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Old 04-27-2014, 10:17 PM   #1
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Battery fuses and engine starters

When I bought my boat, it had an 8d engine start battery and an 8d house battery. These were wired thru a battery selector switch before being wired to the engine starters and house loads. There was no fuse on the batteries. Both batteries were past their prime, and I wanted more house capacity. I now have 8 6V GC's wired in two banks of 4 each. The engine starters are still wired thru the battery switch. I do not have a separate engine start battery and think I don't think need it. I like the simplicity of two matched battery banks for charging purposes. But I still don't have fuses on the batteries and want to add them. I think the starters would not blow a 300 amp SB fuse but they might. I know the code says its OK to run starter wires unfused but that would require a bunch more heavy wire, would mess up the battery switch function, and pose a safety risk of the unfused wire.

So here is my question, What would be wrong with wiring a starter relay parallel to each battery bank fuse so that it only activated when the starter motors are cranked?
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Old 04-28-2014, 01:59 AM   #2
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So here is my question, What would be wrong with wiring a starter relay parallel to each battery bank fuse so that it only activated when the starter motors are cranked?
So you're saying you would use the starter relay (solenoid) to bypass the fuse while starting, as a backup if or to stop the fuse blowing?? I'm guessing you would fuse the circuit to begin with in case there was a starter solenoid sticking and you have a runaway starter or something?? The second solenoid would have to be rated at a high enough amperage to power your starter. I guess you could do that, but you are adding some amount of resistance to the start button/key circuit unless you change that out to a DP switch. I don't have fuses between my starters and batteries either and don't plan on changing that. Why not just simply install a fuse rated with high enough amperage to handle the starter.

But in case you have problems with burning up starter solenoids, I did add a second solenoid to each of my starters to resolve a voltage drop issue. The long distance between the dash key and starter solenoid and the voltage drop created while cranking those 6-354 engines caused the Delco solenoids to chatter. This caused contact pitting and eventually burned up the solenoid contacts. The second solenoid between the key and starter solenoid ensures there will be good current at the starter solenoid directly from the battery and no more solenoid problems.
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Old 04-28-2014, 06:14 AM   #3
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A fused starter line is very rare as it seems not to be needed.


What might be better is two rotary switches in parallel to select the start source , as even the best rotary switches are light on ampacity. This would help the voltage at the starter.
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Old 04-28-2014, 10:40 AM   #4
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I recently surveyed a trawler whose mechanical engineer owner installed a 350 amp fuse on his start circuit that was cranking a Cummins 480CE. Was working fine for him and kind of got my wheels turning. Why not fuse? If cranking circuit is marginal for engine requiring larger fuse than conductor would need for protection kind of moot. Robust cranking circuit with fuse larger than necessary for turning engine over but sized properly (or smaller) for conductor seems like a good program.
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Old 04-28-2014, 12:33 PM   #5
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The engine starters are still wired thru the battery switch. I do not have a separate engine start battery and think I don't think need it.
When starting from a house bank, be sure the inverter/charger is off so no unforeseen voltage drop hits the inverter or 110 volt appliances.

Starting loads can easily hit above 300 amps on some engines. A recording amp meter may be able to tell you the size of fuses to consider.

To run without an engine start battery assumes you'll never deplete the house bank, hummmm?

Your 2x4 banks of 6 V house batteries matches mine, good choice!
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Old 04-28-2014, 01:01 PM   #6
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In our boat our single Cummins 480CE engine has two dedicated 12V Optima D31M Blue Top start batteries wired in parallel. We once load tested one of those batteries and saw 1000 Amps before we backed off the load tester. So our thinking is that two such batteries could do an awful lot of damaging 'welding' if a short happened, therefore leaving that circuit without a fuse did not make sense to us. Next we tested the starting current and saw around 500 Amps for a short time. Hence we installed a 350 Amp fuse, given that these fuses are time dependent we figured that would do, and it matched the wire size, 2/0. Our engine fires up at the first hit on the starter button, if it did not do so that would be an indication something is wrong, we'd not let it crank for a long time. It works for us, 15,000+ miles over 5+ years.
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Old 04-28-2014, 01:34 PM   #7
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When it's time to bleed your fuel system, you might find yourself (or your mechanic) cranking for some time...maybe 15-20 secs. Hopefully that fuse will stand up to the prolonged high current demand.

