Alternator size

[mrsmelly]

Hi Peeps

I know we have had eighteen million threads on batteries but as a long term liveaboard on reading theses threads it seems that the people who know lectrics state that a battery dictates how much charge it takes from the alternator? so quick question if a particular set of leisure batteries lets say 2 of them are lets say 70 percent charged and my 80 amp alternator is putting in say 30 amps at one given time would a 175 amp alternator be putting in a hell of a lot more at that exact same time and state of charge or would it still be the same 30 amps?

Taa

[Robbo]

Depends the alternator is running at the amount of revs to produce it’s maximum.   Check the voltage as well, as this is relevant.

[mrsmelly]

1 minute ago, Robbo said:

Depends the alternator is running at the amount of revs to produce it’s maximum.   Check the voltage as well, as this is relevant.

Thanks but that's not what I meant. My set up runs perfectly with twin 80 amp jobbies but I was wondering if the battery actualy dictates how much charge it receives or wether the alternator size makes a difference. For an exaggerated for instance if the alternator was say a 1000 watts would it still only put 30 amps in at the given time or would it force more into the batteries due to its size?

[cereal tiller]

17 minutes ago, mrsmelly said:

Thanks but that's not what I meant. My set up runs perfectly with twin 80 amp jobbies but I was wondering if the battery actualy dictates how much charge it receives or wether the alternator size makes a difference. For an exaggerated for instance if the alternator was say a 1000 watts would it still only put 30 amps in at the given time or would it force more into the batteries due to its size?

The Former is Spot on.

ETA for dumbness

Edited July 26, 2017 by cereal tiller

[mrsmelly]

16 minutes ago, cereal tiller said:

The Former is Spot on.

ETA for dumbness

So basically a big alternator is only relevant on a very big battery bank or one that is discharged to a great extent each time?

[system 4-50]

6 minutes ago, mrsmelly said:

So basically a big alternator is only relevant on a very big battery bank or one that is discharged to a great extent each time?

or one that is often driving a big non-charging load.

[mrsmelly]

2 minutes ago, system 4-50 said:

or one that is often driving a big non-charging load.

Thanks folks that's all clarified in my tiny brain

[WotEver]

9 minutes ago, mrsmelly said:

So basically a big alternator is only relevant on a very big battery bank or one that is discharged to a great extent each time?

 

2 minutes ago, system 4-50 said:

or one that is often driving a big non-charging load.

Yes to the above

The thing to be concerned about is a hefty great big alternator with a piddly little bank that's heavily discharged. In that scenario the bank could 'demand' more than is good for it. 

[mrsmelly]

1 minute ago, WotEver said:

 

Yes to the above

The thing to be concerned about is a hefty great big alternator with a piddly little bank that's heavily discharged. In that scenario the bank could 'demand' more than is good for it. 

Cool. My set up of twin 80s and a travelpower works superbly and I usualy charge when batts are at about 12.5 so don't ever use the full capacity of the alternators anyway. I just keep reading on threads re peeps having ooge great 175 amp for instance alternators and wondered what the point was but I suppose its to use a big inverter with engine running?

[WotEver]

1 minute ago, mrsmelly said:

Cool. My set up of twin 80s and a travelpower works superbly and I usualy charge when batts are at about 12.5 so don't ever use the full capacity of the alternators anyway. I just keep reading on threads re peeps having ooge great 175 amp for instance alternators and wondered what the point was but I suppose its to use a big inverter with engine running?

Nick is one and he gives various scenarios where with his 175A alternator plus TP he can do the washing, boil a kettle etc. all at the same time. 

[mrsmelly]

2 minutes ago, WotEver said:

Nick is one and he gives various scenarios where with his 175A alternator plus TP he can do the washing, boil a kettle etc. all at the same time. 

mmmmm yes I think I will stick with the dreaded gas kettle as the TP copes with everything else and if a 80 amp alternator goes tits up I have a feeling it will be mooch cheaper to replace than a big one that wouldn't charge any faster anyway.

[cereal tiller]

3 hours ago, mrsmelly said:

mmmmm yes I think I will stick with the dreaded gas kettle as the TP copes with everything else and if a 80 amp alternator goes tits up I have a feeling it will be mooch cheaper to replace than a big one that wouldn't charge any faster anyway.

