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Old 09-12-2012, 14:13   #1
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Battery Capacity

Last year on a thread here, I posted that I'd run a 1500W load on my 1100AH battery bank and that the inverter turned off after just 10 minutes when voltage dropped below 10.5V.

A year later... Same batteries, same load but having been attached to the dock for a season, owner having gotten a bit smarter(!) and having equalized the batteries just a couple of weeks ago, I got a better result... After 1 hour of the load (130A), the voltage started to fall and at 60 minutes I turned off the load when the voltage was 10.9V. Therefore better performance than last year. After recovering for 10 - 15 minutes, the voltage was back to 12.2V (Temperature was 65F)

I believe that 12.2V would indicate about 60% charge level, indicating total capacity at about only 350AH...

Question: Are these batteries shot? Is it a fair evaluation of AH capacity if one pulls at relatively high amperage compared to normal usage? Does the high load effectively lower the apparent AH capacity?

Appreciate your thoughts...
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Old 09-12-2012, 14:38   #2
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Re: Battery Capacity

Quote:
Originally Posted by Bill Balme View Post
Last year on a thread here, I posted that I'd run a 1500W load on my 1100AH battery bank and that the inverter turned off after just 10 minutes when voltage dropped below 10.5V.

A year later... Same batteries, same load but having been attached to the dock for a season, owner having gotten a bit smarter(!) and having equalized the batteries just a couple of weeks ago, I got a better result... After 1 hour of the load (130A), the voltage started to fall and at 60 minutes I turned off the load when the voltage was 10.9V. Therefore better performance than last year. After recovering for 10 - 15 minutes, the voltage was back to 12.2V (Temperature was 65F)

I believe that 12.2V would indicate about 60% charge level, indicating total capacity at about only 350AH...

Question: Are these batteries shot? Is it a fair evaluation of AH capacity if one pulls at relatively high amperage compared to normal usage? Does the high load effectively lower the apparent AH capacity?

Appreciate your thoughts...
Bill,

What is the Peukert # for your batteries...?
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Old 09-12-2012, 14:43   #3
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Re: Battery Capacity

I don't know it... they are Rolls 8D batteries (4).
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Old 09-12-2012, 15:35   #4
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Re: Battery Capacity

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I don't know it... they are Rolls 8D batteries (4).

If they are the HHG type the Peukert is 1.3. This is important for programming a battery monitor.

Also at a 130A load your actual Peukert corrected load on the battery is closer to 170A and your banks capcity at that draw is 850Ah...

Those batteries like to rest a good long time before you get to a "resting voltage". 15 minutes is a drop in the bucket for that bank and not likely very representative of the true SOC. Remember each 0.1V represents roughly 10% of your banks capacity? How accurate is your volt meter etc...

Or better stated each 0.1V = 110Ah's of capacity. This is why an accurate resting voltage is important. It can be pretty easy to be 0.3V off where you really are with volt meter errors or a lack of proper rest for the bank.. If you were 0.3V off, where the batteries really are, that could mean 330Ah's difference.....

The higher the Peukert number the worse the bank will handle high amp discharges. The lower the Peukert the less affected the bank will be.. However on sailboats we usually gain when we have a high Peukert numbers as our average house load/draw is often below the 20 hour rating not above it.

With Peukert any draw or load above the 20 hour rate will yield less total Ah's. Conversely any load less than the 20 hour rate results in more Ah capacity.

The 20 hour draw for your 1100Ah bank is 55A. So you should be able to get 1100Ah's out of your bank with a 55A load when drawing the bank to 10.5V at 80F...


For your bank with a Peukert of 1.3 -

130A = 850 Ah bank
10A = 1835Ah bank
55A = 1100Ah bank

This is where a properly programmed battery monitor can help.....
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Old 09-12-2012, 15:38   #5
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Re: Battery Capacity

Thanks for that - very helpful...
We're planning to head off next June leaving Newport, RI for Europe... want to make sure batteries are up to snuff!
Sounds like these batteries still have life to them...
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Old 09-12-2012, 16:15   #6
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Re: Battery Capacity

Who knows if the batteries are good or bad?

I have a battery meter, my buddy has a battery meter and still another buddy has a battery meter.

On a charged battery(fairly new), my battery meter(voltmeter), indicates 11.5 or so volts . . . battery's shot.

My one buddy's voltmeter indicates 12.7 volts . . . battery must be boiling.

My second buddy's voltmeter indicates 12.3 volts . . . battery appears to be okay.

So I charge the battery fully, then light a little light with a wire for a few seconds.

readings are something like 11.7, 12.8 or so(I remember it was pretty doggoned high) and about 12-1/2 volts.

I know the battery "should" be good and I know that my Guest charger took about 2-3 hours to go through all the charging modes before turning off, which should mean that at this time I should have a full charge of somewhere around 12.4 volts.

I even know I need to either wait a while or put some resistance on the battery before taking my readings.

