35A alternator- Upgrade to 55A or 90A alternator?

[xeitosaphil]

Engine Yanmar 2GM20 existing alternator 35A Mitsibushi.

It would appear I have a problem with my 35A alternator as I ran the engine for 1 ½ hours today charging the batteries up from 78% to 98% @ 14.60v and 4.5amps. The battery voltage and the charge of 4.5amps stayed the same throughout the whole 1½ hrs, hence the problem.

I have two 110A domestic batteries in parallel and a 90A starting battery, both charged via a VSR. I may add an additional 110A battery to the domestic bank if I can find room for it as I have just built a fridge/freezer, but I may have to stay with the 2 x 110A if not possible.

On ringing a local alternator supplier, he said he had a Prestolite 90A alternator he could let me have cheaper than I could buy a replacement Yanmar 55A (which is what I considered as an upgrade anyway).
What do you think of this 90A alternator for my system, is it too much? I assume that it won’t produce more amps than is needed for the battery charge, so shouldn’t cook the batteries, and would only be moderately used, even if on a 50% discharge. It may charge batteries a little quicker, but not expecting much saving in time if any.

Had a quick word also with chap at Adverc who said they fit 90A alternators to 2GM20’s as a package deal with their monitoring system with no ill effects?
He did say at top end of charge rate 60A+ it could take off 1 - 1 1/2 hp off engine, but I think that, that situation would never arise with my discharge/charge rate needs.

I may add an Adverc Control system at a later date, but funds don’t run to that at the moment.

Can you see any problems or issues with using this 90A as opposed to the 55A. Both alternators are virtually the same size, and I can’t see any negatives at the moment, especially as the price is good.

Just wondered what the forum thought.

 

[William_H]

The constant 4.5 amps out of the old alternator may indicate that a diode has gone open circuit. This will limit the current a lot.
It may be cheaper /easier to repair the old alternator.
If you choose a large capacity alternator it will not make any difference to anything if that capacity is not used. If you do want to load it up with high current then you may find problems with belt drive slipping and need an additional belt.
The Adverc controller may demand so much current from the alternator that belt will slip.
Even with the adverc your existing 35A should still do a good job. The bigger one may however be more reliable.
I would suggest you remove the existing alternator and have it checked first off. Good luck olewill

 

[MASH]

The point with battery charging is not what the alternator can produce, it is what the battery bank can absorb. If your batteries (or rather your smart charger) only want to give them say 20A peak charge rate then a 35 A alternator is probably (by 15A) more than enough. Even if the charger wants to give them 40A for a while and then let it down to something more manageable you probably aren't short-changing yourself very much for long. If all your charger demands is say 25A and you have a 35A alternator than what is the point of paying for a 60A alternator? The one you already have does that comfortably.

You need to find out what amperage your charger requires throughout its cycle, and anyway your figure of 4.5A over that range towards the top of your charge sounds fine to me, far from being a problem. The last few % of capacity need to be done slowly. What use is a 90A alternator is all the battery requires is 4.5?

There is no advantage whatsoever in huge over capacity.

The critical figure is what your batteries (or rather your charging system) demand though a shortfall in supply is only going to make a slight difference to your charge rate. Unless the alternator supply is significantly short of that it isn;t going to make much difference.

Anyway, an hour and a half is pocket change in the cycle of a full charge. Far too short a time to be significant.

 

[[2574]]

Something doesn't add up here. If you have 2 x110AH batteries (220AH) and you went from 78% capacity to 98% capacity (adding 44AH) then your charger was charging at a rate 29A not 4.5A as you suggest. I think your battery monitoring system is telling you fibs as I don't believe your charger would really replenish to 98% in that time period. To get above 80% capacity generally the charging is in float mode, effectively trickle charging at <5A. This would suggest that your 44AH replacement up to 98% would take something like 9hrs (overnight) at 5A. I think it might be worthwhile having the system checked over to see what is at fault.

 

[pvb]

It would appear I have a problem with my 35A alternator as I ran the engine for 1 ½ hours today charging the batteries up from 78% to 98% @ 14.60v and 4.5amps. The battery voltage and the charge of 4.5amps stayed the same throughout the whole 1½ hrs, hence the problem.

As Robih has pointed out, these figures don't make sense. As I recall, you have a Smartgauge, which is presumably where you're getting the % capacity figures from. This could be inaccurate. Equally, you've had endless problems with your ammeter, so where are you getting the 4.5A current from, and could this be inaccurate too?

If you still believe your alternator needs replacing, the Prestolite alternator is likely to give you a better charge at low revs than most others, so on that basis it would be the one to go for. But its 90A output won't be best used by only 220Ah of batteries, although it won't harm them.

 

[VicS]

It would appear I have a problem with my 35A alternator

Bearing in mind all the past troubles and the comments above a wise move, I think, would be to get the existing alternator properly tested.

If it turns out to be in good order then you have to look at the system for the "troubles".... simply fitting a bigger alternator wont solve anything.

If it proves to be defective then you have the option to buy the larger one ... perhaps part-exchanging the existing one ... or getting it repaired.

