact car to offer full lighting using light-emitting diodes, better known as LEDs. And I can swear that the only times during the life of the car when the lighting knob has not been in the "AUTO" position, have been when I have been trying to photograph the car with the headlights on. And, to turn on the front fog lights when driving on winding roads late at night, I just pull the knob out a notch, and push it back to the normal position, just like in the Cordoba.

Except for the comment about the fog lights, because I didn't mention it to him, the mechanic had no response to these arguments.

You're probably asking yourselves: "where was the Leon at mile 41,981?" By my calculations, it was either on the motorway, or parked halfway through my 50-mile drive. I'd rather swear it was on the motorway, but the fact that I had to start it twice after I'd done my business makes me doubt it. In any case, there's no way I could have left the lights on while I was running the car, because as soon as you take the key out of the lock, the lights go off, regardless of the position of the remote control; in fact, you can't even leave the high beams on by accident (I've tried it, and they go off). Without the key, you can only turn on the parking lights (left or right side position lights), which makes sense, because they are often used in emergency stop situations, where we may need to get out of the car but need it to remain visible, and the emergency lights are not enough.

As if that wasn't enough, on page 235 of the owner's manual, under "Energy management", it is clearly stated that the car has an intelligent energy management system that constantly monitors the battery charge level and the consumption of the car's electrical consumers, both when the car is moving and when it is stationary. This system always tries to ensure that the car will be able to start the next time the key is turned, so it can cut off the power supply to those consumers that threaten to discharge past that point. In fact, the system is even capable of de-energizing the key's built-in remote control receiver (yes, I know what you're thinking), if the car is left idle long enough. Given the situation, if someone leaves the car with the key in the ignition, is the energy management system going to let such a common occurrence as leaving the lights on drain the battery? If it works as the manual says it should not. And if it does, we're talking about a bug or software error, not user misuse of the vehicle.

What about the mechanic's argument that a battery, after losing 12V, can't get it back?

As soon as I heard that argument it sounded a bit strange to me. I have to admit that battery operation is not a part of the car that excites me; I'm much more interested in engines, turbochargers, suspensions and of course, design. But like any good engineer, I appreciate good solutions to complex problems, and I understand that in engineering there are no absolute truths, only compromises between multiple conditions and variables. That said, I set out to examine the process of charging and discharging a battery to see if that statement was correct.

Understanding how a 12V battery works on the inside

So let's go back to chemistry class for a few minutes: in a typical lead-acid battery we have 6 cells connected in a line, which are separated by blocks or partitions in the body of the battery itself, made primarily of plastic. Each cell is composed of a series of lead plates (Pb) of positive and negative charge, arranged in parallel and in alternating order, separated by a porous material that allows to isolate the plates between them and at the same time to allow the passage of the electrolyte, which is usually sulphuric acid (H2SO4). Bearing in mind that each cell has a nominal voltage of 2.12 V, multiplying we have that 6 * 2.12 = 12.72 V, which is the nominal voltage of the battery.

When the battery is fully charged (12.72 V voltage), the density of the electrolyte is 1.28 g/cm³, and represents 38% of the liquid inside the battery, the rest being distilled water (H2O). As the battery discharges, the sulphuric acid breaks down and its percentage drops, forming more water in its place, while lead sulphate (PbSO4) accumulates on both the positive and negative plates of each cell. As the battery is discharged, the voltage of the battery decreases along with the electrolyte density, the critical point being a state of charge of 20%, as the voltage drops to 11.9V and the density to 1.14 g/cm³. When recharging a battery above the critical point (electrolyte density greater than 1.14 g/cm³ and voltage of 12V or higher), the application of current to the battery easily reverses the chemical reaction produced by the discharge: the lead sulphate and water accumulated in the cells is converted to lead (which is stored in the negative plates), lead dioxide (PbO2, which is stored in the positive plates) and sulphuric acid, thus recovering the electrolyte density.

