thataway":2rarfhoq said:
Sorry for the multiple posts--checking back to the site showed that none had gone thru.
Only part of the thread was addressed specifically to Sunbeam.
I've had that happen before - then you see you posted three times, ergh. I wasn't sure which parts were to me or others.
thataway":2rarfhoq said:
Sunbeam I would fuse for 500 amps as an inrush current for the starter. You might get away with less, if you have immediate start. However, having carburetors, you probably have a longer start time on an average, than an injected engine. (broad statement--depending on may factors)
Hmm, okay. Well I had to research this for fusing on my RV (as built, the 4AWG line from start battery to house bank was unfused, although it was set up for both charging and self-jump-starting). So I looked around to find the starter draw (in my case on the RV, about 150 amps). Then I wondered if I had to account for inrush current. I found an article by Maine Sail, and he said that no, that's too "quick" to fuse for, and that essentially one just fuses for the starter draw. I went with that (used 1/0 Ancor cable, fused at 200 amps; 242 is allowed in an engine space for that size wire, unbundled). Now, I have not had occasion to test that, but a friend with the same RV/engine (gas)/cable run (but only 2 AWG wire, he likes to cut it a bit closer) had a period of time where he was chasing a parasitic load, and did have to "self jump start" a number of times. He was fused at 175 and did blow the fuse once, so that seems to say it was "close" on fusing at that level. He chose to up the fuse to 200 amps and has not blown it since (still an improvement over his original setup, which was a 6 AWG wire that was unfused). Since the starter draws ~150 amps, this seems to say that the inrush current is not affecting the fuse (The spec I found said "typical starter load 130-220 amps; maximum load, 800 amps," which I think is the inrush current. Let me see if I can link to the article. Anyway, that's what gave me the idea that inrush current did not need to be "fused for." Would it be different on an outboard?
Here is the article:
http://www.pbase.com/mainecruising/battery_fusing
The part I'm thinking of is where he says (in reference to fusing starting banks, which he is a proponent of):
Question: "Won't the starters inrush current blow my fuse?"
First, what exactly is "inrush current"? Inrush current is the very brief spike in current that the starter undergoes to get the motor to begin turning over from a stopped state. The inrush duration is usually about 200ms to 250ms long, and not long enough to blow a properly sized fuse. ANL, MRBF or Class T fuses are not sized for the inrush, they are sized to the wire they are protecting.
This video below shows the absolute peak inrush as captured by a Fluke 376 meter. The engine is an older 2QM20 Yanmar. The absolute peak current draw, perhaps 2/10th of a second, is 316 amps yet this motor is protected by a fuse rated well below the inrush. It has never blown nor will it at this inrush capacity. The average starter load during the duration is closer to 150A.
BTW, my Yamaha generally starts right up with no extended cranking. Maybe it's because I don't tend to be trying to start it below around 40ºF, or maybe just lucky. I did have all four carbs rebuilt when I got the boat (it had been sitting) and I drain them all between trips.
thataway":2rarfhoq said:
AIC (Average Inrush current() is difficult to measure unless you have a sophisticated instrument. In the above link the writer has a Fluke 376 meter, about $300. There are multiple variables which determine the AIC.
Generally boaters don't worry about AIC. Builders should, but often they don't either....
Okay, I think we "crossed acronyms" perhaps. I was meaning "Ampere Interrupt Capacity" for the fusing. i.e. not the size of the fuse, but the "toughness level" of the fuse holder or body itself. The Lifeline Group 31 (27 and GC-4 are similar) have a short circuit rating of 3,564 amps. So with two I'd need an fuse/holder AIC of at least 7,128 amps. That's fine for the MRBF which is rated at 10,000 AIC. (On the RV I have three Group 31 XT, so that requires an AIC of 12,771 amps, therefore I used a Class T main fuse (AIC 20,000 amps).
thataway":2rarfhoq said:
You fuse for the current, not the wire size.
Okay, well maybe I've been going at it a little backwards, but the way I've been doing it is I see what size wire I need (usually voltage drop being the deciding factor for the kinds of things I do), and then I see if it has enough/plenty of ampacity (i.e. can I fuse it high enough). If not, I go to a larger wire size, but I still fuse for the wire (and choose the wire based on the load). Whenever the wire size goes down (say at a fuse block or buss bar), I re-fuse if the smaller wire doesn't have enough ampacity to be covered by the fuse on the previous (larger) wire. I have at times just upped a wire size because that was more convenient than re-fusing (space, say at a bus bar). Maybe just another way of saying the same thing (?). If I have something on the end of the wire that needs a special (smaller) fuse, then I'll just use that smaller fuse at the fuse block if the wire is only feeding that item. After all, in that case there is no reason not to fuse smaller and avoid a bunch of fuses or (shudder) inline fuses all over the place.
thataway":2rarfhoq said:
The alternator charging wiring, has no relation to the size of the starter wiring.
Okay, maybe I'm out of the loop here. I was thinking of the cable that would be running from the start battery's bus bars (which is where the outboard connects, and there is no separate wire from the outboard for the alternator) to the house bank's bus bars (if the house bank is moved). In my mind this will be carrying the alternator output to the house bank, and possibly also the starter draw if I combine (or just simply start off the house bank). Is this not correct? This is how it works on the RV. I don't see any cables that would separate the outboard's alternator output from the starter draw in terms of the cables that carry it from my batteries? What am I missing?
thataway":2rarfhoq said:
You say you have 4.8% drop in voltage when starting off an aux battery--measuring both voltage and inrush current is very difficult, unless you have a very sensitive datalogger, the peak current, and low voltage are difficult to read because of the time in microseconds. ABYC calls for no more drop than a voltage to 1.2 per cell, or for a 12 volt battery of 7.2 volts in the battery. Also when using a "booster" which most RV's do--you are using a solenoid which allows the house and start battery to work in parallel. (not sure if you wired your RV differently than what is the convention.)
You've kind of lost me in the first part, but for the latter part, no I don't use a solenoid on the RV. It originally had a Surepower 1315, but I eliminated it for a few reasons. One is that it was recalled (slight matter of them catching on fire). Two is that it was of the old-fashioned type that causes its own voltage drop. Three is that it was bi-directional and I only wanted it to go "aftward." I considered a Blue Sea 7622, but instead went with a Blue Sea On/Off battery switch. I also have a pair of LED lights wired to the switch. When the batteries are combined there is a solid green light on the dashboard, and a flashing red light in the coach. This is so I don't forget and leave them combined. I can reach the switch from the driver's seat or the rear of the coach. As it turns out, I rarely combine any more because with just the solar I only get below 85% charge on rare occasions. The start battery doesn't have any parasitic loads (or at least not any that show up in a period of a few months), so I don't charge that between drives. I've combined the batteries twice in the past 9 months, for about 6 hours each time, so having a manual switch has been perfect for me. If I did it more often I'd likely go with the 7622. I do have a Blue Sea ACR on the boat (but then so far I have not had a robust/separate house charging system on the boat, which I do have on the RV -- so I have relied more on alternator charging, and therefore pretty much always want to combine when underway. That may change when I get my alternate charging methods set up).
(I considered it briefly due to the number of folks who run with one bank (house bank which is also used as start bank), and then just use the start battery as a "reserve" for emergencies - apparently with larger house banks instrument drop-out etc, may not be a problem when starting from house bank. But anyway, at least this go-round, I'm going to stick with a larger house bank (around 200 ah) and the smaller dedicated starting battery. 
