House battery question

[captain ron]

Sean, Would you please explain amp hrs/20 hr rate and how to figure your amp hrs. I'm going to need house batteries very soon and I want to know what I'm looking for when I buy them.

[Jerry Liebler]

Sean,
    I think I may have to get 4 of these.
http://www.hardysolar.com/shop/product.php?printable=Y&productid=17530&cat=47&page=1&js=n
At  $397 for 390 AH they sure beat your prices.
Regards
Jerry 4107 1120

[Jerry Liebler]

Sean,
   Sorry I didn't pay attention to the 12 volt so the L16s do cost quite a bit more Darn.
Regards
Jerry 4107 1120

[Jerry Liebler]

Captain Ron,
     I'll volunteer to answer your questions till Sean gets to it.  To estimate how many amp hours you'll need you'll need to determine how many watts of load you'll be running and how long you want to run them.  If you are running any of the loads through an inverter you'll need to add about 20 % to that portion to account for inverter efficiency.  Then you divide the total watt hours by your house battery voltage (either 12 or 24). The result of this calculation is amp hours you'll be taking from the battery.  For reasonable battery life you'll want a house battery bank with a capacity of 2 times that number, because for good battery life only 1/2 of rated capacity should be used. 

Batteries by their nature have higher capacity if they are discharged more slowly.  Because of this fact it matters how they are rated by the manufacturer.  In order to compare different batteries accurately the time to complete the discharge needs to be known.  The 20 hour discharge rate is available from most deep cycle battery manufacturers though some will feature higher capacity numbers but for longer discharge times.  For example the same battery may have a 390 AH rating at a 20 hr discharge rate and a 420AH rating at 100 hr rate. 
Regards
Jerry 4107 1120

[Sean]

Ron,

This link:

http://www.windsun.com/Batteries/Battery_FAQ.htm#Amp-Hour%20Capacity

which is one of the pages from one of the two excellent references I posted in my first reply here, explains amp-hours pretty well.

To add to it, and to what Jerry said (and as emphasized by Jerry's slip regarding the 6-volt batteries), the concept of Amp-Hours is useful for comparison (because it is commonly published), but only at a specified voltage.  In reality, the energy capacity of a battery is better expressed in watt-hours, which is a measure that takes the voltage into account.  In other words, a 200AH 12-volt battery stores twice as much energy as a 200AH 6-volt battery.  One place where this effect comes into play is when using 12-volt batteries in a 24-volt system.  So while I just bought eight batteries, each with a 230AH rating, my total system capacity will be 920AH because it will be running at 24 volts, and that is what I have to use when programming my battery monitor.

Jerry is right about how to figure your needs.  But I understood your question to mean "how big are my existing batteries?"  Unfortunately, there is no easy way to figure this.  If you know the manufacturer, model, and size, you can probably look up the rated capacity.  The only other way to do it is with a load-tester, which is a long process that requires the batteries to be out of the circuit.  Commercial load testers are available, for a price, but you can make your own from a bank of light bulbs and a couple of meters.

What you might want to do instead is to get an energy monitor, which, really, you ought to have any way.  There are several good ones on the market, such as the Xantrex Link-10 and the Tri-Metric.  These meters actually "count" amp-hours going into and coming out of the batteries.  In order to work properly, in terms of their "gas gauge" and "hours remaining" displays, you need to know the total capacity of your bank.  But there is a trick -- set them to some capacity bigger than you have (by, for example, looking at published capacities of similar batteries, then adding 10-20%).  Fully charge your batteries, and reset the meter.  Then run your coach until the battery voltage drops to the level where the meter reports "low battery" (which it will do based on voltage, irrespective of what you set the capacity to).  Then you can read, from the meter, the total number of amp-hours used, and this will give you an idea of the working capacity of your bank.

HTH.

-Sean

[Jerry Liebler]

I did a bit of shopping, on the www, for AGM batteries.  It looks like the Toyo 6GFM200A is the most economical.  210 AH @ 20 hr. & Sunelec.com is asking $230.  It looks like it's about a 4D size.  19.53"x9.84"x8.15"h.   If I stood them on edge I could almost fit 4 where I have 4 L16s now, fortunately the L16s are still working fine.
Regards
Jerry 4107 1120   

[Lee Bradley]

Sean,
Are those four batteries a complete change out of your house batteries or just part of them?

I am just trying to get an idea of your total pounds of batteries.

Never mind, I misread the posts.

Lee

[Sean]

I'm changing all eight of my house batteries.

I had Xantrex batteries, which we got cheap when Xantrex discontinued their rolling-cart inverter-battery system, can't remember what they called the product.  They had loads of these ~8D size 215AH AGM batteries with weird terminal arrangements, and they dumped them.

We've used them hard for three years, they were on Infinity's shelf for a year, and they were probably on Xantrex's shelf for two years before that.  For the price (a little over $100 apiece, IIRC), we definitely got good value out of them.

We came back here to have them replaced, because we knew that the weird terminal arrangement meant that new cables and tie-downs would have to be fabricated.  It's also turned out that the size was just enough different that we've had to modify the racking slightly as well.

The replacement Trojans are 155# each, so my new house bank weighs 1,240 pounds.  Cabling, racks, and tie-downs probably add another hundred pounds.  I'll try to post some photos of the installation on the blog when they are done.

