So batteries have catastrophic fails releasing all the charge at once. Batteries don't do this, well they do explode if they are lead acid and not well ventilated.
What does happen is batteries loose their ability to hold a charge. All batteries do this.
My suggestion would be that batteries loose .1% capacity each day that they hold current. If the batteries go under 50% charge then they should loose .2% capacity per day.
Just a thought.
Batteries already do this: Small self-discharge of 5W per day. In other words, a battery which is full when uninstalled or disconnected will drop to 300W after an in-game year (60 days). (https://rimworldwiki.com/wiki/Battery)
Also, batteries don't really explode, think of it more as an electrical fault. This is implemented into the game for balance to limit the number of batteries you make. I personally make sure to have two battery banks connected by a power switch so even if this happens I can switch on the non-effect battery bank to resume power.