 

[awol]

My 35A Hitachi feeding 2x110Ah batteries was vastly improved by fitting - dare I say it? - a Sterling alternator controller. Now the batteries actually get somewhere near a full charge instead of the 80% or so after hours of engine running.

 

[xeitosaphil]

Something doesn't add up here. If you have 2 x110AH batteries (220AH) and you went from 78% capacity to 98% capacity (adding 44AH) then your charger was charging at a rate 29A not 4.5A as you suggest. I think your battery monitoring system is telling you fibs as I don't believe your charger would really replenish to 98% in that time period. To get above 80% capacity generally the charging is in float mode, effectively trickle charging at <5A. This would suggest that your 44AH replacement up to 98% would take something like 9hrs (overnight) at 5A. I think it might be worthwhile having the system checked over to see what is at fault.

These figures were taken from the Smart gauge fitted, but to be honest Merlin do say that the SOC reading is not accurate to read when charging is taking place, as normaly it is a calculation of average charge percentages and resting voltages etc ( little bit above my head) so these figures could be wrong.
Having disconnected smart gauge from power source at different times in the course of upgrading battery wiring, I re set SOC % to what I the previous reading was before power loss, as the default setting after power disconnection and reconnection is 70%. Over a period of time this will sort itself out no doubt.

The measurement of charge amps was taken using a new test instrument set to DC amps
http://www.maplin.co.uk/dc-ac-current-clamp-multimeter-629713

as a result of this thread
http://www.ybw.com/forums/showthread.php?353310-Clamp-Meters-anyviews-on-this-one

The charge was from the alternator not a mains battery charger and my concern was that irrespective of the accuracy of the readings taken, my alternator charging at a fixed rate of 14.60v ( also verified by the ST60 Multi at the chart table) was constant, and neither the voltage or the current amps diminish as I would have expected, as charge continued.

With regards to the 90A alternator, as this was cheaper than the 55A Yanmar and with the possibility of additional battery ah I thought it might be a better buy. As previous threads on charging - alternators/sterling/ Adverc in the last few days, wouldn’t it be better to have an alternator charging at say 50% of its capacity rather than having one running at 80%, also it would put in more amps at lower revs if only for a short time.

Pvb you’re absolutely right about the ammeter- still got problems and still trying to sort them out which is why I stared trying to find a starting point for charge rates. Now I seemed to have taken a sideways step – just in the quest to find out if different things are working properly.

 

[Westerly Jetstream]

There have been a couple of excellent threads on this forum in past week or so covering battery charging, both mains, and alternators. We all like to go sailing not endless hrs of engine running to charge batteries.The OP is thinking of increasing his bank size from 310 to 420 as he would like to fit a fridge. So if he runs even the domestic bank planned 330 down to 50% then he's looking to replace 165Ah and like most of us will not want to motor about all day. Now Alternators do Not give out their stated output all the time, Yes a 35 Amp alternator will produce 35amps but only for a short period, Alternators are reckoned to be 60% efficient, so you do Math, also it is good practice to have your Alternator sized at min 20% your bank requirement again do Math, so if it was me I'd be fitting the 90. now thats not allowing anything for engine battery, and presuming he never (doubtful) runs his domestic bank below 50% capacity. Sailors with fridges tend to slaughter their domestic banks.

 

[Plevier]

If your batteries are actually fully charged (and already were at the start of your charging period, contrary to what the SmartGauge said) but your alternator is regulating at 14.6V then that constant current which is about 2% could make sense.
Are the batteries gassing?
If you have a standard alternator without a smart regulator it should be giving most probably 14.0V or 14.2V, maybe 13.8V if it's pretty old.
14.6V is too high (unless it has been meddled with to compensate for a diode splitter, but you said you have a VSR).
So maybe the problem is the alternator regulator.

 

[xeitosaphil]

If your batteries are actually fully charged (and already were at the start of your charging period, contrary to what the SmartGauge said) but your alternator is regulating at 14.6V then that constant current which is about 2% could make sense.
Are the batteries gassing?
If you have a standard alternator without a smart regulator it should be giving most probably 14.0V or 14.2V, maybe 13.8V if it's pretty old.
14.6V is too high (unless it has been meddled with to compensate for a diode splitter, but you said you have a VSR).
So maybe the problem is the alternator regulator.

No the batteries I don't think were gassing and not even warm to touch.

It Doesn't have any additional smart regulation at the moment just standard internal regulator, alternator may have been changed at some stage as it is a Mitsubishi direct earth return and not a Yanmar insulated earth return.

Don't think its been meddled with no additional wires etc, and is the one that came with the boat which didn't have a diode splitter just a 1-2-both switch.

I have fitted a VSR in the course of the present upgrade.

I must add though just thinking about it the charge rate did go up to 5.8A at starting but dropped to 4.5A and stayed there for the period. Half an hour after I shut engine down and before leaving the boat, Smart gauge was showing House bat 12.75v Engine Bat 12.65v which I assumed to be resting voltage.

I wish I had taken reading at both batteries now- re SG accuracy!