I've had to do this several times in the recent past following fuel contamination and filter changes and am now getting better at it. Use of a remote start switch is making it much easier and safer with shorter starter cycles.

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Old 04-28-2014, 01:47 PM   #8
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When I bought my boat, it had an 8d engine start battery and an 8d house battery. These were wired thru a battery selector switch before being wired to the engine starters and house loads. There was no fuse on the batteries. Both batteries were past their prime, and I wanted more house capacity. I now have 8 6V GC's wired in two banks of 4 each. The engine starters are still wired thru the battery switch. I do not have a separate engine start battery and think I don't think need it. I like the simplicity of two matched battery banks for charging purposes. But I still don't have fuses on the batteries and want to add them. I think the starters would not blow a 300 amp SB fuse but they might. I know the code says its OK to run starter wires unfused but that would require a bunch more heavy wire, would mess up the battery switch function, and pose a safety risk of the unfused wire.

So here is my question, What would be wrong with wiring a starter relay parallel to each battery bank fuse so that it only activated when the starter motors are cranked?
You are overthinking things. The ABYC does not require (OK, "suggest") fuses in the starter circuit because any fuse rated high enough for the starter current would do little to protect the wiring.

Dedicated starting batteries are the best solution but if you don't want that, be sure there is appropriate circuit protection for any circuits other than the starter motors. That would be house loads and possibly bilge pumps.
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Old 04-28-2014, 01:52 PM   #9
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A big reason there are no fuses in start wiring is the Alt is frequently connected there in addition.

Remove the batt from an operating Alt , and you might find out how to use a diode press, really well.
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Old 04-28-2014, 01:59 PM   #10
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I don't see the need for a fuse in a starting circuit. As posted above, any fuse capable of carrying starter current is going to carry more than enough current to burn up wires in a fault.

Also, the starter is (usually??!!) only energized with operator paying attention, so if there is a problem, it can be addressed.

Regarding a 350A fuse: It may work fine under normal conditions, but recognize that if for some reason batteries get weak, starter amperage shoots way up, and crank time too. Fuse can then blow at an inopportune time.

More important than the fuse is to have a starter disconnect switch (battery switch) accessible to the operator. Starter contacts can weld and starter continue to crank engine til something burns up. Switch should be accessible to operator.

I've had to deal with a stuck starter and NO batt switch. That was scary.
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Old 04-28-2014, 02:20 PM   #11
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What might be better is two rotary switches in parallel to select the start source , as even the best rotary switches are light on ampacity. This would help the voltage at the starter.
If by "ampacity" you mean to say a battery selector switches amp carrying capacity, the ratings for a Blue Seas heavy duty switch would seem to indicate otherwise:
10 Sec: 2750A
60 Sec: 1150A
5 Min: 700A
Continuous: 500A
Max Voltage: 32V DC

If some one is worried about voltage drops to their starter, either up the wire size and/or add a simple battery paralleling circuit that either operates automatically when you hit the start button or one were you have to hit a momentary switch to activate it when starting an engine. A circuit like that also gives you build in jumper cables.

IMO if you feel the need for a fuse in the starting circuit then just add one of the correct size and be done with it. No need to overly complicate things and add more failure points. A proper sized and installed fuse should not create a significant voltage drop.
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Old 04-28-2014, 02:32 PM   #12
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It does not seem to make sense to us to 'require' a fuse near a battery for near everything yet not instal one for an engine starter. In our case we have a battery switch right at the fuse so the arrangement is batteries/1ft to fuse/3in to switch/3ft to engine starter. Also we have a house/both/engine switch that allows us to start the engine off our 1530 Ahr house battery bank if required, even if the engine batteries fuse had failed, that too runs through a separate 350A fuse. Our engine alternator delivers directly to the house battery bank, the engine batteries being charged via an Xantrex Echo Charger (15A) off the house battery bank. One Cummins Technician wiped our engine's ECM then cranked the engine for quite some time (without blowing the 350A fuse). I kicked him off our boat after asking him where he thought the raw water was going as he cranked the engine over and over. We have a water can exhaust silencer, if the engine were to be cranked for a long time it might be possible to fill the silencer and exhaust loop with water flooding right back to the engine.
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Old 04-29-2014, 12:08 AM   #13
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Thanks to all for your comments. The system I am contemplating has the feed from each battery bank first going thru fuses to the a battery selector switch with the output being fed to both engine starters, the inverter/charger and house loads. Any load being fed by the battery switch with wire smaller than the 2/0 wire between the batt's and switch would need their own appropriately sized protection. While I think the 300 amp fuses would probably start the engines, It would be the very cold instances, with thick motor oil or lower voltage that I would be concerned with. It gets cold in Iowa, the ice just went out about a month ago.

