battery connections

[colin loach]

i have 3 domestic batteries and 1 starter battery. I have connected all negatives to the engine everything is working ok, but when I disconnected the domestic negative buzz bar I still had my led lights working. I traced this back to the earth on the stereo Arial linking both domestic and starter batteries together via the shell, is this normal?. Thanks Colin

[Chris Pink]

i have 3 domestic batteries and 1 starter battery. I have connected all negatives to the engine everything is working ok, but when I disconnected the domestic negative buzz bar I still had my led lights working. I traced this back to the earth on the stereo Arial linking both domestic and starter batteries together via the shell, is this normal?. Thanks Colin

 

It's not good, in the event of a fault - as illustrated by your action - all the DC return current will go through the aerial cable screen, ok for LED light level current but not for anything more powerful - there is the potential for the aerial lead to get rather hot.

 

You should isolate the aerial lead from the hull, either by sitting the aerial on an insulated mount or by modifying the cable near the radio.

[colin loach]

I can see what you mean but will this effect my reception.

[Keeping Up]

Unfortunately this is very common, I have spent a long time tracking down and curing spurious earths which, as Chris says, could potentially be a source of danger.

 

If your aerial is a "car radio" type, it will be difficult to isolate it from the hull (and even if you do, you may get noisy radio reception), so it is easier to isolate the outer sheath of the aerial cable near the radio. I was able to peel back the plug itself and insert a small capacitor in series with the sheath, but it was a fiddly job and it may be easier to cut the cable and use a 'chocolate block' connector to hold the wires and capacitor (something like 1000pF from this range would do).

 

As well as two radio aerials and the TV aerial, I also found spurious earth connections in two exterior bulkhead lights and the two navigation lights. Most unsatisfactory!

[colin loach]

bare in mind I have two alternators dose this make any difference

[colin loach]

Unfortunately this is very common, I have spent a long time tracking down and curing spurious earths which, as Chris says, could potentially be a source of danger.

 

If your aerial is a "car radio" type, it will be difficult to isolate it from the hull (and even if you do, you may get noisy radio reception), so it is easier to isolate the outer sheath of the aerial cable near the radio. I was able to peel back the plug itself and insert a small capacitor in series with the sheath, but it was a fiddly job and it may be easier to cut the cable and use a 'chocolate block' connector to hold the wires and capacitor (something like 1000pF from this range would do).

 

As well as two radio aerials and the TV aerial, I also found spurious earth connections in two exterior bulkhead lights and the two navigation lights. Most unsatisfactory!

dose spurious earths cause any damage in normal circumstances

[Chris Pink]

dose spurious earths cause any damage in normal circumstances

 

Easy answer: Yes, very much so, could be seen as one of the major causes of fire in 12V systems.

 

More complex answer: depends on what you mean by 'normal', damage will only be caused when another fault occurs, for instance if you had disconnected the main supply -ve or it broke and then a water pump or inverter was switched on the whole current for that device would return through the 'spurious earth' connection which is not designed to handle it. Whereas your aerial lead could probably handle a fair amount without catching fire there are many other scenarios where this wouldn't be the case.

 

And you have illustrated one of the major problems; if you don't know which way your current returns, you can't isolate it and fault finding becomes a factor more complicated.

Edited October 7, 2008 by Chris Pink

[colin loach]

that all makes sense. so back to the capacitor, what about using an inline fuse. how dose the capacitor work.

[chris w]

A capacitor will pass ac currents but not dc so the direct earth due to the aerial lead is broken, but the (ac) radio signals will still pass through the capacitor.

 

The capacitor's ability to pass ac currents is dependent on the capacitor's value and the frequency of the ac one is trying to pass. Allann's suggestion of around 1000nF is quite correct as at FM frequencies (say 100MHz), the impedance of the capacitor (its "resistance" in layman's terms) is less than 2 milliohms. At MW frequencies (say 500KHz) the impedance increases to about 0.3 ohms.

 

Chris

Edited October 7, 2008 by chris w

[colin loach]

A capacitor will pass ac currents but not dc so the direct earth due to the aerial lead is broken, but the (ac) radio signals will still pass through the capacitor.

 

The capacitor's ability to pass ac currents is dependent on the capacitor's value and the frequency of the ac one is trying to pass. Allann's suggestion of around 1000pF is quite correct as at FM frequencies (say 100MHz), the impedance of the capacitor (its "resistance" in layman's terms) is less than 2 milliohms. At MW frequencies (say 500KHz) the impedance increases to about 0.3 ohms.

 

Chris

hope you don't mind just one more question. you mention mw is it the same for fm. many thanks colin

[chris w]

hope you don't mind just one more question. you mention mw is it the same for fm. many thanks colin

 

Read through my post again Colin. I mentioned both FM and MW

 

Chris

[colin loach]

sorry missed that bit. there are all sorts of capacitors have you got a picture of the sort I should buy from maplin. thanks.