But, after is all said and done, what I don't know is what if the voltmeter can't be relied upon?
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Old 09-12-2012, 16:23   #7
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To get a consistent usable SOC reading using a voltmeter requires the battery to be resting for about 6-8 hours. Resting means no change or discharge current. This is difficult to do on a boat.

So basically voltmeters are just nonsense when attempting to determine SOC.

Dave
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Old 09-12-2012, 16:37   #8
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Re: Battery Capacity

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To get a consistent usable SOC reading using a voltmeter requires the battery to be resting for about 6-8 hours. Resting means no change or discharge current. This is difficult to do on a boat.

So basically voltmeters are just nonsense when attempting to determine SOC.

Dave
I've got two flooded wet cells on my bench right now that came off an equalization five days ago and just went through a cycle then charge to full at an absorption voltage of 14.7V. They came off charge nearly 7 hours ago and as of about 30 minutes ago were still reading 13.15 and 13.22V respectively.

Fully charged for these batteries is about 12.72V. It takes them nearly 30 hours to fall back to 12.72V - 12.73V, at my barns current temp, which is about 65F... If the barn is warmer they drop voltage slightly faster and when colder, as some bilges are they drop voltage even slower.

When I have had Rolls batteries in my shop for conditioning I find some of them can take even longer to stabilize the voltage than most standard flooded batteries...
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Old 11-12-2012, 10:26   #9
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Re: Battery Capacity

This is not a very accurate way of evaluating your batteries. High load won’t lower the apparent AH capacity but since the load will be drawing the current at a higher rate, it’ll not last as long as expected. Heating issues also contribute to that. As indicated earlier, you need to give batteries a lot of rest before trying to measure amperage.
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Old 11-12-2012, 10:40   #10
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Re: Battery Capacity

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This is not a very accurate way of evaluating your batteries. High load wonít lower the apparent AH capacity but since the load will be drawing the current at a higher rate, itíll not last as long as expected. Heating issues also contribute to that. As indicated earlier, you need to give batteries a lot of rest before trying to measure amperage.
It is Peukerts Law... Unless you have a Peukert of 1.0 then your battery is affected by both high and low loads in the actual Ah capacity... All this means is that with a load above the 20 hour rating you will not get the 20 hour rating out of the battery and with a load below the 20 hour raring you will get more usable Ah's out of the battery..

So a 100Ah battery, at the 20 hour capacity rating, is rated at a 5A load for 20 hours before it hits 10.5V. If that battery has a Peukert # of 1.3 it looks like this.....

15A load for 20 hours to 10.5V and you now have a 72Ah's to use before the battery hits 10.5V.

2A load for 20 hours to 10.5V and you now have a 132Ah's to use before the battery hits 10.5V...

A lower Peukert # means less change under both high and low loads. Big traction batteries like the Rolls 2V cells with a Peukert of 1.5 last a very long time at loads below the 20 hour rating but a lot less long under loads above the 20 hour rating...
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Old 12-12-2012, 08:42   #11
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Re: Battery Capacity

Greetings and welcome aboard the CF, pavel85.

What Main Sail said (post #10).
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Old 12-12-2012, 09:05   #12
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Re: Battery Capacity

Keep in mind that for maximum battery life you should not be discharging below the 50% point so your 1100 AH battery is best for 550 AH loads before charging. Its something like for every 10% you regularly discharge below 50% you cut battery life in half. So your 10 year battery is only 5 years at 40%, 2.5 at 30%, 1.25 at 20%, 8 months at 10% and 4 months if fully discharging.
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Old 12-12-2012, 10:31   #13
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Re: Battery Capacity

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Keep in mind that for maximum battery life you should not be discharging below the 50% point so your 1100 AH battery is best for 550 AH loads before charging. Its something like for every 10% you regularly discharge below 50% you cut battery life in half. So your 10 year battery is only 5 years at 40%, 2.5 at 30%, 1.25 at 20%, 8 months at 10% and 4 months if fully discharging.
Wea should also not forget that while cruisng you will rarely get a bank back much above 85% SOC unless you pull into a marina or turn off loads and let the solar or wind work at it for 10-15+ hours.

So a 1000Ah bank is really a 500Ah bank at 100% SOC but at 85% SOC that 1000Ah bank really only has a usable capacity of 350Ah's before you hit 50% SOC..........
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Old 12-12-2012, 16:54   #14
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Re: Battery Capacity

...and to continue Maine Sail's post:
If you do not frequently fully charge a lead acid battery, the battery's cycle life will be dramatically shortened as it will be chronically undercharged leading to sulfation and electrolyte stratification.

In general: Batteries on boats do not die. They are murdered.
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Old 12-12-2012, 18:06   #15
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Re: Battery Capacity

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...and to continue Maine Sail's post:
If you do not frequently fully charge a lead acid battery, the battery's cycle life will be dramatically shortened as it will be chronically undercharged leading to sulfation and electrolyte stratification.

In general: Batteries on boats do not die. They are murdered.
Amen! I replace LOTS of murdered batteries every year. Some learn, and some spend money......
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