Question re: lipo capacity

[johnmaclean]

Went to charge my batteries this morning before heading to the river expecting that I could charge both of them in less than an hour. I ran the boat last weekend and when I checked the battery terminal voltage I read 7.65 volts per pack (2s-2p 5400 mah). I was surprised to see it that high after running the boat for 3-4 minutes.

I am using a cellpro 4s lipo charger and when I plugged the first battery in the display said that the battery had 3% of its 5400 mah capacity. Got me thinking about the difference between terminal voltage and availble capacity as indicators of pack readiness. A battery's terminal voltage may not be arrpoaching the cutoff value for lipos but the battery may not be able to deliver the power neccessary to run the motor if its capacity is depleted. I'm wondering if the more relaible measure of a pack's readiness is the % which is the combined measure of stored energy and not just the voltage measured at the pack's terminals.

Put both abtteries on the charger this afternoon after a five minute run and again the display showed that the batteries's cpacities had been depleted to about 3% of their total capacity.

Any thoughts on the matter?

[Fluid]

Cell terminal voltage will rebound a bit after use, your 7.65 volts indicates less than half capacity left....but how was it measured? I am not familiar with the CellPro chargers but understand they are a quality unit. The charger appears to have read a different terminal voltage than the other instrument did. If you charged the packs for 5 minutes and the remaining capacity was still 3%, I'd wonder about the accuracy of the charger. What was the charge rate for that 5 minutes?

While I use a Cellmeter-8 or a FlightPower balancer to give me an idea of the capacity left in a pack - what really matters is how much charge the pack takes to fill it. I always compare what the Cellmeter-8 says and what the charger puts back to check accuracy - they are usually very close. How much charge did your packs take? If they took over 5000 mAh then the 3% estimate of remaining charge is about right. If they took only 3500 mAh then the charger's estimate is suspect and I would use another method to determine the remaining charge in your packs.



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[johnmaclean]

Cell terminal voltage will rebound a bit after use, your 7.65 volts indicates less than half capacity left....but how was it measured? I am not familiar with the CellPro chargers but understand they are a quality unit. The charger appears to have read a different terminal voltage than the other instrument did. If you charged the packs for 5 minutes and the remaining capacity was still 3%, I'd wonder about the accuracy of the charger. What was the charge rate for that 5 minutes?

While I use a Cellmeter-8 or a FlightPower balancer to give me an idea of the capacity left in a pack - what really matters is how much charge the pack takes to fill it. I always compare what the Cellmeter-8 says and what the charger puts back to check accuracy - they are usually very close. How much charge did your packs take? If they took over 5000 mAh then the 3% estimate of remaining charge is about right. If they took only 3500 mAh then the charger's estimate is suspect and I would use another method to determine the remaining charge in your packs.



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I'm sure the charger is accurate because each pack took about 5300 mah to restore them to full capacity. They were depleted to 3% after running them for 5 minutes on my setup.

I posted because I wondered how far a pack can be depleted before damaging it. It seems that a pack's capacity can be pretty much depleted without the voltage dropping below the critical 3.0 volt value. Each pack took a full charge from the cellpro 4s and are pretty much ready to go again. I guess the principle is similar to an battery pack eg. at 11.1 volts the deep cycle battery on my full size boat will not turn over the engine but will at 12.1. difference between an afternoon of fun and sitting at the marina watching everyone else having fun.

Perhaps I wasn't clear enough. . . I did not charge the batteries for 5 minutes I ran the boat for 5 minutes and put the batteries back on the charger and it indicated that there was only about 3% capacity remaining in the batteries.

[Fluid]

The terminal voltage will rebound after the boats sits for awhile, under load the voltage will be lower. IME the terminal voltage you measured is wrong - every time I discharge a pack - all my packs BTW - to ~3.8 volts there is ~50% charge left. The pack can sit on the shelf for a month and it remains at 3.85 volts, and takes 50% of the cell capacity to fully recharge. Never anywhere close to 97% discharged.

A terminal voltage of 3.3 volts means virtually no usable charge is remaining - your 3.825 voltage reading makes no sense. Your charger uses terminal voltage to determine the % of charge remaining.

It is never a good idea to run more than 80% of the charge out of any pack, doing so can cause cumulative damage to the cells.




