Full-time planning

[chromisdesigns]

@100sqft:

Several things to remember about lead-acid deep cycle batteries -- you can only use about 50% of their rated capacity on a regular basis if you want them to last any time at all. So your 400 ah bank really has a useable capacity of 200 ah +/-.

Second, when recharging, it takes about 115% of the energy you used during the last discharge cycle to bring batteries back to the same level of charge. Also, you can only "bulk charge" at high amp rates until they reach about 80% charge, when you need to back off the rate considerably. This means the last 20% takes a lot longer to get in. Smart chargers and alternator controllers do this for you. Don't know what kind of alternator controller comes stock on the SMB sprinter, but it might not be a 3 or 4 stage type, rather just an automotive style, which is not really effective for deep cycle use. An auto-type regulator will drop off the charging current way too fast and will take a long time to replenish the charge.

If you are going to be depending on batteries a lot, here is a typical discharge/charge regime you might use, if you are depending on alternator charging, with a good alternator controller:

Plan on using about no more than 50 percent of rated capacity (with good AGM batts, you could stretch that to 60% at the cost of some battery life). Your normal discharge/charge cycling would be between 80% charged, down to 30% charged, then bulk charge them back to 80%. If you did this every day, maybe once a week, try to get them back to near 100%. This will take a while from the alternator, better to find a plug-in and let the charger do it. A 40 amp rated charger might take 3-4 hours or more for the last 20%, as it can't force the high currents into a battery at that level of charge without damage. Solar is another excellent way to get the last 20% charge into batteries -- the low charge rate over extended period of time is ideal.

This gives you an energy budget of about 200 amp hours, give or take, for each discharge/charge cycle. An intelligent alternator controller could put this back into the battery bank by bulk charging in about an hour and a half (figuring the additional 15% on recharge and limiting the alternator to 175 amps max -- really bad idea to push the alternator flat out for very long. Most won't do it anyway, they will heat up and derate due to the heat.) AGM batteries are very tolerant of high-rate bulk charging, so long as you are using an intelligent controller that knows when to drop out of bulk phase into slower charging. You can do it with wet cells too, but you have to be vigilant about checking electrolyte levels, as they WILL gas and if you expose the plates, you have permanently damaged the cells.

However, there is another issue here -- Sprinters REALLY don't like to be idled for long periods of time. Figure for each 100 amps you get from the alternator, taking into account mechanical losses, you are probably drawing only at most 3-4 HP from the engine, which isn't enough load to really make a high-speed diesel happy. You might want to read up on the idling issue over in the Sprinter Forum to see why many (including Mercedes, in the owner's manual) caution against idling for any length of time. Among other things, it plugs up the DPF.

When I had my sailboat, I had 660 AH of batteries, in three banks. I rotated daily between banks, and ran the "80-20" regime outlined above, with a nominal 200 amp alternator and a good controller. I typically ran the Yanmar diesel an hour a day, with the alternator running at about 140 amps, BUT I also had mechanical refrigeration which drew another 5-6 hp easily, and the older marine diesels were much more tolerant of long idling time than the Mercedes, not to mention not having DPF, DEF, and all the other pollution-related stuff on modern high-speed diesels.

Personally, I wouldn't run a Sprinter diesel on even "fast idle" (which isn't, really) on a regular basis.

If you are driving the van every day, that's a different story. Just remember you don't get to use the full alternator output just to recharge your house batteries. The Sprinter is power-hungry for electricity, running all the electronics, the fast-heat electrical heater in cold weather, headlights, stereo, a/c clutch, etc. and also recharging the starting battery. IIRC, someone over on Sprinter Forum calculated it can easily use 30 amps just running down the road. So you need to take that into account, as well. Again, if you want to depend on batteries, it's worth getting an intelligent alternator controller and/or a second alternator with controller, which is an option on Sprinters.

[chromisdesigns]

Originally Posted by 100sqft

I've always been curious about the size and shape of the spaces available underneath the Sprinter for tanks, batteries, etc. Photos I've found that are taken from below are scarce and always taken from a cramped angle. Then it occurred to me that the frame for the cab-chassis model is probably pretty much the same and in easy view. A few quick searches later, I found the two images below.



Even though these are for the 3500, I expect the 2500 is fairly similar. I do have one question: What is the black tank in front of the passenger-side rear wheel. Is this the large DEF tank that is typically deleted from RVs?

