Epic solar boat adventurer - Stress testing my old battery

I've had this big old deep cycle battery for years, and I don't know enough about it so I thought I'd learn some stuff.

It's a conventional, unsealed, 100ah lead acid, deep cycle thing.

At least I think it's 100ah. That's the kind of thing you'd think someone would emboss onto the side rather than adding it as a stamp. That and the voltage.

At least they went to the trouble of embossing the + and - marks.

So with this kind of battery, it seems they don't mind being run down as much as, say... a car battery might. Car batteries hate it when they are run down, and it shortens their lives in a big way.

A deep cycle battery (also called a flooded battery apparently) is a bit more forgiving, but they still see damage with every bit of use. The lower you allow the voltage to get before recharging, the less charge cycles you get from them before they need replacing. 

As I understand it, 11.8v is considered "empty", but that's at rest 6 hours after you stop using it.

I need to know what "empty" is when I'm cruising along drawing 20 amps or 35 amps (that being the amount of juice the motor pulls from the battery at speeds 4 and 5 of 5.

So my education for today turned into more questions than answers. It looks like it's business as usual.



120 Things in 20 years thinks the Internet might not know something I need to know.