The critical point of 20% of the battery, which coincides with the famous "when a battery loses 12 V it is not possible to recover them" of the mechanic of the official workshop, is not because the chemical reaction of load cannot restore the density of the electrolyte with that level, but the amount of lead sulphate accumulated in the plates of the cells is so much, that can cause them to harden, preventing them from accepting a current charge to initiate the chemical reaction of load. To solve this, it is possible to recover a battery that has lost the famous 12 V with a slow charging process (low voltage), to try that the plates begin to consume the lead sulphate and reconvert it into lead, lead dioxide and sulphuric acid, but the chances of this working decrease as time goes by, because the plates become harder due to the accumulated lead sulphate.

Now, the factory battery in the Leon, as you are probably thinking, is not a standard lead-acid battery, but an EFB (Enhanced Flooded Battery) or gel battery. How is it different from a conventional battery? Mainly that the electrolyte has been mixed with a silicon substrate that absorbs the acid and turns the electrolyte into a solid paste that is difficult to spill, so these batteries can be placed in any position, unlike conventional batteries. In these batteries, the oxygen stored in the positive lead plates manages to recombine with the hydrogen in the negative plates, forming water that is recycled in the electrolyte, and that is why they are considered maintenance-free batteries (no need to add water). Some manufacturers also mix phosphoric acid in proportions of 15% to the electrolyte, which increases the capacity of charge and discharge cycles that the battery is able to withstand, also making them more likely to recover after falling to a state of charge of 20% or less.

The battery lost the ability to start the Leon on a Wednesday lunchtime, and 24 hours later it entered SEAT's technical service, where it presumably underwent a slow charging process to be recovered. If the car's electrical system did not excessively discharge the battery (in which case, we already said that it could be considered a software error in the system, and not an incorrect use by the user), and, if we assume that if it was in correct condition the battery could have been recovered by the slow charge process, what could have gone wrong? The only conclusion I can come to is that at least one of the cells died and stopped providing its 2.12V, and since 5 * 2.12 = 10.6V, it is impossible for that battery to restart a car.

The theory of a battery failure is supported by the Club SEAT Leon forums, where there are numerous posts of cars bought in late 2013/early 2014, and whose batteries died at around 2 years and 40,000km; in some cases the cost was covered by SEAT, and in others like ours, it wasn't. I have to admit that the battery was put through a lot of cycles where it barely had time to charge, spending a lot of time on short city drives, but I don't consider it an excuse for it to fail with only 2.5 years of use.

So... new battery and that's it, right?

Almost. We decided not to proceed with the repair at the official garage, which was asking about 190 euros including a special discount of 25%, and go to the local battery distributor who offered us an AGM battery of the AtlasBX brand (subsidiary of Hankook), for about 115 euros. As in EFB batteries, in AGM (Absorbent Glass Mat) batteries, the electrolyte is in a solid state instead of liquid, but in this case it is achieved by using microporous glass fibre mesh between the plates, which absorbs it completely, without becoming saturated in any case. They also have other advantages over gel or EFB batteries:

  • Even lower probabilities of leakage in case of rupture of the battery cover, as the electrolyte is completely absorbed instead of being in solid state.
  • Increased resistance to battery self-discharge, due to the use of lead-calcium alloys
  • Higher cold cranking capacity, due to the lower internal resistance to the chemical reaction between the electrolyte and the plates.

What they do have in common, both gel batteries and fiberglass batteries, is that they suffer in high temperature situations (>55 ºC), so it is curious that they end up in the engine compartment. For this purpose is why they are usually covered with a cover and even with a plastic cover; to protect them from the heat, and not from the cold as some may think. It is also curious that Volkswagen's technical document on batteries states that "gel batteries are not installed in VW brand vehicles", while SEAT installs EFB batteries in petrol models of the Leon, reserving AGM batteries for petrol cars (TDI).

With a new battery, the problem persists...

