Extra batteries

[robmurray]

I have a 70 Amp alternator. I understand that there is some kind of relationship between alternator capacity and amount of battery capacity it will charge. I have 2 x 105Ah service batteries and want to add more battery capacity - how much can I add?

<hr width=100% size=1>

 

[paulrossall]

I think the answer is that the alternator should be at least 10% of the battery capacity. I have a 65amp alternator and 2 110amp/hr service batteries and a 65amp/hr starting battery and have no problem keeping the batteries charged. However I am a motor sailer. I would not worry about at least double-ing (whatever spelling I try does not seem right) your battery capacity. IMHO. Paul

<hr width=100% size=1>" there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats".

 

[William_H]

Go for it. I think you will find that in the worst case with flat batteries the alternator will simply limit it's output current to the rating until batteries are charged.

<hr width=100% size=1>

 

[Talbot]

my 45 amp/hr smart charger is good for up to 500amp/hr of batteries, so you should have no difficulty with a 70 amp/hr alternator

<hr width=100% size=1>

 

[MainlySteam]

Perhaps the relationship you are thinking of is that batteries will generally safely accept a charge rate up to about 20% of their capacity. So, for example, for 220A/Hr of batteries there is not much point in having an alternator too much bigger than 20% of 220 ie approx 45 amps. Or, again, for 400 A/Hr of batteries, say, there is not much point having an alternator too much bigger than 80 amps.

So for your 210 A/hr of batteries your alternator capacity beyond around 45 amps is surplus capacity and I assume is never used (as long as the charge voltage is set correctly). And your alternator will deliver its full capacity into around 350 A/hr of batteries. (Remembering that the charge rate will actually decline during the charge so the full alternator rate will only apply for part of the charge cycle).

However, it does not matter how many batteries you put on an alternator for charging, just that the smaller the alternator is in comparison to the total capacity of the batteries the longer they will take to charge.

(Edit: Note that the 20% mentioned above applies for C20 rated batteries for their A/hr capacity)

John

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 20/07/2004 12:20 (server time).</FONT></P>

 

[ubuysa]

As an aside, how is alternaor output calculated? I only ask since it must be affected by the speed of rotation and thus by engine speed and throttle opening? Would a higher output alternator deliver more current at low revs than a lower output alternator for example??

Just curious.......Tony C.

<hr width=100% size=1>There are 10 kinds of people, those who understand binary and those who don't.

 

[MainlySteam]

Yes the output will rise from nothing at no rotation to full output at some thousands of rpm, dependant on the alternator, and stay at near output from then until its maximum rated revs.

The usual problem for charging in a sail boat is selecting an alternator which can be pulleyed to give high output at slow engine charging speeds, but not over reving it at higher engine throttle speeds when motoring. Some alternators are better at this and it is also obviously much easier to do it with a slower reving engine where the range from idle to full speed, for a small engine, is maybe only 2,800 - 3,000 rpm, than it is for a faster reving engine where the range will be higher.

Manufacturers supply output curves for their alternators.

John

<hr width=100% size=1>