Putting the usual UK/EU tag in here, as several of the screens I’ve seen people using don’t seem to be on the UK amazon site, and as these tend to be a little temperamental I’d prefer buying from somewhere with a decent returns policy!
https://www.amazon.co.uk/Digital-Battery-Capacity-Voltmeter-Waterproof/dp/B07PDJJ3QT/ref=sr_1_1?keywords=Digital+Battery+Tester+lcd&qid=1557628801&s=gateway&sr=8-1
This is one I’ve found so far, seems to be cheap and simple and fairly low profile so it’ll be easy to fit into my small enclosure. Or there’s :
https://www.amazon.co.uk/MICTUNING-Multimeter-6-5-100V-Amperage-Backlight/dp/B01JOUZELG/ref=sxbs_sxwds-stvp?keywords=LCD+Display+Battery+Capacity+Tester+Indicator&pd_rd_i=B01JOUZELG&pd_rd_r=3b5719ef-798c-4342-804b-9a5ac590bf4d&pd_rd_w=K0JjJ&pd_rd_wg=kMrZJ&pf_rd_p=7dc56c0d-8a5f-4d97-9143-7233b106859a&pf_rd_r=QPC3GAVJPM56EZ0Q5H4J&qid=1557628906&s=gateway
This one, which seems to be the same as a DROK one I’ve seen people use on here. Bit bigger though, and seems to be harder to wire up, but might give more info. Could be too big to fit into my enclosure though.
Any thoughts? 
I use the second one…the shunt is HUGE, but its really convenient to use…just look down and see how much power you are using and the energy consumed
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Problem I had with that one, is that going from the description you can only use it to see voltages either during use, or while charging, and not both. Seems you need two of them in order to have it show voltage at all times, and the main reason I’d need the screen is so I can be sure my BMS stops charging at the right time…
It will give the battery voltage when charging, when it’s charging there is a reverse sag that shows higher than the voltage if you let it rest but it does stop at the charged voltage
Ahh I see, thankyou, so it’s useful for both?
As it doesn’t have the usual ‘battery meter’ do you just run it until its hits 33v (or whatever your equivalent is, I’d be on a 10s 30q battery so I believe full is 42v and ‘empty’ is 33v)
What I do is charge the battery, ride till it’s dead, remember the Wh when it’s dead (about 140Wh on a cheap 10s2p) and compare it to that
You have to manually reset it when you charge it fully