Power crazy?

[richardeilers]

My two 120ah lead-acid domestic and one 110ah engine batteries are knacked and I'm thinking of replacing them with four 80ah AGMs (3+1).
I'm worried my 60ah alternator won't cope. Am I right in understanding AGMs need a higher rated alternator? Or has the alternator always been too weedy?
A new 100ah alternator fitted would be about £600.
Perhaps I should just stick with lead-acid or have one fewer domestic AGM and trade power for efficiency?
Any advice appreciated....

 

[kengill]

AGMs will cope with higher charge currents but they don't require them. As with all batterries the lower the charge rate above self discharge trickle the more efficient is the charge . Besides if you deep cycled the lot it's only going to take about six hours to get up to 90% and then you're in the lap of the regulators and how efficient they are.

If you do go for the extra battery make sure you can isolate each of the batteries in case of a failure so you'll, have a slightly complicated switch cascade and/or a very clever charger.

AGMs also operate much better at heeling angles as the acid is held in the mats and doesn't slosh around.
Have fun

 

[Bunny]

Not too sure about the maths involved in this but i do know alternators suffer from "premature alternator shutdown", i.e. after the first initial boost they drop dramatically to a nominal charge rate. fitting a battery sensing charging system will overcome this. If the batteries require charge the unit will tell the alternator not to shut down until the batts are full. A lot of narrowboats use this system effectively. A company called kestron is one company i know of but there are quite a few suppliers out there. Ccst about £150.

 

[pvb]

Your alternator will be fine...

Your alternator will be fine for the batteries you suggest. If you want to maximise charging efficiency, you should fit a "smart" regulator to the alternator - Adverc, Sterling, etc. It would be a waste of money to fit a 100A alternator unless you also invest in a much bigger battery bank.

 

[andy_wilson]

Premature shut down?

It's more a case of the batteries rapidly rising to a voltage that resists charge. Unfortunately this is always a false reading but the alternator knows no better, keeps punching out 14V, and the rate of current flow drops.

You need a charge controller to change the charge characteristic, and force more current in more quickly.

A good charge controller will help a 60Amp alternator keep your proposed battery bank charged up. If you fit 320 Ah, you should be using only 50% of this, 160 Ah. This means that your alternator will need to run for probably 4 hours to charge from this level of discharge. A 100 Ah alternator would cut this to something nearer 2 hours.

 

[pvb]

Think not...

[ QUOTE ]
A 100 Ah alternator would cut this to something nearer 2 hours.

[/ QUOTE ]
I doubt very much that it would - even with a smart regulator you can't push anywhere near 100A into a battery bank that size for long.

 

[richardeilers]

Re: Think not...

All very interesting - if rather baffling for a non-expert like me. It looks like adding a clever controller would make more sense than a beefier alternator.
Think I'll press ahead with the full set of new AGMs tho, unless people think it's just emperor's new clothes?