Thanks for the replies. I'll try to answer some of the questions and give some more information.

Sadly, I already went out and bought the laptop for the trip. There were reasons for choosing a laptop over a tablet, but now that I'm faced with this issue, I certainly wish I had gone with a tablet and dealt with any shortcomings.

Power won't be an issue while I'm in the US, but I'm planning to go through Central and South America, so there will be some longer stretches where I don't have regular access to power.

One of the big reasons for buying a cache battery in the beginning was to ensure that devices get a regular charge when using the dynamo. Using pass through charging with the dynamo connected to the battery connected to the Garmin means the Garmin isn't constantly alerting me every time I slow down or speed up and it stops or starts charging. So I'm looking for a battery that can charge the laptop, but can be charged by the dynamo. I'm beginning to think it doesn't exist, or maybe as suggested I need to find a more specific forum. Thanks for all the replies and suggestions!