boatmike
regular
Reged: 30/06/2002
Posts: 4206
Loc: Solent
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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?
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Captainslarty
regular
Reged: 12/08/2007
Posts: 1905
Loc: Currently La Coruna Spain
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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
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PM me for info re SSB's etc. Bought, sold, repaired, fitted and optimised.
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VicS
regular
Reged: 13/07/2002
Posts: 7646
Loc: Home: Kent. Boat: Chichester
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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.
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Old Chemists never die, they just fail to react
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Captainslarty
regular
Reged: 12/08/2007
Posts: 1905
Loc: Currently La Coruna Spain
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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
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PM me for info re SSB's etc. Bought, sold, repaired, fitted and optimised.
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boatmike
regular
Reged: 30/06/2002
Posts: 4206
Loc: Solent
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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?
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roly_voya
regular
Reged: 05/02/2004
Posts: 923
Loc: Pembrokeshire Wales
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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!!
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boatmike
regular
Reged: 30/06/2002
Posts: 4206
Loc: Solent
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I think you have missed the fact that I only have a 50A alternator currently (no pun intended)
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jerryat
regular
Reged: 20/03/2004
Posts: 2950
Loc: Nr Plymouth
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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.
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john_morris_uk
regular
Reged: 03/07/2002
Posts: 3397
Loc: Plymouth UK
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Quote:
The question they is why is your alternator not achieving close to its max output? It could be a problem with the alternator...
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!
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“When you discover that you are riding a dead horse,
the best strategy is to dismount.”
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jerryat
regular
Reged: 20/03/2004
Posts: 2950
Loc: Nr Plymouth
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Quote:
I think you have missed the fact that I only have a 50A alternator currently (no pun intended)
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!!
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