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Old 02 March 2007, 11:18   #1
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Fusing Batteries

I am in the process of fitting a dual battery system using the Merlin VSR charging relay and isolating switch block. This allows you to parallel the batteries if the crank battery goes flat.

Merlin suggest that fusing both the batteries is a good idea. Is this the case?and if so what size fuse would I need to ensure it does not blow when cranking a cold 90HP engine.

Of course all the instruments and switch panel are fused seperately anyway they are actually talking about fusing the two positive terminals on the batteries.

Chris
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Old 02 March 2007, 12:30   #2
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To be honest, I don't know what the standards are on this (and now you've got me curious.)

I would think that a fuse on each main power feed would cause more problems than it would protect from. After all, you've got the switch to isolate the batteries from everything else.

I'm trying to look up the NMMA and ABYC standards on this, though; my Internet connection is a bit on the flaky side at the moment.

jky

Appears I am wrong. ABYC does state that there should be a fuse on the main power conductor.

Not sure how they size them, but 4/3 your max cranking current should be OK (alternatively, size just below the max current rating of your battery cable.)
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Old 02 March 2007, 13:36   #3
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I have been told 100amp but it needs to be a suitable type i guess like a slow blow

Chris
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Old 02 March 2007, 19:35   #4
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Fusing batteries

Quote:
Originally Posted by Chris1573 View Post
I am in the process of fitting a dual battery system using the Merlin VSR charging relay and isolating switch block. This allows you to parallel the batteries if the crank battery goes flat.

Merlin suggest that fusing both the batteries is a good idea. Is this the case?and if so what size fuse would I need to ensure it does not blow when cranking a cold 90HP engine.

Of course all the instruments and switch panel are fused seperately anyway they are actually talking about fusing the two positive terminals on the batteries.

Chris
Hi Chris, there is no need to fuse batteries for a 90 Hp engine.
Perhaps for an old Diesel with high compression but not a 90 hp petrol.

There is little that the second battery can assist in torquing a 90 Hp petrol engine to start. Furthermore based on little old Kirchoffs law (split currents) and internal resistance of the batteries, You would need both batteries to be in equally good condition.

For what you want, I suggest a Bosch or Lucas Hot start system. These can be charged to near perfect whilst carring on board as back up.
If these cannot start your engine given that any young man can hand start a warm 90 in reasonable condition, then you should not put to sea with such a crock.

You will seriously mess up the batteries by fusing...
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Old 03 March 2007, 03:12   #5
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You will seriously mess up the batteries by fusing... [/quote]

Why?
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Old 03 March 2007, 06:04   #6
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I would have thought there might be a problem if the fuse blew, thus disconnecting the battery from the engine while it's running. May not do the alternator/charging system any good at all. Usually means a blown alternator or rectifier I think.
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Old 03 March 2007, 12:20   #7
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Battery fuses.

Be careful! If the fuse blows whilst engine is running, the alternator output voltage will rise. This could very easily fry any electronics you have on board. As a minimum I would fit a diode accross the fuse to provide a 'sink' just in case the fuse blows.
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Old 03 March 2007, 12:44   #8
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All these answers are making me think.Would you damage your electrics on your outboard if your battery became faulty i.e wouldnt keep a good charge but enough to start your 6 cylinder 150 ficht.
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Old 03 March 2007, 18:15   #9
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Quote:
Originally Posted by Chris1573 View Post
This allows you to parallel the batteries if the crank battery goes flat.
Why would you want to parallel the batteries if one went flat? Just isolate the flat one and use the fully charged one. Keep it simple..........
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Old 03 March 2007, 18:24   #10
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The main battery fuse should be rated to protect the cables. In theory, there should be a main fuse for each connection to the battery (typically the one to the engine and one to switch panel). If you have a main battery switch, putting the fuse between the battery and switch would suffice.

So, the fuse rating depends on what cables are installed. On my boat I've use 25mm2 welding cable to the engine so a 100amp fuse would be fine for that. It's well below the spec for the cable but well above the current required to start the engine or alternator output.

Search for mega-fuses. They do 100 amp upwards.

It is very unlikely that a fuse will blow when engine is running, unless there is a serious fault. If there is a fault, it's probably better the fuse blew (with potential daamge to electronics) than causing a fire on the boat!

(PeterR - A dead battery will not cause any issues with electronics. A properly regulated charge output will not rise much above 14.4v - the issue is the non smoothed voltage from the alternator (it's rectified but is very spikey) - even a dead battery will do a good job of smoothing this out).

Al
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