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Old 08 July 2020, 05:39   #1
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Cobra Isolation switches

Hope somebody on here can help. Was out last night on a very lumpy Solent having a bit of fun with some friends in a couple of boats. On getting back and putting them to bed my friend who is fairly new to ownership was asking about the isolation switches on his Cobra and must admit I was not 100% sure.

There are three Electrical Isolation switches two red and a yellow one in the middle. Obviously the two red ones are for the two systems and the yellow one is effectively a crossover switch between the two systems. He told me that the previous owner had told him to turn on all three switches every time, but in my mind I would have thought you would only turn on the crossover switch if one of the other systems was down i.e. if your starter battery was dead you could turn on the crossover switch and effectively jump start it from the auxiliary battery. This is only a guess though as my knowledge is more from camper vans than boats but slowly learning.

Hope this makes sense I suppose my question is should the yellow switch be on or off in normal running.
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Old 08 July 2020, 06:22   #2
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Hope somebody on here can help. Was out last night on a very lumpy Solent having a bit of fun with some friends in a couple of boats. On getting back and putting them to bed my friend who is fairly new to ownership was asking about the isolation switches on his Cobra and must admit I was not 100% sure.

There are three Electrical Isolation switches two red and a yellow one in the middle. Obviously the two red ones are for the two systems and the yellow one is effectively a crossover switch between the two systems. He told me that the previous owner had told him to turn on all three switches every time, but in my mind I would have thought you would only turn on the crossover switch if one of the other systems was down i.e. if your starter battery was dead you could turn on the crossover switch and effectively jump start it from the auxiliary battery. This is only a guess though as my knowledge is more from camper vans than boats but slowly learning.

Hope this makes sense I suppose my question is should the yellow switch be on or off in normal running.
I'd say your correct - the yellow 'link' switch normally is off, only used if extra amps needed say for the start battery. The issue is if left on without the engine running and charging the system you run the risk of flattening both battery's - worst case scenario but can happen.
Most twin battery set-ups have the engine charging the start batt and as soon as volts are high enough a VSR (voltage sensitive relay ) will then kick-in charging the aux/house batt - that way the start battery should always be charged for spinning the engine over
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Old 09 July 2020, 02:18   #3
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I fitted a twin battery setup on the cobra I had, also fitted was a BEP Marine battery switch cluster, this consisted on two red switches and one yellow switch and a Voltage Regulator.

Normal operation is only switch the two red ones on, these two red ones switch the two batteries on, one a house battery for the electronics etc and one for the engine starting etc. In this mode both batteries are operating independantly from each other and the voltage regulator takes the charge from the engine into whichever battery requires a charge (good protection incase you have one duff battery and one good battery, without the regulator the duff one would bring the good one down).

The yellow switch should be used only in an emergency when experiencing an engine that wont start due to a weak battery, the yellow switch when on effectively joins the two batteries together so it gives the max volts possible from the two batteries to start your engine.
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Old 09 July 2020, 04:36   #4
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Thanks Boristhebold and Fender, as I thought, will let my mate know. Looks like he was given duff info from the guy he bought it from. In my mind from what I knew and what logic told me this was the correct way to operate it but just wanted to make sure that I wasn't giving out false information.
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Old 10 July 2020, 00:20   #5
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Thanks Boristhebold and Fender, as I thought, will let my mate know. Looks like he was given duff info from the guy he bought it from. In my mind from what I knew and what logic told me this was the correct way to operate it but just wanted to make sure that I wasn't giving out false information.
At a guess one of the batteries is knackered hence the seller telling him it’s normal to combine them together to get a sale and save buying a new battery.

I’d test both batteries to check they’re any good.
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Old 11 July 2020, 02:20   #6
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If 'testing' batteries bear in mind it is difficult to properly test a battery unless you have the right kit, a voltage check doesn't really mean alot apart from its low on volt perhaps. Unless battery tested under load etc etc then dont think just because its showing a good voltage that it is OK.

I once had a boat batl that tested fine, luckily I had a second with islotaion switches onboard. Also had a major fualy with an audi s3 once, everylight warning light on dash was flashing, two garages including audi main dealer said I needed a knew dash control unit for mega bucks, I wasn't convinced so bought a new battery from halfords, plugged it in and hey presto no more faults.

So if the batteries onboard of your rib are over 3 or 4 years old then perhaps replace with new ones. The old ones if you know a farmer or horse owner they are oftwen looking for old batteries to use for electric fencing, thats agood way of recycling them.
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Old 11 July 2020, 03:27   #7
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If 'testing' batteries bear in mind it is difficult to properly test a battery unless you have the right kit, a voltage check doesn't really mean alot apart from its low on volt perhaps. Unless battery tested under load etc etc then dont think just because its showing a good voltage that it is OK.

I once had a boat batl that tested fine, luckily I had a second with islotaion switches onboard. Also had a major fualy with an audi s3 once, everylight warning light on dash was flashing, two garages including audi main dealer said I needed a knew dash control unit for mega bucks, I wasn't convinced so bought a new battery from halfords, plugged it in and hey presto no more faults.

So if the batteries onboard of your rib are over 3 or 4 years old then perhaps replace with new ones. The old ones if you know a farmer or horse owner they are oftwen looking for old batteries to use for electric fencing, thats agood way of recycling them.
Correct that testing them with a multimeter will tell you nothing but a drop tester isnít very expensive these days

https://www.ebay.co.uk/itm/6V-12V-10...YAAOSwRolcb7jI

For scrap batteries the last lot I took in for scrap were about 500/ton. So about 15 quid for a 100ah one.
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