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Old 27-07-2013, 17:24   #16
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Re: Battery Capacity Access Problem

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Originally Posted by brian.wagner View Post
So do I pour the beer into the batteries? - wait, that would be a waste of good beer.
Cool

But, Brian, can you just tell us. Was the 12.29 under load or not?
i.e did you have the compressor running and the other electrics running or not? Its vital to know.
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Old 27-07-2013, 22:04   #17
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Re: Battery Capacity Access Problem

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Cool

But, Brian, can you just tell us. Was the 12.29 under load or not?
i.e did you have the compressor running and the other electrics running or not? Its vital to know.
Sorry - Yes the 12.29 was under the same load.
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Old 27-07-2013, 22:05   #18
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Re: Battery Capacity Access Problem

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Originally Posted by smac999 View Post
do a c20 test at the dock. best and only way to fully test battery cap.
Can you explain what a c20 test is and how to perform one?
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Old 28-07-2013, 01:52   #19
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Re: Battery Capacity Access Problem

Hi again Brian,

If your batteries were still under load with a reading of 12.29 after that amount of drawdown I would say you do not have a problem.

I find that the fridge is now the biggest power user on the boat and I am in the process of converting mine from continuous cycling to eutectic with water cooled condenser - I spend a lot of time in the tropics and the air cooled condenser is not very effective and puts a lot of heat into the boat.

I tried out the eutectic thing by putting a false floor in my fridge box then with about 3" of water under it, which turned into a big ice block, I could hold over for 3 - 4 days without cycling.

I then installed a 7 day timer so that the fridge did not turn on until about 10am in the morning and then turned off about 4pm if the thermostat did not turn it off sooner. The refrigerator was also connected to the load terminal on one of my solar regulators and the on voltage was set for about 12.5 volts.

Anyway the system worked quiet well and the refrigerator pretty well always ran from current drawn from the solar panels and not the batteries. My oldest Trojan lasted 7 years and it had been worked pretty hard during it's first couple of years in the boat.

The only downside was that the compressor ran very hot and they have a limited duty cycle. The re-installation now has a cooling fan blowing from under the compressor and hopefully it will now run cooler - if this does not do the job I will put heat pipes on it.

I have 4 banks of 2 X 6V batteries 3 banks of which are Trojan. I run a zapper which is supposed to prevent sulphating and which I rotate around the batteries about a month at a time - don't know if it works but it makes me feel better about things anyway.

I have tried the drawdown test thing and have come to the conclusion that each bank needs to be treated as an individual and now have two smart regulators one of which I will use as a test regulator - it records current in and out and voltage and I can download the monthly data to a spreadsheet. The battery isolation and recollection is carried out with a sort of cross bar assigner which I have built from 1/2" by 3/32 brass bar, connections are made or broken by screwing in or out a brass screw.

I am not sure if any of this will be worth it but anyway I am engaging in firstly trying minimize the daily voltage drawdown of the batteries therefor extending their lives, and secondly, trying to gain an understanding of what is happening regarding storage efficiency with each bank and hopefully I will be able to optimize their useful lives.

And in any event it all helps fill my days.
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Old 28-07-2013, 05:12   #20
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Re: Battery Capacity Access Problem

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Originally Posted by brian.wagner View Post
Can you explain what a c20 test is and how to perform one?
A “C20" battery rating means they are designed to provide 1/20th of the stated AH capacity for 20 hours to 10.5 volts.

The C20 test is a common capacity test; where the battery is discharged at 5% (1/20Th) of its rated capacity (e.g. a 100Ah battery is discharged at a rate of 5A) until its voltage falls to 10.5V.
The capacity is then defined as the 5% rate of discharge multiplied by the time, in hours, it took to get to 10.5V
If it took only 17 hours to discharge to 10.5V , the capacity in this example is 5 x 17 = 85AH, or 85% of rated capacity.

Charles-Augustin de Coulomb's C-Rate for Batteries
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Old 28-07-2013, 08:30   #21
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Re: Battery Capacity Access Problem

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Originally Posted by brian.wagner View Post
Sorry - Yes the 12.29 was under the same load.

