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Topic: Jumping the House battery to the Chassis battery (Read 10 times) previous topic - next topic
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Jumping the House battery to the Chassis battery
Yahoo Message Number: 150017
The two output wires at the diode isolator go directly to the batteries,  with a diode drop for each wire from the alternator. Connecting these two wires  together would cause excessive current flow if one battery (bank) was dead, say  9.0 volts or so. Properly done, a #2 wire from each battery should be connected  with a temporary, high current, solenoid switch, operated by a push button  where the driver can operate it. If also powered by the ignition switch, then  you would be unlikely to leave it on while camped- possibly draining both  battery banks low enough to not be able to start the generator or engine.
Since about 2012 the mothership deleted the diode isolator,  and substituted one of these high current solenoid switches. They have improved  a lot since the ones available 20 years ago. Hopefully they wouldn't weld the  contacts together internally and let you run down all of your batteries at once  like the older designs would do occasionally. RonB and 'Bluebelle' a '99  TKB
RonB (Bostick) living in San Diego
Original owner of "Bluebelle" a '99 TKB
Re: Jumping the House battery to the Chassis battery
Reply #1 –
Yahoo Message Number: 150022
The concern regards high current when connected a fully charged 12 volt lead acid to a very discharged 12 volt lead acid is, in a word, unfounded.

This is so because the internal resistance of a discharged battery is relatively high and this limits initial current, as does the inherent voltage drop across the existing wiring. Keep in mind that the existing wire from the alternator to the battery is not all that small (a 6 AWG wire of up to 5 feet can handle up to 100 amps. However, you will not see anywhere near that kind of current unless the boost battery is itself being charged. This is due to two things, firstly the boost battery voltage will drop as the load is placed on it, typical voltage would be on the order of 12.5 or a little more. As the battery being charged comes up a little, its internal resistance starts to fall (assuming it's not badly sulfated), as this occurs, its voltage rises decreasing the voltage differential between the two batteries, and that tends to drop charge current as well.

If you monitor the voltage and current output of a typical automatic battery charger when connected to a discharged battery you will observe what I'm attempting to explain, but with one difference. The battery charger's output voltage can go as high as 14.8 to 15 volts or so, and in doing so, this higher voltage is able to "push" more current into the battery being charged. Our "boost" battery in the first paragraph does not have the luxury of this higher voltage and so charge current will typically not be so high.

bumper

retired (sort of) CEO alarm company, ex Navy electronics technician, and licensed electrician (dormant) - - with all that, I still would not want to live in a house I wired :c)
bumper
"Yonder" '05 MB
"WLDBLU" glider trailer
Re: Jumping the House battery to the Chassis battery
Reply #2 –
Yahoo Message Number: 150023
More precisely, a 6 AWG cable can handle 100 amps, even if it's 100 feet long (or longer). For 12 volt systems at 100 amps, it's the voltage drop that determines how long a cable should be. But, even a 6 AWG cable could withstand much higher starting currents (which are short duration, intermittent currents) than the 100 amps continuous rating.

bumper's suggestion to use a contactor is fairly common in "newer" motorhomes, I think. My 2005 SOB has one - factory installed. It's safe and easy to use, much more so than jumper cables or even one of the nifty new "jump" boxes.

Eric Greenwell
  [lifewithalazydazerv] wrote, On 12/22/2014 7:39 PM:
2005 Jayco 24SS
Re: Jumping the House battery to the Chassis battery
Reply #3 –
Yahoo Message Number: 150028
"Since about 2012 the mothership deleted the diode isolator, and substituted one of these high current solenoid switches. "

2008 is the first year in which I have seen a HD solenoid installed in a LD. It's a much better idea than the diode.
No idea why LD did not add a jump-start button as Fleetwood and many other RV manufacturers do.

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
 
Re: Jumping the House battery to the Chassis battery
Reply #4 –
Yahoo Message Number: 150051
"This is so because the internal resistance of a discharged battery is relatively high and this limits initial current"
 Assuming the battery is simply in a low state of charge, and undamaged, this is not true. Such a battery will accept a very high charge current, and has a low internal impedance. If the battery is damaged, an open cell will present a high impedance, but a shorted cell will do the opposite, and likely damage the other battery by discharging it, if paralleled.

Steve
2004 FL
2013 Honda Fit