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Old 08-27-2021, 10:20 PM   #1
Tom47
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Default Confirmation of SOC vs. Voltage Chart

I found this chart online for 48 volt, LFP batteries. Although it was pertaining to off-grid systems, I feel it should be applicable to golf carts also.

It was interesting to see the voltage drop from 100% to 99.5% and again to 99%. It appears 99% is what my ECO pack charges to when the charger automatically shuts off before the timer. The last time I watched it, the voltage was 54.65. After 30 minutes or so, it settled to 53.85.

Do you see any red flags in this chart or in my data?

Thank you for your input.

Tom
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Old 08-28-2021, 07:01 AM   #2
Tom47
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Default Re: Confirmation of SOC vs. Voltage Chart

I added my own estimation of amp hours to the chart. I'm not sure about any of these numbers, nor what the 99.5 and 99 Ah would be.
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Old 08-28-2021, 08:41 AM   #3
simicrintz
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Default Re: Confirmation of SOC vs. Voltage Chart

I wish I had an answer, Tom. As much as for your question, but also so it doesn't look like you are talking to yourself......
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Old 08-28-2021, 11:15 AM   #4
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Default Re: Confirmation of SOC vs. Voltage Chart

Quote:
Originally Posted by simicrintz View Post
I wish I had an answer, Tom. As much as for your question, but also so it doesn't look like you are talking to yourself......


Your answer was the same one I would have given! And thanks for keeping the question alive!
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Old 08-28-2021, 11:42 AM   #5
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Default Re: Confirmation of SOC vs. Voltage Chart

The writer of the forum post also mentioned this:

"My self imposed limits are based on what the cells return to after a bulk charge. For the first month, I was bulk charging to 3.55V per cell, they always returned to 3.295V on their own, so why try to force them. Let them be happy at 3.295 and "call it 100%"."

This would indicate 52.72 volts as the settle in point. That seems to be where mine settles in also. By the same chart, this would be 65 to 70% charge. Am I missing something?

Here is a link to the DIY Solar forum for reference. Scroll down to the fifth post:

https://diysolarforum.com/threads/li...ge-chart.3156/
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Old 08-28-2021, 04:50 PM   #6
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Default Re: Confirmation of SOC vs. Voltage Chart

Since a lot of us are in this learning process together here is some food for thought. I watched this video yesterday and it compares 3 different charging voltages. 3.65, 3.55 and 3.45 volts. They all get the job done but to charge at 3.45 it takes twice as long to charge than it take at 3.65v. Maybe I'm looking at something wrong. Got any thoughts on this.

https://www.youtube.com/watch?v=lEW6m-iKH8w
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Old 08-28-2021, 05:26 PM   #7
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Default Re: Confirmation of SOC vs. Voltage Chart

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Originally Posted by WalterM6 View Post
Since a lot of us are in this learning process together here is some food for thought. I watched this video yesterday and it compares 3 different charging voltages. 3.65, 3.55 and 3.45 volts. They all get the job done but to charge at 3.45 it takes twice as long to charge than it take at 3.65v. Maybe I'm looking at something wrong. Got any thoughts on this.

https://www.youtube.com/watch?v=lEW6m-iKH8w
Individual cell voltage was a part of the charging cycle I was not too worried about. Maybe I should be! My ECO charger is 15 amps @ 57 volt output. Other chargers are different amperages and different volt output.

If I run my 72 Ah ECO pack down to 42 AH, then I need to set the timer (you suggested) to 2 hours to replace the 30 Ah (15 x 2=30). Easy enough right?

Not so much. My charger shuts off by itself in about an hour and a half. And I'm magically at 72 Ah and 100% again! How in the world does it do that with a 15 amp output.

It seems every time I see some data online, most of it from home energy systems, I get more confused. The video you linked only gave me more questions.

I swear I will soon grow tired of this and follow Pat911's advice. Stop nit-picking and use the cart and enjoy it! (Paraphrased)

All I am really looking for here is the SOC to Ah to Volt numbers. If these charts are right voltage and Ah wise, then I'm done!

Will I ever be able to assist someone else with these same questions....Never! I seem to know less about lithium now than when I was contemplating the purchase.

But I love the way my cart runs now. Everything is where I can get to it easily. So why worry about it, right? I sure hope so.

Tom
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Old 08-28-2021, 06:03 PM   #8
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Default Re: Confirmation of SOC vs. Voltage Chart

Confirming that charging at a lower voltage (within reason) will still get the job done, albeit with the longer time trade off.

The chart that you posted Tom is correct, but what you have there is the settling voltage, not the max charging voltage. But it is also misleading in that a 100% charged pack will not stay at 58.4v for very long. It will almost instantly drop. The settling process will take time too. To get a accurate settling voltage you will need to wait 24 hours.

Your experience of replenishing 30aH with a 15A charger in 1.5 hours does not compute, no matter how you look at it. The only explanation is that your charger is able to supply greater than 15A or that your battery draw was less than 30aH. Does your SOC meter count up when charging? If so, are you able to monitor it whilst charging? Also, can you measure the open circuit voltage of your charger, that’s when it is turned on but not connected to the battery.

Cheers
Pat.
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Old 08-28-2021, 06:11 PM   #9
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Default Re: Confirmation of SOC vs. Voltage Chart

Quote:
My self imposed limits are based on what the cells return to after a bulk charge.
Here are my thoughts on that. No matter what kind of battery it is they all charge the same through a chemical reaction. With a lead acid battery you would never think of turning the charger off at 50.9v. Instead you charge it to about 56v. This is necessary to saturate the battery and bring it to full charge. The same goes for LiFePo4 batteries. The manufacturers all say to charge to 58.4v. I would think that what you save by not charging them to full capacity on the high end you would lose when you had to discharge them more on the low end. My 2¢.
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Old 08-28-2021, 06:20 PM   #10
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Default Re: Confirmation of SOC vs. Voltage Chart

Tom, I wouldn't worry to much about the backup timer. It's just a peace-of-mind timer. I just use it so it won't stay plugged in all the time. If you think it is going to take 2 hours to charge set the timer to 3 hours.
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