Keep Aux battery in system...or not?

Dave D

Active member
I'm with George wondering if you really want 3 battery banks. 2 house banks add complexity, take more space, and you get diminishing returns from smaller separate banks.
An option to consider is to put your 200 amp battery bank in the under-hood auxiliary battery location to replace the Aux battery. I just did this using the RB Components battery tray with 2 220 amp full river batteries.

I currently have no big loads on it, but have a second alternator to charge it so it will be independent of the MB main electrical system. I'm planning to run larger cable and use the BlueSea battery terminal fuse once the big install begins.



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Mendobob

2014 144" WB 4cyl
I'm with George wondering if you really want 3 battery banks. 2 house banks add complexity, take more space, and you get diminishing returns from smaller separate banks.
An option to consider is to put your 200 amp battery bank in the under-hood auxiliary battery location to replace the Aux battery. I just did this using the RB Components battery tray with 2 220 amp full river batteries.

I currently have no big loads on it, but have a second alternator to charge it so it will be independent of the MB main electrical system. I'm planning to run larger cable and use the BlueSea battery terminal fuse once the big install begins.
Hi Dave - sounds like a good possible plan. Wondering 2 things: 1) did you have OEM Aux under the hood? 2) if yes - will you tap into the relay under drivers seat? I am hoping for a yes on these...it would make a smooth transition when I am ready to upgrade.

Edit: just reread your message and it sounds like you will not tap into the relay since you are going independent of the MB system.
 
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Dave D

Active member
Hi Bob,

I did have the OEM Aux system and am still using the relay under the drivers seat until I do the main build out and add some loads. Right now the only load on the house bank is the motor to raise the pop top. When I put the alternator in in the next few weeks I'll either remove the relay or make it manual to be able to jump start a dead starter battery.
 

camillo

New member
I currently have no big loads on it, but have a second alternator to charge it so it will be independent of the MB main electrical system. I'm planning to run larger cable and use the BlueSea battery terminal fuse once the big install begins.

Sent from my iPhone using Tapatalk
Dave - do you have a 3.0L? Do you have the second alternator in? Would love to see a picture. I bought a dual alternator kit from Nations but haven't had it installed yet. Waiting for Nations to provide instructions (they updated the kit, but the instructions are for the old kit)
 
Hi Dave - sounds like a good possible plan. Wondering 2 things: 1) did you have OEM Aux under the hood? 2) if yes - will you tap into the relay under drivers seat? I am hoping for a yes on these...it would make a smooth transition when I am ready to upgrade.

Edit: just reread your message and it sounds like you will not tap into the relay since you are going independent of the MB system.
This is the exact setup I'm going with too. What is so bad about simply replacing the OEM battery with these 2 running in series to the same charging system and OEM relay? Is the problem the relay or the alternator?
 

DieselFumes

2015 4x4 2500 170 Crew
This is the exact setup I'm going with too. What is so bad about simply replacing the OEM battery with these 2 running in series to the same charging system and OEM relay? Is the problem the relay or the alternator?
The problem is both of those and more.

1) Relay is designed for a certain amperage. Replacing the battery with a higher capacity one and drawing from it at a higher capacity might put stress on the relay.
2) Alternator is specced to have no more than 40A additional draw over what the van's systems use. It sounds like it *can* provide the charge current for higher capacity batteries, but it's not designed to do so.
3) Different battery types need different charging profiles. If you rely on the Mercedes system to regulate charging, it's possibly going to undercharge your new aux battery system or overcharge your house system. Or both.
4) Cable thickness from the aux battery is designed for a 100Ah battery at low draw rates. Adding a higher capacity battery and discharging it at higher rates may exceed the safe rating of that cable.
5) Lead acid batteries take a long time to get from 80% to 100% charge. Do you drive long enough each day to replenish the batteries? Like, 4 hours or more?

I've used a lot of weasel words ("might", "possibly", "could") because other folks seem to have swapped out the OEM aux battery for higher capacity SLA or wet cell batteries and have not reported issues. Yet.
 

ddunaway

Member
The problem is both of those and more.

1) Relay is designed for a certain amperage. Replacing the battery with a higher capacity one and drawing from it at a higher capacity might put stress on the relay.
2) Alternator is specced to have no more than 40A additional draw over what the van's systems use. It sounds like it *can* provide the charge current for higher capacity batteries, but it's not designed to do so.
3) Different battery types need different charging profiles. If you rely on the Mercedes system to regulate charging, it's possibly going to undercharge your new aux battery system or overcharge your house system. Or both.
4) Cable thickness from the aux battery is designed for a 100Ah battery at low draw rates. Adding a higher capacity battery and discharging it at higher rates may exceed the safe rating of that cable.
5) Lead acid batteries take a long time to get from 80% to 100% charge. Do you drive long enough each day to replenish the batteries? Like, 4 hours or more?

I've used a lot of weasel words ("might", "possibly", "could") because other folks seem to have swapped out the OEM aux battery for higher capacity SLA or wet cell batteries and have not reported issues. Yet.
I put 2 Fullriver DC 115s in as essentially a replacement of OEM. EOM relay is fine. Wire is 2 GA. It is fine. Rarely does charge current go above 100A.

When the alternator is charging and batteries connected, Voltage is ~13V....no possibility of charging problems with mixed battery types for house and vehicle...especially with solar to top off....ignore that

This 40A thing is to be ignored too...silly...drivel from conservative MB engineers...they probably modeled in all possible loads at once
 

Mendobob

2014 144" WB 4cyl
I put 2 Fullriver DC 115s in as essentially a replacement of OEM. EOM relay is fine. Wire is 2 GA. It is fine. Rarely does charge current go above 100A.

