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And yet another alternator question: Is bigger always better


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BMC 1800 with I think a 55A alternator, I’ll need to double check. It charges a starter battery and 3x110ah leisure batteries. 
 

So far I’ve not had an issue with electrics because I use little of it but I plan to start using a bit more. (I’m yet to fathom out how much) But soon I’ll be needing to run the engine daily to charge rather than weekly. 

 

simply; what are the size options for a larger alternator for a 1800 BMC

and what are the pitfalls in putting a bigger alternator on an engine?

Thanks

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You cannot run too large an alternator on a single "V" belt.

90A is considered to be the sensible limit without incurring ongoing belt problems.

Unfortunately different crank pulleys for BMC 1.8D engines would have to be specially made at considerable expense as the pulley has to have a rubber bonded harmonic balancer if you are to avoid crankshaft failure.

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1 minute ago, Goliath said:

BMC 1800 with I think a 55A alternator, I’ll need to double check. It charges a starter battery and 3x110ah leisure batteries. 
 

So far I’ve not had an issue with electrics because I use little of it but I plan to start using a bit more. (I’m yet to fathom out how much) But soon I’ll be needing to run the engine daily to charge rather than weekly. 

 

simply; what are the size options for a larger alternator for a 1800 BMC

and what are the pitfalls in putting a bigger alternator on an engine?

Thanks

 

As you are stuck with a single V belt the absolute maximum you should consider fitting is 90 Amps. If you go higher you may well run into short belt life problems. I think Calcutt may fit twin alternators to a 1.8 so if they will sell you the kit you could add another alternator and a bidirectional VSR to combine the outputs when the batteries will accept a high charge.

 

I would probably stick at 70 to 80 amps with a 14.5 regulated voltage.

 

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Ok, thanks Tony.  
Going down a twin alternator ain’t for me at the moment so I’ll check what I have and perhaps go to a 90A. 
 

Swopping for a 90A won’t involve any thing other than taking the old one off and putting the new on will it?

Im good at swopping like for like but if it involves changing some wiring or brackets then it becomes another thing. Especially wiring. 

 

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3 minutes ago, Goliath said:

Ok, thanks Tony.  
Going down a twin alternator ain’t for me at the moment so I’ll check what I have and perhaps go to a 90A. 
 

Swopping for a 90A won’t involve any thing other than taking the old one off and putting the new on will it?

Im good at swopping like for like but if it involves changing some wiring or brackets then it becomes another thing. Especially wiring. 

 

 

If it is a standard 9 diode alternator and the fixing centres are the same then it will just fit. You might have to change the terminals on the wires to fit the new alternator.

 

The back (brush end) fixing usually has a sliding bush that allows the brackets to up to about 1/4" difference. You can carefully slide the bush to allow the alternator to fit but support the bracket on something like a socket while you tap the bush, to prevent  the hammer blows snapping the bracket. 

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11 minutes ago, Goliath said:

Ok, thanks Tony.  
Going down a twin alternator ain’t for me at the moment so I’ll check what I have and perhaps go to a 90A. 
 

Swopping for a 90A won’t involve any thing other than taking the old one off and putting the new on will it?

Im good at swopping like for like but if it involves changing some wiring or brackets then it becomes another thing. Especially wiring. 

 

Agree about the 90A thing and that might be pushing it a bit! But contrary to forum wisdom, a further way to get better charging after fitting a 90A alternator, is to fit a modern alternator controller. Of course it makes no difference when charging is in the “bulk phase” (alternator producing max output) and neither in the end phase (batteries only accepting a few amps) but it makes a big difference in the mid-phase, when with a normal internal regulator the current starts to drop off way before the regulated voltage is approached. But that involves wiring … 😱

Edited by nicknorman
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Its an easy swop.  This one is a bit big @ 105A   https://www.ebay.co.uk/itm/195081818841?hash=item2d6bc852d9:g:VkwAAOSw3 OJijfE6   But will give you an idea as it has a universal mounting bracket and will fit left and right hand mounts.  Its very cheap too.

