Unfortunately I don't have a wattmeter & I'm sure the refresh rate on my voltrmeter isn't good enough but I did get the LoLan bluetooth app working for the battery so it does show realtime voltage, current(a) & power(w) but no datalogging or recording.
FWIW, your voltmeter is likely to have a faster response than the BT app (because there are many more delays in processing the data in the series of many steps to get there than in the meter), and so give you more realtime, lower-latency voltage readings as you ride, enabling you to more closely match a voltage sag with a load event.
That is assuming that it's screen updates fast enough to show it to you before the BT app does. You can put them side by side where you can see them both to find out, if you like.
FWIW, even a cheap wattmeter like this
is worth the investment for troubleshooting things, and testing stuff initially once it's all working to get baseline readings you can compare to later when problems arise. You can also get useful info like amps and watts in particular situations, so that if the system can't quite do what you want you then have readings to go look for new hardware that can provide more power to do it. And Wh/mile to discover how much capacity particular routes, usages, conditions, etc require, so that when you replace this battery for whatever reason you can get one with enough to handle range needs, either smaller and lighter for less range needed, or bigger for more range..
My battery only charges to less than 82v before I get a cell overcharge warning. 1 or 2 cells were just over 3.7v but most of the other cells were around 3.37v.
That usually indicates a battery made of mismatched cells that aren't staying balanced throughout the discharge. Most likely the higher voltage cells have lower capacity and/or higher internal resistance than the others. (or if it's made of many paralleled cells, some of the ones in the higher voltage groups could have become disconnected due to poor build quality).
3.37v for LFP is perhaps 3/4 full or more (you'd have to look at the capacity / discharge / charge curve for your cells to know exactly what it is, but you can look up any similar-capacity LFP cell and use that curve to guesstimate), and 3.7v is well over full, which is quite a disparity. lgyte-info.dk is a good place to find small cell testing data if you don't find anything for similar-capacity cells as yours.
Some BMS will shutdown the output if there is more than a 0.1v difference between cells, because it means something is wrong with them and the pack needs repair. (replacement of the mismatched cells).
Since LFP has a very flat discharge curve (vs the typical Li-Ion) so any significant difference in voltage between cells means a significant difference in cell characteristics. (a problem that only gets worse over time as the cells age and become even more different).
If your BMS has a balancing function (not all of them do) you can temporarily "fix" the problem by leaving the pack on the charger for at least hours / overnight, but since it is a high capacity pack and a BMS's balancing capability is small (usually around 50mA), it could take days to weeks to rebalance depending on how many Ah difference there is between the lowest and highest cells. Let's say there's a 5Ah difference...5000mAh / 50mA = 100 hours. If there's a 25Ah difference (half the capacity of the pack) it'll take 5 times that long.
When the charger has stopped turning on and off for at least a few hours, it's probably done rebalancing the pack.
But this only makes all the cells the same voltage--it does not fix the differences in cell characteristics (capacity, internal resistance, etc)--those can't be changed except by replacing any cells that are different with cells that match the remaining ones (even this only gets you close, because you probably can't get exact matches, and even the remaining cells are probably not matched to each other....it requires building a pack of completely matched cells to start with to "prevent" this problem).
