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Old 15-01-2019, 23:47   #1
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Need help how to make a power calculation

450ah batteries bank , 97% soc 12.6v with a small load of 1.5A draw .
If the draw was increased to 17A turning on all my equipment I would used on a over night sail
1... what would I expect to find my voltages to be after 5 hours used ,
with the load still at 17A .?
2....Also How would I do the calculation ?
Thanks
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Old 16-01-2019, 00:26   #2
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Re: Need help how to make a power calculation

I think you are over complicating the question.

Assuming you start with a fully charged battery 5hr * 17 amp = 85 amp-hr.

You can typically use around 50% of the rated amp-hr or about 225 amp-hr without risk to the battery...so it should be fine.

17amps is small enough that the peurket effect shouldn't have a big impact (if you were pulling 100amps, it would be a concern that needs to be accounted for).

Now if your batteries are near the end of life and you really don't have 450amp-hr anymore, it's anyone's guess.
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Old 16-01-2019, 00:44   #3
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Need help how to make a power calculation

Thank for the reply, maybe I can put it another way .
I discharge my batteries at the rate above and after around four hour with the load on the voltage was down to 12.2v the soc was 87% , and I am trying to work out why my Voltage was so low when i only use 85amps .
The voltage reading was with the load still on
Batteries only a few months old T125
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Old 16-01-2019, 01:25   #4
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Re: Need help how to make a power calculation

Quote:
Originally Posted by medsailor1 View Post
Thank for the reply, maybe I can put it another way .
I discharge my batteries at the rate above and after around four hour with the load on the voltage was down to 12.2v the soc was 87% , and I am trying to work out why my Voltage was so low when i only use 85amps .
The voltage reading was with the load still on
Batteries only a few months old T125
To get an accurate estimate of the state of charge via voltage, you need to stop drawing from them as that will throw off the voltage (as a measure of charge state). Really you need let them rest for a few hours to be really accurate but even a few minutes will get you a better estimate than while the draw is still going on.

Rule of thumb with no load, 12.6v is around 90% and 12.2v is 55%...if your measurements included a decent rest period, that's around 35% or a little over 150amp-hr. My guess is you measured the 12.2v with the load on which could easily explain the lower voltage.

Easiest is to get an amp-hr meter. Temperature, size of the load and few other things make using voltage difficult for accurate measurements of what you really used. It's a decent method to get a rough idea of the battery state but not to do any accurate measurements.
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Old 16-01-2019, 02:42   #5
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Re: Need help how to make a power calculation

Read this article by Perez published in the Aug/Sept. 1993 issue of Home Power. Remember that you would have to repeat it with your specific battery, because the charge and discharge curves depend on the type and make of the battery. But it should explain the principle of how to go about it.
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Old 16-01-2019, 05:18   #6
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Need help how to make a power calculation

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Originally Posted by chemist View Post
Read this article by Perez published in the Aug/Sept. 1993 issue of Home Power. Remember that you would have to repeat it with your specific battery, because the charge and discharge curves depend on the type and make of the battery. But it should explain the principle of how to go about it.


Hi link can't find the page .

I was asking the question because .i am puzzle when it's became unsafe to keep drawing power .
my Batteries only 4 months old . 4x T125 .

Here where I am confused , we are told that not let your batteries go below 12.1 v 50%soc ,
What does that mean when it comes to voltages ,
12.1v with a load on or without a load?
And if it means without a load , what a safe margin with a load on ?

