[SDspeed]

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).

 

[SDspeed]

I am no engineer, but my understanding is that lithium batteries really need a different trickle charger over AGM or Acid batteries. It is not clear to me on the T6 how that car would handle one trickle charger hooked up to the one battery in the rear when the owner's manual says to jump the car from the posts under the hood. If a trickle charge so hooked up would even charge the other support battery.

It seems to me that for the T6 the positive terminal under the hood must be directly connected to the main battery in the rear of the vehicle without passing through the support battery or any other sensitive electronics. So the positive terminal under the hood is probably just an extension from the positive terminal of the start battery and the negative terminal is grounded in the engine compartment. If the connection worked any other way, there would be a risk of damage while jump starting. My jump starter outputs 100A, and you wouldn't want to send that through the support battery or other electronics. I do know that the voltage measured at the terminals under the hood is exactly the same as the voltage measured from the main battery terminals.

I assume the support battery is downstream of the main battery and it recharges by drawing current from the main battery. So the main battery acts as a buffer for the support battery and other electronics during jump starting. That would also explain how the support battery eventually charges when you connect a charger to the terminals under the hood.