Terrible Construction on Mercedes Charge Relay

I wanted to be able to manually connect my car battery/alternator to my RV house battery pack with a switch. There is a factory option with docs available on how to do that here:

http://www.sprinter-rv.com/wp-content/uploads/2010/09/Aux-battery-retrofit-guideline-NCV3.pdf

...which, overall, is pretty good.

However, I've had *two* separate failures on my relay, poorly pictured here:

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I haven't been able to find a better pic of the text, but it says "Made in Hungary" on the side. I'm not taking the seat apart to get a better one at the moment.

I *think* this is where I ordered it.

https://www.mbpartscenter.com/oem-parts/mercedes-benz-relay-0025424019

First, the power input to the relay has a diode in it, and definitely has a correct and incorrect polarity, but that polarity is not marked anywhere on the relay. I was only able to determine it by trial and error, and then later when I was forced to take the relay apart. As you can see from the same picture above, the terminal closer to the power terminals is the negative, with the positive on the bottom. I scratched the polarity markings onto the case for future reference.


Second, when I tried to push my blade connector onto the actuator terminal, whatever was backstopping the blade broke, and the blade retracted back into the casing. I have to admit some responsibility here, because I did crimp down the female connector a bit to make sure I had good contact, as it was initially a bit loose. However, I still wasn't pressing that hard when it broke.

At this point I should have returned the part for a replacement, but I was on deadline and broken down in Abilene, so I instead drilled out the rivet holding the box shut, inserted shims of various kinds to defeat the clips holding the box together, and took the relay apart.

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Inside I found that the only thing preventing exactly the problem I'd experienced was one tiny, poorly applied daub of black plastic! It takes *very* little force on the blade to crack it off, and now your blade connector has pushed back into the house, contacting who-knows-what.

I wanted to put a daub of epoxy on it or something to hold it back into place, but the parts were so packed in there I wasn't confident I could do it without gumming something else up. Instead I think I tried to melt the same little daub back into place, then installed the female connector with the relay still apart so I could manually backstop it with my hands.

I snapped it all back together after all that drama, and, finally, it worked. I installed mine on a manual switch, because I would much rather have my Renology solar charger, which is a full 4-stage MPPT charger, do the last stages of charging to make my batteries last longer. The default installation automatically connects the two battery systems whenever the engine is running, and I didn't want the alternator bulling in at 14.1 VDC when the house pack was already mostly charged and overboiling my electrolytes. At the time my serpentine belt was missing, and I was waiting on parts, so I was able use my house batteries and solar charger as my alternator for a few days. That didn't help with the lack of power steering, but luckily no one dove unexpectedly in front of the van during the days it was down.

Fast forward to now when I'm field testing my RV setup in Portland, Oregon. I'm planning on getting together with a full-time van dweller for some shop talk over lunch, but first I want to go to the PSU Farmer's Market to fill up my new Vitrifrigo fridge. I was up until 4 AM replacing my door speakers (another story), but still got up at 10 AM to try to hit the market. But when I try to start the engine, the battery is dead. I throw on my iCarsoft OBD, and I'm dropping down into the 8V range when cranking. Well, I think, that sucks, but I'm *prepared* for this situation! I flip the switch, but see no increase in voltage on the iCarSoft. I try to crank, and nothing happens. There's no connection!

Now, I can *hear* the relay clicking into place when I flip the switch, so I know both that it's getting power and at least *trying* to make contact. It then occurs to me that maybe the 150A fuse I installed as part of the power system is blown. That lives on the positive bus bar with the battery, so I dig in there, which requires unbolting the lock handle from the back of the battery (which it now occurs to me is still unbolted...oops) and check. It looks blown, but when I replace it with my spare, nothing changes. I do a continuity test on both fuses, and they're both ok.

So *that's* not it.

I can't do much more to troubleshoot the situation since it required pulling off the driver's seat. I'm running on maybe 5 hours of sleep, and have somewhere to be, so I swallow my pride and re-create my original project by manually hooking up my jumper cables from the house to the car batteries. Immediately I see the power flow in from my Trimetric meter, the voltage go up, and *BAM*, the engine starts.

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Neil offers me a driveway where I can finally take off the driver's seat. I help him with some projects on his rig, then pull it off. Testing the terminals, I can tell that while the relay is clicking, it's never making contact. Once again, I start stabbing the edges with flatheads to pry it open. I try activating it when I can see what's happening. The contactors are snapping shut when I apply power, but the connection is inconsistent. Sometimes it flows through, sometimes not. What a great recipe for arcing!

