Alternator size

[boatmike]

A question for the leccie experts.
I have been running a 50Amp alternator which is standard on the Vetus (Mitsubishi) 4.14 that I have with a Sterling smart charger. The battery bank is 390 Amp Hours. It always got pretty hot and has I think now expired. I am considering fitting a Driftgate 110 Amp alt to replace it.
The question is this:
I guess the rating of an alternator is it's theoretical maximum output, which is seldom drawn. A 25 Amp charge rate has been commonly seen on my battery management panel however which is 50% maximum.
If I fit a 110 Amp it will probably not charge at a greater rate than this (I think?)
with the Stirling attached, but will be running at less than 25% of maximum.
Have I got this right? Or will the 110 amp alt give a higher charge rate?
Wouldn't more than 25 amp charge cook my batteries? Am I doing the right thing fitting a bigger alt which will be working less hard and will therefore last longer and even allow me to fit more battery capacity if I want to at a later date?
The cost at £165 is considerably less than the cost of a direct replacement from Vetus by the way. I know I could probably get the existing one fixed but would not a bigger one be better value?

 

[captainslarty]

Hi Mike
It will charge at a higher rate, the charge acceptance rate of your battery bank could safely be up to 25% of your bank capacity as a guide, so the 110 is well suited.
there is a HOT rating for an alternator which on most is lower than the nominal rating.
Also, most alternators are for automobile use at an upper limit of 14.2, they seldom run high loads for any length of time. Your stirling will push the voltage up to around 15V from the alternator depending on battery type and cable loss. this also lowers max output but so you should still see a healthy output.

The charge will also drop off as the battery charges up, this is normal and your smart reg does a good job of charging to near capaicty if given time.

Joe

 

[VicS]

If the larger alternator is controlled to the same voltage, by the same Sterling regulator why wont the charging current of the same battery bank be the same, just higher maybe initially.

 

[captainslarty]

Hi Vic, 25 amps for the old 50 amp automotive alternator - now running at a higher voltage and rather hot - is about right.
So yes, the new one should give a far higher initial output on bulk. I dont beleive the 25 amp limit is has seen is down to the reg or batteries. also depends where he was seeing the current ?.. is it pure charge or charge plus load of equipment in use etc.
Joe

 

[boatmike]

When starting engine after a period of discharge (say overnight with lights on, fridge running, eberspacher etc.) The Stirling would ramp up the charge to 25 amps on boost. Are you saying that with the same stirling on the same settings it would be higher with a bigger alternator?

 

[roly_voya]

Something is definately blocking your charge rate. I have a similar setup to the one you are contemplating 400a/h batteries, 100a alternator, Sterling regulator. even when topping up the house batteries with less than 50a/h used so at about 85% charged I get 50-60a initially falling to about 30a untill the regulator cuts down to 13.8v. The alternator is frequentlyputting out 70-80a at 1500rpm at these charge rates as it is also charging the electronics battery, start batt and running the ship.
The question they is why is your alternator not achieving close to its max output? It could be a problem with the alternator, often the drive pully is to small so the alternator is not turning fast enough. Ideally you should get max output at about 2000rpm so if your output keeps increasing as the reves go up much beyond this the the pully setup might be wrong. The other problems can be poor battery condition, how old/good are the batteries. Poor wiring/faulty connections, check the voltage at the alternator output compared to at the battery terminals, after taking out any drop accross a diode spliter if you have one you should have neglagable volatage drop at 25a.

If it was me I would hope to find out what is wrong befor changing the alternator sothat you know what else to change, otherwise you could spend all that dosh and get little improvement if it is for, example your wiring strangling the output. If you do change to a 100a+alternator you WILL need to do a fare bit of rewiring as the cable will need to be abot 8mm dia accross the conductor with some serious terminals, ordinary crimps wont take 100a!!