When the alternator is charging and batteries connected, Voltage is ~13V....no possibility of charging problems with mixed battery types for house and vehicle...especially with solar to top off....ignore that

This 40A thing is to be ignored too...silly...drivel from conservative MB engineers...they probably modeled in all possible loads at once
Excellent! - good to know - thanks!
 

GeorgeRa

2013 Sprinter DIY 144WB, Portland OR
I have 2 x 115 Ah Fullriver batteries connected to auxiliary battery circuit since 2013. Before a long trip my plan is to install the Magnum ME-SBC in series with factory relay with a manual override. ME-SBC will limit current and connect the alternator at specific adjustable voltages. Is it a perfect solution? - no, does it work for many? - yes. Most if not all RV motorhomes are connected that way from the factory.

With Li batteries using alternator directly likely will not change except a high voltage manual or automatic disconnect will be required.

George.
 

Graphite Dave

Dave Orton
The problem is both of those and more.

1) Relay is designed for a certain amperage. Replacing the battery with a higher capacity one and drawing from it at a higher capacity might put stress on the relay.
2) Alternator is specced to have no more than 40A additional draw over what the van's systems use. It sounds like it *can* provide the charge current for higher capacity batteries, but it's not designed to do so.
3) Different battery types need different charging profiles. If you rely on the Mercedes system to regulate charging, it's possibly going to undercharge your new aux battery system or overcharge your house system. Or both.
4) Cable thickness from the aux battery is designed for a 100Ah battery at low draw rates. Adding a higher capacity battery and discharging it at higher rates may exceed the safe rating of that cable.
5) Lead acid batteries take a long time to get from 80% to 100% charge. Do you drive long enough each day to replenish the batteries? Like, 4 hours or more?

I've used a lot of weasel words ("might", "possibly", "could") because other folks seem to have swapped out the OEM aux battery for higher capacity SLA or wet cell batteries and have not reported issues. Yet.
Hate to keep repeating my comments about a two inverter system.

But:

1. No transfer relay.

2. If you have 120 volt power available the shore power charger can be programed (at least mine) to provide just 40 amps for charging. My Magnum's max. charging rate is 50 amps but it is programmable with the remote to whatever amperage you want. Mine is set at 80% of the full rate or 40 amps. I have no need for more amperage for my application. The size of the vehicle inverter limits the maximum amperage that can be drawn from the vehicle system.

3. House battery and vehicle battery are never directly connected together so battery types is not an issue.

4. No heavy cables between the vehicle battery and the house battery. The power from the vehicle powered inverter is at 120 volts instead of 12 volts so 14/3 is all that is required.

5. The solar system provides the last portion of the house battery charging.

Using 120 volts AC with a 3 stage solar charger and a 3 stage shore power charger always charges the house battery with a 3 stage charge with the correct charge profile for your particular house battery.

The house charging system is completely separate from the vehicle charging system.

The 120 volt power from a vehicle powered inverter is available for other 120 volt loads in addition to house battery charging. I use it to power a simple shower water heater and to power a 120 volt AC air heater. You can charge or heat water or heat air with the engine running.

Two inverter block diagram:
 

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jamesthomsen

New member
Last year I followed Graphite Dave's suggestions for a two inverter system and have just returned from a 30 day travel and camping trip. Thank you! The system worked perfectly.
 

HarryN

Well-known member
Hate to keep repeating my comments about a two inverter system.

But:

1. No transfer relay.

2. If you have 120 volt power available the shore power charger can be programed (at least mine) to provide just 40 amps for charging. My Magnum's max. charging rate is 50 amps but it is programmable with the remote to whatever amperage you want. Mine is set at 80% of the full rate or 40 amps. I have no need for more amperage for my application. The size of the vehicle inverter limits the maximum amperage that can be drawn from the vehicle system.

3. House battery and vehicle battery are never directly connected together so battery types is not an issue.

4. No heavy cables between the vehicle battery and the house battery. The power from the vehicle powered inverter is at 120 volts instead of 12 volts so 14/3 is all that is required.

5. The solar system provides the last portion of the house battery charging.
m:
Dave, one additional value to this approach - Since it is AC, it is easy to send power to a trailer using standard twist lock cords used in RV and marine applications. I didn't really appreciate this aspect until I was trying to figure out how to send a bunch of DC amps to a trailer.
 

Graphite Dave

Dave Orton
Dave, one additional value to this approach - Since it is AC, it is easy to send power to a trailer using standard twist lock cords used in RV and marine applications. I didn't really appreciate this aspect until I was trying to figure out how to send a bunch of DC amps to a trailer.
Some people on this site have a problem doing the electrical in a different way than the standard RV method of connecting the house battery directly to the vehicle battery for charging. There are multiple reasons IMO why the two inverter system is a better solution.

Two inverter system worked very well in the Sprinter build. Duplicated the system in the Transit build. With my 300 watt single solar panel as the primary charging method, I have no problem traveling indefinitely without shore power or a generator. (My power usage and climate).

Another choice that works is a single inverter that can get 12 volt power from the vehicle or the house battery. Just uses a selector switch to select which source. I think Evil Patrick on this site was the first one to do that design. Same advantage of always charging the house battery with a proper 3 stage charge profile.
 

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