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Old 09-30-2018, 10:48 AM   #11
scottyb
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Motor efficiency is a good point. I think the original question would be influenced by what voltage the motor in question is most efficient.
Since the cell AH remains the same and just the pack voltage rises when adding another cell in series.
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Old 09-30-2018, 11:42 AM   #12
sunking
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Quote:
Originally Posted by bronsonj View Post
Hello all,

If I have 14 cells charged to the full 4.25v I'll have 59.5v
If I have 15 cells charged to 3.97v I'll have 59.5v

My gut says the 15 would have greater capacity but I don't know.
Real easy to figure out. First thing you have to do and understand is you use NOMINAL BATTERY VOLTAGE, which has nothing to do with charge and discharge voltages. Sounds like you have LMO cells with a 3.7 volt nominal. In 14S configuration is a 51.8 volt pack and 15S is 55.5 volts.

To determine a batteries energy capacity Watt Hours = Nominal Voltage x Amp Hour. .

So to answer you question you know 3 of the 4 variables to answer your question. The one missing variable you have to find, we cannot. Example say these are 100 AH cells. Well you know in 14S fully charged is 51.8 volts x 100 AH = 5180 AH. A 5th grader can figure that out. On 15S we already know the voltage to be 55.5 volts. What is missing? You do not know the amp hours because you are in a Partial State of Charge. It is less than 100 AH. Lets say it is at 70% SOC or 70 AH, then 55.5 volts x 70 AH = 3885 watt hours which is significantly less than 5180 watt hours.

FWIW LMO cells charged to 3.9 volts is roughly 90% SOC. One mistake people make, especially DIY is fool themselves into thinking voltage gives you the State of Charge which is complete nonsense.
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Old 09-30-2018, 05:12 PM   #13
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Thanks Sunking. I kept reading this thinking, “work is not measured in amps, work is measured in Watts.”

I always thought

Volts is how fast the river is flowing

Amps is how wide the river is

Watts is the volume of water flowing down the river

By adding another cell you Must be increasing the total pack energy capacity. As you stated, you just don’t know how much of that capacity will be realized if not fully charged.
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Old 09-30-2018, 05:34 PM   #14
sunking
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Well the Forum Software cut me off from an Edit so here goes again.

To compare battery capacity, you do not use Amp Hours because Amp Hours is meaningless without a voltage specified. The common denominator is Heat Energy aka Energy Consumed or Supplied With electric power Energy is Watt Hours:

Watt Hours = Nominal Battery Voltage x Amp Hours.

48 volts x 200 AH = 9600 Watt Hours
36 volts x 267 AH = 9600 Watt Hours
24 volts x 400 AH = 9600 Watt Hours.

So go back to your example of a fully charged 14S battery. Say it is 100 AH cells so nominal is 51.8 volts x 100 AH = 5180 wh. I cannot tell you what voltage would be requires to equal the same energy as a 14S pack, no one can. But I can tell you exactly how many AH would be required, by factoring AH out. I know 5180 wh is equivilent and I know the nominal voltage of 15S = 55.5 volts, so there for:

Amp Hours = Watt Hours / Battery Nominal Voltage

5180 wh / 55.5 volts = 93 AH.

Only way to know the SOC i snot by voltage, but by coulomb counting Amp Hours In and Amp Hours Out. The difference in voltage between 93% SOC and 100% SOC is so small, you cannot use voltage with any accuracy.

Now if you have a charger you can program an algorithm, you can get real close. Lithium batteries are super simple to charge, much easier than PB. All you need is a simple CC/CV charger set to the correct voltage, and have the ability to terminate charge when a specific current level is reached.

Example a 15S LMO battery Set Point Voltage is 63 volts. You hold 63 volts on the battery until current tapers off to 3% of C where C = the battery Amp Hour capacity. On a 100 AH would be 3 Amps. That gets you to 100%. Want a little less is easy. Make the Finnish Current a higher value like 10 amps.
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Old 09-30-2018, 06:37 PM   #15
sunking
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Quote:
Originally Posted by ag4ever View Post

Volts is how fast the river is flowing

Amps is how wide the river is

Watts is the volume of water flowing down the river
Try this analogy.

Volts is the Water Pressure

Amps is the current flow rate

Watts is the force of water coming out of the hose. Not energy.

Important to understand the difference between Power and Energy.

Power = Watts = Volts x Current at any given moment in time, or Rate at which Energy is being consumed or generated. Example a 100 watt light bulb or 10,000 watt golf cart motor.

Energy, Watt Hours = Watts x Hours is the amount of work done in a specified amount of time. Example a 100 watt light bulb would consume 1000 watt hours if turned on for 10 hours. 100 watts x 10 hours = 1000 watt hours. I do not need to know what the voltage and current is. Voltage and current is just a result and numbers. Watts and Watt Hours are what you are doing. I can make 100 watts

10 volts x 10 amps = 100 watts
100 volts x 1 amp = 100 watts.

Given the choice above you would use the highest voltage you can because it is the most efficient, easiest, and least expensive option.
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Old 09-30-2018, 09:50 PM   #16
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

I don't have a charger that will do the columb counting, so I can't answer that. On paper, these are 47ah cells, but that doesn't tell us what we need to know to answer this.
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Old 09-30-2018, 10:48 PM   #17
ag4ever
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Here is a Chevy volt battery chart I found.



The voltage is for the complete 96 cell pack, so you would need to interpolate the individual cell voltage.

Also, to determine actual energy capacity, you need to define your bottom limit as well.
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Old 10-08-2019, 04:14 PM   #18
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Default Re: lithium capacity - 14 fully charged vs 15 partially charged cells

Sorry for the delay. I don't know why I never replied...

We did end up doing the test for this and it was exceedingly dramatic. (spoiler alert, the experts were right!)

While not the 14S/15S I originally asked about we did a 12S/11S test and I am sure the results would scale.

The test mule here was a 36v Taylor Dunn utility cart. The packs were Chevy Volt LMO 12S and 11S packs charged to 44.0v (3.66vpc and 4.00vpc respectively). The packs were measured by the seller as 47ah pack capacity. Range on 12S was 2 miles (maybe a little less) The 11S pack we never measured the range on but I know we've drove it 5+ miles and it was not in need of a charge when we finished. For the Taylor Dunn the final pack was a bit different (3P11S) and the owner is quite satisfied.

So the test mirrored what all the experts said... I just wanted to reply in case anybody found this in the future with a forum search.
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