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Old 03-13-2015, 07:03 PM   #1
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How long will batteries last?

if you have fully charged batteries (12.8v) and they are rated at 1450 ah and are using 100 amps according to your amp meter. how long will the batteries last until they hit 50 percent level? 7.25 hours? thanks Tim
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Old 03-13-2015, 07:20 PM   #2
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If you really had 1450 AH batteries (I'm pretty sure you don't), they would probably last about 4-5 hours at that rate. That because the AH rating is normally quoted at the industry-standard "20 hour rate", which means an amp load that lets the batteries run for 20 hours before "dead". To get a 1450 AH rating, that would be around 70A continuous for 20 hours. At 100A, the rate of discharge would be much faster, so I'm guessing maybe 5 hours to 50% SOC.

The bottom line is that the discharge rate isn't linear (Puckert's Law). You might expect that doubling the amp draw cuts the time in half, but it actually cuts it something like 60-70%, depending on a bunch of factors.

To get 1450 AH from a lead-acid battery, the battery (or set of batteries) would weigh about 1000 lbs and require a humongous amount of space. Are you sure that 1450 isn't the CCA rating?
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Old 03-13-2015, 08:25 PM   #3
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thanks for the answer. they weigh 1050 pounds. yes 2 in the battery tray, and two 12 ga steel boxes under mh left side of driveshaft, approx. under the table. 4 batteries in each box. they are 12" off the ground, same height as steps. i will post pictures tomorrow, but the boxes are sealed and have a shield in front so not sure how much will be seen.
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Old 03-13-2015, 08:35 PM   #4
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Quote:
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thanks for the answer. they weigh 1050 pounds. yes 2 in the battery tray, and two 12 ga steel boxes under mh left side of driveshaft, approx. under the table. 4 batteries in each box. they are 12" off the ground, same height as steps. i will post pictures tomorrow, but the boxes are sealed and have a shield in front so not sure how much will be seen.
i guess i should add the boxes sit on top of 3 pieces of 1 5/8 unistrut ( which will hold 1200 lbs. every 2') which are supported by 9 5/8" threaded rods( each will support 1000 lbs.) to another set of unistrut which is attached to the frame.
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Old 03-13-2015, 10:04 PM   #5
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Okay...this is stupid simple...sort of...

First you need to get a copy of the manufacturer data sheets for each battery type.

Next we will assume your 100 amps is being drawn equally on each "string" of batteries (26 volt pairs are each one string)(each 12 volt battery also a string)

Say you have 2 strings of batteries in parallel and 100 amps total load then you have 50 amps per string load.

Now look in your data sheet for a line or column relating to 50 amp loading and you need to round up to closest line.

If you have 50 and choice is 45 and 55 you select 55.

Somewhere on that line or column it will indicate the rate or time for that load.

Pay attention to the end voltage as it can be from 1.65 vpc (Volts Per Cell) to 1.9 vpc.

Confused?

Look close at most storage batteries (not starting) and you may see something like 80 amp hr capacity @8hr.

This means if you place a 10 amp load on the battery it will maintain a voltage above 1.75 vpc for 8 hours.

The 1.75 vpc is the "standard" end point most commonly used but we used 1.8 vpc to allow for some error in adjustments for alarm and control plus to leave room for battery health.

Back to yours...

When you find the place where your load per string resides you then select your end point voltage to determine your designed actual capacity.

Divide the capacity by the load and the result is your run time.

Simple...

Now it gets interesting...

What if you have different battery types installed?

The smaller ones will have less capacity but they will not load down the big ones.

They will all play together well but the run time calculations can get complicated and they still may not be real close.

My software tools would calculate cell site batteries to within minutes but that is a whole different animal.

For mixed plant you could calculate load per string based on battery size.

If a 100 amp battery and 300 amp battery in parallel then with the 100 amp load place 25 amps on the 100 amp battery and 75 amps on the 300 amp battery and see how the hour rates line up.

This is enough to confuse many but just read it twice then review the data sheets and read the operation instructions and it will make sense.

Forget about only using 50% of the charge...

The calculations above are for full discharge of the battery's installed capacity.

If you only wish to discharge 50% then take the calculated time and divide by 2.
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Old 03-13-2015, 10:28 PM   #6
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Another bit for calculating...

As the battery discharges the voltage drops.

Resistive loads will have decreased amps or watts but regulated loads will have increased loads due to the constant output of the regulators.

