http://www.surepower.com/pdf/180031c.pdf
I can't seem to embed it..., perhaps because it's a pdf.
Note how:
The negatives are common, eg; tied together.
The "bar" end of the diodes = cathodes go to the separate + terminals on the batts.
The "butt" ends of the diodes are common, eg; tied together
When we are charging the batts, this setup WITHOUT the selector switch *automatically* charges the weaker battery....REGARDLESS of whether we happen to have left the selector switch in "A" or "B" position. That sounds good and it probably is in most cases. (Ideally, we would really like to be fully aware & know how much current is being drawn by batt A and then switch to batt B so we are always in a position to compare them. But then again, sometimes "automatic" works very well.
I say again, notice how the charging current goes to BOTH batts, REGARDLESS of which position the switch happens to be left in. Hmmmm.
When we are DRAWING power from the batts, that same switch (The one whose center pole is within the dashed box labeled "Sure Power Isolator #31822 or #31922" and which connects to the anode or "butt" ends of BOTH diodes) >>ONLY<< lets us draw from one battery at a time.
You want it this way on a boat!! Because being able to start an engine can be very critical, when a current is washing you up on some rocks. So, if you have been using "batt A" all day long, drinking beers in the sun and are plowed when the current comes up and you suddenly you realize you had better freaking start your engine and scoot; your ability to start your engine off batt "B" is not a matter of whether you were a goofball and let the radio drain BOTH batteries: You didn't have the choice. You would have had to switch that big fat switch over to "B" and that big KACHUNK would have triggered those remaining functioning neurons to think something like "whoa, I better not run the radio off batt "B" because I'm gonna need to start this thing later.
The diodes have to be current and voltage rated to handle your maximum charging current, which, if you are using solar cells, is miniscule. Very, very few diodes will have a problem with 12 volts. OTOH if you're using an automotive alternator that can pump out 85 amps, you will need some FAR more serious diodes and heatsinks. You could need that (eg; high-current diodes) if you have hundreds and hundreds of sq feet of solar panels, as well. These pictured are prolly 35-50 AMP diodes, and, would require mounting on a heatsink to truly be able to handle that amount of power.