The you have agreat battery bank and are doing VERY well!

8 hours of that load and you are down to 12.29 UNDER load is great. Without that load you would be 12.40 or better.

Excellent!

I run mine down to 12.15 under load. thats about 12.24 without load, but my load is very light compared to yours.

So you can go down to about 12.15 before you are at 50% (12.24)


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Old 28-07-2013, 09:04   #22
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Re: Battery Capacity Access Problem

I used to think we could cruise more economically under sail until I realized that we needed to run the engine most of the time anyway just to keep up with electrical demand. The fridge is the biggest culprit but we also ran running lights, radar, laptop, radios, instruments, autopilot, cabin lights, cell phone chargers, handheld vhf charger, Electra-san, and probably a few more things that I don't recall right now. It all adds up. Of course we had a 140watt solar panel that helped some, a few led lights that helped some, a well insulated fridge, etc. but the fact is that there's a lot of demand and limited capacity. Conservation helps some but there's no free lunch as they say.

Bigger batteries require bigger charging systems, there's no way around that. We settled for 220 amp hours of capacity and ended up running the engine between 1 and 2 hours a day (at anchor) plus whatever the solar panel contributed. Doing it again, I would add more solar panel capacity to try to reduce the engine run time. Some people love their gensets but we preferred the main engine and a nice big Balfour Alternator.

I had a friend with a 60ft trawler that used forklift batteries and had a 80hp diesel charger/genset but then he ran everything off the batteries including an electric range in the galley and A/C ... great committee boat.

Our consumption deficit averaged maybe 80-100 amp hours. By deficit, I mean that's what we had to replace by running the engine. Also we were on Gel Cells which have a higher acceptance rate than wet cells so even though charging voltage was set to 14.1VDC the batteries, at least initially, would take up to about 60amps.

It was a good system but next time I'd add a couple more solar panels.
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Old 28-07-2013, 22:16   #23
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Re: Battery Capacity Access Problem

Hi again,

Along with increasing the amp hour capacity of the battery banks I have also increased the wattage ratings of the solar panels and now have 2 x 120 and 2 x 130 watt panels. I am tending to feel that if I can implement the eutectic fridge and daylight only running of it thing I may be on top of the electrical storage thing at last.

I also have a wind generator but find it pretty useless in the tropics.

I carry an inverter type welder and have occasionally used it to top up the house batteries using an AC genset. I have also used it to emergency start the main engine however this usually blows the excitation capacitor.
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Old 29-07-2013, 16:51   #24
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Re: Battery Capacity Access Problem

Thanks again guys, but I still just must be pretty dense because I can't get around the idea that if I have a 1200 amp capacity bank I should be able to take more than 138 amps out of it before it needs to be recharged. There should be 600 useable amps right? If all I'm gonna get is 11% whats the point of having the 1200 amp hour bank and what happened to the other 39%?

I realize that there is merit to what some of you are saying about the 12.29 being under load and that without the load the voltage would be higher, maybe in the 12.40 range.

Still seems screwy to me and Trojan so far hasn't been very helpful. Once they saw that my SG's were good, I haven't heard from them again.
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Old 29-07-2013, 17:00   #25
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Re: Battery Capacity Access Problem

Good point, and as I said earlier, what's the point of the big bank even if you could pull out 600 amps..... how do you put all that back in?
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Old 29-07-2013, 17:01   #26
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Re: Battery Capacity Access Problem

Specific gravity is the only accurate way to determine state of charge. Your specific gravity was right when you started your test. What was it when you finished? What was your resting voltage not with a draw?
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Old 29-07-2013, 17:35   #27
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Re: Battery Capacity Access Problem

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Still seems screwy to me and Trojan so far hasn't been very helpful. Once they saw that my SG's were good, I haven't heard from them again.

You still haven't told us what the SG was after the 135AH out. And if you turn everything off and wait 15 minutes what does the voltage go back up to?

