instruments turn off starting the engine

[wetstuff]

Hi there,

I have just changed my engine and now have a rotary selector switch for the two batteries.

One battery was replaced a month ago. Now when I start the engine the GPS turns off. I seem to remember that a capacitor can be fitted across the GPS power feed to keep it going while the volt drop due to starting takes place. Can anyone confrim this?

I am thinking that the second battery also probably needs changing? Any thoughts?

Last but not least, I was wondering what I would need to do to the alterntor output to allow it to charge another battery ( I have two currently) - I thinking of fitting a dedicated engine battery.

Cheers for now.

John

 

[Tranona]

Suggest you have a look at fitting a switch cluster with a VSR to create two separate banks, one engine start and the other service which will include instruments. Merlin at www.power-store.com (among others) can supply the bits and design the layout.

 

[pampas]

start on No1 battery and connect your domestics to No2 direct (Via fuse) when started then select both or No2 . Looking at your profile I am suprised that you ask such a question. /forums/images/graemlins/grin.gif

 

[VicS]

[ QUOTE ]
I am thinking that the second battery also probably needs changing? Any thoughts?

[/ QUOTE ] If it is not capable of starting the engine on its own or if it does not hold its charge then it needs changing but apart from that is is impossible to say from this distance.

Although you can designate one of your batteries as engine start and the other as services, switching over as appropriate Tranona's suggestion is a good one which allows you to make one battery a dedicated engine start battery and the other domestic services (but with the capability of linking then if necessary) and always charging both but with priority automatically being given to the starter battery. If you are thinking of adding a extra battery for engine starting it is almost certainly the best way forward. Presumably you will then replace your existing 2nd battery and make a large domestic bank from it an its current companion.

Tranona's suggestion will involve a bit of wiring alteration, most probably, to separate the two systems so a bit of professional help in planning the layout would be wise if you are uncertain about things.

BTW with your present system do not forget that when you change from one battery to the other while the engine is running it is absolutely vital that you switch via the "BOTH ON" position. If you disconnect both by going via the "OFF" position you will destroy the alternator diodes very probably.

 

[MarkGrubb]

Yeah a capacitor will keep the volts up while you start the engine. Couldn't tell you what size and type though (which is terrible because I'm an electronic engineer). I'll take a guess at a nice big electrolytic. It will need to store enough energy to hold the supply up for the duration of the cranking time, so a couple of seconds I guess.

 

[halcyon]

The following diagram allows you to run two seperate banks, engine and service, start the engine from the service bank, or if you have a solar panel also charge the engine battery.

Brian

 

[davidpbo]

[ QUOTE ]
Yeah a capacitor will keep the volts up while you start the engine.

[/ QUOTE ]

IMHO You would amost certainly need a correctly rated diode in the circuit to stop the capacitor being drained by the starter motor. Hoepfully the voltage drop accross it should not cause a problem.

 

[CPD]

Hi John, maybe try a fully charged normal car battery (known to be in good condition) to start the engine to see if it is voltage drop that is the problem. As a side note, I did away with the rotary 1/2/both/off switch as it died and now use the 2 battery isolators to do the battery selecting, always taking care to make sure at least 1 is on when the engine is running. If your rotary selector knob is dying, then this could be the cause too. Sorry didnt see you recently. WIll try and drop in on the way back - end of July sometime. Hope all is well otherwise. Have a look at the crewing section - you may be getting a call soon !. Cheers, Alan

 

[philip_stevens]

I had this same problem on a previous boat. The way I overcame the problem was to fit a very small intruder alarm battery that all the instruments were fed from. This battery was itself fed from the common terminal of the 1/2/both/off switch via a Schottky diode - very low forward resistance of less than quarter volt.

When the engine was started from either battery, the instruments were still receiving over 12volts from the intruder alarm battery, but as soon as the engine was running, full voltage was resumed from the main batteries.

No further shutting down of instruments when engine starting.

 

[wetstuff]

Hi Philip,

Thanks for your suggestion - I am finally off the water and can look into it. A few questions I am afraid....

I don't suppose you have any more details on the diode you used do you? I am guessing that I ought to be able to get it from Maplins once I know which one to get.

Do you have a wiring diagram to show how the battery , diode, instruments and original power are connected?

Is the alarm battery charged via the boat supply, or do you have to top its charge up externally from time to time?

Thanks in advance for your help.

All the best,

John

 

[wetstuff]

Hi Folks,

Thanks to everyone who has made suggestions. Will be changing the domestic battery at the end of this week and then see how we get on.

Cheers for now,

John

 

[philip_stevens]

John,
the diode I used, because I had one, was a Schottky stud diode. Any Schottky diode of about 4 to 8 amps should suffice, but you could use two or three of these - Maplin/1N5820TSC - in parallel.

All instruments are fed from this source, via the alarm battery, which is constantly being fed from the main battery. When the engine is being started and the voltage drops, all voltage is from the alarm battery, as the diode is blocking any reverse feed. No external charging is necessary.

I don't have the unit with me at the moment, as I am in St. Peter Port, but can send a diagram of it you when we get back home - unless "Will" from Western Aussie can send you a diagram.

Basically, the negative is taken from the normal connection, and the positive is taken from the alarm battery positive. The positive feed to the alarm battery is from the main battery via the diodes, and then to the alarm battery, where the instrument positives are taken from.

I cannot draw it out for you as I don't have a CAD program on my Mac, and I don't have a scanner with me if I was to draw it by hand.

 

[halcyon]

If you do not want to make a diode pack, look here .

Brian