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Old 06-18-2019, 08:33 PM   #1
fjc
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Default Jump start chassis battery with house battery

Hi On my 2018 CS Adventurous, there is a house AGM battery in the driver side engine bay. Can I connect a jumper cable to start a dead chassis battery?
The house battery (with 400W Lithium) has over 13 volt.
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Old 06-18-2019, 09:28 PM   #2
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On my 2013 I did that when the auxiliary battery negative was connected via (I think it was brow)n cable to the ground stud next to the battery. Since, my auxiliary one is wired differently. So, you need to connect auxiliary plus to engine battery plus connection stud under the hood, assuming 2018 has this stud.
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Old 06-30-2019, 05:18 PM   #3
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I've successfully jumped the chassis battery off my engine bay AGM house battery before, HOWEVER, I would caution you about trying a direct jumper cable between a lithium and AGM battery (Mercedes chassis battery)

The lithium battery is capable of far faster charge and discharge rates than AGM. You DID mention that the engine bay battery is an AGM. I assume that's there to reboot your ecotrek lithiums? It would probably be safe, but the wiring in Roadtreks can get complex. (I have a 2015 CS Adventurous with 8 AGM batteries in a hybrid 12/24 volt system)

A MUCH BETTER solution would be the TRIK-L-START charger from LSLProducts.net .

I now have one on my unit and it keeps the chassis battery fully topped up all the time, which will greatly extend its life. I just wired it directly from the engine bay house battery to the jumper terminals in the engine compartment. Easy peasy.
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Old 06-30-2019, 05:18 PM   #4
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There are a number of factors. Coach batteries are typically deep cycle, designed for longer draws at lower amperage, while vehicle starting batteries are designed for short bursts at higher amperage. Using the wrong batter y risks damage. If the proximity allows the coach battery to reach the starting battery with one set of jumpers it may be ok, but I would recommend totally disconnecting the coach battery. The two systems are isolated from each other and jumping them together with the coach battery connected could damage battery monitor, charging or isolation devices.
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Old 06-30-2019, 06:38 PM   #5
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Quote:
Originally Posted by engnrsrule View Post
There are a number of factors. Coach batteries are typically deep cycle, designed for longer draws at lower amperage, while vehicle starting batteries are designed for short bursts at higher amperage. Using the wrong batter y risks damage. If the proximity allows the coach battery to reach the starting battery with one set of jumpers it may be ok, but I would recommend totally disconnecting the coach battery. The two systems are isolated from each other and jumping them together with the coach battery connected could damage battery monitor, charging or isolation devices.
My intuition suggests exactly the opposite. I don't see how putting. a starter battery and a deep-cycle battery (or even a lithium) in parallel for a short period is going to risk damage in any way. This is especially true if you are using the house battery to "boost" a weak starter battery, as opposed to replacing a totally dead battery. Volts are volts and amps are amps. I don't see how it matters what chemistry is producing them, and you aren't going to be doing this long enough for the discharge profile to matter.

Most importantly, however, please note that you must never run the engine with no battery connected, even briefly. You will almost certainly destroy your alternator. So, if you were to completely disconnect your starting battery and then use the house battery to start the engine, how are you going to get the starting battery back in the system (so it will charge) without ever connecting them together??

P.S. -- I agree with ProseMan. Trik-L-Start is the best.
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Old 06-30-2019, 06:42 PM   #6
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The Mercedes Sprinter has a button on the dash to give the chassis battery a boost from the coach battery, if needed.
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Old 06-30-2019, 06:48 PM   #7
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The Mercedes Sprinter has a button on the dash to give the chassis battery a boost from the coach battery, if needed.
Yes. When I installed my second engine alternator, I recycled my isolation relay to install such a switch.

It is not a Mercedes thing, though. Your upfitter installed it. Usually, the wires they use aren't big enough to actually let you start the car that way if the chassis battery is completely dead, but for a "boost" or as a way to charge the chassis battery it is fine and very handy.
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Old 06-30-2019, 07:58 PM   #8
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Damage is not likely, but if the chassis battery is very flat and the coach battery is not sufficient it could just get sucked down too. It is also a good idea to carry a booster pack. They also add benefit of an air compressor, portable lighting, 110v inverter and USB charging ports.
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Old 06-30-2019, 08:23 PM   #9
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Quote:
Originally Posted by avanti View Post
It is not a Mercedes thing, though. Your upfitter installed it. Usually, the wires they use aren't big enough to actually let you start the car that way if the chassis battery is completely dead, but for a "boost" or as a way to charge the chassis battery it is fine and very handy.
I agree, my 1999 Xplorer/Dodge has a button on the dash that actuates a starter type solenoid switch. it also has heavy enough cables that when the chassis battery is totally dead, it starts with the same gusto as normal (wasn't necessarily wanting to, but had an occasion to test this function a couple of years ago in the 'middle of nowhere', Yukon Territory).
I have not worried about depleting the chassis batteries (two deep cycle flooded) because if the chassis batteries were low, I could fire up the genny for a while, then jump start the engine.

Anyway, it is a very handy feature - much easier than getting out the jumper cables - if I didn't already have one, this is an upgrade I would definitely do.
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Old 07-01-2019, 02:20 AM   #10
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Trik-L-Start charges the chassis battery at 5 amps max. Xantrex Echostart charges the chassis battery at 15 amps max.

Trik-L-Start is the easier install. Echo start is not environmentally sealed so I put it aft of the firewall. I have both on different vehicles and they both work very well.

When I had a dead chassis battery with the Echo Start I let it charge the chassis battery at 15 amps for 20 minutes and was able to start the engine. Not as good as shorting the two batteries together but it worked.
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