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I was thinking about making a simple circuit with LED, solar cell and battery and put it in resin mold to make it weather proof.

I wonder is there anything wrong (chemically) with putting a battery into resin?

I know battery will die at some point and I wont be able to replace it, but it's ok since the cost of the whole thing will be low.

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    \$\begingroup\$ It depends greatly on the type of resin. Once it's solid you're fine, but the battery may be damaged while the mass cures, depending on the resin. I can't see anything going wrong too badly, though. I would try it (excercising normal safety precautions) \$\endgroup\$ – felixphew Sep 22 '14 at 1:32
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    \$\begingroup\$ If you were trying this with a more complicated/expensive circuit, though, you could have the battery on the outside, in a waterproof case. Same goes for any part that is consumable or likely to fail. \$\endgroup\$ – felixphew Sep 22 '14 at 1:34
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    \$\begingroup\$ Chemically it should be okay if it is not polyester resin which attacks many plastics. Epoxy resins are safe for most materials. But the main concern is shrinkage which can rip the potted device apart, therefore it is advisable to use resins specifically designated for use in electronics. \$\endgroup\$ – venny Sep 22 '14 at 1:59
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You can put these items into resin. Most resins will work, however polyester is more aggressive, and care must be taken.

For example the battery is wrapped in a plastic shell, if this softens and there's a solder-spike connected to a positive (+) this may pierce the plastic and connect to the battery shell. Often the shell is negative (-) and this will create a short circuit.

Also badly mixed polyester is much more likely to get incredibly hot at the core, potentially severely damaging your battery chemistry.

The best choice is a low aggression resin like epoxy, to be sure.

Make sure your electrical connections are well made, most cheap epoxies do shrink a little, by far not enough to stress the battery or solar cell beyond their limits - that is, if you do mix them accurately enough. If your connections are half hearted they may experience just enough stress to introduce unwanted extra resistance. For example battery terminals can be plated for low-corrosion, which is hard to solder to, while initially it may cling, it is very easy to tear off. You're probably best off getting a battery with solder-tabs for battery packs or PCB mounting.

All that said, I have potted many many things containing any combination of electronics, including all of your mentioned parts, into cheap resins, epoxyd and others, and never had major failure. If you want the "block" to stick neatly inside a box after curing, shrinking is an issue. You may need special potting compound, or, alternatively, get a bulk package of decent glue epoxy.

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