Electrical ignorance, please help

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Andrew_Fanner

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My port engine is allowing the relevant batteries to go flat. On startup the ammeter reads 8A or so and the alternator charge light goes out quite happily. After a few minutes the ammeter drops to zero but the charge light stays off. The belt seems OK to the best of my unskilled understanding so beyond that I'd be grateful for advice. As that side runs the fridge its seen as vital for chilled wine, ice in G&T, kid's lollies etc:-)

Rather aged 24V system to two 12V batteries, (unwisely?) centre tapped for 12V by a previous owner and not yet altered to a separate 12V setup by the idle current owner. New batteries last May which charged quite happily to capacity on the mains charger from shorepower.

We are Thames based but its not apparently alternator speed related, experimented with high revs out of gear and no change in ammeter behaviour.
 
It looks to me as though the fridge is the cause of the problem.

One battery is being drawn down by the fridge, but the alternator is seeing both in series to make 24v. As the other battery is still almost fully charged after starting, the alternator reduces the charge current after just a short burst.
Ideally you need to charge both batteries with 12v, then they will fully charge. As it is, they're unevenly balanced without any means of getting more charge back into the one that needs it most.

There are several options, one would be to run the fridge from 2x12v batteries in parallel, thereby using both batteries evenly. It would bother me that the starter batteries weren't stand alone though, far better to have completely segregated starter and domestic batteries so you know where you are!

Hope this helps explain the problem, even if fixing it is more complicated..
 
Andrew_Fanner said:
As that side runs the fridge its seen as vital for chilled wine, ice in G&T, kid's lollies etc:-)

Rather aged 24V system to two 12V batteries, (unwisely?) centre tapped for 12V by a previous owner and not yet altered to a separate 12V setup by the idle current owner. QUOTE]

Can you expand on 12 volt, are you using this tapping ? if so what for ? as this could be your problem.

Brian
Click to expand...
 
Starboard side has standard 2 x 12V in series to make 24V for the engine start and 24V bus. Port side has 2 x 12V in series to make 24V for the engine start and can be #1/#2/both selected 12V for the 12V bus, on a centre tap. The fridge uses the 24V bus (and the 12V one for the switch relay 'cos I had a 12V relay to hand!) Fridge is a fancy Shoreline one with minimal current drain and the issue arose middle of last week.

I have always assumed if you put a nominal 24V charge across 2 x 12V batteries they balance up on charge after a short while, and I run the both setting on the selector to even out drain.
 
are the batteries of the same capacity and standards ? If they are different, then - basically - the larger one will not charge fully.
 
Andrew_Fanner said:
I have always assumed if you put a nominal 24V charge across 2 x 12V batteries they balance up on charge after a short while, and I run the both setting on the selector to even out drain.
Click to expand...

No.

Assume you have drained one battery 50% from using 12 volt supply, and the alternator regulates at 28 volt.

The current through one battery in series must go through the second.

So the full battery will take a little charge and the voltage could rise to 15 volt across it. The second one being a lower capacity has a lower voltage across it anyway, so sees 13 volt, 28 - 15. So the first battery is happy with a very low current it's full, the second one is happy with a low current because that is all it will take at 13 volt.

Thus you end up with a full battery and a half charged battery, it's more complicated, but that's the simple answer. The battery level sets the voltage.

Check what the voltages are across the each battery off load, thay should be approximatley the same, 12,5 and 12,52, not 12.8 and 12.4.

Then do the same with engine running, again you should see 28 volts approx, and 14 volt across each battery.

If so, and the batteries are getting low start by checking the alternator.

Brian
 
How about using bank A in 24v mode as the starting bank for both engines and bank B in parallel as the 12v system? One presumes that as a motor vessel you run the engines at the same time so one alternator would charge the engine batteries and one the highly important 12v bank. ( might need the alternator changing or some jiggery pokery to drop high current to the 12v bank perhaps a 24 to 12v charger )

With a couple of isolator switches you could rig the 12v bank as 24v for an emergency start circuit.

Just a thought if you can select #1,#2 or both in 12v mode then what happens to the 24v charge across the battery when both are selected? There must be some diodes involved there.
 
I always leave the 12V selector on "both" to try and ensure equal use of the batteries. IMO its probably a system needing a serious rewire, but that's time I don't have at the moment. Looks like pulling the relevant alternator out and testing same would be a good plan as it was all fine and dandy beforehand. Ideally the end product would be a 24V engine start pair, a 24V domestic bus pair and a 12V domestic bus pair charged by 24-12V chargers but that needs to wait a wee while.
 
If the battery bank has a solid lead from one battery positive to the other battery negative terminal that does not go via a switch then there is no way that you can get a '12v both' ( ie parallel ) arrangement working. if the 4 leads go from the batteries to the switch you may be able to work it but i have never seen an arrangment like that.

Somethings amiss with your setup or your interpretation of the wiring!
 
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