 

If you want a good price and good advice have a word with Lancashire Rotating Electrics in Leyland but they will send next day for £10.

Edited by Tracy D'arth
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6 minutes ago, nicknorman said:

Agree about the 90A thing and that might be pushing it a bit! But contrary to forum wisdom, a further way to get better charging after fitting a 90A alternator, is to fit a modern alternator controller. Of course it makes no difference when charging is in the “bulk phase” (alternator producing max output) and neither in the end phase (batteries only accepting a few amps) but it makes a big difference in the mid-phase, when with a normal internal regulator the current starts to drop off way before the regulated voltage is approached. But that involves wiring … 😱


Yeah wiring, I do struggle with wiring. 
Learning bit by bit but not me favourite. 
What I was  thinking is simply a bigger alternator to give a quicker bulk charge in the morning if I’m sitting moored up, then let the solar do the rest through the day (summer time anyway). 
 

Maybe I’ll come back to the issue of using a modern alternator controller,  I understand what you’re saying. 
But bit by bit and slowly for my noggin. 
 

Firstly I’ll get a straight swop 90A alternator and go from there. 
 

Thanks. 

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2 minutes ago, Goliath said:


Yeah wiring, I do struggle with wiring. 
Learning bit by bit but not me favourite. 
What I was  thinking is simply a bigger alternator to give a quicker bulk charge in the morning if I’m sitting moored up, then let the solar do the rest through the day (summer time anyway). 
 

Maybe I’ll come back to the issue of using a modern alternator controller,  I understand what you’re saying. 
But bit by bit and slowly for my noggin. 
 

Firstly I’ll get a straight swop 90A alternator and go from there. 
 

Thanks. 

If you are going to fit an alternator controller later, get the supplier to fit the extra wire into the brush gear now. You don't have to connect it to anything yet.

LRE will do this for free I think.

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In my case the alternator controller makes a very big improvement in how quickly I can charge the batteries. It likely depends on how bad the alternators built in regulator is but I suspect most of them are quite bad because they are not intended for fast charging of boat batteries. In the worse case you might even find that the new bigger alternator does not give much improvement.

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26 minutes ago, dmr said:

In my case the alternator controller makes a very big improvement in how quickly I can charge the batteries. It likely depends on how bad the alternators built in regulator is but I suspect most of them are quite bad because they are not intended for fast charging of boat batteries. In the worse case you might even find that the new bigger alternator does not give much improvement.


Are there down sides to charging faster?


In theory: Why shouldn’t I get a 120A alternator?

 

 

 

Edited by Goliath
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23 minutes ago, Goliath said:


Are there down sides to charging faster?


In theory: Why shouldn’t I get a 120A alternator?

 


No downsides to moderately fast charging. 90A is not fast charging unless you have a very small battery bank!

 

If you fit a 120A alternator the belt will slip, screech and shred itself fairly quickly. We have a 175A alternator but it is run by a flat polyvee belt than can transfer much more power than a v belt.

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22 minutes ago, Goliath said:


Are there down sides to charging faster?


In theory: Why shouldn’t I get a 120A alternator?

 

 

 

 

Faster charging is a big subject, there are no big downsides to charging faster than you do now, but too much and you need fatter cables etc, the engine might struggle, and most of all it becomes diminishing returns because the batteries will only take a very big charge for a short time.  As you are becomong a bigger user of the 'leccy then its probably worth fitting an ammeter (at least) so that you know what is going on.

 

As for a 120amp alternator....you only have a single V belt and it would slip and so wear out quickly

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2 minutes ago, dmr said:

 

Faster charging is a big subject, there are no big downsides to charging faster than you do now, but too much and you need fatter cables etc, the engine might struggle, and most of all it becomes diminishing returns because the batteries will only take a very big charge for a short time.  As you are becomong a bigger user of the 'leccy then its probably worth fitting an ammeter (at least) so that you know what is going on.

 

As for a 120amp alternator....you only have a single V belt and it would slip and so wear out quickly

Ok 

Just wanted to know the extremes 

 

What do you mean by an engine will struggle ?