On the way home after installing the new battery (a few kilometers through the city and highway), the Start/Stop still didn't work. That same afternoon we returned to the workshop, where we proceeded to eliminate the errors in the various control units of the car, the result of having lost power during the battery change, and to re-program the new battery to the car with its data, only that the new battery lacked the label with the QR code to read directly the data from it (you can see this label in photo of the factory battery), so it was re-programmed with the "Custom Battery" option of the diagnostic software, and all the data was entered by hand. After leaving the shop, we did a highway run of about 30 miles, stopping for about an hour in the middle, to try to recharge the battery in case that was the problem (gets repetitive, doesn't it?), but the Start/Stop refused to engage, and in the Start/Stop trouble menu, we were concerned to read a message stating "vehicle power consumption is high".

The message was not at all reassuring, considering that the battery failure occurred during a mainly motorway journey, and that another 50km journey had failed to charge the battery sufficiently for the Start/Stop system to work. Did this indicate a possible alternator malfunction? Had the alternator allowed the factory battery to fully discharge? Or had its poor management of the battery charge caused one of the cells to fail? Not really.

One of the theories we had as to why the alternator had not charged the battery, during the 30-mile drive, is that perhaps the alternator was governed by the engine control unit, and that the engine control unit would disengage the alternator at intervals to reduce consumption, as there was one less element stealing power from the timing belt. As a result, the battery would power the vehicle's electrical system during these phases, and it would not be easy for it to reach 100% charge. But this idea seemed unlikely because of the increased complexity and cost that such a system would entail. We didn't get it right, but we weren't wrong either.

The alternator current regulator is connected via the LIN-Bus Charge to the battery regulation module, which is integrated in the on-board network gateway or control unit under the instrument panel. The LIN-Bus is controlled by the gateway, which acts as the master, and uses this bus to communicate with the devices connected to it, which act as slaves (maximum 16), via a single cable. The LIN-Bus is used as a complement to the better known and more expensive CAN-Bus, as it allows to build sub-systems that can communicate with a fixed response time, high flexibility, communication error detection, error detection in the slave devices and low cost as far as silicon is concerned. The gateway however, assumes many more responsibilities, being also master of the LIN-Bus Flywheel, being connected to all CAN-Bus of the vehicle (except the private ones), and being responsible for translating messages between different CAN-Bus lines, among other functions.

Depending on the signal it receives from the gateway, the alternator can:

  • Generate 12.5 V: This occurs at low alternator load phase, and happens when the accelerator pedal is depressed or when the cruise control is active. When the alternator produces 12.5 V, fuel consumption is reduced as there is less load on the engine, and it is the battery that supplies most of the energy to the car's electrical consumers.
  • Generate 13.5 V: This happens when the vehicle is in a low alternator load phase, but it is necessary to generate a higher voltage, for example because the driver requests a high engine load and the engine speed exceeds 2,500 revolutions, because the coolant is very cold (13-15 ºC) or very hot (105-110 ºC), or because the ventilation turbine is operating at high intensity, among others.
  • Generate 14.5 V: when the vehicle is braking and/or the engine is in a holding phase, the alternator enters a high alternator charging phase, which allows the battery to be charged (more than 12.7 V are necessary for a full charge due to chemical reaction losses), and to supply current to all the car's consumers. This increase in alternator voltage has the added benefit of an increase in engine braking which, in any case, my father and I agreed that the amount of engine braking provided by the Cordoba's 1.8 single-shaft petrol engine was greater.

Of course, there are exceptions where the control unit requests the alternator regulator to produce 14.5 volts: when the battery is not charged or there is a danger of discharging it, when the battery discharge during the journey is more than 15%, when the alternator provides very little electricity to the consumers, when the transport mode is active, when the battery temperature is extreme (very low or very high) or when the outside temperature is very low. The control unit can also request the alternator to generate 14.5 V if the idling speed has been increased, which is often the case when the Leon starts completely cold.

This is applicable to all VAG Group vehicles with Start/Stop technology, and it is likely that similar, if not the same, ideas will be applied in other car groups.

OK, that's good. Very interesting but... what happened to that Start/Stop?

