Undersized battery in Tiguan, quits on startup

[foopy]

Hey All,
I bought my used 2009 Tiguan TDI (2.0L, 4Motion, 103kW) which had an undersized battery installed. Before I discovered it was undersized, it was borrowed by someone who experienced a dead battery, and replaced it with the same undersized battery. Now I have this near-new battery:
* Yuasa DIN53LH which has 500 CCA and 55Ah capacity.
I discovered that the correct battery should be:
* Yuasa DIN65LH which has 680 CCA and 68Ah capacity (or equivalent, up to 740 or 800 CCA)

My question is: how BAD is it that I have this undersized battery?

This Tiguan TDI starts easily: and fires up after 2 to 5 turns of the crank, but it does seem to crank rather slowly. It has done this since I had the car, so i'm not sure what to expect, but my Touareg TDI cranks much faster with its massive 95R battery.

One thing I observed, is an issue where the TDI engine will turn over, fire up twice, and immediately quit. The engine runs for 1/4 of a second and quits. It doesn't do this when it's cold, but when warm, it usually does this once or twice, then it starts up normally. The car runs perfectly well otherwise.

Has anyone observed this issue (engine fires up and immediately quits)?
I was wondering: could it be because of the undersized battery?

That is, if the starter-motor doesn't get enough power, it struggles to properly turn over the (high-compression diesel) engine, and fires up, but barely, and can quit randomly. Once running, it doesn't quit, only quits immediately after startup. That's why it seems like an issue with battery + starter-motor.

I'd love to know if anyone out there can provide some insight.

 

[ZippyNH]

Add a jump pack when you are starting too see if it still dies after cranking....be useful in troubleshooting....or simply change the battery, you NEED one anyway. If the problem goes away...no need to chase issues, you found it.

Guessing that the voltage drop during start is causing the computer to reboot or shut off...bet if you watch the system voltage with a volt meter, it drops to less than 9 volts during the crank....
Most often, an undersized battery results in a VERY short battery life cause is so badly depleted by every start that they fail very fast.... typically a lead acid battery drained below 50-60% wears quickly....

 

[TurboABA]

Voltage drop test starter circuit.

https://static.nhtsa.gov/odi/tsbs/2020/MC-10174460-0001.pdf

 

[turbodieseldyke]

Has anyone observed this issue (engine fires up and immediately quits)?

Sounds like immobilizer kicking in. Look for that light on your dash next time it happens.

I dont know why anyone would intentionally install a smaller battery. I try to oversize mine when possible.

 

[foopy]

Thank you all so much for the very helpful information. It helped me solve this problem.

>> Add a jump pack when you are starting too see if it still dies after cranking
An excellent suggestion! I happen to carry a jump starter with me. For the last several days, over 15 trips, I used the jumpstarter each time I started.
With the jumpstarter, it cranks quickly and starts up immediately, and it NEVER quit once after startup. Wow! for the first time in 5 years I've owned this car, it cranks like a normal car!

Thus I can confidently say that the issue of "quitting after startup" was caused by a weak undersized battery.

>>Guessing that the voltage drop during start is causing the computer to reboot or shut off.
This makes sense, and explains why the engine quit immediately after firing up.

>> Most often, an undersized battery results in a VERY short battery life
I expect that this would be the case. Next week I will use a 'battery tester' to measure its "state of health" but I expect it is basically dead by now. I'm curious to know just how quickly this near-new battery was killed, and I look forward to replacing this battery immediately after that.

>>I dont know why anyone would intentionally install a smaller battery.
It's interesting. The previous owner had it since new in 2009, and they were meticulous, and had service records for all servicing at the VW dealership. Yet somehow, (probably unknown to them) it had that undersized battery in it. Maybe they had the battery replaced by a "roadside assistance" service who made a dumb mistake.
When I got the car, the battery sounded weak to begin with. It was garaged 9 months a year and not driven much. Battery failed when my brother borrowed it, and he took the battery to an auto-parts store and got another battery just like it. That was 1 year ago.

>> Voltage drop test starter circuit.
I didn't try this. But I don't expect there will be any resistance in the cables because the jumpstarter gave it the extra current it needed, and cured the issue.

I will post an update after testing the battery, and after final replacement of the battery next week.