12volt alternator and 24volt starter

[claredelune]

It is a long story, but i now know why there were two new batteries installed when we took ownership of our Westerly Centaur. The starting circuit makes sense to me with a solinoid that connects two 12 volt batteries in series to give 24 volts for starting. However the charging circuit only goes to one battery. The second battery has the -ve terminal connected via the solinoid to the +ve terminal of the first battery to give the 24 volt and thus cannot be connected to the main earth or the first battery will short out on starting.

Has anyone a solution that doesnt involve periodically swapping the batteries about so they charge turn about. I dont really want to connect another solinoid that will make / break the second battery to earth connection at starting.

Pleased help as we want to take advantage of every second of decentish weather this year!

 

[VicS]

Ugh what a ghastly lash up.
Given that a Centaur only has a small engine (MD2 or Md11?) and does not need the benefits of a 24 volt starting system Get a 12 v starter motor to fit and convert the whole thing to 12v with two 12v batteries that can be designated starter and services if you wish.

Any attempt to introduce addition switching to parallel the batteries for charging is likely to end in disaster when one is accidentally shorted out. It could be done though.

Follow the wiring diagram in the original Centaur owners handbook. Maybe the WOA or its forum can help if you don't have the book.

 

[claredelune]

yeah, its a bit ugly. the engine is a seapanther watermota and at approx 32hp is massivly overpowered. But it starts first hit every time. a 12 volt starter might not have the guts.... (and i dont have the cash for a new engine and all that entails)

 

[VicS]

As you say that is overpowered. But even an engine that size should start Ok on 12volts. You dont find many cars with 24 volt starters trucks maybe. Another forumite mentions starting 200hp engines on 12volt with no problems. You may need a bigger battery than the 60Ah normally specified for an MD 11 but you could parallel two smaller ones.

That's a marinised Ford engine isn't it? What is its size in cc?


I see you have also posted on the WOA forum. Have you also tried the discussion group?

 

[Gunfleet]

You have measured 24 volts, yes? Because your description sounds like paralleling 2 12v batts to give one big battery. And I don't really see why someone would go to all the trouble of setting up even a 32 hp motor with 24 volts. It just doesn't need it. My last diesel car had 140 hp and still managed with 12 volts electrics.

 

[CPD]

My advice would be as per VicS, although you need to be sure that a 12v starter would do the trick, and without buying/installing, I cant think of how you would be absolutely sure. Personally, i would go down the road of asking in as many places as you can, then go with gut feel. I cant think either of why a 24v starting system would be needed. 32hp is overpowered for a centaur, but then 32hp isnt a lot compared to car diesels where compressions are higher etc.

 

[VicS]

[ QUOTE ]
You have measured 24 volts, yes? Because your description sounds like paralleling 2 12v batts to give one big battery

[/ QUOTE ] Checking for 24 volts is a sensible suggestion but i am afraid the description he gives, namely a solenoid connecting the negative of one battery to the positive of the other, very definitely indicates to me that the two are connected in series not in parallel.

 

[Gunfleet]

Vic's right of course. If it's just solenoids and starters receiving 24 volts while the alternator outputs 12 it's a peculiar system. Where's the advantage? Are the batteries a long way away from the starter? I don't see what the difference would be between the 12 volt and 24 volt starter anyway... they're just windings and magnets. If the cables are thick enough I'd give it a spin at 12 volts and see what happens.

 

[William_H]

As I understand the set up you have a few options.
The solenoid in the negative of the battery (presumably the only solenoid does sound a bit strange. It would be reasonable to fit another solenoid to connect that battery negative to ground (engine negative) and a diode (but you will be stuck with voltage drop) or better still another solenoid to connect the first 24v starting +ve to the other battery +ve for charging. This would acheive charge at 12 volts on both batteries in paralell. It will require that 2 solenoids be activated for charging and 2 activated for cranking. A VSR would be usefull to turn it all off when not charging. Decent solenoid (high current relays) rated at 500 amps or more are not that expensive.

It seems strange that the system was left as you describe as apparently there is no provision for automatic charging of the 24v battery. I think you should check again how it is wired and look out for diodes or other trickery.

Anyway another option is to replace the starter and do everything at 12 volts. However before you do that just try the 24 volt starter on 12 volts. You may find it saitisfactory. It may even be a 12 volt starter. The current drain of a starter depends on the rotation speed of the starter so a 12 v starter on 24 volts will not be damaged as you might imagine because the current falls rapidly as it starts to turn. Likewise a 24 volt stater will draw a lot of current until it starts to turn so generating power to start even at the lower voltage. So it is worth a try.

Oviously if you go for 12v starting then you use the 2 batteries for isolated domestic services and you need a paralelling switch for charging or a VSR.
If you stick with 24 v starting don't be tempted to use switches to paralell the circuit for charging as if you then hit the stater button without setting the switches you will have a real short circuit.

good luck (especially with the weather) Our TV news has pictures of your flooding every bulletin) (Here it is raining but not enougyh and we love it) olewill

 

[roly_voya]

If you confirm that you definatly need 24v for the starter an altenative is to use the existing 12v batteries in parrallel for the house bank then fit 2 seperate batteries in series for the 24v system and charge them via a 12-24v converter. Does not need a big one, 10a from 12v side should be plenty as the starter draws very few amp hours (lots of amps but only for a very short time) If you wanted it could be conected via a voltage sensative relay so it only connected when alternator was running outherwise it will keep charging all the time and put some drain on the 12v battery.

