low voltage to alternate battery for power inverter

sydkyd

New member
I have a 2005 sprinter with a 5000w power inverter. Under the drivers seat is a relay with the mercedes logo and reads

A 002 542 40 19
89 9322 000
12v
>PBT-GF20<

the power into this relay reads 13+ volts as it should but the output was 6 and is now down to 3

Im not sure what this part is and cant seem to find anything online for it
 

d_bertko

Active member
I can't help you on the relay but what size is your inverter?

5000w would be an enormous load and require maybe 6 batteries to run...
 

icarus

Well-known member
5 kw draw from 12 vdc is in excess of 400 amps! (not counting inverter loses!). Even 6 T-105 batteries wired for 12 vdc would not deliver 400 amps for very long!

Icarus

PS running such a large inverter for smaller RV loads is VERY inefficient. Consier a smaller inverter.
 

sydkyd

New member
I use the van for air quality testing and have several analyzers, computers, pumps and recording systems running for 8-12hrs at a time. I usually operate with my van running at all times so it holds up well at that end. IT seems as though the system was factory installed as everything is tagged with mercedes logos down to the 2nd battery even. Running the power is not a problem and hasnt been for over a year just dont know what this part is and it is a bit frustrating ive had to bypass it and everything works fine but now i dont have any protection on my first battery so i must be absolutely sure inverter as well as back cabin lights (run off 2nd battery) are off so as not to drain BOTH batteries and leave me unable to start my ride.

I will make it into a dealership late next week to find out about the part but am testing every weekday until then so was hoping to find out more about part online and maybe order it but I cant seem to find it by googleing

At the moment the problem is the power supply coming to the 2nd battery. Even with the inverter off its only getting 3volts from the main battery when the engine is running. (should be 13+ volts) i have traced the line to a relay under the drivers seat which has the 13v cable coming in from main battery but the line coming out of relay is only 3v on a 4guage wire this should still be 13v just protects the draw when the engine is off.
 

cedarsanctum

re: Member
Is that relay the battery isolation relay for the auxiliary battery? This will disconnect the 2nd battery from the main battery when the engine is not running.
It sounds like you may have fried that relay. Replacement is the only option.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
I have this isolation relay in my LTV 2012 MB. 2 big terminals for the contacts and a plug for 2 small leads to the coil. Does anybody happen to know what these 2 leads are? I'm building a charging control circuit - I put LiFePO4 batteries in the MB and I want to be able to stop both charger and alternator charging when my monitor says the batteries are full (or at whatever setpoint I pick). It appears that the easiest way to stop alternator charging is to prevent the isolator from closing, and I can do that by breaking one of those 2 coil leads. It probably doesn't make any difference which side of the coil I open, but I'd love to know where those wires go. I'm guessing that one is switched +12 from the ignition (always +12 when key is on) or +12 from some M-B sensing system that decides when the isolator should be closed. I'm guessing that the other side goes to chassis ground. Does anybody know?
 

Spike HG

Member
Connect circuit switching wire to the EK1 Terminal 1 D+ (s. Page 10) and use only existing ground terminals on driver seat base floor (1)



I would install a contact/relay/output in the D+ loop (wire) to drop out the alt from charging the bank.

If one needs to use an output (power monitor relay) for two devices, cutoff relay and charger. One may need to share the output, one way, install interposing relay with a two contacts like an Dayton 1EGX1 or(similar relay). One contact would break D+ to cutoff relay, other to disable charger.

I set up BMV-700 power monitor to change the state of an Dayton 1EGX1 the other day, have notes see post #6:
https://sprinter-source.com/forums/showthread.php?t=42735
 
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Peter Tourin

2020 Unity RL, ex 2012 Unity MB
Jay - I'm starting to pull this together. I found the M-B PDF, and I see all the components. I'm used to working from schematics so I want to be sure I understand the PDF. Are you saying that the single wire off the D+ terminal goes to the relay coil, so that if I lift the only wire that's on the D+ terminal, that disables the relay? So the wire off the D+ terminal becomes my switch point for disabling alternator charging?

And yes, I do intend to put a relay there - the control relay in my Victron monitor will only handle 1A @ 60V, and I have 2 circuits to switch, the alternator circuit that we're discussing and my RV charger. The alternator looks like it'll be easy to switch - I assume that wire to the isolator coil only draws maybe a amp - I'll measure it unless you happen to know what it draws.

I'm thinking the easiest way to control the charger is to switch the 120vac input to the charger - I'm thinking that'll require a 2nd relay that'll switch 120vac and handle 1000w.
 
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Peter Tourin

2020 Unity RL, ex 2012 Unity MB
I looked at your Victron thread - all very familiar! I have a BMV702 monitoring 200Ah of LiFePO4's. I swapped the OEM charger for a Progressive Dynamics PD9260AL. This is all working pretty well as of last October. At this point I just got the RV out of storage (we're in northern VT) and I'm monitoring, but I have no charge switching yet for either the charger or the alternator.
 

Spike HG

Member
>way to control the charger is to switch the 120vac input to the charger - I'm thinking that'll require a 2nd relay that'll switch 120vac and handle 1000w.

One using DPDT like Dayton 1EGX1 relay (there are many), one could program output on the BMV to change state of the interposing relay, disconnect the 12vdc D+ cutoff relay and the charger (120VAC + wire that feeds the charger) with one relay. 1EGX1 Relay has a 10 amp rating, most likely if charger is less than 100 amp, 12 VDC, should work. P=1XE 1200=120X10

http://www.learningaboutelectronics.com/Articles/DPDT-relay-wiring-diagram
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
Yeah, in an earlier post I proposed using a single DPDT to save on coil current, and someone, I forget who, didn't like the idea much - worried about isolation between the 2 sets of contacts since I'd have 12vdc on one set and 120vac on the other. I hadn't really thought that would be a problem - the contacts are supposed to be isolated!! But I started thinking about switching to 2 separate relays. Also, it might be handy to have the alternator relay up under the seat - there are all those unused extra positions for relays - I don't know yet what you need to order for relay bases in order to use those, but it would be a neat setup. It might make more sense to have a relay for the charger back near the charger. I have my BMV702 mounted near the side door where the light switch and step defeat switch are mounted (under the counter where the TV pops up), so it's about halfway from each.

The charger says input 1000w @ 120v. It's a 60 amp charger. So a 10A relay should cut it, though not with a lot of headroom...

BTW - I have a pretty good understanding of basic electric power generation and distribution, I understand simple logic circuitry - it's the practical details that slow me down - what relays, what bases, how to route the wiring when I don't know the RV very well, and making sure I understand the specifics of working with Lith batteries (if anybody actually does understand it well <g>...). I have some practical experience - built a 45KW hydro site and ran it for 15 years, was involved with installing and servicing data loggers for small independent power producers in VT who sold to the directly to VELCO, the VT power distribution entity. But I don't have any sort of EE training and I've never had a Sprinter before so everything in it is new to me. I also haven't done a lot of modern car work tho' I know something about vintage motorcycles and cars. So it's a bit of an adventure...

For the sake of getting the RV movable again, I lifted that D+ wire and extended it - also put a wire on the D+ terminal. So for the moment, both of them are reachable with the seat back in place, and I've got them tied together. As expected, the alternator charges the house batteries when I start the engine. I assume that if I untie them, the isolator will stay open and I won't charge the batteries, but I was waiting to hear from you - wanted to make sure I understood the M-B description correctly (beginning of post 8).

Thanks for the guidance - it's dark out now so maybe I'll look into the M-B upfitters site for a while.
 

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