TAfter about 20 mins, up and down hill, power reduces, eventually went down to 40.2v under load and 44v not under load. Bike then stopped at 40.2v voltage and then mutimeter went up to 44v while stationary.
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The controller is 48v 7A, low voltage protection 40v +/- 0.5v Max current 14+/-1A.
THis makes it very likely that it is just the battery running down far enough that under load it simply can't supply the current and voltage the controller needs at that point. So the controller cuts out at that voltage to protect the battery from damage.
The battery doesn't seem be starting at full charge, either. The battery should charge to around 54v full, for a 48v (13s Li-Ion) battery pack. If it doesn't charge that full, it either isn't a fully 48v pack (13s) or it is not working correctly, or the charger is not working correctly and unable to reach full voltage.
What voltage does your charger say on it's label?
If the charger is the correct voltage (on it's label, and measured at it's output pins) then it's probably the battery internally not working correctly. The most common reason is one or some of the cells (or groups of cells) are not the same capacity / etc as the rest, so they reach full charge voltage sooner than the others, and the battery stops charging. If it has balancers in the BMS, it will eventually rebalance the voltages of all the cells so that it will fully charge, but the problematic cell(s) will still have the same lower capacity, so they will simply reach empty sooner than the rest, and it will stop discharging sooner because of those.
But you can first try leaving the battery on the charger longer after it first says it's done, and see if it restarts charging after a while (could be minutes to hours to do this). If it does, then it should cycle on and off like this until all the cells are the same voltage (not the same capacity; that can't happen anymore once this problem begins, as it is the cause). Leaving the pack on the charger until this completes might give you a bit more range, but depending on how bad the imbalance is it can take hours, days, or even weeks if it's really bad.
Some batteries do not have a balance function in the BMS, so they cannot do anything at all about this problem, and either have to have the battery repaired or replaced, or be manually rebalanced by opening it up and individually discharging high cells to match low ones, or individually charging low cells to match high ones. (either one has risks, anytime you're working insiide the pack where things can be shorted out).