Beneteau/Battery connections

[kdf]

Was on a friends Ben 44.7 last night and when he started the engine the house batteries voltage went way down (computer rebooted!). When I checked the connections on the batteries it appears that the engine and house banks are connected in parallel so when the engine started it used the house power as well and hence the dip. Surely this isn't normal wiring for a boat. I would have thought that the enginer bat would be completely separate from the house banks?

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[VicS]

The installation needs a battery switching system. Either a "1,2, both, off" type of switch or simply separate isolating swiches for each bank.

Are you sure the latter was not installed but your friend merely had both swiches on?

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[ozzyb]

I have just recently purchased a Beneteau 361 and the system is the same. There is only 2x 70 amp Batts on my yacht but with fridge etc. it doesn't seem enough. Is there any one with sugestions as to fit a separate additional 110 amp batt or modify the setup.

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[bedouin]

On my system (Not a B*****u) I have separate Domestic and Starting banks but they share a common Earth return.

Starting the Engine can sometimes cause a large enough voltage to be dropped on the negative return to trigger the low voltage alarms on the instruments, even though they are connected to the separate domestic battery bank.

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[kdf]

I didn't see which way the switches were - there were three. Black (master) and two other reds which are isolation switches rather than selectors.

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[Sunnyseeker]

Best solution, and safest in terms of not ending up with a flat starter, is three islolator switches. 1.Isolates domestics 2.Isolates Engine start 3.Links domestic and engine together in the event of the start battery becoming flat.
In this way the link switch can be mounted away from other switches and on our boat cable tied to prevent anyone linking batteries by accident. In this way you know each bank is seperate. You do need a split charger to supply each bank independantly.
Rotary switches in my experience pose the risk of flattening batteries, as it is common for people to switch to both to start then forget to switch back to domestic only when the engine is stopped... leave the fridge on all night or lights etc and wake up with dead batteries all round.
Capacity depends on how long between charges you plan on, we found two 170 domestics and a 90 start were fine, using towed generator when sailing and wind when anchored we never needed to run the engine to charge the system.

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[VicS]

It sounds as though there is a master switch which will isolate the whole of the boats electrical system from the batteries and two separate switches, one for each battery bank. The latter enable either of the battery banks to be brought into service or, if and when necessary, both. I suspect your friend had both batteries switched on. Normally one would just switch on the starter battery start the engine and then after a period long enough for that to be recharged switch the domestic bank on and the starter battery off. BUT its is VITAL that if the engine is running the new battery is switched on before the old one is switched off. If you disconnect all batteries while the engine is running you will almost certainly destroy the diodes in the alternator. If the engine is not running it doesn't matter but of course going through an "all off" situation will cause the computer to reboot.

What really prompted your question in the first place was that the computer rebooted when the engine was cranked. I'm afraid this is almost certainly just down to the voltage drop when such a large load is applied. While it should be possible to minimise this by ensuring that all the connections to the batteries and the various isolating switches are clean and as good as humanly possible it may not eliminate this problem. It will re-occur when the batteries get a bit past their best any way.

To overcome this the computer power supply should come directly from the domestic bank (+ve and -ve), via a suitable isolating switch and a fuse. The latter being located as close as possible to the battery. (It is usually recommended that the VHF is connected like this as well).

You could get into far more complex arrangements but this I feel is the simplest solution. It is not totally fool proof, for example the problem with the computer will return if the engine is started in a hurry without switching back to the starter battery or if someone just forgets.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[StugeronSteve]

Standard Beneteau wiring will have a black isolater switch, which will kill all 12v electrics and two red isolators, one for each battery. As suggested elsewhere in this thread I only use the engine battery for starting up and then isolate it, leaving the domestic battery to carry the instrument / lighting load on passage. Then if you happen to leave the fridge on you don't find youself unable to re-start at the end of the day. I do not have radar (yet) and up to now I have found the two batteries have proven to be well up to the duty required, including overnight passage with a good 11hrs of dark.

<hr width=100% size=1>Semper Bufo

 

[kdf]

Yes this is the same. I understand the need to isolate the engine batt once started so that it doesn't get drawn down but I guess I need to figure a way to stop the voltage drop on the house bank when starting the engine. The only way I can see it working is to only have the engine battery available for starting (re-wire) and not linked to the house bank at all. We may still have a problem as someone else suggested a drop on the negative return causing a problem.

Thanks for all the replies. Got some thinking to do.

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[StugeronSteve]

It's hard to comment as it is not a problem that I have suffered, even when we have spent a good few days away from shore power. I often start with both batteries "on line" without detrimental effect. Could well be a common earth problem, or maybe the batteries are about sh***ed. I do intend to get a smart alternator controller wired in to get a more efficient charge from the engine.

<hr width=100% size=1>Semper Bufo

 

[Anthony]

Are you sure suggesting the placement of a 3rd switch between the two banks so as to be able to start in emergancy? If the engine battery is really dead, then personally I would hesitate connectinng it to the dommestice and then starting, chances are it would drain the domestic . Surley better to have a selector switch that normall connects engine to engine bank, but can be turned to connect to to dommestic only?

<hr width=100% size=1>The difference between men and boys, is just the price of their toys...

 

[cliff]

"Yes this is the same. I understand the need to isolate the engine batt once started so that it doesn't get drawn down but I guess I need to figure a way to stop the voltage drop on the house bank when starting the engine."

You DO NOT isolate the start battery after starting - it needs charging again.

Isolators are not recommended unless they are interlocked or the charging system (alternator) is connected directly to at least one battery.

If the voltage drops on the domestic batteries when cranking the engine on the starter battery then you need to find the fault. A common earth does not cause that. It sounds like all the positives are connected to a common bus. Either that or the feed for the instruments is taken from the starter battery as is the feed for your computer. Apart from maybe the VHF, nothing else except the starter should be fed from the starter battery.

Good idea to check all connections are clean and tight.

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Stay put when I hit you!

 

[bedouin]

"A common earth does not cause that"

Sorry to contradict you - but if you check my posting above you will see that it can/does. Any voltage dropped in the common part of the negative circuit is dropped equally in both domestic and engine circuits. So if you are dropping e.g. 1.5V in the engine starting circuit between the battery and the common negative point then you will also lose that 1.5V in the domestic circuit.

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