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[cybercrxt]

I will go ahead and say this, discharging your 5400mah packs where they require 5300mah to chrage them back is a pretty bad thing to be doing on a consistant basis. You should only be putting back in a max of 80% of the 5400mah's. I am suprised you are not asking why your batteries are so hot or why they have a puff to them. I really wish everyone would read more about proper lipo care, it could save your house, or your life someday. I just purchased 18 new polyquest packs after using zippys for a couple years and you better believe I am changing my bad habits! Its just not worth the risk. Mike

[cybercrxt]

Yeah, I would question a static charge of 3.87 volts with 5300mah withdrawl, that indead sounds like a 3.3volt situation. Getting 5 minute runs also sounds a little off unless u are running 3p or 4p.

[johnmaclean]

Battery temps were about 103 degrees just warm to the touch, and were not and are not puffed. 5 minutes would have been the very outside. I have literally spent dozens of hours reading everything I can on lipos and have followed every piece of advice I've found including charging and storage. I've been timing my runs and keeping them under 5 minutes, checking temps after 2 or 3.

As far as measuring the terminal voltage I am using the little device I purchased from Kintec racing. . . digitial readout of the pack total as well as the individual cells. It agreed with the numbers I saw on the cellpro 4s. That is precisely why I posted this . . I didn't understand how the mah capacity corresponded with the terminal voltage. I wanted a better understanding of how these lipos functioned.

[johnmaclean]

The pack can sit on the shelf for a month and it remains at 3.85 volts, and takes 50%[/I][/U] of the cell capacity to fully recharge. Never anywhere close to 97% discharged.

A terminal voltage of 3.3 volts means virtually no usable charge is remaining - your 3.825 voltage reading makes no sense. Your charger uses terminal voltage to determine the % of charge remaining.

It is never a good idea to run more than 80% of the charge out of any pack, doing so can cause cumulative damage to the cells.




.[/QUOTE]


Thanks for the input. . . That may be a clue. I've only had these since July. . . . bought them used. On a couple of occasions I've charged the batteries to 100% in anticipation of running them the next day but couldn't get to the lkae for one reason or another. A few days later I'd check the batteries and their capacity had dropped to 95%. But from what I've read Lipos hod their charge during storage. Perhaps I've got a couple of packs that are on their way out.

[Fluid]

Edit based on last post:

Yes, it is possible you have funky cells with high resistance internal connections. That makes it very tough to diagnose from 1000 miles away.




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[lectriglide]

Thanks for the input. . . That may be a clue. I've only had these since July. . . . bought them used. On a couple of occasions I've charged the batteries to 100% in anticipation of running them the next day but couldn't get to the lkae for one reason or another. A few days later I'd check the batteries and their capacity had dropped to 95%. But from what I've read Lipos hod their charge during storage. Perhaps I've got a couple of packs that are on their way out.

I've got a couple Cellpro 4S's. Looking at the cell balance view should tell you if a cell is going bad. My experience has been that the bad cell will both charge faster and deplete sooner than the good cell. At your 95% capacity after letting them sit for a couple days, are both the cells at the same voltage?

[johnmaclean]

I've got a couple Cellpro 4S's. Looking at the cell balance view should tell you if a cell is going bad. My experience has been that the bad cell will both charge faster and deplete sooner than the good cell. At your 95% capacity after letting them sit for a couple days, are both the cells at the same voltage?

It has been a while since I checked that but I will over the next couple of days. When I am charging I constantly switch between display screens, monitoring the cell baance voltage, amp draw and mah capacity. One other thing I have noticed with the packs is that shortly after I pulug them in for a charge the capactiy will often drop a couple of percent (eg. from 95 to 91%) before the pack capacity started increasing again. I assumed it to be a function of the charger as it went into its balance charge mode.

Aslo found the following in a thread relating to swordfish ESC's. I understand that Aquastar and Swrodfish are produced by the same manufacturer so I'm wondering if there are parallels between steven's observation and my own. Mine is a 90 amp ESC and he was reporting on a 120. . . I don't know the details of an n2 setup so I won't even pretend to state anything with any degree of certainty. . . I'm just curious. . . that's all

"Glad you liked it Ken. I have had great luck with them. . . . The Swordfish 120 has been outstanding. My son and I beat the hell out of a 120 this weekend in his n2 mono. He was draining a 6500 hyperion 35c pack to 5% in 170-180 seconds. Right at the rating on that esc and it went as smooth as silk. Previously we had only been able to run a Hydra 240 on that setup." (Steven Vaccaro)