So going back to my figure I give , I started with a fully charge batteries 100 % soc , some hours later while still drawing 17A and the SOC was say 80% but voltages showing 12.2v at that stage while there was still a load on ,
at that point do I turn everything off or can I continue using it ?
and if so to what voltage can I let it drop too before it's no longer safe to do so .
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Old 16-01-2019, 07:53   #7
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Re: Need help how to make a power calculation

12.1V without a load
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Old 16-01-2019, 09:51   #8
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Re: Need help how to make a power calculation

medsailor1, what do you see as the voltage when the batteries are full? 12.2V for 80% SOC is too low. Where are you measuring it from? To get a proper measurement the battery should be in a steady state (no current draws) and you should do it as close to the battery as possible, unless the cable sizings to the measurement point are done properly. Check if there are any voltage drops/differences between the battery terminals and the measuring device/points.
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Old 16-01-2019, 10:07   #9
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Re: Need help how to make a power calculation

To estimate what is left in your battery there must be no load on the battery at all. As has been said, the battery should be left for a few hours to let it stabilise before you have an accurate reading of the voltage, but usually, 5 minutes will give you a near enough figure to work with.
So if you started with 12.6v and then added your 85A load for 5 hours and get a reading with the load of 12.2 A on your battery bank, they look to me to be rather in good condition. Left without load for 5 minutes and you should see 12.4v ish.
12.8 full battery -11.8v dead battery. The value for a fully charged battery will deteriorate with the age of the battery. So charge your batteries fully and then you will get the fully charged reading of voltage for your set of batteries only. Others will vary slightly. Draw a graph to illustrate voltage versus charge. This graph will be a useful approximation of the batteries state of charge. Not foolproof but a very good guide. I have a set of cheap led voltage gauges in my saloon which tell me the voltages of the service batteries, starting batteries and AC voltage. Hence I always have a picture of the state of the electricity on the boat as I walk past or sit in my chair.
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Old 16-01-2019, 11:37   #10
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Re: Need help how to make a power calculation

Get a cheap ammeter ($20 Amazon) and a hydrometer ($15 auto parts store). Make sure there is some load that you can apply that can be varied, a 25w halogen light in a dimmer would be perfect. Find out what the various loads on the boat draw in amps. Fans at different speeds would be good too.

Fully charge the batteries. Check voltage for the battery and specific gravity for each cell. Record values.

Turn on loads for 18amps (that’s 4% of capacity). After 10min record voltage.

Continues drawing at 18a for about 2hr. Use the dimmer to maintain the draw at 18a. It will vary over time so check it every 5-10min and adjust.

With 37amp-hr discharged record voltage then up the draw to 27amps. After 13m20s you should be at 43amp-he discharges so record voltage and drop load to 9amps. After another 13m20s you should be at 45a-hr discharged, record voltage and turn off ALL loads. Wait 1-2hr then record resting voltage and specific gravity for each cell.

Continue at 18a draw until you get to 82a-hr out then repeat the previous cycle.

After 5 cycles you should be at about 50% SoC.

You will now have enough data to draw 5 curves:
4 for voltage vs SoC for 0C, .02C, .04C, .06C
1 curve for SpGrav vs SoC.

Use Excel to plot the data and draw trend lines.

With this you can check amps being drawn and voltage and make an educated guess as to state of charge.
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Old 16-01-2019, 12:28   #11
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Re: Need help how to make a power calculation

If my memory serves me correctly, I think we may be a little too concerned about deep discharges on batteries like the T105 and T125. I remember that the T105 batteries can withstand 700 discharges to 5.25V (10.5V in series) before having substantial deterioration. That's why they are used in golf carts, where there's no real protection against deep discharges other than having the cart slow down too much.

Yes, I understand that these cycles are under laboratory conditions (80 degree water baths and so forth), but my point is that I think we get all caught up on the 50% "rule" and we shouldn't, especially with classic deep cycle batteries like T105s.

The curve which plots cycles against reserve minutes shows a consistent capacity for 500-600 cycles or so, then a pretty precipitous drop after 650-700 cycles. Again this is from memory, and lots of communication with William Brecht at Trojan in the 1990s.

Faster discharges (higher % of C) will result in more cycles, since the batteries will hit 5.25V before they are as discharged as they would be with slow discharges.

This doesn't really answer your question, but I wouldn't sweat occasional or even frequent deeper discharges presuming you follow them up with a full charge.

Cheers,

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