[chris w]

sorry missed that bit. there are all sorts of capacitors have you got a picture of the sort I should buy from maplin. thanks.

 

Allan gave a link to the Maplin site in his original post above. Here it is again for convenience http://www.maplin.co.uk/Module.aspx?ModuleNo=304 The type you need is Part No.WX68Y @ £0.11

 

Chris

Edited October 7, 2008 by chris w

[Guest TerryL]

i have 3 domestic batteries and 1 starter battery. I have connected all negatives to the engine everything is working ok, but when I disconnected the domestic negative buzz bar I still had my led lights working. I traced this back to the earth on the stereo Arial linking both domestic and starter batteries together via the shell, is this normal?. Thanks Colin

 

Perhaps I'm missing something here but why would you want to disconnect the negative bus bar only and create a problem? Keep all negatives connected and switch off only on the positive and no problem!

[chris w]

Perhaps I'm missing something here but why would you want to disconnect the negative bus bar only and create a problem? Keep all negatives connected and switch off only on the positive and no problem!

 

He's not creating a problem, he's identified an existing problem which is real but normally hidden. The capacitor will preclude the problem ever becoming an issue. It's a safety thing.

 

Chris

[chris w]

BTW, I mistakenly read Allan's original post as a suggestion of a 1000nF capacitor and did my calculations on that basis. In fact Allan suggested a 1000pF capacitor. IMHO, I feel a 1000pF will cause problems on MW as it will have too high an impedance (300 ohms). I believe (with all due respect to Allan) that a 1000nF would be a better choice. In fact anything in the range of 100nF to 1000nF should work fine.

 

Chris

[colin loach]

thanks for all your time and information, its been a great help. my problems started when I turned off the cut off switch to leave the boat and noticed the stereo stayed on for about 3 seconds. I checked the volt meter and saw the voltage was draining down slowly. so one by one I disconnected each appliance, and that is when I noticed that my lights were still on and everything was disconnected. you know the rest.i think my inverters some our although off are storing a small amount of current for a short while, because when I disconnected them the volt metre dropped straight away.

[chris w]

thanks for all your time and information, its been a great help. my problems started when I turned off the cut off switch to leave the boat and noticed the stereo stayed on for about 3 seconds. I checked the volt meter and saw the voltage was draining down slowly. so one by one I disconnected each appliance, and that is when I noticed that my lights were still on and everything was disconnected. you know the rest.i think my inverters some our although off are storing a small amount of current for a short while, because when I disconnected them the volt metre dropped straight away.

 

I'm slightly confused as I think you may mean charger NOT inverter (the inverter provides 240vac). But whatever, both inverters and chargers will drop their output over a couple of seconds at switch-off.

 

Chris

[colin loach]

Chris I have two inverters one 600 and the other 2000 watt both connected to the neutral buzz bar with all the other items, pumps etc.

[Guest TerryL]

He's not creating a problem, he's identified an existing problem which is real but normally hidden. The capacitor will preclude the problem ever becoming an issue. It's a safety thing.

 

Chris

 

Who asked you? If it should be connected permanently then it will create a problem same as disconnecting anything on the wrong side. Safety is not your thing is it?

[chris w]

Who asked you? If it should be connected permanently then it will create a problem same as disconnecting anything on the wrong side. Safety is not your thing is it?

 

You don't know where you are in the world do you? On another post, you advocated not using an isolation transformer in case a lead broke and made it unsafe. Now you are saying that this guy doesn't need to protect against a lead's coming off? Make up your mind. I imagine this contrariness of advice of yours is why you were so unpopular that you were thrown off the other boating forum???

[bottle]

Who asked you? SNIP>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

 

No one, this is a forum and we also have freespeech in this country.

 

TerryL

 

I realise that you have a problem with Chris w but may I ask you to desist from being aggressive it does you no favours and is getting a little tiresome.

[Keeping Up]

BTW, I mistakenly read Allan's original post as a suggestion of a 1000nF capacitor and did my calculations on that basis. In fact Allan suggested a 1000pF capacitor. IMHO, I feel a 1000pF will cause problems on MW as it will have too high an impedance (300 ohms). I believe (with all due respect to Allan) that a 1000nF would be a better choice. In fact anything in the range of 100nF to 1000nF should work fine.

 

Chris

Agreed; I used 1000pF because I never listen to Medium Wave

[Guest TerryL]

You don't know where you are in the world do you? On another post, you advocated not using an isolation transformer in case a lead broke and made it unsafe. Now you are saying that this guy doesn't need to protect against a lead's coming off? Make up your mind. I imagine this contrariness of advice of yours is why you were so unpopular that you were thrown off the other boating forum???

 

Wrong again.

[chris w]

Wrong again.

 

What WAS the reason you were thrown off it then??