I do not think that you have pushed the Deck too hard by using software. Actually everything should be sufficiently secured, however, I have now read several times about faulty batteries. I myself try to use my Deck between 20% and 80% and hold, however, I recently had no charger with me and after the message, I had only 10% battery, I thought to myself - well 10% at 11W are still a few minutes. Nope, after 30 seconds the 5% message appeared and after another 20 seconds the Deck shut off.
So either the batteries are really susceptible, or the software is screwed up.
In your case, it sounds like a defect and I would definitely inform the steam support about it and in case there is no software solution, claim a RMA.
If you don't want to wait hours or sometimes days for steam to respond, there are a few things you can do yourself to isolate or fix errors.
At first, try to switch off you Deck completetly when it's on or in sleep-mode by holding the power-button for ~10 seconds and then connect it to the charger for a few hours.
You could also first try another charger that you are sure will work to rule out a fault here.
If you also have a USB voltmeter at home, I would check if the Deck is drawing power and charging at all.
If you are confident to open the Deck, you can also follow this tutorial
up to step 7. So disconnect the battery once, wait a few seconds, reconnect and reassemble the Deck, of course while it is not connected to the charger!
With this you have completely disconnected the hardware from power once and the Deck can then reboot "freshly".
If all this doesn't help, you could also resort to more drastic means to rule out a software conflict, and reset the Deck.
Here's the official How-To
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