Battery Capacity

[Jabberwock]

I have just replaced my service batteries with some new and larger AGM batteries.
The original 2 batteries had been descibed to me as 95Ah each when I bought the boat 3 or 4 years ago. When removed the desciption on them was 85Ah/95Ah Solar. Does anybody know what the meaning of the 95Ah Solar is as surely a battery has a certain capacity regardless of whether or not solar cells are topping it up (which they are)

 

[tobble]

your hunch, that here the word solar is meaningless, is correct.

However there are other factors which affect the capacity of a battery. the capacity is usually stated in Ah at the '20 hour rate' which means if you have 80 Ah you can draw 4 Amps for 20 hours after which the battery will be dead flat. but if you draw more amps, you will run it down disporportionately faster, i.e. reduce the effective capacity, and vice-versa. Also batterys aren't perfectly efficient (you get out about 80% of what you put in) and the way in which it is charged (how many volts are applied compared to the state of charge time, temp) has an effect on this - trying to push to many amps in will just generate heat, wasting energy and potentially damaging the batt.

There's never a simple answer is there....

 

[2nd_apprentice]

Now that you brought that one up...
Mine reads:
Battery Capacity [Ah] 125
Battery Capacity 5h (Ah) 100

So that's 6.25 Amps for 20 hours or 20 for 5 hours?
Doesn't seem to make much sense but then I'm mathematically challenged

/forums/images/graemlins/confused.gif

 

[tobble]

yes, that makes sense. Trust me, I'm a... trust worthy person...

If you are drawing a higher current (in your case 20 rather than 6.25), the battery is less 'efficient' so you dont get as many Ah (less energy) out in total...

If you pulled out say 2 amps until it was dead flat it might have a capacity of say 140Ah, and would take 70 hours...

any clearer?!

 

[2nd_apprentice]

Hmm rule of three doesn't work here I take.
So is there any way of calculating how long the battery will last when I know how many amps I'm drawing?

 

[tobble]

have a look here. (I've been doing some more reading!)
I worked out your battery's Peukert's number at about 1.2, and its theoretical capacity to be 177 Ah (that's the same as capacity at 1 Amp). You can use these numbers to work out what you were asking based on T=C/I^n, but you'd have to know the state of charge, which would envolve keeping a running tally of how many amps you took ou for how long; possible but not easy just buy a nasa BM1!

 

[2nd_apprentice]

I thought of getting he BM1 but found it not to be a top priority at the moment (read: budget). Thanks a lot though for your explanation!

 

[William_H]

Back to the Solar rating. A battery is usually charged with a constant voltage relying on the current to diminish as the charge state rises. This voltage is usually chosen to avert overcharge when constantly being applied. (charged)

A simple solar panel without regulator charges a battery with a high potential voltage and usually a small current such that no harm is done with full time charging.

It is conceivable therefor that they consider it can be more thoroughly charged or have a greater capacity when charged with small current non voltage regulated than is available with voltage regulated charge.

Note I am disregarding stepped or stage charging which may provide a more total charge than simple voltage regulated charging. Much here depending on the float voltage.
But then AH rating is simply a way of describing the size of a battery and at best is only a vague indication of actual capacity depending on condition of the battery and more to the point the degree you are willing to discharge it.
Just speculating here as to the "solar rating" olewill

 

[tobble]

Sorry I have a tendency to over anayse things... you're quite right that the age/condition of the battery will play a large part. I don't know exactly about this, but I would guess that one symptom of the battery getting tired would be an increase in Peukert's number andreduction in Pukert's capacity, but it may be that as you get sulphation of the plates or other damage it deviates from the theoretical behaviour.

with regard to your coments on the solar 'optimised' battery, that might in theory be possible, but I've never seen one and would guess that if it's been stuck in a boat it's not going to be anything particularly special. Interesting concept though, and I'd be happy/intrigued to be proved wrong!

 

[gavina]

The design and capacity of a lead acid battery depends on its application. For turning over an engine for example high cranking power may be required and this is achieved by using more plates to give increased active surface area. Where high discharge performance is not required the same amount of active plate material could be used on a lower number of (thicker) plates. If the high cranking power is not required then the second design could give longer life as the thicker plates will not be damaged so quickly by the corrosion that occurs during the life of the battery. The best batteries for the domestic applications are usually heavy because they are using thick plates – generally for a given design the heavier the better!
The ‘capacity’ of batteries generally increases a little during early use and then the falls away depending on the way in which the battery is cycled. For long life avoid deep discharging and never leave the battery in a discharged state - overcharging can also be a problem as the gas recombination that takes place in modern sealed designs is not 100% efficient and eventually the battery electrolyte could dry out. With a good modern charger however this should not be a problem - the Sonnenschein Dryfit batteries on our yacht were still operating well after ten years of life mostly on charge in the marina.
I must declare an interest here as we sell Sonnenschien batteries through our business.