Why did the battery blow out?
I have a bed for which the headrest can be adjusted. There are 2 separate power inputs for each side like in the picture:
In addition, there is a set of 9V batteries on each side, presumably for backup:
Yesterday I heard a loud bang and found one battery on the floor:
By looking at the battery, is it possible to determine what happened to it? There has been no water damage or anything else I can think of. The batteries were old, I don't even know how old. Is this a normal failure mode for old batteries?
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These 9V batteries don't fail like it in normal use, not even when they are very old.
They do this only when they are non-rechargeable but are being recharged. There may be a fault in the bed circuit somewhere that causes the power supply to inadvertently recharge the battery. This would only happen due to a fault if the device was not designed for rechargeable batteries.
You may want to check in the manual to make sure that regular (non-rechargeable) 9V batteries can be used. Perhaps these controllers were made for rechargeable batteries, and someone at some point goofed and replaced them with non-rechargeables?
If manual is not available, and googling the model number and manufacturer etc. doesn't yield a manual online, have a look at the metal shell of the battery that blew up, and the other one. See if they were rechargeable or not.
If non-rechargeable batteries were meant to be used, then something failed in a way that is recharging the battery with some gusto. Only then will it explode. Very slow "trickle" recharging usually doesn't lead to such spectacular results.
Rechargeable batteries may also explode if there is an external or external short, but there would be some signs of it.
The bed should work fine without the batteries I hope. So there shouldn't be a problem just leaving them out, as long as the bed works.
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Those batteries must be for powering electronics that receives commands from your controller or remote. They don't have anywhere near the energy to raise something like the headrest of a normal bed.
However, that has nothing to do with why one of them failed. I've never seen a 9 V battery fail like that. All I can think of is that one of the cells had a fault that grew over time, eventually shorting the battery and failing in a spectacular way. Even that surprises me since these seem to be primary cells, so there wasn't extra energy available from a charger.
In short, an old battery failed. It is hard to say why, and it really doesn't seem that it is due to anything you did.
If those actually are rechargeable, then possibly the charger failed and over-voltaged the batteries.
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