Jerry -- the dimensions, specs, and even the terminal arrangement on those Toyos look almost identical to the Xantrex units we just took out.  (Xantrex, of course, simply OEMed the batteries from someone, maybe Toyo.)  And the price is right.

-Sean
http://ourodyssey.blogspot.com

[Sean]

I found an on-line copy of the specs on my old Xantrex batteries, here:

http://www.partsonsale.com/xantrexbattery.pdf

After comparing them, I have to guess that the Toyo's you found are the same battery.  (I note Toyo claims a 210AH 20-hr rate, whereas Xantrex claimed 215AH.)  The dimensions, terminals, and power curves are identical.

So, probably, I could have just bought eight of those and had drop-in replacements.  Three years ago, I had done considerable research to try to find out who was making the batteries for Xantrex and how to get direct replacements, and came up empty-handed.  Oh well.

-Sean
http://ourodyssey.blogspot.com

[Jerry Liebler]

Sean,
     Dallas asked me how to compute the Peukert exponent, I didn't know but I do now.  Peukert defined battery capacity as the hours a battery could supply one amp and discovered that that discharge time  at any other current was the product of time, in hours,  and the actual current raised to the ' Peukert' exponent.  If one has two discharge conditions for any battery one can compute both the 'Peukert capacity' and the 'Peukert exponent'.  The ideal battery would have a 'Peukert exponent of 1, typical lead acid batteries are from 1.05 to 1.3.  The lowest exponent I've seen (1.05) is An MK AGM in size 8D.

Who cares?  Well if your usual loads take more than 20 hours to discharge then finding a battery with a higher exponent will mean you actually get more than you paid for, in capacity.  While the battery with the lowest exponent will most closely approach the predicted performance when discharge currents exceed the 20 hr rate.  BTW AGMs  generally have lower exponents than flooded batteries.  I calculate that your new Trojan AGMs have an exponent of 1.22
Regards
Jerry 4107 1120

[Sean]

Jerry,

I was trying not to open the whole Peukert can of worms here :-)

The Peukert exponent is one of the parameters that also needs to be programmed into your battery monitor in order to get an accurate energy-remaining indicator.  My Link-10 has a default setting for this, IIRC, of 1.15.

One of my pet peeves is that battery manufacturers should, but usually do not, publish the Peukert exponent for their batteries.

An on-line Peukert calculator (along with some other good battery info) can be found here:
http://www.geocities.com/CapeCanaveral/lab/8679/battery.html

A lengthy technical explanation of Peukert's equation, and how it is often misused, is here:
http://www.smartgauge.co.uk/peukert2.html

And, yes, the Peukert Exponent of my new Trojans is about 1.221.  But, I can go around telling people that I have some of the largest Trojans around, or, alternatively, I use a bigger Trojan than so-and-so.  FWIW.

-Sean
http://ourodyssey.blogspot.com

[Dallas]

Thanks Jerry,

I have been looking for the Puekert exponent for my batteries (Walmart Marine) with an Ah of 125. Unfortunately, I've had no luck. They are built by Johnson controls who also make Optima, Diehard and many others.

To maybe make it a little easier for everyone to understand the importance of the Peukert Equation is to put it this way:

If you have a battery with a capacity of 100Ah that's what you can expect out of it under certain circumstances. The lower your current draw, the more actual useable amp hours there is.

Now if you add another battery to your first battery, it seems like you would have 200Ah. Well, not quite. What you may see is something on the order of 230Ah.

From my understanding, and it may be flawed, as usual, the reason for this is, instead of drawing say, 50A at the 20 hour rate, you are actually only drawing 25A per battery at the 20 hour rate. Since we know that the lower the current draw, the more Ah you get, you have effectively more than doubled you capacity by adding one battery.

There is a problem with Puekerts exponent as it is normally written, but I will leave that up to you dear reader to find.

For those interested here is the equation as copied form Smart gauge Electronics.com:

Iⁿ T = C

Where:

I = the discharge current in amps
T = the time in hours
C = the capacity of the battery in amp hours
ⁿ = Peukert's exponent for that particular battery type

More on this at:

http://www.smartgauge.co.uk/peukert2.html

And another Puekert battery life calculator:

http://www.csgnetwork.com/batterylifecalc.html

Dallas

(Sean Posted before I got done typing)

[Jerry Liebler]

Dallas,
     The equation is fine BUT the capacity is the hours at 1 amp load NOT what is specified on most batteries.
That is the missuse Sean is referring to.  Be careful. 
Regards
Jerry 4107 1120

[Jerry Liebler]

Sean, 
     How does your battery monitor handle charging?  Is Peukert's equation appropriate to charging as well?
On my long list of someday projects is designing building a battery monitor.
Regards
Jerry 4107 1120

[Dallas]

Jerry,

Here is the Modified equation Smartgauge uses.

T = C/(I/(C/R))ⁿ X (R/C)

Where:
I = the discharge current
T = the time
C = capacity of the battery
ⁿ = Peukert's exponent for that particular battery type
R = the battery hour rating, i.e. 100 hour rating, 20 hour rating, 10 hour rating etc.

I tried it and it seems to be much closer to real life than the original.

Of course, that is using my cheapo Wally world batteries.