So with a tablet as an always-on display, what’s the best way to power it?
I can automate, for example, switching the mains power on when the battery drops to a certain level, and off again after recharging to a certain level. Or I can just leave the mains on all the time, so the battery will remain permanently full.
If I cared about battery life, which would be better?
I’ve not been able to find the specific source I’m referring to in this post but I recall seeing a study a few years ago that concluded that for optimal battery health/longevity, lithium ion batteries should avoid being charged to 100%, 80-90% is better, and should not be discharged below 50%. Lead acid are different and other lithium-based batteries may also differ.
In practice for devices I care to preserve the battery health of, I avoid charging above 80% as much as possible unless I need that extra 20%. For a phone, sometimes exceeding this range is unavoidable but for a wall-mounted tablet you’ll only occasionally remove for short periods, I’m sure even starting at 50% battery it would be fine for the duration of use.
The general consensus from a safety point of view is to have the device on a smart plug so it can be turned off. This stops the battery from swelling.
The other option is a recent Lenovo or Samsung which have battery protection built in so that it stops charging at a certain percentage.
The other option is a battery less tablet such as the Lenovo K10 or something designed for use in business/retail.
This is only software and should be possible to do with any device that’s not totally locked down.
Exactly the consensus I’ve researched as well. My Galaxy tablet once asked me if I wanted to enable battery life extension by stopping charging at 90% (it would then show that was fully charged). I accepted the suggestion. Anyways I keep all my wall panel tablets between 50 and 90%.
Some mixed/twisted information here. 
The “best” or less stressed for an Lithium-Ion or Polymer battery is 50% - why is that? Because of chemistry! The level of electrons between the cathode and anode are mostly equal which is the perfect state to store a Li-Ion battery 
When it is about charge and discharge the extreme ends in both directions putting maximum stress on the battery, so getting the last 20% out of a battery or topping it up the last 20% is degrading the battery faster. Again it’s the chemistry! 
Another thing to avoid (if someone wants long lasting batteries) is fast charging as this degrades batteries faster 
With the previously linked solution from @3ATIVE it is easy to have the desired charge level on your phone even if the manufacture doesn’t want you too!
Also extra slow charging (the opposite of fast charging) like only 0.1 or 0.2C can drastically increase round-trip energy efficiency (or simply avoid losses) 
Very interesting info. I suppose then the absolute “ideal” range would be keeping the charge % somewhere between 40 and 60%? This only really applies if you have constant charging on-hand. If you’re out and about potentially without an outlet (like a phone often is) charging it to 90% would be worth the trade off of reduced battery health (although still better than 100% charge)
Thanks all. It’s a new Samsung tablet. I’ve now switched on the “limit charge to 85%” option.
I also have a new Samsung tablet and enabled the 85% charge limit. Is this enough to maintain battery health and prevent overheating, or should I also use a smart plug to manage charging?
Thanks
I had a couple of fails using the obvious strategy -- turn on charging at 50, turn off at 85 -- associated with a sensor becoming unavailable. So I've turned to a "belt and suspenders" approach which (so far) has been fully robust, and not run the tablet down. Here is the code:
Automation to cycle charging:
alias: Tablet Office
description: >-
Controls the charging switch for the tablet based on battery level and system
restarts.
triggers:
- entity_id: sensor.office_tablet_battery_level
below: 50
id: turn_on_low_battery_office
trigger: numeric_state
- event: start
id: turn_on_restart_office
trigger: homeassistant
- entity_id: sensor.office_tablet_battery_level
to:
- unavailable
- unknown
for:
minutes: 5
id: turn_on_error_office
trigger: state
- entity_id: sensor.office_tablet_battery_level
above: 85
id: turn_off_high_battery_office
trigger: numeric_state
- entity_id: sensor.office_tablet_battery_state
to: full
id: turn_off_full_office
trigger: state
actions:
- choose:
- conditions:
- condition: state
entity_id: switch.office_tablet
state: "off"
- condition: or
conditions:
- condition: trigger
id: turn_on_low_battery_office
- condition: trigger
id: turn_on_error_office
- condition: and
conditions:
- condition: trigger
id: turn_on_restart_office
- condition: numeric_state
entity_id: sensor.office_tablet_battery_level
below: 85
sequence:
- action: switch.turn_on
target:
entity_id: switch.office_tablet
- conditions:
- condition: state
entity_id: switch.office_tablet
state: "on"
- condition: or
conditions:
- condition: trigger
id: turn_off_high_battery_office
- condition: trigger
id: turn_off_full_office
sequence:
- action: switch.turn_off
target:
entity_id: switch.office_tablet
mode: single
Obviously your switch and tablet names will be different. Any tablet that has Home Companion on it (e.g. used as an interface to Home Assistant -- for android tablets, you can replace the launcher with home assistant, which cuts out the middleman) will report its battery state to Home Assistant.
As a failsafe, if that becomes unavailable or unknown, turn on charging, or if the battery percentage falls below 50. (You can set this wherever you like -- some commentators recommend cycling between 20 and 80, but we sometimes unplug the tablet to use it elsewhere, so having more than 20 is helpful.) Turning on charging after an HA restart protects against missing the transition, as does turning off when full.
Many IoT devices, especially $5 smart plugs, have wonky wifi. Since a run-down battery means the tablet won't recover without manual intervention, while too much charging has a downside three or four years in the future (when many of these tablets won't be getting security updates anyway), protecting against the battery running out is useful. Moreover, older tablets often have unreliable state of charge stats -- I see a lot of implausible patterns in the history.
Indeed, I would just keep charging the tablet and wreck its battery, except that doing so creates not just a risk of a tablet that can't run on battery, but also of a fire. Cycling the battery also reduces the risk of a swelling battery, which in turn reduces the risk of a dangerous lithium fire.