 

[boatmike]

I think you have missed the fact that I only have a 50A alternator currently (no pun intended)

 

[jerryat]

I agree with you. I doubt that the alternator, in good condition and properly wired, wouldn't keep the batteries in good condition.

We too have a 400+ service battery bank and a 65 amp starter battery, all kept in excellent condition by a Lucas 70 amp alternator. Yes, the alt. does run hot after heavy use of the services, but we've had no porblem over many years, a lot of which was full time cruising.

The alternator should be capable of out-putting it's full designed load and I suggest it's well worth whipping it off and having it tested. For example, Lucas Marine tested my 70 amp unit (it is a standard 'car' refurbished one, and I wanted to know it was ok!) and it topped out at 69.4 amps, which I thought was near enough!!

As others have said, it's sensible to check the wiring thoroughly, get the alternator checked and check the Sterling, before rushing out to buy a larger unit.

 

[john_morris_uk]

[ QUOTE ]
The question they is why is your alternator not achieving close to its max output? It could be a problem with the alternator...

[/ QUOTE ] There might not be anything much wrong with the alternator. I agree with Joe (Captainslarty). The rating published for an alternator is the cold rating. As soon as the thing warms up, they can produce far less than the headline figure.

Limiting factors are: 1. Battery internal resistance (which rises as they accept charge and which some people call the acceptance rate.) 2. Size of alternator (and how warm it is!) 3. Size of cable between alternator and battery.

Asuming that the cable size is appropriate (in the positive and return circut) and that there are no poor connections, the discharged 390 Ah battery bank will accept a lot more than 25 amps when it starts charging. The figure will reduce as the batteries charge up. We have a similar size battery bank and it accepts 40 or 50 amps at first.

However, I reiterate, there might not have been much wrong with the 55 amp alternator. It's just being asked to do too much, gets warm and self limits. The alternative suggested sounds fine. Just check the cabling and connections!

 

[jerryat]

[ QUOTE ]
I think you have missed the fact that I only have a 50A alternator currently (no pun intended)

[/ QUOTE ]

I agree 50 amp a touch on the small side, though it does depend on what load you're expecting from it. If, for example, you are discharging the batteries by 40-50% overnight or between charging, it probably is too small. Again, as others have said, if your 'operating load' i.e. load during charging, is also high, it adds to the problem.

As I said in my previous post, get the alternator checked out. Lucas did it for me for nothing when I popped in, but it only takes a few minutes on a special rig, and even a few pounds may save you a hundred!!

 

[boatmike]

Joe,
Thanks for the input so far. Are you able to say what the output of the 110A alternator would likely be in practice? Is 50A reasonable?
I have a conventional diode splitter with the sensing wire at the batteries on the 50A and have been getting voltages of 16.5 - 17 indicated with about 25A on boost which seems to suggest the wiring is indeed marginal anyway. I would expect 14.8 plus about 1.5 for diode plus cable maximum (?). If I fit a 110A I will seriously examine/replace the cabling but how big should it be? In am assuming for that amperage I will need 25mm2 battery lead. Also would you expect the single 1/2" belt drive to be man enough? I see the driftgate alternator is fitted with one as standard but am considering fitting twin pulleys is this OTT?
If I assume 50A at 15.5V thats 775W which is a little over 1HP assuming I have got my sums right (?) assuming 100% efficiency. 110A would be 2-2.5 HP theoretically but I think the sterling will need the trip switches re-set for the bigger alternator (I will have to look up the installation manual again) and will not allow a higher rate of charge than the batteries will tolerate which will probably be no more than 50A. Either way I will fit a switch in the activation line of the stirling to allow me to reduce the charge rate when I need maximum output of the engine to drive the boat.

 

[boatmike]

Jerry,
I hear what you say but I believe the alternator is "fried" due to running too hot for too long and it's on the small size anyway. I would rather err on the side of a new bigger one if the bill comes to less than £200. I do have things like fridge running constantly and use the eberspacher and cabin lights a lot in cold evenings so tend to hammer the batteries.

