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Re: Gel battery weakness
A shunt should not reduice the voltage any. A shunt, in battery charging systems, is just a very precisely machined resistor with a very low resistance. It is placed inline with the alternator (either in the hot or ground side, depending on philosophy) and then a very sensitive voltmeter is connected to the two sides of the shunt.
That voltmeter just measure the voltage consumed by (lost in) the shunt, typically in millivolts. When the shunt and voltmeter have been matched, they are called an ammeter, because the amount of amperage flowing through the shunt (and therefore, through your charging system) can indded be measured by the amount of millivolts consumed by that shunt.
So if you have an ammeter on the boat, or a "battery monitor" of some kind, there normally will be two small wires, in a twisted pair, that go from the shunt to the "ammeter" or monitor. And then two more wires that actually supply power to the ammeter or monitor.
Typically, for every 50 or 75 millivolts that a shunt consumes, one amp is flowing through it.
For smaller systems, a 20-60 amp "ammeter" is designed with the shunt actually internal in the meter, and the whole system output is run through the meter directly.
A shunt kinda looks like a six inch long piece of bus bar, with two extra screw terminals set in from the ends, and one cable lug/bolt at each end. And in theory, there's a plastic cover over it so you can't make big sparks. (That's one reason they are sometimes put in the ground side of a system, less chance of making the big zap.)
As to whose battery to buy...No really, are there are battery DISTRIBUTORS near you? As opposed to retail stores that have to mark 'em up high? Or a Sams' Club?