Unbalanced - new Renogy Bluetooth 100ah - keep or return?

JoeHike

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Apr 28, 2022
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I received my 100 amp hour Renogy with Bluetooth battery 5 days ago, and have been trying to top balance the cells. No luck!
Screenshot_20220510-122851.png
Opened a case, but support insists that these cells are " normal ". Supposedly, it is possible that I could return the battery and get my money back. Should I do so? Am I being too picky?Screenshot_20220511-204855.png
 

MisterSandals

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I received my 100 amp hour Renogy with Bluetooth battery 5 days ago, and have been trying to top balance the cells.
How are trying to top balance the cells? What is your reasoning to top balance the cells in your battery?

And what is happening that you felt you needed to contact Renogy?
 

Substrate

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Ah, I see it - the cell voltages coming from a BT app, with only 1 digit of precision. Um, I don't totally trust it.

@JoeHike - how have you been trying to top balance? Is this just the very first charge? Have you been running it through a few charge / discharge cycles (doesn't have to be deep, but pull at least 10-20ah from it) to give the bleeder resistors a chance to tighten things up with successive charges?

Just saying - with only 1 digit of accuracy, you could be in a position where you are actually not as badly balanced as you think they are. Also, a true balance is only measured when you are actually in a state of charge, not at static rest. (the present current reading zero tipped me off)
 

JoeHike

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Substrate.
Thank you for your interest and observations.
I have run probably two dozen discharge/charge cycles. At first, I discharged fairly deeply - to 50% - 70% SOC. The last dozen or so cycles, the discharge at 17.5 amps was down to 89.9% to 99.0%.
Yesterday, Renogy released an update to their BT app that promised, among other things, more precision. I installed the update, and ran a new discharge charge cycle this morning.

Regardless of SOC, when my 10 amp charger is first turned on, it begins charging at around 4 amps for the first 15-18 minutes. On the BT app, all four cells are reporting to be at 3.3 volts. No additional absorption is reported by the cells until SOC has been at 99.1% for about 5 minutes. At that point pack voltage is at 13.5, and beginning to rise. Cell number three is the first to reach 3.4 volts followed by cell number 4, about 3 minutes later. It is quickly joined at 3.4 by cell 2. Things speed up after that. As pack voltage reaches 13.8, cell 3 goes to 3.5, and at 13.9 volts to 3.6 and cell 4 to 3.5 volts. A few seconds after the pack reaching 14.0 volts, cell 4 displayed 3.7 volts, and charging was terminated.
 

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Substrate

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Cool! Thanks for the detailed info.

But man, something isn't right. A ten-amp charger should be hitting 10amps of charge, since you assured us of a discharge, until the very very end when CV naturally causes a smaller tail current.

Seeing only 4A no matter what worries me. I have the feeling that this app is the only testing instrument available to you. Either the charger *IS* delivering 10a, but the app is wrong! Which makes me not trust the other readings either.

How do things look near the bottom of capacity? Oh, I see you've taken it well down in capacity. Are the voltages totally ridiculous, like with one cell doing a deep deep dive well before the others.

BTW, what exactly is your 10a charger? It just freaks me out that you aren't pulling 10a from it, indicating that either the charger is funky, or something is wrong internally with the battery or the app itself!

I really don't want to say it - but maybe without using other testing equipment, but having the ability to pull full /nearly full capacity, this may be almost a cosmetic issue. A frustrating one to be sure.
 

mikefitz

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I would not worry about charge current too much at this stage.

What you are seeing is slightly unbalanced cells, this is not unusual in ready built batteries .

Once the cells reach 3.4 to 3.5 volts with a charger delivering a low charge current, they are mostly full, it will only take small amounts of power to push the volts quickly to cell over volts and cause the bms to terminate charging.

There is a great deal of overthinking on cell balance, whilst its ideal to get all cells at 3.65 volts with no deviation under charge, its not a requirement to get a useful battery.

I suggest charging the battery and carrying out a capacity test. Even without test equipment you can get a useful estimate by measuring the time the battery powers a known load, for example a car headlamp bulb.

If you get near to capacity, and my guess is that you will , then continue to use the battery. Over time the ballance will improve.
To help this lower the charge voltage to less than 14 volts.

If you can show the battery capacity is very different to specification then raise a dispute with Renogy.

Mike
 

JoeHike

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Substrate -
Here is a link to the automatic charger that I am using. I believe a slow ramp up to 10 amps is a safety precaution for lithium batteries. When the auto charger displays 9.9 amps, the Renogy app shows 7.7 or 7.8 amps. My clamp-on meter shows around 9 amps at the charger's clamps.

 

JoeHike

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Mike - thank you for your post! I am totally new to lithium batteries, and my primary source of information about them has been a friend who has assembled several packs with the components sourced from China. He believes that I should have less variation between cells, and keeps suggesting different charging regimes that might accomplish that. Also, on the forums which I began viewing a couple of weeks ago, it appears that people are very concerned about top balancing cells and packs. However, Renogy keeps reassuring me that the variation in cell voltage in my battery is normal. So, it is very interesting that someone besides the manufacturer would agree with them. That is why I posted on this forum, to get a better idea how important variation in cell voltage is.

I have only done two deeper discharges, at 17.5 amps, down to 50% and 70% SOC. Unfortunately, I did not time the discharges. How deep of a discharge do you think would be informative but not harmful to the battery pack? Would a larger discharge load - 50 or 80 amps, be harmful?

I was hoping to use this battery as a replacement for two 11-year 100 amp hour agm's. The biggest loads it would experience, would be a 85 to 90 AMP discharge for a max of 15 minutes, from my use of a small microwave or air fryer.
 

mikefitz

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To establish that the battery has the capacity claimed , discharging down to BMS cut off would be my method, one deep discharge will have little effect on battery life.
If you feel this is too severe then using a combination of battery voltage and the battery SOC may give you an idea of capacity.
A resting battery, no load or charge, will settle to 12.0 volts at around10% capacity.
You don't need a high current discharge, a 5 amp or 10 amp load would require a longer test time, ( need not be a continious test period).
Just use whatever is convenient and makes the sums easy. A more accurate and low cost method would be to use a Dork load monitor with a hall effect sensor, ( shows a UK source but available from Ebay or Amazon).
With the Dork it just measures power for any load current.


Other points, your 100Ah battery is limited to 100 A continuous discharge and 50 amps charge

When you are testing the battery at high loads, review the cell volts. They should not have a significant voltage difference between each cell over the 20% to 90% state of charge range.

Mike
 
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JoeHike

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Thanks Mike! Very helpful guidelines. Basically, I just want a battery where the SOC on the app is reasonably in the ballpark, and that lasts at least 5 to 7 years. Monitoring individual cell voltages is not what I want to do when I'm camping.
 

time2roll

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A few seconds after the pack reaching 14.0 volts, cell 4 displayed 3.7 volts, and charging was terminated.
Set the daily charging at 13.8-13.9 and put the battery in service. Check in a month and see if the cells have balanced out a bit.
 

JoeHike

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Thanks. That sounds like good advice! My DC to DC charger has the lowest setting of 13.9 volts. I'll use that.
 
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