Battery testing - choices for series circuits

[AriM]

Comment on your cell selection criteria.
Peak voltage may indicate something, but mid charge voltage shows more about the entire cell, internal interconnects, cell resistance, etc. A partial dump from full charge and under load voltage might be better to sort cells. AC impedance testing is used by the industry to measure internal resistance but voltage with a medium load vs time (2 amps for 5 minutes for example) would be a quick way to weed out poor cells. If a new cell gives a voltage outside the group distribution at 70 to 80% charge, it is an outlier.

I have the ability to graph/log all of the above data. Including internal resistance. Good suggestions. How about this. I will do a full charge/discharge analysis of the cells (at 1c). I will then post the data, and we can all pick the cells as a "team". That way every ones concerns are addressed. LiFePO4's mid level charge characteristics are really quirky, compared to "standard" Li-Ion cells. They really do jump around a lot....another option would be to take the cells to "storage voltage" (70%) and allow them to sit for a few days. Whichever cells are closest to the average at that level, could make the cut?

[AriM]

here is a great example that addresses your concerns Rocketman. Look how fast LiFe cells flatten, and then look at how consistent the average mid-charge voltage is. The part that always freaks me out about LiFe cells, is their "jumpy" behavior at mid charge levels....have a look (yes yes I know we are talking tenths of a volt here)

[DaveP]

I'd like to see a "safe" cell list adopted by ECF. By safe, I mean a cell that can take a shorted atty or high drain within limits and not experience a rapid catastrophic failure that blows up in someone's face. I may be mistaken, but IMR cells are described as being likely to produce high heat and gas release, but not a rapid compression that results in explosion. Is that right? If so, then a mod that suddenly gets hot enough to make you drop it and move away would be much safer than one that vents noxious gases and flame followed by a BOOM!

I see the value of slots in the side of a tube, but wouldn't it be better if all batteries were required to be inserted with vents toward the cap end? Compression buildup in a mod that is blocked by a swelled battery wouldn't be so much a safety issue if the battery vents were still next to the cap vent. At least there would still be an avenue for gas release if the positive end were next to the vent hole location.

I've thought about a Lavatube or Vtube type mod for myself. The IMR battery gives me pause because of the lack of built-in fault protection, though. What are your thoughts on those? The electronics limit the voltage output as you lower the resistance and/or raise the voltage. This prevents dual coils from being used over 4v and limits the voltage by sensing the current load. I think this is a good feature for safety.

If my metal tube mod gets hot and produces a blister in my palm, that's one thing. If it removes a finger or two from venting flames or exploding, I won't be a happy camper. I'm in full agreement that testing is a good thing and user education is absolutely necessary for safety.

Continue on, guys! This is good stuff.

[AriM]

Tenergy 30200 LiFePO4 Li-Ion batteries:

Rechargeable LiFePO4 RCR123A 3.0V 750mAhBattery
Life cycle: 2000 times (Traditional Li-ion: 500 times)
Working Voltage: 3.2V
Peak Voltage: 3.6V
Charging cut-off voltage: 3.6V
Discharge cut-off voltage: 2.2V
Please never overdischarge battery below 2.2V/Cell
Charge the this battery with LiFePO4 RCR123A smart charger
Capacity: 750 mAh
Maximum discharging rate: < 550 mA
Maximum charge current: < 550 mA
Cell's dimension: 17mm Dia. x 34.5mm H.
Weight: 0.6 oz (18 grams).
Manufacturer's Part Number: 30200

Tenergy 30200 LiFePO4 RCR123A 3.0V (3.2V) 750mAh Rechargeable Battery

this is what scares me about the tenergy life cells....every source lists different specs for the same cell....this is was my concern about ratings above 2c

[Rocketman]

WOW
That looks like a Dyno plot of a race engine with too small a fuel supply
Like the cell is the gas tank, and electrical power delivered to the load is 'starving' both between load pulses and during each load pulse. Cell chemistry seems to be 'starving'.

[AriM]

WOW
That looks like a Dyno plot of a race engine with too small a fuel supply
Like the cell is the gas tank, and electrical power delivered to the load is 'starving' both between load pulses and during each load pulse. Cell chemistry seems to be 'starving'.

LiFePO4's are "freaky" cells. By all common logic, that cell should NOT give you vapor. I have no valid explanation for why they work in PV's. You want to have a good laugh? Here is the exact same test on a tenergy LiFePO4.

I hate to taint the test (I have done them already...a long while back) but have a look....

same test...

peak charged....cells sat for about a week.

2 amp CC. Duty cycle 5 on 20 off....LOLOLOLOLOL

I did not cut that cell of manually....same test....cutoff was 3 volts. it made it through 1.5 cycles

[Rocketman]

That looks like a PTC cutting out, cooling off, coming back on. Looks like it worked as advertised.

[AriM]

That looks like a PTC cutting out, cooling off, coming back on. Looks like it worked as advertised.

Those were both unprotected cells....that was the duty cycle test ("pulsed" discharge). The part I wanted you to see was the erratic drop out under load. Li-Co's / Li-Mn have a nice smooth drop on each cycle....consistently lower on each cycle (except for those weird blips we saw on the IMR's in the other thread). The LiFePO4's are all over the place. Sometimes they hit the same peak under load at cycle 20, as they did on cycle 5. Freaky stuff man....and they say "life batteries are the future"....

I for one ain't buyin' it....how about we just skip LiFe chemistry (as a human race) and move straight to Li-Ti

P.S. I got your 18650's in the mail last week. I have our "battle of the button tops" shootout all lined up....I might run it this weekend. Should I start the new thread for it? or do you want the honor (chore)?

[Rocketman]

You can start it

The LiFePO4 cells don't have a PTC in the positive end?
or some kind of temperature switch?

That sounds scary.

[AriM]

You can start it

The LiFePO4 cells don't have a PTC in the positive end?
or some kind of temperature switch?

That sounds scary.

The LiFePO4's have no PTC at all, just like the IMR/Li-Mn. They consider them "safe" chemistry. Which is a laugh in my book. There is no "safe" chemistry, just safer

I can pull current from a LiFe cell until the shrink wrap ignites, and there is no safeguard to stop it...thankfully they don't build internal pressure the same way as Li-CO. The worst byproduct I have seen is runaway discharge and venting/leaking.

there are BMS/PTC/BPC boards available for LiFe cells though....I just can't figure out why no manufacturer is including them in the finished battery. At the very least to stop over discharge