Lots of possibilities out there at a lot of different quality and price points (and the two are not necessarily related).
One good option are the SOK server rack battery packs which are now UL1973 Certified.
SOK Battery | 100Ah 48v | User-Serviceable Rack Mount Battery – Current Connected
$1,650 each at the time of this writing. 48v, 100Ah, 5.2kWh. Lots of videos tearing these open and looking at components, testing, etc.
SOK sells a lot of other batteries if one wants 12v or 24v. I use two 12v 206ah (2,636wh) in my fishing boat to run my electric bow mount motor. Had the same setup on my previous boat, too (sold those batteries when I sold that boat). For this most recent boat, I sold the 9.9hp gas kicker outboard because I could troll so well with the electric. These batteries are monsters. If I drained them from 100% to 0%, calculations based on rear-world amp draw shows I can troll 2mph for 27 hours or 3mph for over 9 hours. I typically keep them at around 80% State of Charge (SOC), go and troll for 5+ hours and come home having used less than 20% capacity, so I only charge every 2 or 3 trips to bring them back to 80% SOC. Each 206ah battery weighs 48lbs which is less than the typical 75ah lead acid battery that are only supposed to be discharged to 50% SOC (so their capacity was really 37.5ah). All the weight is mid-boat which is great for weight distribution plus, I got rid of the 100lb kicker off the transom and don't need to do annual engine oil changes and lower unit oil change.
The lifepo4 typically don't like huge loads like for starting applications, but Dakota brand makes a 60ah and 100ah lifepo4 battery that are rated to start outboards. I use the 60ah as my starter battery and to run all my 12v electronics (fish finder, livewell and bilge pumps, windshield wipers, radio, wash down pump, etc.) and the battery weights 16.5 lbs! Lead acid starter battery was 50lb and had 1/2 the useable amp hours.
Our slide-in, pop-up pickup camper uses BattleBorne 100ah as that is the only battery that would fit in the compartment (SOK are wider than typical or I'd have used SOK). Battleborne has performed great also. More expensive for the same thing. I know a few people using Battleborne "270Ah 12V LiFePO4 Deep Cycle GC3 Battery" and they are very happy with them.
I've only used these brands so will use them as an example since I have personal experience with these three brands and trust them. Lots of other good brands out there (and, some really horrible brands out there as far as internal construction). But thought I'd use these brands for cost comparison to the powerwall battery in the original post and just to compare to each other (since I was curious).
To get to about 10kwh would be as follows. Bikes only have 6.6kWh battery, so if you didn't need to fully charge and were just topping off at a track between sessions, maybe one would only need 5kWh. But I used 10kWh just for comparison. Cut price in half for 5kWh, which really isn't too bad.
SOK 48v 100ah 5.2kWh - $1,650 x 2 = $3,300 for 10.4kWh.
SOK Battery | 100Ah 48v | User-Serviceable Rack Mount Battery – Current Connected
SOK 12v 206ah 2.6kWh - $1,029 x 4 = $4,116 for 10.4kWh
12V 206Ah LiFePO4 Lithium Iron Phosphate Battery Pack | SOK Battery USA
Dakota 48v 96ah 4.9kWh - $2,699 x 2 = $5,398 for 9.8kWh
Dakota Lithium 48v 96Ah Deep Cycle LiFePO4 Battery
Battleborne G3 12v 270ah 3.4kWh - $2,375 x 3 = $7,125 for 10.2kWh
270Ah 12V LiFePO4 Deep Cycle GC3 Battery
Besides the battery, then you just need, at a minimum, a battery charger and an inverter. Inverter should be a 240v inverter faster Varg charge times over the 120v charging voltage. Charger would be programable, lifepo4-compatible and could be a relatively small/economical one since one can likely afford the charge cycle to take 20 to 40 hours unless you are doing back-to-back riding days. If you want to get fancy, you can add solar panel(s) and charge controller and charge battery back over over the course a week so it's ready to go for the next weekend. And/or add a DC/DC charger to your vehicle. Keep in mind solar panels don't capture very much energy. A typical 12v 200w panel will recharge a battery bank about 0.6kWh - 0.8kWh in an entire day depending on sun exposure, angle, atmosphere (clouds), and charge controller efficiency.
For those that want everything built into one unit, Bluetti makes some good "Solar Generators" (goofy name but that is the common nomenclature for a unit with a built in battery, AC inverter, DC converters, AC charger, and solar controller with the intent that they are typically paired with solar panels). Some of their units have all of the components plus the battery built in and some have all the components except for the battery. One adds on battery modules to those units. And some units have 240v Split Phase Bonding so you combine two units using an accessory Fusion box to get 240v. By far the most expensive solution, but an option. We have one of the medium sized Bluetti all-in-ones; the AC200. It's onboard battery is only about 2kWh, but it's been good enough for occasional home power outages and portable enough to easily move around the house or throw in a vehicle (Plus, I bought back when they were 1/2 the price as a backer introductory price, which helped).
Regarding the Varg battery, it will be interesting to hear cost for a second battery and to see someone swap it. Plenty of swap out info has been shared, but seeing one done will help people determine if that is an option for them. My opinion is your money is better spent on batteries/inverter as a means to charge the Varg rather than a second Varg battery since the batteries can be used for other things rather than spending your money on a proprietary Varg battery which isn't usable for anything else. Plus, in many riding scenarios, you won't have fully discharged battery 1. Being able to just add some juice to a 1/2 depleted battery is a better alternative to running battery 1 down to 10% - 20% then swapping to battery 2 exactly at that time.
I imagine the recharge scenario will be something like: Arrive with bike at 100% SOC and go ride. First pitstop with 60% SOC and charge up to 75%, Second pitstop with 35% SOD and charge up to 50%, last session run it down to 10%. Or however it works out. Basic point is you will not be charging back up to 80% - 100% each time but simply putting in a fraction of the battery (15% - 25%?) each pitstop.
