Slow Voltage Drop in Battery Pack

I have a battery pack that I pulled form a power station that didn't work and had been sitting quite a while. It is 18 - 18650s in a 3S6P configuration.
When I pulled it out, it measured about 8.7V. I put it on a charger and it was 13.38V when I took it off.
I've been watching it for five days, and it has dropped to 13.13V.
Is this normal? Is there a problem with it? Is there any reason I couldn't hook it up to some small solar panels and a charge controller?

You've likely destroyed it.

First thing, if this is a 3S non-LFP pack, you've damaged it. 12.6V is the MAXIMUM charge for a 3S NMC/NCA/LMO Lithium pack.

Second thing, if this is a 3S LFP pack, you've damaged it. 10.95V is the MAXIMUM charge for a 3S LFP pack.

sunshine_eggo said:

You've likely destroyed it.

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What signs would indicate that I have ruined it. Is there a way to test it?

I am here to say that you are lucky your house didnt burn down if you had LiCo cells at 4.46v x 3
You need to know WTF you are doing.....

I would get that thing out of my house as using it, even to test, could be explosive.

iverp said:

What signs would indicate that I have ruined it.

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Voltage.
The maximum voltage for LFP is 3.65 volts per cell. a 3S battery would top out at 10.95 as pointed out earlier. Apply the same concept to the max voltage of a NMC cell of 4.2 volts for the same conclusion. Voltage above battery specs is damaging.

iverp said:

What signs would indicate that I have ruined it. Is there a way to test it?

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Knowingly charging to or measuring greater than 12.6V for any lithium. LFP CAN be charged to 4.2V/cell, but it's not good for it.

PotusGobbleBottom said:

I am here to say that you are lucky your house didnt burn down if you had LiCo cells at 4.46v x 3
You need to know WTF you are doing.....

I would get that thing out of my house as using it, even to test, could be explosive.

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This.