Ah. It was funny. After returning to the trusted garage, we suspected that perhaps not being able to program the new battery into the Leon's ECU, automatically via the QR code, might have something to do with it, so we did a battery swap with the garage owner's Start/Stop vehicle; this served a dual purpose in that if the owner's car was able to rely on our new battery for the S/S functions, perhaps the problem was with our vehicle. However, the consensus was that the Lion needed to do more miles to rely on the new battery. So, after programming the Varta Silver Dynamic AGM battery from the garage owner's car, identical to ours in the Leon, by reading its QR tag in the diagnostic software, I took the car and did my homework: I did 250 kilometres in two afternoons, mainly on motorways, but with some steep sections to make it easier to charge the battery.

The next day we were going back to the garage, this time to drop the car off to see if there really was an electrical problem, but I gradually gained confidence that the Start/Stop would eventually work; the car's cold start was smoother than with the factory battery, and the Start/Stop worked with the battery we bought from the garage owner's car after a day's use. I also confess that I wanted to believe, blindly, that whoever wrote the code for the Leon's on-board network control unit did their job correctly.

The car literally stopped automatically at the garage door as we were about to drop it off. So we retrieved our battery, cleared the errors in the control unit from the new disconnect, and the Start/Stop continued to work without needing to do another 250 kilometers.

According to SEAT's own documentation on battery replacement in vehicles with Start/Stop, after a replacement the battery must be re-programmed in the control unit for battery monitoring. This causes the control unit to carry out a charge determination process for the new battery, which requires approximately 10 engine start cycles. Adding up the many journeys that were made during those 250 kilometres, it works out that the engine was started approximately 10 times.

So why didn't the Start/Stop work as soon as the new battery was inserted, if we hadn't reprogrammed the engine?

This probably happened because the dealer who carried out the installation did not eliminate the errors in the various control units of the vehicle, and with these errors present the Start/Stop system was deactivated. Once the battery control unit took the measurement of the borrowed battery, we got our battery back and did not proceed with a re-programming because the characteristics were the same. And after eliminating the errors in the control unit, we went out and at the first opportunity, the Start/Stop worked normally.

What if you run out of battery in a vehicle with Start/Stop?


In a very, very strict way, vehicles with Start/Stop that take advantage of engine retention to charge the battery can be considered vehicles with electric KERS, because they transform the kinetic energy of the crankshaft rotation into current that is stored in the 12 V battery. I don't go into whether they are micro-hybrids or not, because some of you require a reversible alternator, which is able to act as a starter motor, to consider a vehicle as such. But in that case you can rest assured; the electrification of non-hybrid gasoline vehicles will continue to advance, as Bosch announced a couple of years ago with the announcement of its Start/Stop system with "sailing" driving mode, capable of turning off the combustion engine when we leave the car in neutral without accelerating or braking.

This in turn, could be improved with a regenerative braking system that allows not turn on the engine to stop the vehicle, and charge a small battery of about 5 kW, which could be used to start the vehicle through the reversible alternator or an electric motor which, in turn, could feed a mechatronic turbo. There are plenty of possibilities.

Would you have saved all this, if you had paid what the official workshop was asking?

It is possible. The battery distributor listed the Varta Silver Dynamic AGM as the one from the owner of the garage we went to at about 178 euros; with a bigger discount to buy that battery and adding about 25-30 euros of labor, it would not be unreasonable to think that the official garage would have installed that battery, re-programmed the control unit and told us that the Start/Stop will take a week to work again, and that if it doesn't, we should go there again. It is also possible that they installed another gel battery, with the same specifications, and that when you leave the workshop the Start/Stop would be working because there would be no need to reprogram it.

The official workshop could also avoid the errors caused by the loss of power to the control units by supplying electricity to the vehicle while the battery is being replaced. But what is clear is that they would have offered us the security of having found and solved the problem, and not having to worry about why the Start/Stop was not working. So what we saved in money, we ended up paying for in suffering and anxiety, but at least the article you have in front of you came out of it, and luckily I still have a car to tell the tale.

What if you run out of battery in a vehicle with Start/Stop?

Now, if you'll excuse me, I've been informed that this morning the Lion was given dinosaur juice to drink until his stomach was full, so I think I'm going to help him digest a little.

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