Sunchaser: Yes if I depleted both battery banks I would be at risk of not starting the engines. I would run the two banks one at a time and switch when at 50% capacity. Four 6V's even at 50% would have no trouble starting the engines. But if I did get in trouble, my genet has a completely independent battery and with the 125 amp charger in the inverter, I would have enough juice to start the engines in short order. I do have two backup chargers in case the inverter goes south. Your comment about the chance of triggering a low voltage shutdown in the inverter if left on when cranking has me thinking. First I don't think that is likely since I would only be running one bank at a time, but I don't hunk that would pose a problem since I do not have anything AC powered that would be hurt by a shutdown. Did you have something else in mind I haven't thought about?

Ski: I agree that a fuse sized to reliably carry the engine start load would be too big to adequately protect in the event of a short. What I am proposing is to use is a fuse appropriate to protect the wiring but use the starter relays to jumper the fuses and carry the excess current when the starters are engaged. When the key is released, the 300A fuses would again offer protection. In the event of a starter runaway I would need to be jimminy quick to turn the switch off if the fuse didn't blow. But that would be no different If I wired the starters without fuses.

Flywright, Your idea of carrying a remote starter switch is an excellent one. I will have one soon. It's a real drag to have climb out of the engine room to the helm to click the starter then climb back down again. That's hard on an old guy.

Craig
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Old 04-29-2014, 07:08 AM   #14
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When the key is released, the 300A fuses would again offer protection.

Other than the starter what would usually need 300A to operate?
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Old 04-29-2014, 07:45 AM   #15
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If the fuse blows, would not the system still work the same? Then why bother putting the fuse in at all? So the protection you are getting is from moving the starter solenoid closer to the battery? Unless battery has a long cable run to the engine (it should not), I don't see significant protection being offered. Are engine auxiliaries still fed from starter lug? Maybe fuse for that current draw instead of 300A. If starter hangs after engine start, it probably will not blow a 300A until after it starts melting.
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Old 04-29-2014, 11:20 AM   #16
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>Unless battery has a long cable run to the engine (it should not),<

The cooler batts are kept , the longer their life.

Far better to have 15-20 ft of heavy cabling to the engine than cook the set in the engine room.
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Old 04-29-2014, 11:41 AM   #17
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................ Far better to have 15-20 ft of heavy cabling to the engine than cook the set in the engine room.
I would not agree with that as a "blanket statement". The voltage drop over that length of cable will be pretty significant at a couple hundred amps or more.
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Old 04-29-2014, 05:17 PM   #18
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>Unless battery has a long cable run to the engine (it should not),<

The cooler batts are kept , the longer their life.

Far better to have 15-20 ft of heavy cabling to the engine than cook the set in the engine room.
A 15 to 20 foot run turns into a 30 to 40 foot run when you double the distance the current travels in a DC circuit. You'd need very, very large cable for that to work.

I would think you could get away with it using 3/0 or 4/0 if you could keep the total run to 20 feet or less and the starter amp load at or below say 350.
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Old 04-30-2014, 07:38 AM   #19
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After rereading all this I understand you want to crank your engines using existing house service cabling. If the cabling will handle engine cranking current OK then you should be able to install a fuse that will protect it and start your engines. If the fuse required to crank the engine is too large for the conductor then shouldn't you have larger conductor(s) for your cranking circuit? If your house wiring is too small to safely crank your engines then you have two choices. Resize them to do that job or install dedicated cranking battery(s).
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Old 04-30-2014, 10:57 AM   #20
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Resize them to do that job or install dedicated cranking battery(s).
There you go, it took awhile.
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