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Old 01-02-2014, 03:18 AM   #1
ECU
 
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Default Using the Aux Batt to jump start

I'm still working on my Aux battery install. Two AGMs under the passenger seat of my T1n. I will install a relay where the factory one seemed to go, under the driver seat. I picked up the PDF for factory install (T1N_Retrofit_Aux_Battery.pdf).
Two kind of related questions.
I want to put a fuse at the battery location, but what rating?
I have been stuck a few times from dead batteries, so I want to be able to jump start from the aux batteries. I would think the easiest way is to force the relay closed with power from a switch, only to do the starting then flip switch off. So the fuse at the aux battery would have to be enough to take the starter draw.
Only a guess, but I'd think that 200 to 250 amp fuse would be needed.
I've searched to site but haven't found anyone setting up to do the jump starting. Is it a bad idea?
PS. I'm putting the inverter in the back in a wheel well box I'll build. With a 118, it will be a shorter run.
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Old 01-02-2014, 03:38 AM   #2
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Default Re: Using the Aux Batt to jump start

A trick whenever you're faced with long wire runs for starting is to attach the jumper cables...
and wait a couple of minutes.
During that time, the "good" battery will be charging the "bad" battery.

After the "couple of minutes" you then try starting the Sprinter (car), with the jumper cables still attached.

Since you've already shoveled a bunch of amps into the usual starter battery, the helper battery (and cables) doesn't have to carry the full load of the starter.

(feel free to substitute "isolator relay" for "jumper cables" in the above description)
--dick
p.s. your isolator relay should also be sized to carry the expected starter load... so 200 amps continuous
(since you may be cranking for 15 seconds or more). The contacts don't have to switch 200 amps, but they need to be able to carry it without melting (or welding together). Good relay spec sheets will specify both switching and continuous ratings.
p.p.s. don't forget that even the glow plugs draw 95 amps when cold (drops to 65-ish when they're hot)
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Get YOUR Sprinter's full configuration datacard by entering your VIN to https://www.datamb.com/
http://diysprinter.co.uk/reference <-- lots of service documentation, Thanks to Jens Moller and Arnie_Oli
((as always: this post may go through a couple of post-posting edits... so maybe give it ten minutes before commenting))

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Old 01-02-2014, 04:13 AM   #3
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Default Re: Using the Aux Batt to jump start

That sounds good. I don't know alternators much, but mine is 150 amp. So while charging, the relay would have to be heavy duty as well. I'll look for a 200 amp relay. Should I bother with getting OEM? I can get the mechanic's discount on it. But I can get the same discounts at NAPA too.
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Old 01-02-2014, 05:04 PM   #4
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Default Re: Using the Aux Batt to jump start

Quote:
Originally Posted by ECU View Post
That sounds good. I don't know alternators much, but mine is 150 amp. So while charging, the relay would have to be heavy duty as well. I'll look for a 200 amp relay. Should I bother with getting OEM? I can get the mechanic's discount on it. But I can get the same discounts at NAPA too.
I'd try to find the manufacturers' specifications for the candidate relays.

German engineering being what it is, the OEM relay is probably rated at whatever current MB uses for that fuse position... it certainly will not be expecting to carry starter current.

Hmmm... i just dug through the 2006 service manual ... and page 8w-20-3 shows that the connection between the two batteries (via the relay) is NOT fused!
(unless you count the melting point of 6- or 4-gauge wire)
(likewise, page 8w-21-2 shows that the starter isn't fused, either)

So the relay is "it".

For comparison, the 2007 NCV3 has a 150 amp fuse between the starter and aux battery relay.
The battery to starter link is still unfused, but then from the starter they have a 300 amp in-line fuse to the alternator, and then from there they have a 100 amp in-line fuse to the glow plug module... jeesh...

--dick
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2005 T1N 118" Freightliner 2500 Passenger Wagon (2.7L, 15" tires, standard (short) roof)
Get YOUR Sprinter's full configuration datacard by entering your VIN to https://www.datamb.com/
http://diysprinter.co.uk/reference <-- lots of service documentation, Thanks to Jens Moller and Arnie_Oli
((as always: this post may go through a couple of post-posting edits... so maybe give it ten minutes before commenting))

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Old 01-03-2014, 03:15 AM   #5
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Default Re: Using the Aux Batt to jump start

Thanks for that info. I was looking for a 200 amp CB today and could only find a 150. The 200 would have to be something off the internet.
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Last edited by ECU; 01-03-2014 at 03:47 AM.
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Old 01-03-2014, 07:37 PM   #6
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Default Re: Using the Aux Batt to jump start

My Surepower 1315-200 bi-directional battery separator has a lot to recommend it. It gives alternator priority to the starting battery until its voltage comes up high enough and then feeds my agm house bank. (The 1314 uni-directional is a little cheaper if you'll never charge the other direction from shore power/genset.)

It also has an override switch to temporarily combine both banks to help with a low starting battery.

I used a 150a dc cb instead of a fuse because I wanted an easy recovery from any fault. I did not think hard enough about starting use and only have a 2 gauge wire running to the house agms. So I also use the wait-a-minute approach to cut down on the starting surge. It does a great job as a starting boost in any case if I let the starting battery run low from a mistake.

My 2000w inverter is as close as possible (18") to my agm bank and uses 00 cable. You definitely want a large, short cable run until the 100a of dc becomes 10a of ac.

Still important, but less so, if your ac needs are under 500w and not many ah's are consumed.

Always a shame to derate those expensive agms with the voltage drop over a longer run.

Dan
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Old 01-05-2014, 11:22 PM   #7
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Default Re: Using the Aux Batt to jump start

I have another related question. I have a sunforce 11240 inverter with a 12 cig lighter output.
I am wondering if I should pull 12v off of that (20 amp) to feed some aux devices around the van? Or should I tap directly from the battery?

This 12v output is hot all the time, not just when the inverter is on. It seems to be linked to a bank of capacitors. When you pull power, it slowly dies.

Thanks for the previous info. Finally got around to looking at the swivel adapter. I didn't know it went down over an inch in to the battery room. It will be a tight fit.
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