 

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As you are into extremes (which can be a very good way to understand things) in an extreme case of a big alternator on a small engine, the engine might not be able to get above tickover until the charge current had dropped a bit.    A very few boaters have a switch to turn the alternator off should they ever need every last bit of power from the engine. With your BMC you should fine.

 

A 120 amp alternaor could be producing over 1.5kW so allowing for how inefficient alternators are could be sucking well over 2kW from the engine which is about 3 horsepower. Thats 3 horses less available at the prop.  

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32 minutes ago, dmr said:

As you are into extremes (which can be a very good way to understand things) in an extreme case of a big alternator on a small engine, the engine might not be able to get above tickover until the charge current had dropped a bit.    A very few boaters have a switch to turn the alternator off should they ever need every last bit of power from the engine. With your BMC you should fine.

 

A 120 amp alternaor could be producing over 1.5kW so allowing for how inefficient alternators are could be sucking well over 2kW from the engine which is about 3 horsepower. Thats 3 horses less available at the prop.  

And at tick-over that is all the engine will produce, it may even stall.

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17 minutes ago, Tracy D'arth said:

And at tick-over that is all the engine will produce, it may even stall.

 

Its possible, but I think the more likely course of events is that it will tickover ok because the alternator has not really got going, but as speed increases the alternator output really ramps up so the engine can't go much above tickover. We've got a TravelPower and when the washing goes onto heat I can really feel the engine having to work hard.

I sometimes think I've got something round the prop so drop into neutral, if the engines is still working hard I know its the washing machine 😀

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I did a similar thing a few years ago. If the replacement alternator has a bigger "frame" you will also need a slightly longer belt (you were going to fit a new one anyway weren't you) - check the dimension between the centre of the pulley and the pair of mounting eyes compared with the existing one. Also, this may not apply but, in my case it moved the outside of the alternator perilously close to the sound deadening on the side cover panel (cured with a bit of work with a stanley knife). Don't be tempted to over tension the belt to make up for the extra load - it will wear out the water pump bearing (DAMHIK).

 

springy

 

 

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1 hour ago, Goliath said:

Yes

That bit I can understand. 

The other factor in that equation is that the input power to an alternator comprises the belt force x the rpm. So as the engine rpm reduces, the belt force (which is torque by another name) increases. So an alternator which is pulleyed up to produce most of its output at idle, will present the maximum torque demand from the engine at idle. And of course the engine’s maximum torque output is at a minimum at idle. So it’s not too hard to envisage a situation where the torque demand from the alternator equals or exceeds the maximum torque available from the engine at idle, especially when adding the torque required to turn the propellor. The engine gets stuck at idle or stalls.

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10 hours ago, nicknorman said:

The other factor in that equation is that the input power to an alternator comprises the belt force x the rpm. So as the engine rpm reduces, the belt force (which is torque by another name) increases. So an alternator which is pulleyed up to produce most of its output at idle, will present the maximum torque demand from the engine at idle. And of course the engine’s maximum torque output is at a minimum at idle. So it’s not too hard to envisage a situation where the torque demand from the alternator equals or exceeds the maximum torque available from the engine at idle, especially when adding the torque required to turn the propellor. The engine gets stuck at idle or stalls.

 

As I understand  was found out by the AA when they equipped their Mini patrol vans with alternators. Also, as said above, some vintage low revving engines with belt ratios that allow decent charging at cruising revs in 100's RPM. All worse with well discharged batteries. Hence, the reference to a switch that allows the boater to prevent the alternator charging while the engine is cold.

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14 hours ago, Goliath said:

soon I’ll be needing to run the engine daily to charge rather than weekly. 

 

That's a bit negative a month before the longest day of the year!  More solar ... preferably into a lithium battery, so you can store all the sunshine.

 

 

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I see there are lots of A127 70A alternators that appear to be a straight swop for my A127 55A. 
With the same size pulley and all. 
I will need to check. 
Am I going to notice much difference going up to 70A?

 

The 90A alternators I see look like I will have to make modifications to brackets at least. 
 

 

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