Safe lowest voltage under load

[ezhitz]

On my P-hydro I'm running 3700mah 60c 2p and I notice looking at some of my data logs from the SF 220 pro+ that the min voltage ranges were from 13.6-14.3 which is about 3.4-3.57v per cell. Now three of the four batteries I use have had one cell go bad. The first one had a cell start getting warm and out of balance when I was discharging it into storage the second one had a cell drop to almost zero setting on the table and now the third one has cell at 4.13 while the other three are at 3.82. Is it a question of bad luck with these batteries or not enough mah for P-hydro. The batteries where always balanced charged and would have about 30% left after each run.

[Fluid]

So much depends on the quality of the cells. Just because the label says 60C does not mean that is the true value - there is NO industry standard for measurement so each pack assembler can print whatever he wants on the label.

That said, voltage depression to 3.4 volts is not extreme for decent cells. (And the 7400 mAh should be plenty if the ripple current is under 2 volts.) I set my LVC to 3.2 volts and even my SAW record holders have never hit it. My packs last for years, but then I buy quality cells. I had two "bargain" packs which were way out of balance initially but cycling three to four times with a balancer seemed to fix the problem. OTOH I did have two packs (an unfamiliar brand) which came highly recommended and which I "assumed" were high quality, but both refused to ever reach a balance even after two weeks of cycling. $130 paper weights.

If one cell is dead then toss the pack. Cycling by fully charging to a balance, discharging to around 3.6-3.7 volts, balancing, the repeating may help to salvage the packs. Bottom line - high quality packs are much less likely to give problems, but you can't always believe the hype. Bad packs can be found anywhere.



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[ezhitz]

Fluid thanks for your answer. How can I find out what the ripple current is. Is this something you see with the castle data logging! I didn't mention the brand because I didn't want somebody just replying putting the blame on the batteries without looking at the set up. They where Revolectrix which I do have 4 new ones that I will break in for next summer. Also the three bad ones have already been discharged and safely disposed.

[Fluid]

The ripple is recorded by Castle's logging. I am not sure how you could find it from your logged data. I thought that the Revolectrix cells were okay, but since you got three faulty packs now I'm to so sure. Usually it takes cell voltage under 3.0 volts to cause damage.


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[NativePaul]

3V was the safe cutoff back when the best cells were 6C, but the improvements to the IR (that gives us higher "C ratings") mean that the loaded voltage drops less and is now closer to the resting voltage than it used to be, so the higher the cutoff has to be higher in order to keep the cells at a safe resting voltage (3.7v resting is around 20-25% remaining capacity, which is often touted as being a good usable capacity to cell longevity compromise).

With today's cells if you run down to 3V pulling any FE current you will only get close to the stated capacity once as the pack will have degraded significantly if not puffed, at 3.2v you may get a half dozen good runs, 3.3v if your lucky you may get a season and with 3.4v you might get a probably a full year with testing too, 3.5v probably leaves about 25% in a pack (those figures are not set in stone, the higher the load the lower the LVC can be as the voltage is getting pulled further from its resting voltage at that state of charge (and I generally run lower amps than you guys so you may get away with a little lower, but I also generally run low "C" higher IR packs, so maybe not)).

I've seen recommended LVCs increase from 2.8v-3v-3.2v-3.4v-3.5v, hopefully in a few years it will be 3.6v, and when I can find a unicorn to fart on my LiPos making them the perfect superconducting pack, I will have to set it to 3.7v!

[ezhitz]

I know I don't see any mention in the logging about ripple current. Would adding a couple caps. to the esc help with ripple current? I do have some revo 5200 mah 40c packs that I have run for the last two summers in my lsh that seem to be ok so I was a little surprised with the 3700 all packs are their 420 blend.

[NativePaul]

Yes, caps do help with ripple.

[ezhitz]

Thanks Paul

[urbs00007]

3V was the safe cutoff back when the best cells were 6C, but the improvements to the IR (that gives us higher "C ratings") mean that the loaded voltage drops less and is now closer to the resting voltage than it used to be, so the higher the cutoff has to be higher in order to keep the cells at a safe resting voltage (3.7v resting is around 20-25% remaining capacity, which is often touted as being a good usable capacity to cell longevity compromise).

Fluid :With today's cells if you run down to 3V pulling any FE current you will only get close to the stated capacity once as the pack will have degraded significantly if not puffed, at 3.2v you may get a half dozen good runs, 3.3v if your lucky you may get a season and with 3.4v you might get a probably a full year with testing too, 3.5v probably leaves about 25% in a pack (those figures are not set in stone, the higher the load the lower the LVC can be as the voltage is getting pulled further from its resting voltage at that state of charge (and I generally run lower amps than you guys so you may get away with a little lower, but I also generally run low "C" higher IR packs, so maybe not)).

I've seen recommended LVCs increase from 2.8v-3v-3.2v-3.4v-3.5v, hopefully in a few years it will be 3.6v, and when I can find a unicorn to fart on my LiPos making them the perfect superconducting pack, I will have to set it to 3.7v!

So much depends on the quality of the cells. Just because the label says 60C does not mean that is the true value - there is NO industry standard for measurement so each pack assembler can print whatever he wants on the label.

That said, voltage depression to 3.4 volts is not extreme for decent cells. (And the 7400 mAh should be plenty if the ripple current is under 2 volts.) I set my LVC to 3.2 volts and even my SAW record holders have never hit it. My packs last for years, but then I buy quality cells. I had two "bargain" packs which were way out of balance initially but cycling three to four times with a balancer seemed to fix the problem. OTOH I did have two packs (an unfamiliar brand) which came highly recommended and which I "assumed" were high quality, but both refused to ever reach a balance even after two weeks of cycling. $130 paper weights.

If one cell is dead then toss the pack. Cycling by fully charging to a balance, discharging to around 3.6-3.7 volts, balancing, the repeating may help to salvage the packs. Bottom line - high quality packs are much less likely to give problems, but you can't always believe the hype. Bad packs can be found anywhere.
2 different answers from experienced modelers. which is it ? 3.2 or 3.5 ???????

[NativePaul]

Jay said he set his at 3.2 but has never hit it, so it must be above there. I don't think that there is necessarily much disagreement.

[urbs00007]

Jay said he set his at 3.2 but has never hit it, so it must be above there. I don't think that there is necessarily much disagreement.

my bad. I use telemetry on my expensive builds. going to use low voltage alarm ( a good one) on one of my lesser expensive builds. looking for the correct voltage setting. sorry for my mistake.