60A alternator ..... whats that mean?

Bav34

Bav34

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This is what I have gleaned from various battery questions:

AmpHours ...... divide watts by volts .... 25W bulb 12v system = 2AH.

Batteries will only supply half their rated AmpHours so my newish service battery (140AH) will only supply 70AH.

Batteries will accept charge more slowly the nearer they get to being fully charged.

OK, if that's all basically true I pretty much know how much power I will take from my 70 available AH's by adding up the wattage of everything I'm using. (Probably very simplistic but that's the way I like it /forums/images/graemlins/ooo.gif)

BUTTTTT

What's with the 60A charging bit?

I know that if I run my engine for an hour that I haven't mystically thrown another 60AH back in ..... if it was that simple why bother with wind gennys and smart charging systems?

So what does an hour of engine running with a 60A 14v alternator actually give me?

And why?

How long would it take to get a battery showing say 12.5 volts up to (fully charged???) 12.7/8 volts at rest?

Please be gentle with me.
 
Re: 60A alternator ..... what\'s that mean?

when your running you are running on alternator not battery, if you use more than 60a your battery will be discharging, if you go below 60a your battery will charge by spare unused amps. your battery amp/hour is its reserve power without charging.
 
A 60A alternator is capable of pushing out a maximum of 60A. If your battery was very low then the alternator would indeed start to recharge it at 60A. However, as the charge in the battery increases so it's voltage increases but, the voltage from the alternator is effectively fixed by the regulator so the amps into the battery drop.

If you have a smart alternator controller, this increases the voltage from the alternator (from around the standard regulator value of 14.2 to maybe 14.6V) which increases the current into the battery. However, this voltage is still insufficient to drive 60A into a battery that is fairly well charged and the charging current into the battery will continue to reduce to almost nothing when the battery is fully charged. This is why it takes much longer to fully recharge a battery than the maximum alternator output would suggest.

Note: A smart charge controller does more than just increase the alternator voltage; it cycles the voltage increase periods because this helps the battery to attain full charge quicker (and I'm sure someone better versed in battery technology will be along shortly to us tell why!).
 
Yes I think that alternators are built for vehicles and are capable of generating no more than 60amps without overheating. It is an advertising type number so is made to look as high as possible.
So it is likely that the 60 amps is flat out such that the actual voltage generated is barely 12v which of course won't charge a battery. (there is some argument about this statement and it might depend on manufacturers.)
ie 60 amps into lighting or similar.
It is a bit like claimed HP for a car engine not like a wattage rating for a lamp which is a specific rating (not a claim) for a given input voltage.

So for battery charging the alternator may be at best able to deliver 50 amps into a very large very flat battery.

A battery takes charge depending on it's charge state and the voltage pushing the charge in.

If you had an amp meter and monitored charge current into the battery you would be dismayed to find it starts well but diminishes rapidly. As said a smart charger can improve this situation.

Bottom line is a 60amp alternator will if asked to deliver more current with less internal heating than a 40 amp alternator and less than a 100A alternator.
I reckon for most boats however that a 60 amp alternator is seldom asked to reach its capability. And if it does reach its limit while charging it won't be for long. It should be happily self limiting. However some feel the heating might be detrimental. olewill
 
The simple answer is that under ideal conditions it will generate 60 amps into 12 volts.

However like in most things, the simple answer isn't necessarily the correct answer. A 60 amp alternator will only produce 60 amps under exceptionally favourable circumstances. When the batteries are completely flat it might get somewhere near that figure - but only for a short period.

Firstly as soon as the battery voltage starts to rise, the internal regulator of a normal charger will reduce the current output.

Secondly the batteries that are on most boats very quickly develop some internal resistance to being charged and just won't accept a high current. Nothing wrong with this - its just the physics of charging batteries.

Finally, the alternator heats up and this significantly reduces its output.
 
The max output of the alternator will also reduce at lower rpm. More modern alternators are optimised to produce good current at low-medium rpm, as modern cars use a lot of current and don't get revved that often. At tickover/slow pottering out of marina rpm, my old 55A alternator was only good for about 15A, of which the fridge took a good handful.
 
Thanks (as usual) to all but I guess there is no simple answer to my questions:

<span style="color:blue"> So what does an hour of engine running with a 60A 14v alternator actually give me?

How long would it take to get a battery showing say 12.5 volts up to (fully charged???) 12.7/8 volts at rest?
</span>

I was hoping for something simplistic like '' one hours charging will put 'x' number of amps/amphours back into your battery''

As no-one has contradicted my usage formula I guess that I understand that well enough (25w / 12v = 2ah) .

Still not sure how long I would have to run my engine for just to replace that simple calculation /forums/images/graemlins/confused.gif
 
[ QUOTE ]
Still not sure how long I would have to run my engine for just to replace that simple calculation

[/ QUOTE ] You run the engine until the beer is cold in the fridge ... what more do you need to know?!! /forums/images/graemlins/tongue.gif
 
[ QUOTE ]
Thanks (as usual) to all but I guess there is no simple answer to my questions:

<span style="color:blue"> So what does an hour of engine running with a 60A 14v alternator actually give me?

How long would it take to get a battery showing say 12.5 volts up to (fully charged???) 12.7/8 volts at rest?
</span>

I was hoping for something simplistic like '' one hours charging will put 'x' number of amps/amphours back into your battery''

As no-one has contradicted my usage formula I guess that I understand that well enough (25w / 12v = 2ah) .