Regarding the DEF tank, actually that IS the large DEF tank location for many RVs that are built on the cab-chassis Sprinter (which SMB is not, of course). Mercedes offers 3 options for DEF tanks, IIRC, on the cab/chassis -- standard under the hood tank, smaller relocated tank, and the large one you see in that picture. Up to the manufacturer which one they order.

[Chance]

Optional larger alternators. Seem to be engine choice specific.

[Chance]

A second alternator bracket is available for stand-alone charging system. Capacity seems limited mostly by alternator weight.

[Chance]

Small tank may be engine size specific. Worth checking into.

[100sqft]

chromisdesigns: Thanks! That is truly valuable information. It certainly cements my intention to get 400A-h of AGM batteries. I've always been hazy on rated capacity vs usable capacity. I will look into the fast idle situation. I may decide against getting that option.

Chance - Interesting information on the alternator options. I take it that there is no MBz option for a higher output alternator compatible with the V-6 engine. I'll see how well I get along with the standard equipment before I consider something like a second alternator.

[Chance]

Originally Posted by 100sqft

Chance - Interesting information on the alternator options. I take it that there is no MBz option for a higher output alternator compatible with the V-6 engine. I'll see how well I get along with the standard equipment before I consider something like a second alternator.

No doubt it's expensive. And I don't know cost if done later. Also don't know to what degree a stand-alone second system for house batteries would offset other costs like larger alternator or isolator.

For me personally (can't speak to others needs, preferences or concerns) the idea of not messing with auto charging system has some appeal. If not too expensive overall I'll eventually look into a higher voltage separate circuit before making a final decision. At least 24 Volt nominal like on larger RVs. But high cost could change my mind quickly. I typically pursue value.

Good luck with your build.

[100sqft]

Just a quick update to say that after what seems to be an eternity of planning, I sent Sportsmobile a deposit check for a Sprinter today. It will be a 2500 EB with a V6. The paint color is Brilliant Silver. Major options include "Options Package A", Active Safety Plus, roof rack rails, idle control, HD suspension, and the Trailer Tow Group. I did negotiate with a local MBz dealer to get a similarly equipped van. I got them to slice about $3K off their original quote. That put them slightly below the Sportsmobile price, but not by enough to overcome the hassle and uncertainty involved with getting a third party involved. Also, the MBz salesman couldn't guarantee that I would get MBz swivels on the cab seats - that's a fleet-only option.

As a belated answer to an issue that Chance raised: I had an opportunity to look under a 2500 chassis, and the rear leaf springs are immediately behind the single rear wheels; set wider than the springs on a 3500.

[Chance]

Congratulations. And please keep us updated on progress.

What tipped the decision to go with 2500 instead of 3500? I recall you were struggling with chassis weight capacity versus all you wanted to include on your RV. Did you get the weight down?

By the way, thanks for confirming the spring spacing. The wider stance should provide added roll resistance for a given amount of spring stiffness.

[tonyvoudy]

Originally Posted by 100sqft

Just a quick update to say that after what seems to be an eternity of planning, I sent Sportsmobile a deposit check for a Sprinter today. It will be a 2500 EB with a V6. The paint color is Brilliant Silver. Major options include "Options Package A", Active Safety Plus, roof rack rails, idle control, HD suspension, and the Trailer Tow Group. I did negotiate with a local MBz dealer to get a similarly equipped van. I got them to slice about $3K off their original quote. That put them slightly below the Sportsmobile price, but not by enough to overcome the hassle and uncertainty involved with getting a third party involved. Also, the MBz salesman couldn't guarantee that I would get MBz swivels on the cab seats - that's a fleet-only option.

As a belated answer to an issue that Chance raised: I had an opportunity to look under a 2500 chassis, and the rear leaf springs are immediately behind the single rear wheels; set wider than the springs on a 3500.

Awesome man congrats! Just a quick note, I am not sure how long you want your roof rack, but when I asked for one, I assumed the Thule rack would cover the length of the roof. Indiana had another idea and installed it on 1/2 the length of the van. Make sure you settle on exactly measurements since they can only bolt it to the roof of the van before the install of the internal ceiling. I had 1/2 the roof rack bolted, and 1/2 of it screwed in.