Only getting 10.5 volts

[Saddletramp]

Just bought a new boat - hooray!

I was checking her over yesterday prior to delivery.
With the engine running I checked the voltage across the negative bus bar and the various wires for lights, autohelm etc. I was only getting 10.5 volts on average. Should it be 12 volts?

The wiring does not look too good, most of the wires look too thin and connected through small choc box connectors.

The output from the alternator was also getting hot. Again that has a join in it, through a rusty connector.

I need to decide if I have to go through everything checking and replacing before she goae in the water. It will need to be done at some point anyway.

 

[richardabeattie]

Oh dear! You are going to have a busy winter.

 

[tobble]

Have you checked the rested voltage of the battery? (leave it for an hour or two to 'settle' after charging)

poor connections and skinny wire will present more resistance than good connections and beefy wire, so may well be that the installation is causing the low voltage.

 

[Kieran_Breheny]

If you suspect the wiring at this stage, and I mean even 'suspect' it, you will always be concerned about it on the water. So bite the bullet and get a proper installation engineer to check it over. Most, but not all, marine navigation electronics problems are caused by poor wiring.

 

[cmedsailor]

Hot wires is an indication of wires that need to be replaced. A friend of mine couldn't start the engine. Brand new battery and no problem with the engine. Just by coincidence he found out that the "thin and hot" battery wires was the problem for this.

 

[najsmith]

Ohms Law in full effect here. Basically, the thinner your wire, the higher the resistance & voltage drop.

Engine to Battery & Battery to starter motor need to be MONSTER thick heavy duty cables with as short runs as possible to reduce resitance & voltage drops.

Marine Engineer not necesserily required provided you know someone who *really* knows what they're doing/talking about. That said - if what you can see is bad - I'd haet to think what the hidden issues might be.

You do NOT want to find this out at Sea

 

[AlanPound]

Sounds like it might be a duff cell on the battery (or one of the batteries if there is more than one). This needs sorting ASAP as it probably will cause any remaining batteries to go sick too.

If so, then replacement may well be the only feasible option.

 

[BobnLesley]

... I was only getting 10.5 volts on average. Should it be 12 volts? - It should be more than twelve; check the output where it leaves the alternator, if you're getting 13.8 - 14.8V there, the chances are that you've a knackered battery; try charging the battery on a mains-charger to check.

...The wiring does not look too good... What is it with yotties and wiring? When I was learning to sail, I went on eight different yachts and repaired a basic electrical fault on six of them! I'm not any sort of Auto Electrician, I just spent several years owning Italian motorcycles - Magnetti Marelli those boys can make electricity do amazing things even without the benefit of adding salt water! The accepted method seems to be:
1.Buy a new gizmo at the boat show 2. Wire it in 'temporarily' just to see if it works 3. Yes it does; we'll leave it for now and go sailing then wire it in properly... 4. Repeat steps 1-3 with every other electronic gizmo that's been developed 5. get upset/annoyed/surprised when electrics fail to function. /forums/images/graemlins/tongue.gif I've been guilty too, but usually manage to make step three brief and generally skip step five all together /forums/images/graemlins/wink.gif

...The wiring does not look too good... It's a DIY job, work through it slowly & logically over the winter, secure/support wires properly, tried to avoid routes through particularly wet areas, create a proper bespoke wiring diagram as you carry out the work.

The output from the alternator was also getting hot... I presume you mean the cable rather than the alternator terminal itself? Check the connections and/or renew cable as part of the winter re-wire.

I need to decide if I have to go through everything checking and replacing before she gose in the water... It depends on how far you plan to go, if you're just weekending make temporary repairs - as item 2 of the yachties wiring method (At worst you can always charge a battery ashore and work on a total-loss system - other than starting, a diesel motor uses little power, nor do basic electronics, just don't run lots of lights or turn on radars & fridges. There's no reason that you can't fully re-wire once afloat; there'll be no work to do that's both outside and below the water-line

Have fun and enjoy /forums/images/graemlins/ooo.gif

 

[whipper_snapper]

With the engine running you shoud see around 14V across the battery. With the battery charged and engine stopped you should see around 12.8V

The alternator itself will get hot and that may not indicate a problem.

When you say 10V between -ve busbar and 'wires for' various things do you mean with the various things switched on and running ? In the resting state and measured with a simple digital voltmeter, wiring thickness and dodgy connections will not normally reveal themselves. It is only when a current is pulled and you see the voltage fall, or a volatge 'dropped' across a connection that you can see a problem. To test that, measure the voltage that appears across 2 points in the positive or negative supply. It should be zero at rest and rise to only a fraction of a volt when current is drawn.

I suspect that you probably do have dodgy wiring (chock blocks are morally wrong!) BUT you are probably not detecting it with the test you are doing.

 

[Saddletramp]

Thanks for all your replies.
I identified the problem whenI was trying to find why the autohelm would not work. (evertyhing else, lights etc all seem to work) So with the engine running and everything switched on I checked across the -Ve bus bar and each live wire coming out of a choc box. 10.5 volts
2x 110 amp hr batts were showing 12.5 volts on each. So they were down but they started the yanmar 2gm ok.

 

[matelot]

You have to listen to BobnLesley - any man who can conquer Italian motorcycle electrics had been through the mill. Joe Lucas may have been the Prince or Darkness but Dottore Marelli ( everyone in Italian engineering seems to be a Dottore) was in a different league altogether.

Anyway, BobnLesley is a bit wrong on one thing. The nominal voltage of a fully charged and rested battery is about 12.7. When this battery is being held at float voltage by a running alternator it should be somewhere between 13.4 and 13.7 across the battery terminals. When the battery is being charged at full whack, the voltage across the terminals will rise to something like 14.5, but that does not necessarily mean that the battery is duff. Possible just a bit flat. If the voltage gets above 15, then the voltage control on the alternator has gone phut.

All feed wires have some resisitance and some voltage loss, but you should see voltages at the bus bars within a few tenths of a volt of those at the battery terminals. If you are losing more volts then you have a high resistance in either thin wire or dodgy connections. This resistance means that heat is being generated and at the extreme it can and does cause fires.

At one time I would have been casual even sloppy about such things but having seen a boat fire, I am no longer. So I would do something about the wiring reasonably quickly. The plus point is that it is easy to \DIY if you are handy and prepared to take care.

 

[Saddletramp]

[ QUOTE ]
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you should see voltages at the bus bars within a few tenths of a volt of those at the battery terminals.

That's helpful.
Suspect rusty choc box connectors and wires doubled up.

 

[Bilgediver]

Sounds to me that you were charging one battery and using the other to measure....However the battery you were measuring was off charge and moreover has a duff cell so you were measuring a 5 cell battery....No big problem....Need a new battery and a clear set of instructions on how the charging and battery use is controlled

 

[Saddletramp]

Not quite clear what you mean. I did not check the batts before I started which would have helped. They both showed the same, 12.5 volts, which I would have thought unlikely. Are you saying that one is topping the other up?

Still does not explain the voltage drop. I expect I have 2 problems together.