I was afraid of that. 0.7v to 1.0v at the isolator, and then more voltage loss in the cable and connections before it finally gets back to the batteries, and yeah, 12.7, 12.8, sometimes as much at 13 volts is pretty much what I figured.
13.2 volts is considered a minimum charging voltage. Here's the thing. It takes about 5 times longer to fully recharge a lead acid battery than it does to discharge it (not nearly as long for AGM batteries, tho). Meaning, if you discharge a battery at a rate of 25 amps, however long that takes, it will take 5 times longer to put a full charge back into it at a current of 25 amps. The first 70% will charge up very quickly, but it will drop off dramatically after 70%, though, and then will take a few hours for those last few amps to get pushed back into the battery. And that's at the proper voltage. It takes quite a bit of voltage to punch those last 30% through. At lower voltage (less pressure pushing the amps back into the battery), it will take much longer. Much. If at all.
13.2-13.5 is actually considered a "float" charge, and will keep a battery topped off, after being fully charged at 13.8-14.1 or even 14.4 (temperature depending).
Trying to charge a battery at 13 volts or less is, well, for one, gonna take days to do, but mainly it's just a nifty way of efficiently sulfating the batteries (a.k.a. killing them). At 13 volts or less, what you're doing is, at best, charging the batteries up to about 70%. No more. The unused 30% will quickly sulfate, resulting in a rather dramatic loss in capacity with each charge/discharge cycle.
If your batteries will last 12 hours between charges, within a few weeks, couple of months at most, they'll soon only last 10 or 8 hours. Soon it's 5 hours, then 4, then within another month or so maybe an hour or two. Before you know it, 6 or 8 months have gone by and you're knockin' on Crown's door looking to get some replacement batteries.
The battery monitor may very well lie to you and tell you the batteries are fully charged when they're not. That's because of how the Xantrex determines the battery is fully charged. If you're charging them at 13 volts or less, eventually the inflow of amps will trickle down to nothing, and finally the monitor will see that the battery isn't accepting any more amps, and must therefore be fully charged up. It knows how many amps went in, and will thus adjust your capacity accordingly. When your capacity begins to noticeably drop down to less than what you think it should be, you'll be shocked at just how little time (in days or weeks) that it will take for them to be useless.
I can tell you all of this with great authority, because I have a Sprinter and I used to have... that... same... exact... isolator.
Simply put, it doesn't work. There's just too much voltage loss at the isolator, and then in the cable back to the battery bank. You need a near-zero loss isolator and then at a minimum of #2 cable to keep the voltage loss to a minimum. I use a much heavier 2/0 cable, to keep that resistence as low as economically practical. 4/0 would be better, but that crap costs more than the van did.
What you need is this:
Battery Separator 1315-200 sure power
It works. the voltage at the battery is virtually the same that the alternator puts put, with the only difference being a 0.1v or so loss from the cable and the various lug connections, busbars and fuses, that add up to slight resistance.
The link above for the 1315-200 is a bi-directional separator, meaning it will engage if the starting battery is being charged by the alternator, as well as engage if the house bank is being charged by a shore power charger (to keep the starting battery charged up at the same time the shore power is charging the house bank). The 200 means it's a 200 amp unit.
For almost no difference in money, you can get a uni-directional unit that will engage only when the alternator is charging the starting battery, and will not engage if the house bank is being charged by shore power.
Sure Power battery separator 1314-200
The links provided are merely to where I got mine, 'cause that was the best price I could find at the time, and there may be a better price somewhere else. I can't remember where RLENT got his, but he's got the same one. He'll chime in if he feels so inclined.
These separators do have a "start assist" where they compare the voltages of the starting battery and the house bank, and if the house bank's voltage is higher than that of the starting battery, it will engage and combine the batteries to allow the house bank to help with the starting. They have a spade terminal that gets connected to the ignition position of the ignition switch, so that it only looks at and compares the voltages only when the key is turned to the ignition position (and not all the time, like when the key is in the #2 RUN position). I don't recall what year your have, and I'm not sure about the 07/08 Sprinters, but on the 06's and earlier, connecting it to the ignition switch is harder'n Chinese 'rithmetic. It's somewhere up there in the steering column and I decided it wasn't that big a deal. RLENT came up with a more creative and elegant solution for it, which he may explain if you ask him real nice like.
But you really need to ditch that isolator and get a battery separator, and I'd do it toot sweet before you kill those batteries dead.
