That's not normal and it's odd that the dealership (Volvo of Austin?) keeps replacing the batteries instead of investigating the root cause (e.g., software fix).

 

Please forgive me, I am a newbie. Does the XC90 T6 non-hybrid have two batteries? I read the manual and it says:

The start battery is used to start the electrical
system and power electrical equipment in the
vehicle. The hybrid battery is used to start the
combustion engine.

But I don't have a hybrid but do I still have a hybrid battery that is just used to start the engine? I thought only the Hybrid T8 had two batteries. I know there is one "traditional" car battery in the rear hatch driver side and there are charging posts in the engine compartment but where is the second battery that starts the engine on the non-hybrid?

Not on Volvo, but I have used the Deltran battery tender for 20 years on another car that is a weekend only driver and has a difficult time keeping a charge if left idle for a week. The key to these trickle chargers is to make them easy to hookup. I did not want to have to open the hood or other compartments to connect them. And I wanted to make it obvious that it is plugged in so it is impossible to drive away with it attached. The trickle charge companies sell adapters that converts the normal terminal clips to a cigarette lighter connector. The only problem is that some modern cars do not supply +12V to the accessory cigarette lighter sockets when the car is not running. For those cars, I mounted a permanent wire from the battery terminals and mounted a cigarette lighter connector on the B pillar of the driver's door so it is very easy to access and always has power present. It is impossible to not notice it is connected and impossible to forget to disconnect it before driving away. So I enter the car I can easily remove the trickle charger connection and put it back after I park it.

Trickle chargers have improved over the decades and now Deltran is not as advanced as it was decades ago. If I were to have to buy another, I might look at the CTEK ones.

 

Please forgive me, I am a newbie. Does the XC90 T6 non-hybrid have two batteries? I read the manual and it says:

Yes. There's two, a primary and what they call a "support battery". It's a different part number. I know the support batteries main function is for auto start/stop. I'm not sure if it has other duties. I believe the battery under the hood is the support battery (I don't think it's just charging posts). The primary and larger battery is in the rear corner of the car.

 

I use the Battery Tender brand on my "fun" cars which I don't drive for long periods. Seems like it would be easy to hardwire one to the support battery in the rear of the XC90. This tender has a 2-prong plug in the cord with a cap on it....feed the cord through a gap in the battery cover upholstry and then plug in as needed.

It's perplexing how a vehicle with a 600 pound battery that you charge with 220v power has a standard battery that is not charged from anything but the engine when running correct? There are some incredible designs and engineering that are way ahead of the industry in the T8 but that aspect seems like that could have been designed better.

 

The OP was asking about the T6. My experience is with the T8.

Others know more than me regarding non-Hybrids. With that said, the T6s et al are likely similarly configured as the T8, except for:
1. the additional 12v start/stop battery: and
2. the hybrid battery that is only in the T8. (That will change in future mild hybrids in MY2020 and beyond).

On the T8, the hybrid battery charging system also charges the 12v battery as long the hybrid charger is connected and the hybrid battery is still accepting a charge (i.e., not full).

A 12v battery tender is not normally needed during frequent usage if the hybrid battery charging is being regularly used. I see two uses for a battery tend on a T8:
1. Extended use of Sensus, with electrical system on such as for installing software updates, familiarization, et al; and
2. Extended periods of non-use where the hybrid battery charger is not in connected and in use (as recommended by Volvo). Upon returning from non-use, the hybrid battery should be at ~25% and would be plugged into the hybrid battery charging system and would contemporaneously charge the 12v batteries.

At least for the T8, the 12v battery tender would be typically attached to the "jump posts" near the Support battery under the front [hood] and it charges or maintains the Main battery in the rear. (I supposed one could implement a permanent connection since most battery tenders have quick connect/disconnects.)

 

My experience has been that in all of our electric cars they have a 12 volt battery for accessories and the traction battery. The charging system does not affect the 12 volt accessory battery. If I recall the traction batteries are somewhere around 400 volts. So there is a compatibility issue I believe. This was also the case on my three hybrid vehicles from Toyota. They all had separate 12 volt systems as well as the smaller traction battery. They keep them separate perhaps from a safety standpoint? They would probably need a separate inverter for the 12 volt in addition to the traction battery inverter.

 

Assuming the OP has a T6,

You have two batteries. The main start battery is located in the cargo area and is used for starting the vehicle as well as powering the electronics during operation. The second battery is a smaller "support" battery located under the hood that provides uninterrupted power to the electronics during start/stop when the main battery is being used to restart the engine.

If the voltage of either of these batteries drops too low, start/stop will be disabled. That's the first indication that voltage is dropping. If you don't charge the batteries externally or by driving a longer distance, you may eventually see a low voltage warning message on your dashboard.

The terminals under the hood are used for jump starting the car or charging the batteries. Volvo specifically warns against charging the batteries or jump starting the car by connecting directly to the main battery terminals in the cargo area (they don't want you to connect to the negative battery terminal, but instead use the negative grounding connection under the hood).

Based on my experience, if you connect a charger to the terminals under the hood, the start battery in the cargo area will be charged first. If you disconnect the charger as soon as the main battery is charged to 100%, then the support battery under the hood will not have a chance to fully charge and start/stop may not be reactivated. If you leave your charger connected overnight, you are more likely to fully charge both batteries and reactivate start/stop.

I charge our T6 overnight about once per month and have been doing so since it was new. The car is mostly used for urban trips shorter than 15 minutes, so start/stop frequently stops working due to low voltage. I use a Schumacher charger that initially charges at 30A and eventually switches to a 2A float charge. It also has a 100A jump starter function that I've never used. We are still using our original batteries. Volvo warns against using a quick charge function, but I haven't run into any problems charging at 30A.

If you don't have a garage or don't want to open your hood to charge the batteries, there are float chargers that come with pigtails that you can leave connected to your charging terminals under your hood. The other end of the pigtail is intended to be left accessible from outside the vehicle, and you connect the pigtail to your charger instead of opening the hood. When I've used pigtails on other vehicles, I also make sure to throw a reminder note on the driver's seat so I don't drive away with the charger still connected. Battery Tender is one brand that offers pigtails on some chargers (or as they describe it, ring terminals and a quick-disconnect plug).

 

Given we're not driving much during these crazy times, I'm going to battery tender once a week. I'm still hopeful the conclusion that using the charge terminals under the hood charges both. I'm using this charger: https://www.amazon.com/BMK-Maintainer-Detachable-Alligator-Waterproof/dp/B06XK7PM9F

It does have intelligence and it does properly cycle through reporting various state of charges, but not sure if it stops when one battery is full versus the other.

All that said, since I originally posted, my January 2019 battery replacements have held up. We've had consistent availability even as of earlier this week of auto start/stop, something the car rarely supported prior. That said, we moved in mid 2018 and my wife's commute grew, but we also don't drive the car on the weekends since we take the Tesla.

Here's a picture of the tender on the car:
View attachment 59771