Mahindra XUV 300: A puzzling case of key fob battery failures

I was completely befuddled. How can I have a dead battery in BOTH my keys despite the complete opposite use?!

BHPian ashishk29 recently shared this with other enthusiasts.

About 3 days back, I noticed that my usual key fob didn't work by using the request sensor on the door, but unlocked fine using the buttons on the fob. I didn't think it was a big deal.

A day later, even that didn't work anymore. I had to unlock the car using the physical key within the fob, and of course that set off the alarm

Why? If it's an authorised key, the alarm shouldn't be based off of how the car was locked the last time around. (For the benefit of any who were unaware of this as I was, the car will sound the alarm if you don't use the same method to unlock as the last lock. So if I lock using the fob, and then unlock using the same physical key, it fires the alarm)

So now I had to get in the car, hold the fob at the start/stop button to get it on. Ok. So far so good. Car started fine. And then it kept beeping for "Key not in car" for the next several minutes. I stop at a signal, power the car off, and then on again, and this warning goes away. (WHY?!)

I reach my office, come back to the parking an hour later, and somehow the damn fob works?! HOW?! And somehow in the evening, the fob doesn't work again. Surprise surprise.

All is fine, I come back home, and my first assumption was, yeah, fob battery must've died. So I went inside my home, got my spare fob, which was unused for the last 2 years by the way! And whaddaya know. Same problem even with THIS key.

I was completely befuddled. How can I have a dead battery in BOTH my keys despite the complete opposite use?! I was almost convinced there was something wrong with my car.

Took it down to the showroom, and of course the service assistant suggests the first thing to try, change the fob batteries. "Anyway workshop will ask for the same thing" he says. Fair enough.

At first he changed the battery in one fob. And whadday know, it worked. I asked him to get in the car and fire it up to confirm. He does that, then powers off. However, as he was doing this, I accidentally clicked the button on the other fob which was in my hand, which did NOT have its battery replaced. And it worked?! (Again, WHY?!)

Anyway, he replaced the battery in the second fob as well, and so far things have been ok.

But this raises several concerns for me:

1. Why did the fobs work intermittently for some time?
2. WHY this stupid notion that a physical key unlock will sound the alarm? They have RFID chips inside the keys, can't the damn car detect it's a valid key?!
3. Why can't the car warn me when the fob battery is running low? I talked to a couple of my colleagues, and they said their car cluster warns them of low fob battery. In start/stop type cars, this needs to be mandatory.
4. I am still not convinced that somehow BOTH my fobs were dead at the same time.

Apart from this, got the ECU/EMS updated flashed at the same place. 15 minutes job, I was in and out before I knew it.

Does anyone know what this update was supposed to do/fix? A change-log of sorts?

Here's what BHPian subramanyam84 had to say on the matter:

1. That is simply how the cell electrochemistry works. It simply has caught you at a wrong threshold, where the cell voltage is at the borderline between 'good' and 'bad' for the fob transmitter to work. So it has appeared mystical to you. The discharge curve is not a smooth straight line. But a jagged line over 10's of millivolts. This would happen for a couple of days and the fobs stop working altogether. (I've noticed this pattern of cell death in Honda City, Wagon R, Ford Ikon. Of course there the keys were only to lock/unlock).

2. That goes down to the circuitry. Mechanical intrusion is seen as a mechanical intrusion. I'm not saying it's an intelligent way of addressing the issue. It's just a cheap way I guess. But even if a system were there to detect the original key, your cell was dead, so there's no way of telling. Maybe I should experiment with mine and check, one early morning

3. I'm surprised that the car didn't warn you. Hope someone from Mahindra reads this!

4. You could find literature that talks about constant drain (spare key fob) killing the primary cell faster than a similar primary cell subjected to the same constant drain with occasional discharge spikes (main key fob). It's not intuitive but you've to go into the working of a lithium primary cell to understand this phenomenon. Bottom-line, it's completely normal.

BHPian ashishk29 had the following update to share on the matter:

Update on my key fob situation:

One of the key fobs started giving a weak response today itself.

For reference, I got the battery in both fobs replaced on Dec 22/23. So it's been hardly 2 weeks.

I would've suspected a vehicle issue, but then why did it work for the last 2 weeks?

I am yet to try out the second key with the new battery, but I suspect it'll behave the same.

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