BATTERY DIODE?

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Formaldehyde

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I have to fit a dedicated battery for my VHF for Irish Coding requirements.
I plan to fit a house alarm battery wired into the VHF 12V supply, which should mean the battery is charged along with the domestic batteries in normal usage?
I am told however that I must fit a diode to the positive supply to the alarm battery to stop it discharging in the event that the domestic battery bank goes flat or short circuits in the event of flooding.
Can anyone advise if this system will work, and what spec the diode should be?
Richard
 
I think the diode should be rated for at least the max current of the radio. The radio battery will be charged at a slightly lower voltage than the main batteries, which is fine. The diode drop of say a 15A rectifier will be a lot less than the normally quoted 0.6V when the radio battery is float charging.
The scheme will work fine, because most of the time you are not transmitting, just drawing a small current when listening.
If anything I would suspect a chance of overcharging the radio battery, which will probably be sealed.
The diode might be dissipating 10W when transmitting with the engine running, so a heat sink may be required.
I would want a fuse in series with the diode and another protecting the radio battery.
 
[ QUOTE ]
What is the differance

[/ QUOTE ] The difference is that one has a full-stop at the end the other doesn't.

It's something people do without thinking, they come to the end of the sentence so they add a full-stop. Then the link won't work.

The trick is to always test links, even if you've copied and pasted them, in preview.
 
[/ QUOTE ] I hope your gear works better than your links

[/ QUOTE ]

We have supplied over 3,000 to one boat builder without a fault, so I'm happy.

http://www.kddpowercentre.com/page33.html

Above is the link as copied, no full stop.

Have tested again in preview, and link worked okay, so who put your full stop in, I did not.

Brian
 
[ QUOTE ]
so who put your full stop in, I did not

[/ QUOTE ] It's in your post so nobody but you could have put it there!
 
Returning to the original question, if you want to reduce volt drop across the diode you could choose a Schottky type. I use one of these to "slave charge" my windlass battery from the domestic battery and it seems to work well.
 
Sorry the volt drop of a silicon diode is always in excess of .6volt. Often more depending on temperature. The volt drop then can increase with a lot of current in a more resistive manner. The volt drop of the diode is a liottle akin to a spring pressure on a non return valve (in flow direction). Nothing gets through until the spring pressure is overcome and then that pressure drop is always there.

Anyway the suggestion of a schotky diode is the best. A silicon diode may in practice be OK with its volt drop but the alarm battery will never be fully charged.

I have found the best source of schotky diodes is old coputer power supply. They come in a plastic package bolted to an aluminium heat sink with 3 wires looking much like a transistor. These type come in a 2 diode package. They have a little diode symbol 2 symbols facing one another. If you have a multimeter with diode test range you cna measure the volt drop directly. If you have a spare silicon diode you can compare. If you don't have a multimeter with diode test then use low ohms range. Again compare to a silicon diode you will find if it is a schotky that forward resistance will show a lot less. Don't let the resitance reading confuse you. It is not a measure of actual resistance.

Or you could just buy a schotky for very little money. Yopu want a few amps capability. good luck olewill
 
Maplin sell silicon and Schottky diodes with a range of specifications. You will also find a fair bit of technical stuff on the website. Graphs showing various aspects of performance including forward volts drops vs current.


www.maplin.co.uk
 
[ QUOTE ]
Sorry the volt drop of a silicon diode is always in ...ess of .6volt. ...diode may in practice be OK with its volt drop but the alarm battery will never be fully charged.



[/ QUOTE ]

When I was at uni, it was an exponential function, giving a lower forward drop at currents very much below the design rating. OK the above is generally true, but I was thinking in terms of some 50Amp diodes I have here, which measure about 0.4V on this here meter.
The radio battery can be fully charged by this arrangement, if the house battery is being charged at 14.5V, then the radio battery will be around 13.9V. It won't charge quickly at this voltage, but its above the float charge level (Yuasa quote 13.65V). It depends whether the battery is required to start from discharged, or the radio is normally powered from the house battery. Also the ratio of time float charging to high current use, e.g Tx or loud audio Rx. If you run down the radio batt by switching off the house batt, it may take ages to recharge.
The diode must be rated for the max current the wireless will draw. A schottky diode is a good option I agree.
I suspect the radio battery will eventually suffer from over charging if anything, if the boat runs up a lot of engine hours, but it will be cheap to replace every couple of years.
Whatever system, the radio battery needs to be monitored occasionally, it would be very easy to ignore it!
The diode and battery is a good simple scheme, go for it.
I used something similar to keep my GPS going during engine start.
 
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