Yep the Lucas A127 type Alternators can be replaced for just 50-60 Drinking Vouchers,and even the Far eastern Clones can be durable if they are are Modified with Field Temperature Limiting,as one should.

[DHutch]

As said, the size of battery bank will determine the rate at which it will accept charge. A large alternator will simply raise the voltage a little quicker and then throttle back to the rate the chemical reaction with the cells of each battery can occur.

Unless you have a large load, such as an inverter running a washing machine. At which point most of the alternators output will be consumed by that.

 

Daniel

[Neil2]

Thinking about these boats with jumbo alternators and travelpower, presumably there is a way of working out how much engine power (in HP) they consume?  It seems to me that if someone has enough hardware to boil a 3kw kettle and run a washing machine, that must be consuming an awful lot of horsepower.   

 

[Robbo]

2 minutes ago, Neil2 said:

Thinking about these boats with jumbo alternators and travelpower, presumably there is a way of working out how much engine power (in HP) they consume?  It seems to me that if someone has enough hardware to boil a 3kw kettle and run a washing machine, that must be consuming an awful lot of horsepower.   

Not a lot of HP compared to the engine that they are usually attached to.  One of the reasons a dedicated generator is better if you can afford the space n cost.

[WotEver]

1 hour ago, Robbo said:

Not a lot of HP compared to the engine that they are usually attached to.

By way of reference, 3kW equates to 4hp. 

So a 3.6kW Travel Power working flat out feeding a dishwasher, microwave and hairdryer uses less than 5hp. 

A 12V 150A alternator working flat out into deeply discharged batteries (1.8kW) uses around 2.4hp. 

[OldGoat]

For discussion -

I think if you're going to have a large alternator then it's sensible to have an Adverc type of battery manager as - in some measure - it will try to limit either the current or the overall battery voltage by managing the battery voltage.

Just slapping a dumb alternator isn't a good solution?

 

[Robbo]

3 minutes ago, OldGoat said:

For discussion -

I think if you're going to have a large alternator then it's sensible to have an Adverc type of battery manager as - in some measure - it will try to limit either the current or the overall battery voltage by managing the battery voltage.

Just slapping a dumb alternator isn't a good solution?

 

The only advantage is that it will (or should) have temperature monitoring and monitor the voltage at the batteries.  Alternators like from Mastetvolt already come with a advanced regulator.

[Neil2]

10 hours ago, WotEver said:

By way of reference, 3kW equates to 4hp. 

So a 3.6kW Travel Power working flat out feeding a dishwasher, microwave and hairdryer uses less than 5hp. 

A 12V 150A alternator working flat out into deeply discharged batteries (1.8kW) uses around 2.4hp. 

So one horsepower is about 750watts.

So my 50A alternator even working flat out, which it never is, will never consume more than one horsepower?   I've always been led to believe that any alternator will rob an engine of a couple of horsepower at least, which is why I was curious about all these generating devices attached to the engines of some boats.  

Just as in the process of the power of an engine getting to the prop some of that power is lost, surely it's the same with alternators etc. ie there has to be a loss of efficiency factor in there somewhere?  

[WotEver]

10 minutes ago, Neil2 said:

Just as in the process of the power of an engine getting to the prop some of that power is lost, surely it's the same with alternators etc. ie there has to be a loss of efficiency factor in there somewhere?  

Oh absolutely. I didn't mean to suggest that a 150A alternator would only rob the engine of 2.4hp, I was simply trying to point out the approximate 'order' of power required. As in "around 3hp" as opposed to "around 30hp". 

The alternator itself is inefficient - maybe as little as 55% for an old design according to Delco. There are also losses in the belt (ever noticed that the belt gets hot?). So overall efficiency (discounting engine inefficiency) can be as low as 45%. However, modern alternators are MUCH more efficient than that, but still with losses of course. 

Edited July 27, 2017 by WotEver
Added a bit of detail

[cereal tiller]

Agreed,I do Calcs. on 45% efficiency.

[Tony Brooks]

and I use 50% because:

a. I only want indicative figures for illustration.

b. Its easier and I can do that in my head.