I bought a couple of GT Power LCD RC 130A Watt Meter Power Analyzer Ver.2 High Precision off ebay a couple of months ago off Ebay for $16.50 shipped.
This unit is quite handy to use as a troubleshooting device as well as just using it as a low budget fuel gauge. If the battery cuts out on your BBS02 often it is not immediately obvious if the battery pack has hit the low voltage cutoff or if the BMS has shut down. If you have this device in between the battery and the controller then it is obvious what has failed because if the battery is still producing power then the display will be on, if the display has gone dark then the battery is your issue.
This unit is only good up to 60v and my 48v packs are frequently at 59v when I pull them off the charger. This means that you can’t use this meter with anything over 48v rated nominally. If you plug it in and the screen flashes on and off that is just showing that there is too much voltage. Once you start producing a load on the battery the display should come up and read properly.
The way I use it is to crimp Anderson Powerpole connectors on the pigtails then you can hang it off the power line wherever you want to keep track of what kind of power you are using. If you have a 20Ah battery it should be able to deliver close to that before the BMS shuts down. You can also use it backwards to see how much juice you battery can take before it stops taking a charge, which is pretty cool.
It has some issues in the cold which would be expected of a $15 watt meter, sometimes the display is hard to read, other times the display is really, really hard to read especially when the temps approach 0 degrees F. For the most part it works as advertised, the Volts, Amps and Watts are always displayed and the bottom left hand corner switches between Ah, Vm and Wh which can be a little annoying.
It’s no Cycle Analyst that is for sure, but if you’re a cheap skate like me and have a whole fleet of ebikes this little watt meter might work great to help you keep an eye on how much power you’ve used and how much you’ve got left for a lot less than a $130 Cycle Analyst. The cheapest I can find it for now is around $20 shipped on ebay, search for “GT Power LCD RC 130A Watt Meter Power Analyzer Ver.2” and it should come up. Better yet buy one for $30 with XT90 connectors and 2 extra XT90 connectors included from Lunacycle.com right here.