I was really curious about this, and keep in mind that i have the OLED steam deck so it has a bigger battery, if you have a LCD steam deck in theory, it should add another 25% to it. (50/40)
So I tried a bauses 65w 20000mah power bank for this test. the results will change slightly depending on the efficiency of your power bank but ones with high voltage support are usually better (assuming they use the cells in series with a step down etc)
I managed to charge my steam deck 1.25-1.2 times with this, meaning that for a 20,000mAh power bank:
- 3H total with no power bank -> of total 7.3H of play time~ (steam deck and power bank both at 100%$)
- 4H turns into 8.5H
- 6H turns into 13.4H
for a 10,000mAh power bank (assuming same efficiency, should be slightly lower):
- 3H turns to 4.8H
- 4H turns into 6.4H
- 6H turns into 9.6H
This is not an ideal scenario however. Your power bank will attempt to recharge the Steam Deck as fast as possible and lithium ion batteries lose efficiency as draw increases. The most efficient route would to instead run the deck, charged at 100%, off your battery bank until that bank is depleted, then allow the deck to run off internal batteries.
That said, as the Deck charges in the 20-25w range, this efficiency gain would be eliminated if your game demanded 20-25w system power.