 

[boatmike]

Crossed posts there John but you will see from my recent posts that you are confirming my view. Alternator too small. Cabling probably needs replacing with bigger especially as its quite a long run of cable, and Sterling will need to be set up for new alternator. How big is your alternator and does it run off a single 1/2" belt OK?

 

[captainslarty]

You have a problem with your cabling Mike. 17v is too high really.
for the 50 A unit, 25 mm2 is adequate, for the 110, 35 as a minimum, I use 50mm2 on a balmar 130. with SHORT runs.. you say you have a lot of cable length ? if so, definitely 50mm2.

a lot of your problem, knowing the voltage now (And presuming that is AT THE ALTERNATOR - not at the batteries !) then the cabling is inadequate.
The diode splitter will drop 0.6 to 1v. easilly made up at the alternator. I would aim for a max voltage at the alternator of 16v. assuming 14.8 max at the batteries i(f wet cell)

The other thing to consider, is how are you reading the output current - with a splitter you have two banks, are you monitoring just one, or both ?.

Max recommended size of alternator on a single belt is 130 Ah.

the output from the alternater to a disharged bank (Assuming a 110 unit and proper cabling - ) should peak up to 90 amps for a short period (You will very rarely if ever get the maximum due to heat and the fact the alternator was not designed for marine use - ie, a higher voltage than automotive and much more use)

My 130ah alt can peak at 115 for short periods - that is controlled by a balmar reg also.

The stirling unit should not need any adjustment - afaik the dip swiches are only for the battery bank TYPE. there is also a fuse that can be fitted in two positions for either neg or pos field reg alternators.

No problem on the switch, there are deatails in the stirling reg manual afaik.

Hope that helps..
I would firstly change to 50mm2 cable, keep the runs as short as possible. try the older alternator first.. the cable will be oversized a tad for your existing alt, but thats better than undersized and allows you to go up in size on the alternator without changing calbing again.

Hope that helps Mike.

Ps Charles Stirlings instruction re voltage drop and cabling are very comprehensive (Assuming the pro digital unirt) - have a browse through that as well.

Cheers n beers

Joe

 

[boatmike]

Thats very helpful Joe thank you. I am sure you are right about the cabling. You can see from my earlier posts that I had already reached that conclusion. I just looked up current ratings for cable and agree with your sizing. The charge cable will end up bigger than the starter motor cables but I guess they only have to pass current for a short burst. The charging line cables have to pass current for extended periods....
Yes the voltage mentioned is at the alternator and showing up on the engine panel. I actually have three batteries on a 3 way split diode. There is a shunt on each line from the diode splitter to each battery which is also showing a high voltage (15V) but I am advised that this is probably due to the sensor wire going to the battery side of the diode rather than the battery terminal itself. This I shall also change. The shunts are wired directly to a Stirling battery management panel which allows me to see volts and amps to each battery. The sensing wire goes to the battery supplying the fridge, eberspacher, lights etc, which is going to be the highest discharged. The second battery serves instruments and essential services, The third battery is for engine cranking only and seldom gets heavily discharged.
There are 2 sets of dip switches on the Stirling. One is set for voltage re wet or gel batteries. The other controls the charging cycle. for a small alt on big bats it extends the boost cycle time. The other way around reduces it. They are the ones I will need to reset to avoid boiling batteries with a bigger alternator.
I can't try the old alternator Joe, it does not work! The whole point of the exercise is to aviod spending money on repairing it when it's too small anyway. I think in a way it's a blessing in disguise that it has failed as I now have an excuse for buying a bigger one and upgrading the installation generally.
Thanks a whole lot for your very helpful input (and that of others of course) that in true forum tradition has helped me to get the details clear in my mind. I shall post again when the project is completed and tell you how I got on.

 

[captainslarty]

Nice one Mike.
Hope you get it all sorted out - I am sure you will.
happy charging and have fun

Joe