I wouldn't waste money on the $2 hydrometer, for ten bucks you get one a foot long that is much easier to read and includes a CASE. A hydrometer is a turkey
baster full of acid, it will drip and it will eat clothes and wood trim and whatever else it can find, a case should not be optional. Treat it as a hazmat source and you may only need to replace a t-shirt and old pair of jeans every year.
And IIRC, the hydrometer still isn't perfect because it can take 24 hours for the acid in the battery to circulate and even out. That's literally the surface charge being redistributed, and the SG will vary depending on how much acid you slurp up and 'average out' in each cell. Personally, I'm convinced the clothing
industry invented the hydrometer.<G>
Mark, if you have a three-stage regulator locate it. If it isn't hardwired in between the alternator and the battery, it is in a box someplace uninstalled. Should be a gizmo between the size of a cigarrette pack and a paperback book with 4(?) wires going in and out, to the alternator and battery.
It certainly sounds like your alternator is not putting out 100% power. Most of them will put out 14.4 volts under normal cruising speed, i.e. 2000 rpm
on the engine. 13.6 is at the very low end of adequate, I'd expect that at low idle speeds only. There are some tests you can run on the alternator, but it may be simpler to take it out to a shop and have them check it. If the alternator is typical, there is one output lead (by itself, connected hot to the battery, so make sure to disconnect the battery power before you work on that output lead!) and then a 2-3 wire "pack" connector with the other leads, which are for the idiot light (if any), the field coil power supply, and a voltage sense lead. If the external regulator isn't wired into that "pack" of wires--it's off watch and needs to be roused!
With 2x55AH for a house bank (those are wee little things!) and one SLI battery, an 80AH alternator should be plenty good enough. But if it only has an internal regulator--it needs some help, unless you're motoring from time to time.