Yesterday, I took the Barth out for a 20 mile local run to warm it up for an oil change...My neighbor & SOB owner went along for the ride.

Engine start procedure was normal - for me. Before turning the key to start, I brought the house battery on line, using both batteries to supply starter power. My neighbor shook his head and gave me a tsk-tsk, but couldn't explain the reasons why to my satisfaction.

I guess my rationale has always been:

1. Consistent w/ wire size, length, capacity of switches, relays, etc, the more CC amps available, the better.

2. If the depth of amp draw-down on a battery is ONE of the factors that determine battery longevity, it seems to me that this procedure allows for a shallower draw-down on the chassis battery, as well as an insignificant draw down for the deep cycle house battery.

3. Since starting loads are the only real big ticket loads on the chassis battery, it seems like paying for a top-shelf, high CCA capacity chassis battery is unneccesary.....a mid-level and lower cost battery can easily handle routine lighting, radio, fans & other loads (assuming the alternator is keeping up).

Been doing it this way (incl. genset starts) with two coaches for years....Now I'm told it's been a no-no all along (:

So, what's the REAL scoop?

PS: We're talkin' 454 gasser here......

quote:

What's the REAL scoop?

Short answer:

It depends. Your neighbor could be right.

--------------------------------------------------------------------------------------------------------


Long answer:

What is your house battery?

How many amps does your starter draw?

Once we have your answers, we can shilly-shally, equivocate, prevaricate, obfuscate, generally and dance around the issue much more accurately.

The smaller your coach battery and the larger your starter amp draw, the less advisable it is to use a deep cycle battery as a starting battery. The more powerful your starting battery, the less damage will be done to your deep cycle battery. The skinnier the wire connecting the two, the less damage will be done to the deep cycle battery.

The quicker your engine lights off, the less damage will be done to your deep cycle battery. An ancillary consideration is that it is possible that a slow starting generator could do more harm to a deep cycle battery than an engine that lights right off. Even if the engine draws more currant, it would be over a shorter period of time. For engine starting, I depress the throttle once to set the choke before the float bowl is filled, then let the electric fuel pump fill the carb, then crank and give it about three shots of Lunkenheimer during cranking. That seems to offer me the quickest start with the least amount of cylinder washing. But every engine is a little different.

It is a general rule that batteries are "horses for courses". That is, starting batteries do not like deep cycling and deep cycle batteries do not like starting, i.e. short heavy discharges. Has to do with the physical design of the plates and internal connectors. There are horror stories of plates buckling, etc.

The truth of the matter is that, like so many things, something that is not good in theory often works OK in the real world. But, we really don't know for sure. I have discussed this with the folks at Trojan, and their answers vary. If you ask a battery expert about starting with deep cycle batteries, they will say no. If you ask them how much inverter load a particular battery can provide without damage, the answer is a little different. There are some pretty big inverters out there. And how about starting the generator off the coach battery? How much is too much? I know boaters who routinely start their engines from one of their deep cycle battery banks, and have done so for years.

At some point, a bank of deep cycle batteries is large enough to start an engine. Some folks who use just one 12 volt coach battery use a combo battery that has a CCA rating on the side. A battery like that in single use or in parallel with a starting battery would be OK. A single 12 volt true deep cycle size 24 or 27 would be less good. A size 27 would be better than a size 24. The bigger the deep cycle battery, the better it can withstand heavy starting loads. It is a curve, not a step, so the answers you get will be subjective and judgmental, empirical and intuitive, rather than hard science. It would take a long term science fair project to come up with a good answer.

Unnamed persons from an unnamed battery company have been known to mutter, sotto voce, that a 6 volt golf cart battery was good for 500 CCA.

I can tell you that I often take the starting battery out of my Barth to carry as a spare when off-roading in our 4WD. As a result I often start the Barth engine with the two 6 volt coach batteries. There seems to have been no permanent damage, as I am in year 5 or 7 of the batteries. Of course, my 502 has a mini starter that draws only 225 amps, so that is a factor. Jim Le, an engineer at Trojan at the time, has confirmed that my combination of batteries and amp draw was no problem. Since you say "battery" in the singular, and I suspect your starter has a heavier draw than mine, you might be shortening your battery life.

If you use a battery of the recommended size, the starting draw is nearly inconsequential, so there's no need to parallel the house batteries with the chassis battery for starting.

The two things that are most damaging to any flooded cell battery are low electrolyte level and prolonged complete discharge; deep-cycle batteries are designed to tolerate deep discharge. Starting batteries aren't.

Another reason to use only the chassis battery for starting is thet the surge of charge current after starting will help in desulfating.

Are you hurting anything by using the emergency switch when starting? Probably not. But using it may mask an incipient problem with the chassis battery or starting system.

If it were I, I'd leave the switch alone unless the chassis battery were dead.

However, like bill h mentioned, starting from a deep-cycle battery is No Big Thing - when I had boats, since everything ran off the same battery bank, I had all deep-cycle batteries.