Still not sure how long I would have to run my engine for just to replace that simple calculation /forums/images/graemlins/confused.gif

[/ QUOTE ]
It just isn't that simple, OK?
25w / 12v = 2 amps, by the way, not 2 ah. But if you deliver 2 amps for an hour you deliver 2 amp-hours.
"An hour of engine running with a 60A 14v alternator" will give you something that is totally dependant on several factors, chiefly the state of charge of the battery. As others have already explained, the more the battery is charged, the greater its internal electrical resistance, and smaller will therefore be the current that the alternator can push through it, because Volts=amps x resistance.
I beg to suggest that you may not understand this properly unless you read a book about it!
 
[ QUOTE ]
I beg to suggest that you may not understand this properly unless you read a book about it!

[/ QUOTE ] Or someone can explain it in clear english on here .... which I guess you're not about to do?
 
[ QUOTE ]
[ QUOTE ]
I beg to suggest that you may not understand this properly unless you read a book about it!

[/ QUOTE ] Or someone can explain it in clear english on here .... which I guess you're not about to do?

[/ QUOTE ]
I can't!! /forums/images/graemlins/confused.gif I did do A-level Physics but that was 45 years ago! /forums/images/graemlins/tongue.gif
 
So did I ... but even though it was a lot more recently than yours I still can't (quickly) answer the question!
 
How about: 'If your battery is significantly discharged, say down to 50% of capacity, running the engine for 2 - 3 hours will restore it to something like 75% capacity. The last 25% capacity will require many more hours of engine charging and given the limitations of the basic regulator you will nevere get it beyond say 90%. The only way to do that is to use a more sophisticated charger.' ?
(The numbers in this idiot's guide were invented by this idiot and should not be thought of as reliable although the broad principles are correct.)
 
As has been said already the amp rating of the alternator is the maximum output given at a specified speed. This is useful in matching alternator output to system demand. For cars lights wipers and on some cars various electric heaters have meant that larger alternators have become the norm in order to carry the running load without discharging the battery. The output voltage of the alternator is regulated to prevent damage to the system including the battery.

BatteryChargevVoltagechart.jpg


Charge and discharge rates as a fraction of battery capacity (Capacity/5 C/40 etc.) are shown in the chart. Increasing the charging voltage by using a different type of regulator reduces charge time but also causes the battery temperature to rise and increases gassing.

Simply increasing alternator size will not cause the battery to charge more quickly as the voltage, assuming the rest of the system on a boat is relatively small that is within the capacity of the alternator, will not increase the rate of charge.

In order to prolong life lead acid batteries should not be deeply discharged as a fraction of their capacity. 50% is commonly used as a good rule of thumb.
 
Am certainly no expert at these matters, but having installed the battery monitor, you can see whats going in and whats going out.

With a battery used at anchor for nearly 24 hours , using a fridge at its lowest level, gps vhf, and echo sounder, the battery will be down to 12.3 volts or thereabouts or maybe slightly lower.

My little 35 a/h alternator will start charging at nearly 35 amps, but will quickly reduce to 30 then 25. Probably after an hour it will be down something like 4 or maybe less amps. So to calculate the actual amp input it would take a graph plotted every 5 or 10 minutes. The amps could then be added up.

If I charge for longer the rate just goes down to a few points of an amp, which means the battery is resiting the final % of the charge.

Point of my words is that a battery monitor will show you whats going on, or in and out!
 
[ QUOTE ]
Thanks (as usual) to all but I guess there is no simple answer to my questions:

<span style="color:blue"> So what does an hour of engine running with a 60A 14v alternator actually give me?

How long would it take to get a battery showing say 12.5 volts up to (fully charged???) 12.7/8 volts at rest?
</span>

I was hoping for something simplistic like '' one hours charging will put 'x' number of amps/amphours back into your battery''

[/ QUOTE ]

The reason that no-one has given you an answer is that its impossible to give you an answer! If your alternator regulator is the standard one, your batteries take days to get to 100% If you have asmart charge regulator, its a bit quicker, but the physics of the battery stops you getting the last bit in very quicky. (You can't force the last bit of chemical reaction to take place any quicker than its going to.)

We have a battery monitor that tells us how many amp-hours have gone out and how many have gone in. I give up trying to get it to a net zero as even with a smart charger it takes for ever. Sometimes if we are plugged in overnight in a marina, then the batteries go back to 100% charged.

Sorry that's not any more help, but after one hour of charging, you might have put a few amp hours back in...
 
[ QUOTE ]

I know that if I run my engine for an hour that I haven't mystically thrown another 60AH back in ..... if it was that simple why bother with wind gennys and smart charging systems?



[/ QUOTE ]

Thats a fair question to ask, and the answer is that for most sailors wind gennies and smart chargers are a waste of money. They are simply bells and whistles - bit like the TV built into my car! After all, we first buy a boat and then we buy toys for it.

Reason is that most of us ( long distance cruisers and liveaboards apart) spend far more time under engine than we like to admit or even realise, and the alternator is way better at charging the batteries than a wind genny or solar panels. Whats more the extra you gain from a smart charger is real but not great if your basic system is well designed in the first place.

In one year when I kept records (bit sad!) I used my engine for 50% of the time that I was off a mooring, and that was cruising a decent distance. But most people arent anal enough to keep records so they dont know what they do do.

Lets put it another way. With fridge and heater and TV and no inclination to switch lights off, I have never managed to flatten a domestic battery on the boat (or even get near) in 20 years. With nothing more than the engine to charge it up - and I never run the engine for that reason alone.
 
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