Converting to watts solves this problem somewhat.

First tske your load in amps and convert to watts.

Multiply load in amp by measured voltage in volts.

Say 100 amps at 12.6 volts will have 1260 watts.

Now determine your bottom endpoint voltage.

Subtract that voltage from your starting voltage and divide that by 2.

Subtract that from your starting voltage.

Now convert your watts back to amps at that voltage by dividing the 1260 watts by whatever that voltage was.

This is now the load in amps you use to calculate your run time.

You basically take the middle of your discharge voltage range to then determine the average discharge amperage to then calculate the run times.

It will be plenty close enough...

And this is considering your load is constant.

If you turn things off then the run time can be called "minimum run time of" xxx
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Old 03-14-2015, 01:46 AM   #7
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At the risk of be scolded by TQ60, who knows a lot more then I do, here is a simplify formula. I know it won't be as accurate as TQ60's way but it will get you in the ball park. Assuming the 100 Amps is DC Amps,
100 DC Amps X 6 Hrs Runtime X 1.2 Inefficiency Rating = 720 Amp - Hours.

The best way to monitor you batteries and useage is to have a TriMetrics battery monitor.
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Old 03-14-2015, 01:56 AM   #8
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Would not scold you for what you have here.

You just calculated how many amp hours it may consume in 6 hours and not sure about the 1.2 part.

Calculating amp hours consumed is easy.

Time times load.

Determining capacity of the battery is the tricky part that even the engineers have issue with until they learn how to read the data sheet.

A couple lives ago there were many a few folks who got hollered at for not getting a generator deployed in time when the batteries were to have longer runs...engineers yelling were basing times on battery label of 125 ah per string (4) but the actual design based on loading was actually only about 65 ah so 4 hours becomes less than 2.

Lots of folks make that mistake and then blame the batteries when they are doing exactly what they are designed to do.

The fun part is doing the evaluation to determine a run time of x time then test it to see if you were close.

If new it sets the benchmark for load test later and if old it still creates a benchmark and you can determine what the general condition of the batteries are by how they compare to the data sheet.

If one is testing for condition one should only test with one type of battery connected if in a mixed plant so they remove the guessing.
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Old 03-14-2015, 02:22 AM   #9
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What is the source for your 1.2 ?

Some folks use a term "constant" or "correction factor" or my favorite..."fudge factor" to explain something added to correct for some error someplace or for naming something needed but unknown.

Temperature correction factor is a known variable used in the charging algorithm so no issue there.

Multiplying a result by .8 because the result seems to high so we use the 0.8 as a correction is bunk.

Batteries output current and you can add a correction (guess) to accommodate dropping voltage or like I did selected the center voltage to create an average.

In my past lives we had constant power loads so results were consistant.

In an rv where no two are alike all bets are off.

Maybe the 1.2 works for that...
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Old 03-14-2015, 02:35 PM   #10
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Thank you for the information guys. here are some pictures of batteries, battery boxes, and monitors I use. the one picture is the shield next to one of the battery boxes.
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Old 03-15-2015, 12:23 AM   #11
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The 1.2 inefficiency rating was in the formula in the manual of my Tripp-Lite inverter for estimating Amp Hr usage and battery recharge time. They didn't explain the 1.2 but I assumed it was for internal losses in the batteries. As you know, actual amp-hour capacities are less when batteries are discharged at faster rates. I probably should have stayed out of the conversation. The formula I posted would get you in the ball park, your's would get you to home base.
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Old 03-15-2015, 12:49 AM   #12
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Quote:
Originally Posted by tlaffourtit View Post
if you have fully charged batteries (12.8v) and they are rated at 1450 ah and are using 100 amps according to your amp meter. how long will the batteries last until they hit 50 percent level? 7.25 hours? thanks Tim
Was the 100 amp question actual usage or just theoretical? Just curious of the need for that size battery bank on an RV. Thanks for the pictures.
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Old 03-15-2015, 01:25 AM   #13
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Dont know how long your wheelbase is, but you better be carefull going over some raised railroad track intersections.
I cross one yearly that would take off my Aquahot exhaust pipe, if I couldn't raise my entire coach with the air leveling system.
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Old 03-15-2015, 11:02 AM   #14
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for the last question. how many watts (solar panels) would it take to recharge the same batteries? assuming the best conditions. presently I have a mppt-60 controller. I am mounting 3 300 watt panels. I will have room for 2 more if needed knowing another controller would be needed also.
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