I have an Aqara no-neutral Zigbee double light switch wired in, and the bulb it controls is a yeelight
I’ve disabled the relays on the light switch as I don’t use them (effectively creating a permanent connection to the bulb), but the problem I have is that sometimes the lightswitch will simple die
It seems over time it loses power when the Yeelight bulb is off
Is there any way to ensure that it gets enough power even when the bulb is not on?
Think of how the no-neutral switch is connected:
~AC Mains~------[Aquara]------[Yeelight]-------~AC Mains~
If the Yeelight isn’t passing current, the Aquara is toast.
Some no-neutral switches support a bypass module (e.g. Fibaro FGB-002) to give a guaranteed trickle of current but don’t know about Aquara.
The modules are typically a specially rated capacitor (X1/ X2) and a resistor but might not be compatible with other switches.
The old hack was to fit a small incandescent bulb (resistive load) in parallel with LEDs to guarantee some current flow for a switch.
If this helps, 
this post!
Thank you - what’s very strange is that the Yeelight is still functioning. I can control it from the app. It’s still getting current and it’s still works. But the aqara switch does not.
It’s not solely down to it being off either - crucially it has to be off for some time - it’s not immediate. And then if the switch loses connectivity, turning the yeelight on from somewhere else (app or alexa or even HA), the switch magically springs back to life!
Are you sure this is the case? Or is it possible that ZHA is waiting for the device to time out before setting the state as unavailable?
Have a look at the ZHA config page, right at the bottom there is a text box where you can enter how many seconds a mains powered device must be offline before HA sets their state as unavailable. It might be the switch disconnects instantly but HA doesn’t report that for “timeout period”.
Mine is set to 30 seconds for mains powered devices because I have lots of dumb switches powering smart bulbs and need them offline pretty quick. Consider shortening the interval and testing again.
EDIT: Sorry, I thought I’d read that you were using ZHA but there is actually no mention of which Zigbee integration you’re using. I imagine Z2M has similar settings in case you are using that!
I’m using Zigbee2MQTT - you might be right, I’m still able to virtually control via the Aqara switch a few mins after it’s off tho (but not out of the question that this is simply some powerdown of the yeelight)
What you are seeing is a function of the impedance and minumum power needs of each device (AC mains, so reactance or impedance != resistance).
I’d assume the Aquara device needs more current (mechanical relay) than the Yeelight at quiescent conditions. The Yeelight probably doesn’t need much to run its control circuit… until you turn the light on.
If this helps, this post!
So, seems fair to say that my options are:
I either need to
Am I missing any options?
Remember the bypass module I mentioned? 
You mentioned the Yeelight has an app - why not look at controlling it directly?
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Will any brand work with any brand? (assuming it’s just a load and nothing more complex) - ie. could I try the fibaro or Shelly on in this scenario?
It is controlled directly - the switch calls yeelight via Home Assistant. Can also control with Alexa. But need a switch of some sort there
The manufacturer will say “NO”, but all I can say is a resistive load like a small mains incandescent bulb in parallel with a complex LED load has worked for me with Fibaro kit in the past.
Personally, I’d remove the mains switch and replace it with a cheap Zigbee remote button and a cover plate over the “real” switch.
There are lots of 3d printed designs out there and on the usual stores for Hue:
https://www.printables.com/model/82781-philips-hue-dimmer-switch-plate-cover-us-version
https://www.printables.com/model/192288-switch-cover-for-hue-dimmer-v2
If this helps, this post!
So coming back to this… definitely something odd going on. I’ve not messed with the switch in 2 days, and the bulb has been off, yet switch still has power…
Weird! You might be best going with @FloatingBoater suggestion and trying a bypass module to keep a small load on the line for better stability. Maybe check first if the switch manufacturer has their own. Better yet, check with an electrician because I am not one! 