For the last month during the hottest part of the year for us, I have had the above Weego JS12 12000 mAh Jump Starter Heavy Duty Battery Pack installed in my Gen1 basically replacing the stock 12V AGM battery and it has worked just fine. I have it wired between chassis ground and positive terminal of the stock battery thru a 100A fuse.
I haven't committed to pulling out out the heavy 12V AGM battery just yet. Instead, I just have the negative terminal pulled off from the AGM eliminating it completely from the circuit for the past month. This small 120000 mAh battery pack has no issue starting and running my Gen1 at all. I have wired the 14V input into the pack from the switched Accessory Power so the battery pack will only charge when the Volt is powered on from the 14V APM. Operating this way, the battery pack always gets a charge from the APM and normally stays fully charged with all 5 indicators lit when operating the pushbutton.
For those who like to keep their headlamps on with the Volt turned off, you may want to hold onto your AGM. I was able to flatten this battery pack after about 20min with my Gen1 turned off and my aftermarket LED headlamps on. Since the Gen1 is smart enough to shut off the headlamps and interior lamps after 10min, I had to turn off then back on my headlamps in order to get the small battery pack fully drained. I suppose leaving the hazard flashers turned on would also drain this pack alot faster than if the stock AGM battery were used.
Although this battery pack has worked fine even in the extreme 100+F temps with the Volt sitting outside all day in the sun for the month of July, it will be interesting in seeing how this pack would perform in the dead of winter cold temps.
I do admit that I overpayed for the Weego battery pack as the GOOLOO 400A Peak 12000mAh Portable Car Jump Starter Power Pack GP01 appears to be identical and more than likely made by the same Chinese manufacturer for half the price!