If you do keep a 24v system it might be worth looking at running the mast head light from it as the length of cable run often causes voltage drop problems for 12v but not for 24v. Dont be tempted to put much stuff on 24v though as bigger converters get very expensive and you lose 5-15% of power through them.

 

[ccscott49]

I think if you have started the engiena few times and the battery is not flat, then it's being charged somehow.
Check the voltage at each set of battery terminals with the engine stopped, then start engine and check voltage again, if the volatge is higher on both batteries, then there is a hidden diode somewhere that's doing the job. I'll bet there is.
24v starting is not as strange as some folk think, gives a much better whoomf to start a diesel engine, used to have 24v starting on an old rally car I had.

 

[jwilson]

Look at the engine on this boat: http://www.yachtsnet.co.uk/boats/s6737/s6737.htm - so your Centaur having 24v starting is not unique. This one though has two alternators - one for each engine.

It ought to be possible to wire up a single alternator to automatically charge two batteries in series via diodes (but I'm not a good enough electrician to guarantee exactly how this would be set up), but you can't do it safely via ordinary switching.

 

[pvb]

There is a solution...

[ QUOTE ]
Has anyone a solution that doesnt involve periodically swapping the batteries about so they charge turn about.

[/ QUOTE ]Well, I agree with others that it all sounds a bit of a lash-up, and there's no reason why an engine that small needs a 24v starter. However, there's a quick and easy solution if all you want to do is keep the second battery charged. Take a look here at this 24v set-up which would meet your needs. It was designed to run a bowthruster, but your starter circuit is similarly an occasional use item. I'm assuming the solenoid you mentioned is only activated during starting.

 

[philip_stevens]

I've copied this, from a reply to the Westerly Group in June 2005 -

[ QUOTE ]
--- In Westerly-Owners@yahoogroups.com, "cy" <asyt47@....> wrote:
> The reason you have a 24volt system is that you have a MK 1 Sea
Panther fitted, these engines have no pre heaters and therefore rely
on spinning up the starter to high revs in order to start.
Incidentally the starter motor is a 12volt one which has 24v applied
across it to spin it faster !! It therefore is very important that
you do not wind it over for long periods as it will quickly burn out,
5 secs max was the advice given to me by the "worlds expert" on Sea
Panthers, a chap who built most of them up and until recently was
still working down in Devon somewhere.
>
> My 74 model did not have a battery selector at all and all my
lighting and nav lights were 24v.

[/ QUOTE ]

 

[ccscott49]

That clears it up a bit, so you dont have to use the 24v shorting solenoid to start the engine? So somewhere there is a charging device that charges both batteries, I suspected it was to give it more oomph! Do as I said above and all may well be revealed.

 

[claredelune]

Thanks to all that have taken the time to reply. The update is thus.
1. it is 100% 24volts at the starter
2. i have checked with a multimeter and the 2nd battery is not charging
3. the engine does not appear to be original but then...
4. if the starter is a 12v one and the engine is starting first hit as it is it wont take long to rewire the batteries to 12v in parallel and see if it starts. with no pre heater, winter starting may become a problem.
5. the engine started fine for one season, but then the draw on the uncharged battery is minimal.
6. there is a delay on the charge light going out and the ammeter showing charge so there may be a diode / relay fitted somewhere that is causing this but i cant find it and the second battery stays at 12v when the light is out. (the first gets to 13/14v)

I will try the 12v starting option and with two new batteries in parallel this will hopefully work.

thanks again for all the input

David

 

[VicS]

Re: There is a solution...

[ QUOTE ]
However, there's a quick and easy solution

[/ QUOTE ] I dont like it but it is an ingeneous solution that would work. The switch could be a solenoid, capable of carrying the full starter current, activated from the starter keyswitch. A refinement could be to include a diode in series with one of the headlamp bulbs so that the 12v domestic services battery could never drain any current from the other one. The down side would be that it would impose a 0.6 volt drop and therefore limit the charging of the second battery.

There are some aspects of this design though that do deserve comment. It is suggested that the highest wattage lamps that can be found are used and that each is wired to bring both filaments into use. Now then typically head lamp bulbs are 60 + 55 watts ie a total of 115 watts. That means in 24 volt mode each battery would be supplying nearly 10amps to one of the bulbs, considerably more than the "couple of amps" refered to. That might be enough to affect starting especially when the gods conspire to make things difficult. In addition modern bulbs are halogens which run very hot. That means that not only would care be needed to mount them so that the heat causes no problems but running both filaments would shorten their lives. Therefore I would suggest that both filaments are not used, or single filament bulbs are used, that ordinary bulbs are used rather than halogens. and that finding the highest available wattage may not be the best of ideas.

The circuit could be "improved" by putting relays in series with the bulbs that are also energised by the starter switch so that when the starter is operated the bulbs are disconnected and so do not draw any current. (one relay with two sets of contacts could be used)

That is only a short step away from a system without the bubs, just a relay, in which the solenoid to connect the two batteries together to provide 24v is energised by a third set of contacts on the relay that connects/disconnects the batteries in parallel for charging/ series for starting.

I dont like any of it. I would fit a 12volt starter motor if it were mine, upgrading the battery cables if necessary.

 

[brianrunyard]

It'll be a 12volt starter run at 24volts to give extra oomph.

 

[celandine]

I have the slightly later Watamotor Seapanther that starts on 12volts but has pre-heaters. I am sorry to say, re your plan, that mine will not start without the pre-heater - summer or winter.

.

 

[pampas]

Quite a bit in windings between 12 to 24 volt starters. I expect the starter is 12 voly and has the 24 just to give it the extra punch for starting, simular to the rally boys.