My SO32, when I bought it from its first owner, came with two 75Ah batteries, each in a plastic battery box under the stern berth. The battery boxes had been screwed down THROUGH the plastic, which punctured them and made them vulnerable for acid leaks, the prevention of which is precisely the object of a plastic battery box:
Note the acid stains on the bottom of the battery space:
After having installed a diesel heater and finding that the battery was too limited in capacity to feed a refrigerator all night and provide power for the heater the next morning, I decided I needed a bigger house battery. This was to be a 105Ah battery of the same make of the old one. Disregarding the downside of charging two dissimilar batteries on one generator over a connecting relay (which proves to have limited effect) the changeover was a good one.
However, I still wanted an acid proof battery box. So I decided to make one myself from plywood, woven glass and epoxy. I had researched the acid resistance of epoxy and found it will resist battery acid, but not concentrated sulphuric acid. The concentration of battery acid is well within its capability. I set to work, getting some 6mm ply and glass cloth:
First I glued together the bottom and the sides of the box, then I added the partition and the ends. The pieces were pinned together with brass pins and glued with epoxy.
The inside edges of the box were finished with an epoxy fillet to reinforce and seal the seams between the sides, ends and bottom. The fillet is made using ordinary epoxy and wood dust which I got from a boatyard building plywood boats. The epoxy is first mixed from two containers with measuring pumps to get the ratio right, then the wood dust is mixed in:
Next was the glassing-in of the structure, which would provide an armoured surface and also impregnate the ply with epoxy. The glass cloth was cut to size, during which my hands got the itch from the fibres penetrating my ski n. It is better to use protective gloves:
Using a roller with epoxy resin over the glass firmly stuck the matting to the surface. Here is the result:
After another episode with the epoxy roller, the box was ready. Note the dissimilar compartments - there is just enough space for a 75Ah battery and a 105Ah battery:
Here the box is installed in the compartment under the berth. It is a close fit and will be locked in by wedges, so the integrity of the box is preserved and any acid leaking out will stay in the box.
After fitting the batteries in the box, the cables are reattached. The supply and charging cables feed on to the same clamp of each unit. They are tied together in such a way that they can be released to take out the battery for replacement.
This arrangement has been in place for several years now without any problem.