I do multi-day MTB rides, often 12-16hrs of use per day, so …
Two smaller powerbanks are better than one large one since if one fails you can still use the other. Find the mAh of the 820’s battery, you lose around 10% charge moving that charge from one point to another, so if the 820’s battery is 2000mAh then you need somewhere around 2200mAh in your powerbank to fully charge the 820.
Look for “Angle up” (or maybe “Angle down”, can’t remember which one but it’s obvious from an image) cables - these have the mini-B connectors at right-angles so you the cable doesn’t stick out as far from the back of the unit. This means the cable can often stay in place whilst the unit is on the bike and also that it puts less strain on the connector mounting on the internal circuit board.
Anker are one of the better brands, I’ve also used Raav (sp?). Occasionally you come across a powerbank/device combination that simply doesn’t work: either the powerbank isn’t producing enough current to overcome the device’s internal resistance or the device doesn’t draw enough current to make the powerbank think there’s something attached! They are all based on 18650 cells so just tend to be multiples of the weight of one of those plus the main casing. Don’t bother with pass-through (where the battery acts as both source and sink at the same time) as they don’t last as long.
If you start with a fully charged unit and assuming that it lasts a whole day then one of the single cell powerbanks is likely to be enough to charge things for the second day.