 

[[2574]]

Don't forget that if you uprate the alternator you'll have to uprate all of the cabling atwixt it and the batteries as well; the existing cables would melt if you tried shoving 90A down the line. Sorry if that was plainly obvious to you.

rh

 

[pvb]

The charge was from the alternator not a mains battery charger and my concern was that irrespective of the accuracy of the readings taken, my alternator charging at a fixed rate of 14.60v ( also verified by the ST60 Multi at the chart table) was constant, and neither the voltage or the current amps diminish as I would have expected, as charge continued.

The voltage doesn't diminish as the batteries are charged by an alternator. The alternator is set to give a certain voltage (14.6v in your case apparently) and it just gives that voltage forever. The current diminishes as the batteries approach full charge.

 

[xeitosaphil]

Don't forget that if you uprate the alternator you'll have to uprate all of the cabling atwixt it and the batteries as well; the existing cables would melt if you tried shoving 90A down the line. Sorry if that was plainly obvious to you.

rh

Thanks Robih,

I must admit I did think about this as I have only just replaced the original cables with 10mm2 Tri rated cable rated at 70amps. I did wonder if it was safe to assume that the alternator would only ever reach maybe 80% of its total output just around the 70A as I read somewhere that they never ever reach full output?

 

[ianj99]

View attachment 30167The best way to get an idea of the true state of charge of the batteries is either by measuring the specific gravity or removing all loads and note the voltage after about 15mins.

Trjoan batteries publish a very uesful table for their batteries http://www.trojanbattery.com/batterymaintenance/testing.aspx

Only fit larger batteries if you have the means to recharge them unless you are prepared to run the engine for many hours.

 

[pvb]

I did wonder if it was safe to assume that the alternator would only ever reach maybe 80% of its total output just around the 70A as I read somewhere that they never ever reach full output?

Alternators do indeed reach their full rated output if an appropriate load is present. If you only have 220Ah of batteries, and if you don't ever use high-load items such as an inverter, windlass or thruster, it's unlikely your charge current will ever exceed around 50A. However, it's obviously not an ideal situation to have theoretically undersized cable.

 

[Westerly Jetstream]

Ah but he's planning on having 330 Ah of batteries excluding the engine battery and just how he's going to get 50 amps out of a 35 amp alternator puzzles me.Yes alternators do reach their full rated output but can not stay at it for ever and a day, they are at best to repeat myself 60% efficient.

I run a 110 Amp alternator for 330 house and 110 engine and it works very well batteries always healthy and more sailing less motorsailing.

 

[Plevier]

No the batteries I don't think were gassing and not even warm to touch.

Smart gauge was showing House bat 12.75v Engine Bat 12.65v which I assumed to be resting voltage.

I wouldn't expect them to be warm.
Gassing at this voltage and current won't be vigorous.
For comparison I've got a 70Ah on charge in my garage at the moment, at 14.4V it's taking about 1A and bubbling just very slightly.
The OCV is about right, a little bit lower than I would have expected unless rested for a while (or is there a bit of load on?).

A quick dig shows that some Mitsubishi alternators have 14.5V regulators which is unusually high (in my experience).

I'm not sure you have anything wrong at all. (That's not to say a bigger alt might not be a good idea especially if you're adding more batteries.)

 

[xeitosaphil]

View attachment 30167The best way to get an idea of the true state of charge of the batteries is either by measuring the specific gravity or removing all loads and note the voltage after about 15mins.

Trjoan batteries publish a very uesful table for their batteries http://www.trojanbattery.com/batterymaintenance/testing.aspx

Only fit larger batteries if you have the means to recharge them unless you are prepared to run the engine for many hours.

Thanks Ian for the table and links, great site with loads of information which will be very useful. I assume it works for all batteries not just Trojan?

Don't like the idea of having to run the engine for many hours to re charge batteries, if I can get the system right I shouldn't have to.

Will look at additonal alternative solutions later once engine charge side is sorted.

 

[xeitosaphil]

Alternators do indeed reach their full rated output if an appropriate load is present. If you only have 220Ah of batteries, and if you don't ever use high-load items such as an inverter, windlass or thruster, it's unlikely your charge current will ever exceed around 50A. However, it's obviously not an ideal situation to have theoretically undersized cable.

I hear what you are saying, but as I don't have high-load items such as an inverter or thruster, and windlass is supplied by engine battery only when the engine is running, maybe it is unlikely the charge current will ever exceed around 50A, and the new cable only being rated at 70A is obviously not an ideal situation to have, but I could reviewed the situation once the 50% discharge alternator charge rates have been established, and should the need arise I could change it then.

One thing I did wonder about was, if I had to put bigger rated cable in for the alternator, could I disregard the present direct alternator charge wire from alternator to battery , and use some left over 50mm2 from alternator to the engine battery side of the engine battery isolator. This would be a permanent link using the engine start supply cable as the alternator charge feed to engine battery. This would be a quite short distance and maybe solve the problem.

This wouldn’t cause any issues would it? - Disregard that idea as it would mean that alternator current would be protected by a250A engine start fuse, not a good idea!