But I believe you need to run the batteries 500-600AH out and then recharge and try again. It took 2 years for my batteries to start acting close to their AH rating.

I saw a curve of cycles verse AH on a site (think it was US battery) when they showed that it took qute a few cycles till the batteries could perform to rated AH capacity.
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Old 29-07-2013, 17:39   #28
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Re: Battery Capacity Access Problem

As others have said, reading voltage on an active battery bank will always show a lower voltage then whats stored in the battery. Partly this is due to wire losses. But also due to voltage sump, while the battery is under load.

The more amps coming out the bigger the sump. I've found on my 400 amp wet cell bank that I'll have at least 0.2V more in the batteries then the volt meter indicates under light load. Nature of the beast.

Think your batteries are fine and running more or less normal.
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Old 29-07-2013, 19:55   #29
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Re: Battery Capacity Access Problem

brian-
A little knowledge is a dangerous thing.
"Thanks again guys, but I still just must be pretty dense because I can't get around the idea that if I have a 1200 amp capacity bank I should be able to take more than 138 amps out of it before it needs to be recharged."

You've only got bits and pieces of the whole picture there. Like not knowing a wet lead battery needs to stand, to equalize charges, to physically circulate and equalize the electrolyte, to burn off float charges, etc.

Which is why your 13.5 volts was bogus (float charge) and you need to figure from 12.6 or 12.7 volts to 11.6 volts. Or down to 12.1 for a 50% discharge.

So yes, with a 600AH capacity to half charge, you should be able to pull 300AH out of the bank while just pulling it down 25%, i.e. from 12.7 to 12.55 volts. But again--that's voltage measured with no load (or a very small load) and the battery "rested" to get an accurate reading. If your load was 5-10 amps...I'd call that probably insignificant on a bank that size but your best bet is to make measurements with a rested battery and no load.

Leave a good voltmeter connected, with no other load, and see how the battery voltage changes over the course of an hour. You'll be surprised.

Then there's also the question of how you're measuring voltage. Is your voltmeter calibrated, and reliably reading tenths of a volt? Many can't, and that can also throw you.

You can use a 55W or similar automobile high beam to perform discharge testing on your battery bank for that "c20" type test, if you've got no convenient loads on the boat.
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Old 29-07-2013, 20:23   #30
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Re: Battery Capacity Access Problem

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Originally Posted by brian.wagner View Post
Thanks again guys, but I still just must be pretty dense because I can't get around the idea that if I have a 1200 amp capacity bank I should be able to take more than 138 amps out of it before it needs to be recharged. There should be 600 useable amps right?
Yes, that's right. But you haven't measured the charge level of your bank so you don't know what you have used. You have a huge bank so a C20 discharge is 1200/20=60A. That's about 750 watts continuously for 10 hours. After you do that then you have to let the batteries rest for several hours and check the voltage. It will probably be between 12.1 and 12.2 which is about 50% charge. Measuring the voltage with even 1A load will not give an accurate indication of charge. The resting period is very important to get an idea of state of charge using voltage.

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If all I'm gonna get is 11% whats the point of having the 1200 amp hour bank and what happened to the other 39%?
Probably nothing happened to the other 39% because it's still in there.

In my view there is little point to a 1200AH bank unless you have at least C/10 an preferably C/5 charging capacity. For that you need minimum 120A charge source and preferably 200A charge source. That means multiple alternators, huge wire and big fuses. What good is a battery bank that lasts for 2-3 days and takes another day to recharge it? Answer: Not much.

Another point that was made earlier that is worth repeating is about recharge times. If the recharge time (12-16 hours) is longer than you are willing to wait there is a strong chance the bank will spend little time above 80% charge. This destroys almost as many batteries as over charging does.

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Still seems screwy to me and Trojan so far hasn't been very helpful. Once they saw that my SG's were good, I haven't heard from them again.
If the SG's are good then almost certainly the batteries are working correctly. It's hard to understand what you want Trojan to do?
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