So why is this happening? I look more closely, and guess what I find trying and failing to hold the coil in place? You can see it here at the end of the chopstick (best mechanic's helper ever!).
Another one of those crappy plastic welds! Except this one wasn't even melted into place! It was perfectly cylindrical, and therefore doing precisely *nothing* to prevent the coil from rocking backward, thereby reducing contact pressure and causing the problem.

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Once again I dragged out my butane soldering iron and melted down the cylinder so it binds "properly." Once the coil is immobilized, the connection is immediately fixed and makes full contact 100% of the time.

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It's raining fried trees here in Portland, which is both directly and indirectly blocking my solar charging, so the Return of the Interconnect is well-timed. However, as you might guess, I have *very* little confidence in one of the highest electrical flow parts inside my rig that lives between my legs. I could have gotten any number of other relays to solve this problem, but I paid extra for the Mercedes part because I wanted to do it right. I was punished for my confidence and brand loyalty.

It seems like I've seen some other Mercedes versions of this relay labeled at Siemens, which tends to be a pretty good company. Has anyone else seen this problem? Any recommendations on other mechanical 12V relays rated for this fairly aggressive level of power flow (at least 50 A)? I don't feel safe with this Mercedes part in my van.

I suggest upgrading to the Blue Sea Systems SI-ACR. It is a voltage-sensing relay that will allow your solar to charge your vehicle battery, and your alternator to charge your house battery. Rated at 120 amps continous.

Highly recommended.

sprint2freedom said:

I suggest upgrading to the Blue Sea Systems SI-ACR. It is a voltage-sensing relay that will allow your solar to charge your vehicle battery, and your alternator to charge your house battery. Rated at 120 amps continous.

Highly recommended.

Click to expand...

Thanks for the recommendation! This looks really nice and automatic, and would be good for a lot of applications. The automatic feature is actually a problem, though, because I *only* want the interconnect to work when the house batteries are heavily discharged, which is pretty uncommon. If the batteries are mostly full, they can be damaged by connection to the alternator because it stupidly tries to maintain 14.1V no matter what. My solar charger is smart enough to step the voltage down to float the batteries when they are almost full, which makes them last a lot longer. What I *really* want is to be able to pull the State of Charge from my Trimetric meter so that the interconnect happens automatically *only* if the house pack is below a certain level, say 80% or something. I'm working on that project now.

In the meantime, what I want is a simple relay like the Mercedes one, but with rigorous construction.

thebitmaster said:

Thanks for the recommendation! This looks really nice and automatic, and would be good for a lot of applications. The automatic feature is actually a problem, though, because I *only* want the interconnect to work when the house batteries are heavily discharged, which is pretty uncommon. If the batteries are mostly full, they can be damaged by connection to the alternator because it stupidly tries to maintain 14.1V no matter what. My solar charger is smart enough to step the voltage down to float the batteries when they are almost full, which makes them last a lot longer. What I *really* want is to be able to pull the State of Charge from my Trimetric meter so that the interconnect happens automatically *only* if the house pack is below a certain level, say 80% or something. I'm working on that project now.

In the meantime, what I want is a simple relay like the Mercedes one, but with rigorous construction.

Click to expand...

While I don't actually agree that there's a risk of damaging the batteries by connecting them to the alternator (they will gradually stop accepting charge as they approach 100% SoC, even if held at 14.1V), there is a simple way to accomplish what you're looking to do with the SI-ACR:

The ACR has a ground wire that needs to be connected in order for it to function. To prevent it from combining the batteries you could simply wire a switch in-line with that ground wire, leaving the switch in the off position until a time or circumstance of your choosing. Or, you could instead wire a relay that is activated by your battery monitor below a particular SoC.

I'm puzzled by the "14.1v" number... on my 2005 T1N Sprinter (admittedly not a LINbus alternator), the 14.1 output is only temporary after starting.
Within a few minutes it drops down to 13.6 to 13.8 (depending upon accessories and headlights).

At best/worst, you could use a diode isolator instead of a relay, depending upon it's (roughly) half-volt forward drop to keep your house batteries happier.
(normally i'd never recommend a diode isolator, but you seem to be asking for "special case" circumstances)

--dick

The Scan gage on my sold 08 NCV3 always showed 13.9-14.1 volts.

sprint2freedom said:

While I don't actually agree that there's a risk of damaging the batteries by connecting them to the alternator (they will gradually stop accepting charge as they approach 100% SoC, even if held at 14.1V), there is a simple way to accomplish what you're looking to do with the SI-ACR:

Click to expand...

