Tie in to alternator for aux battery

I have an 07 long/high cargo van, slowly being converted to a minimalist overnight camper and toy hauler. I need to tie in a battery isolator relay so I can charge my aux batteries from the engine. If anyone has done this, advice would be welcome, as would pics.

question:

Where will your house/aux batteries be mounted?

They are above the drivers side rear wheel well now. I have the #4 cable ready to go, just need to determine the best way to mount the isolator relay and tie into the alt.

michaelrking said:

I have an 07 long/high cargo van, slowly being converted to a minimalist overnight camper and toy hauler. I need to tie in a battery isolator relay so I can charge my aux batteries from the engine. If anyone has done this, advice would be welcome, as would pics.

Click to expand...

The factory connection to the vehicle is made at the starter battery fuse terminal under the drivers feet, with a relay under the drivers seat providing the isolation and a large fuse on the cables that then go to the auxiliary battery under the hood. I made my house connections at the large fuse under the seat.
Hope this helps.
Jef

Michael -

What sized batteries are you considering for house use? What size alternator does your truck have?

Near as I can tell, 180amp alternator. Plan to use 2x group 27 (I think) deep cycle batteries. I have an Elixer 60 amp converter/AC/DC load center installed, and now use shore power to recharge the single battery I have in there now.

michaelrking said:

Near as I can tell, 180amp alternator. Plan to use 2x group 27 (I think) deep cycle batteries. I have an Elixer 60 amp converter/AC/DC load center installed, and now use shore power to recharge the single battery I have in there now.

Click to expand...

These fellas are worth checking out:

http://www.perfectswitch.com/

(I have no connection with them, I've been looking at isolators, and had a phone conversation with a fella there - VERY helpful, and the power-gate product they sell has some good reviews.)

Check your amp hour rating of your batteries carefully and figure out your electrical budget.

Here's our recent battery upgrade - I'm in the midst of trying to make sure how we can set things up to have the bank charge from the truck running. I keep running into concern with overcharging the bank or frying the alternator!!

https://sprinter-source.com/forums/showthread.php?t=11298

Make sure of your amp rating on your alternator.

You're not tying into the alternator, per se, you are tying into your staring battery. The alternator charges this battery via a smaller gage wire, as the amount of current flowing between alternator and battery is small relative to what flows between the battery and other things such as the starter.

Really, this is pretty simple. There are dozens of images here and many have already put a fair amount of effort into the answers and probably aren't going to repeat that effort every time someone asks, so do some searches and surf and maybe pick some other things up on the way.

Don't spend too much on your isolator. Even though a bunch of unnecessary voodoo high end options exist, their benefit is near zero. The Mercedes isolator found in vehicles with the aux battery option can be found in the aftermarket equivalent for about $20.

Basically, as posted above, you are hooking into your staring battery, the positive post, with an isolator and ~250 amp fuse in between. Mercedes uses 2 gage wire for this.

You have 3 posts under the driver's seat with full time power, and switched power. Do some homework on these terminals. They are meant for all accessories to be wired into. Run your isolator off one of these 3 posts.

Good luck,

You need to be very careful choosing the correct power source to energize the relay. You do not want the relay turning on before the engine is running. I believe my '07 Sprinter based motorhome's isolation relay is energized by Terminal D+ power.


Cal

I would also consider using larger wire. I used 3 ought for my 2000 watt magnum energy inverter. You can find a wire sizing chart in the owners manual found here.

http://www.magnumenergy.com/Doc-Lib-SineInverterChargers.htm

I use anchor marine cable like this

http://www.google.com/products?q=an...ent=firefox-a&um=1&ie=UTF-8&sa=N&hl=en&tab=wf

You don't need 3/0 cable to charge the battery. Connecting an inverter to the battery is the only place you need cable that thick. Mercedes uses 2 gage. The 4 gage he bought will probably work fine as well. If he hooks an inverter down stream, then the thicker cable is recommended.

NBB said:

You don't need 3/0 cable to charge the battery. Connecting an inverter to the battery is the only place you need cable that thick. Mercedes uses 2 gage. The 4 gage he bought will probably work fine as well. If he hooks an inverter down stream, then the thicker cable is recommended.

Click to expand...

Alternator/Isolator to House Battery Cable:

Charging Amps: 60 (Possible when battery is more than 80% SOC or 20% DOD)
Wire lenght(Both Ways): 20ft
Voltage Drop: 3%

Recommended Wire: AWG 4 (Voltage Drop) or AWG 10 (Ampacity)


Capacity per ABYC Standards: 160 amps.


The wire you select must be large enough to meet both ampacity and voltage drop requirements for adequate charging.

I used a Sure Power separator/interconnect when I built my similar diy about 5 years ago. Popular with the commercial Sprinter rv converters. My 150a alternator has had no trouble keeping four extra house batteries fed all this time.

I think I used a 1314(?) unidirectional unit. The optional override switch allows for an emergency jump start from the house bank as well as the ability to charge the coach battery from my inverter-charger under shore power. A number 2 cable is sufficient for an emergency start---just wait a minute or two after you connect and a lot of charge will transfer to the starting battery.

Get the bidirectional unit if you plan to maintain the starting battery this way---typically for a less-used rv. Not really needed if you have an override switch. Nothing wrong with the more capable bidirectional unit either.

Dan

OrioN said:

Alternator/Isolator to House Battery Cable:

Charging Amps: 60 (Possible when battery is more than 80% SOC or 20% DOD)
Wire lenght(Both Ways): 20ft
Voltage Drop: 3%

Recommended Wire: AWG 4 (Voltage Drop) or AWG 10 (Ampacity)


Capacity per ABYC Standards: 160 amps.


The wire you select must be large enough to meet both ampacity and voltage drop requirements for adequate charging.

Click to expand...

+1

I have about 250 AH worth of AGM batteries in my van. When they are half depleted (down to about 12V) my inverter-charger maxes out at about 100A charging them. When fully discharged (about 11.6V) the charging voltage drops significantly suggesting that over 100A can be absorbed by these batteries.

I would suggest calculating the various loads and making sure you stay well under your alternator's maximum current rating. I think I've told my story of alternator failures elsewhere and have switched to the largest alternator available (200A for my T1N) and am much more careful about how I load it. But this is mostly as a precaution. It is likely, but I've not proven, that the prior alternator failures were caused by asking it to provide more current than it was capable of.

An isolator that has current limiting or disconnects when the vehicle voltage drops below a certain amount will at least provide current limiting or turn a large load into an intermittent one. Short of properly engineering a solution, should the alternator fail with these measures in place, a larger alternator as a replacement may do the trick. If not, then more complex load limiting measures would be necessary. I do not recall seeing any posts on failed alternators from people who are using isolators that have some kind of load limiting mechanism.

I've noticed that in the diagram found in the link below, the D+ terminal is given the label "10amps". I have a 220 amp alternator....which rating do I need to consider when choosing a battery isolator? The blue/yellow wire attaching to D+ terminal hardly seems big enough to handle 220 amps.

https://sprinter-source.com/forums/showthread.php?t=10047