Battery capacity

[tony t]

Hi,

I am used to dealing with batteries with their values expressed as say 12 volt 200AH.

Battery capable of suppling 200Amps for one hour or 1 amp for 200 hrs or any other combination.

I have been looking at rolls batteries for a new pv system, either the 4000 or 5000 series but im not sure about battery capacity.

Can anyone assist.

Best Regards

Tony

[stephent]

Go here...

http://www.rollsbattery.com/

click on "site map" on the left side. and then under solar click on specifications.

[SamoaPower]

"Battery capable of suppling 200Amps for one hour or 1 amp for 200 hrs or any other combination."

Unfortunately, it's not quite as simple as this. The actual capacity depends on the discharge rate over a certain time period with the most common time period being 20 hours. Your 200Ah battery at a 20 hour rate should supply an average current of 10A for 20 hours. If you discharge at a higher rate, the actual capacity will decrease and the opposite is also true.

Also, keep in mind that the depth of discharge has a large bearing on the number of charge-discharge cycles you will receive from a given battery, thus the lifetime. Some say a limit of 50% discharge depth but I prefer to use 25% for a longer life.

A piece of advice - keep them fully charged as often as possible. Chronic undercharge is the great battery killer.

[radiantboy]

SamoaPower, you said,"Some say a limit of 50% discharge depth but I prefer to use 25% for a longer life."

I've always heard that it would be best to limit one's discharge to 20%, and even better to a mere 10% (approximately 12.4 and 12.5 volts respectively).  Of course, I know this depends on the application.  An electric car, for example, obviously wouldn't get very far at all on a mere 10% discharge.  And I guess a sealed battery would be more sensitive to deeper discharges  than a wet one.

But one thing I've always been curious about, but have never asked, is this: When they say a 10% discharge (or about 12.5 volts), do they mean that one's voltmeter would read

12.5 when it is still under load?  For example, if I'm using my battery to power some stuff, and it reads 12.42 volts, but immediately jumps up to say, 12.68 volts when I

disconnect the load(s) (by turning off the inverter), have I discharged my battery to almost 20% (12.4) or to less than 10%?

[SamoaPower]

Sorry for the delay - I just now noticed your comment.

All references to remaining battery capacity by terminal voltage measurement is the resting voltage with no charge or discharge and preferably with a waiting period before measurement. This is why using voltage as a capacity indicator is such a pain and less than precise.

Much better is an Amp-hour meter that keeps track of energy both in and out of the battery continously.

After an initial full charge, the one I use resets the read-out to zero upon the start of discharge, so the subsequent numbers are negative and increase with continuing discharge. When the net current is positive, indicating charging, the numbers decrease toward zero and if continued, go positive and increasing, indicating overcharge. Actually, you want to see some overcharge since the battery charge efficiency is not 100%.

The result is a continous indicator of where you stand on capacity. Please note that this should be refered to the ACTUAL capacity, not necessarily the new rated capacity. A timed discharge test should be done periodically to determine the actual capacity or you could be kidding yourself as to the percent of discharge.

[radiantboy]

Hello SomoaPower.  This is even more of a delayed message, as I have been away for some time from the board.



You stated "After an initial full charge, the one I use resets the read-out to zero upon the start of discharge, so the subsequent numbers are negative and increase with continuing discharge."



What brand and model number is your amp-hour meter?  I like that set up.  I have never used an amp-hour meter, but I have perused a few models online.  



One other question: is the model you use compatible with either sealed AGM batteries or wet cell batteries?  Or is it just for the latter?



Thanks!