I'm not a power expert, but if there weren't valid reasons to by fussy about controlling charge voltage and current when the battery nears full, then three or four stage chargers wouldn't be the industry standard for increasing battery life. How much it helps is open to debate, and I wish I had more hard data to back it up. My friend Chris, who used to convert cars from gas to electric for a living, could probably do a better job of explaining why it's helpful. I'm chasing the elusive 10-15 year lifespan for flooded lead acid, not the more common 3-5 years.

The ACR has a ground wire that needs to be connected in order for it to function. To prevent it from combining the batteries you could simply wire a switch in-line with that ground wire, leaving the switch in the off position until a time or circumstance of your choosing. Or, you could instead wire a relay that is activated by your battery monitor below a particular SoC.

Click to expand...

That's a good idea and would totally work. What I'm working on is getting the SOC data out of my Trimetric into a Rasberry Pi or similar to do exactly that...connect them when the SOC is below a certain point. In this case, what I'm really looking for is "exactly the same type of mechanical relay as the Mercedes one, but actually well-built and reliable, while still being pretty cheap." I paid $35 for the relay. If the extra features the BlueSea had were applicable here, it's totally the way to go. They make great gear.

Right now the Eagle Creek Pass areas up in the gorge is burning, and it's raining fried forest. I'm down to 7 A coming in through the solar, the sky is hazy, and there's no point in trying to clean the panels until the ash falls off. In the meantime, at 68% SOC, I'm getting about 40 A from the newly repaired alternator interlink.

autostaretx said:

I'm puzzled by the "14.1v" number... on my 2005 T1N Sprinter (admittedly not a LINbus alternator), the 14.1 output is only temporary after starting.
Within a few minutes it drops down to 13.6 to 13.8 (depending upon accessories and headlights).

Click to expand...

My iCarSoft reader has it between 13.9 - 14.1 V most of the time, with some brief dips below. I have a 2007 NCV3 3.0 diesel.

At best/worst, you could use a diode isolator instead of a relay, depending upon it's (roughly) half-volt forward drop to keep your house batteries happier.
(normally i'd never recommend a diode isolator, but you seem to be asking for "special case" circumstances)

--dick

Click to expand...

That's another good option, thanks Dick.

autostaretx said:

I'm puzzled by the "14.1v" number... on my 2005 T1N Sprinter (admittedly not a LINbus alternator), the 14.1 output is only temporary after starting.
Within a few minutes it drops down to 13.6 to 13.8 (depending upon accessories and headlights).

Click to expand...

The chassis electrical bus on the NCV3/V6 is normally 14.1 +/- 0.1 volt and if it's any lower than that during normal cruise operation then you need to start looking for a problem (although it can temporarily drop to around 13.8-13.9 under heavy load such as immediately after starting, PTC heater on, etc.)

14.1 volts while underway do no harm whatsoever to typical lead-acid batteries. FLA cells at a full state of charge may gas a little under those conditions but it's harmless.

smiller said:

14.1 volts while underway do no harm whatsoever to typical lead-acid batteries. FLA cells at a full state of charge may gas a little under those conditions but it's harmless.

Click to expand...

Agree. A far bigger problem and a sure way to kill your batteries quickly is having too low of a charging voltage, or leaving the batteries in a discharged state for any significant length of time (more than a few days) as sulfation will occur.

sprint2freedom said:

Agree. A far bigger problem and a sure way to kill your batteries quickly is having too low of a charging voltage

Click to expand...

Yes, if there's anything wrong with 14.1 is that it's a rather low absorption charge voltage and while it will do the job it will not provide optimal cell equalization and maximum cycle life. It won't hurt anything over a few charge cycles but if your normal way of recharging house batteries is driving on the road then it's a good idea to give them a full charge with a proper 3/4-stage charger every once in a while, or perform a manual equalization.

I used the Mercedes relay without a problem on the sold Sprinter. What was funny was the design of the actuating relay connectors. Not designed to connect a wire to the relay. Was told had to buy adapters. Bought the adapters which did connect to the relay. Only problem was I still could not connect wires to the adapters. As I recall I discarded the adapters and jammed a spade wire terminal into the opening. Found the design quite hilarious.

I also believe a battery charged with a correct 3 stage charge provile does last longer than a battery directly connected to the starting battery. Unfortunately I have not read any proof that it is true or how the battery life will change. I get a proper 3 stage charge from my MPPT solar controller. For a backup if weather condition force its use, I power the shore power 3 stage charger with 120 volt AC from a pure sine vehicle powered inverter.