Hypothetical Jumpstart Question

[Lin]

Those that have the same house voltage and coach voltage are able to link the battery banks for charging or an emergency start.  I like many, have a 12v house bank and 24v start batteries.  I presently can charge my house bank on the road using a Vanner Equalizer as a battery charger.  My question here is about emergency jump starts.  I use the term "hypothetical" because it is likely I will never actually put this into practice.  Anyway, over the years I have needed to jump start the bus.  Since I did not have a 24v supply, I used the 12v house bank with jumper cables to one of the start batteries.  Therefore, really just jumping one half of the 24v system.  It has worked several times.

My question then is if I wanted to have a hard wired emergency start system, what would be wrong with connecting the house bank to one start battery through a relay and a momentary dash switch?

[Oonrahnjay]

... I have needed to jump start the bus.  Since I did not have a 24v supply, I used the 12v house bank with jumper cables to one of the start batteries.  Therefore, really just jumping one half of the 24v system.  It has worked several times.  ... 

    Lin, I have no info on your idea of a solenoid-switch crossover, but my bus is parked at a charter bus operations lot.  The buses have high watt entertainment and lighting systems and the drivers (most of whom are part-time school bus drivers) as forever forgetting to turn off the bus master switches.  Man, that business goes through 8-D's like Sherman went through Georgia.  But when they catch the situation when the batteries are just down and not totally killed, they put a 12V jump box on one battery.  In almost all cases, that starts the buses right up. 

    So, I'm thinking that one good, high-capacity jump box (kept fully charged, of course) would probably do us for most of our practical low start battery conditions.  If someone were to be really concerned, you could have a pair of jump boxes. 

    And the way I have it set up, I could always put one of the jump boxes on the 12V battery for my generator, get the generator going, and put the battery chargers on the start batteries.  When the chargers read "Floating" on the digital readout, my bus always starts easily right down to freezing.  But I don't know if you have a separate battery for starting your gennie.

    But, as to your question of boosting just one battery, yeah, that seems to work fine under most conditions.   But, I'd also be thinking about those critical conditions where I needed to add juice to both batteries.    BH   NC   USA

(PS - The buses I've seen involved in this are -- 1]  Cummins - M11;  3-4] 8v-Something DD 2-smokes's;  1] International DT-455 (or something like that).  Works on all of them.)

[bobofthenorth]

Lin more amps is more amps.  What you suggest isn't going to be as effective as a 24v setup but if the bus starts when it wouldn't have otherwise, who cares?

[bevans6]

I had a 12V house bank for a while, four golf-cart 6V batteries.  I could, by switching some jumpers, make it into a 24V bank in about 5 minutes so that I could use it to jump the starts.  Never actually did it, but I could have.

Brian

[gumpy]

Quite often, when you need a jump, it is because one of the start batteries is low, not necessarily both. This can happen if someone rewired the headlights
to feed directly off the 12v center tap, or if you are charging a 12v house bank off the center tap. In this case, it is preferable to connect the 12v
jumper battery to the low start battery. This is typically the one that has ground connected to the frame (left on most MCIs).

Connecting up a 12v house bank to one of the start batteries through a relay is not a bad idea. However, for the number of times you would actually
use it, I would be more inclined to just keep the jumper cables in the bus and save the expense and work for something else you need in the bus.

It should be noted, too, that if both start batteries are low, jumping only one may not immediately provide the juice necessary to get the bus
started. However, if the house bank is left connected to the single battery for a period of time, eventually the charge would raise on both batteries
and might then be enough to start it. It takes time, but the batteries will try to equalize.

[Lin]

I agree that it might not be worth the effort.  Also, since both banks are chassis grounded, only one cable from positive to positive is needed.  That's why I said it was hypothetical.  I did want to find out if there was anything that would be downright destructive about the idea and practice though.

[TomC]

When I was driving truck, my truck had a series/parallel switch to make the 12v system into a 24v system just for the starter. The truck would not start on just a 12v jump. But-I had an Olds Diesel with 2 batteries. I carried two jumpers and would jump each set of batteries and that would start the truck after about 10 minutes of charging from the car.]

If you're going to make a jumper, I would wire a jumper individually from one 12v deep to one 12v start, using two sets to make 24v from your 12v deep cycles. Just would have to have two solenoid jumpers. Good Luck, TomC

[PP]

Time before last, we pulled into a park and shut down while talking to manager and selecting a site. When I got ready to move, it wouldn't start. I got out my jumpers and used the ground cable from the house bank, which is 12V to the negative terminal on just one of the 8D starter batteries, which are connected for 24V. Then I connected the positive terminal of the house batteries to the positive terminal of the same 8D and hit the rear starter button. She fired right up. I disconnected the jumper cables and moved into the site. When we were settled, i discovered that one of the 8Ds had dropped a cell and it just so happened to be the same one that I hooked the jumpers to. 50/50 odds of getting the right one.
Will