Lipo arrives at 1.27V. Is that a deal breaker?

[Peabody]

My pretend constant current 50 ma charger worked fine.  I got the battery up to about 3V, and am letting it rest for an hour.  Then I'll try to take it up in stages, then switch over to using a real lipo charger to finish off.  Then I'll have to do some discharge/recharge cycles and see how it performs.  But at this point I'm enouraged that it may work out ok.  Thanks for everyone's help.

[james_s]

Is it worth a house fire to save a $2 battery? Granted one that small is not going to create a big fireball, but I certainly would not leave it on any flammable surfaces.

[timb]

You may want to read https://www.eevblog.com/forum/projects/lithium-ion-charging/msg624770/#msg624770

Also read the rest of his posts in https://www.eevblog.com/forum/projects/diagnosing-lithium-cells/ , it's someone who researches lion cells.

I don't see anyone using 18650s to run a 700 size heli on 150 Amps.

18650’s are commonly used for electronic cigarettes. For this, they’re commonly used in single and dual (parallel, occasionally series connected) configurations. In the simple mechanical mods, the batteries are hooked directly to the coils (a 0.1 to 5 Ohm Load) through a switch; in the more advanced configurations they power a buck/boost converter, which is capable of delivering up to 200W for 5 to 10 seconds at a time.

That’s a lot of power out of what you call a low current cell. (In fact, I’ve got some IMR18650 cells here rated for 30A continues output.)

Laptop battery packs also traditionally used 18650 cells and were capable of putting out tens of amps, and that was 20 years ago!

Tesla battery packs are made out of 18650 cells and can be fast charged.

By the way, the aforementioned IMR18650 cells are pretty cool. These don’t have built in protection circuits as they’re Lithium Manganese. This chemistry means the batteries don’t violently explode if damaged or overheated; instead they heat up a bit, maybe put out some smoke and then slowly break down. There’s also the newer INR cells, which are Lithium Manganese Nickel. These have the thermal stability of IMR cells, but can put out over 50A!

In fact, the only real advantage Lithium Polymer cells have is that they can be custom manufactured in a specific size and shape, allowing you to pack more capacity into a given area then you could with 18650 cells. (Which is why high end laptops now use internal LiPo packs, instead of external packs filled with 18650s.)

The other advantage LiPo has is in the number of discharge/recharge cycles (500 to 1000 for high end cells). Though, modern Lithium Ion chemistries are catching up—if not surpassing—LiPo in that area.

So no, 18650 cells aren’t relegated to low current usage.

[james_s]

It's worth remembering that 18650 is a physical form factor, cells of widely varying characteristics are available in that form factor. It's been a while since I've looked at a datasheet for a typical laptop cell but I recall seeing a spec of something like 2C max continuous discharge rate and 1C fast charge.

There's a reason that electric RC aircraft and other vehicles almost universally use LiPo batteries rather than 18650 type cells. It's very common to have single LiPo cells that are rated to supply a continuous 30C or more, that can be charged at 5C or in some cases 10C while still offering a few hundred cycles.

The price to pay for this is they are somewhat temperamental, if you abuse them by drawing too much current or discharging excessively they puff up and the IR increases dramatically. If you overcharge or physically abuse them they burst into flames, I've seen it happen before, somebody crashed a large electric helicopter at the field while I was flying planes, the thing made a fireball shortly after impact. There have been a number of house fires posted on the RC forum, mostly charging accidents.

[paulca]

So no, 18650 cells aren’t relegated to low current usage.

RC LiPo batteries are rated in 30C constant 50C burst and some even claim 50C constant.   You can get 8Ah cells that are rated 50C.  So thats 400A without parallel cells.  Pro 700 size 3D heli motor can pull 3kW.  They often use 2x6S in series (44V) to try and lower the amps and prevent melting wires etc.

The pack will be depleted in a single 6 minute display.  It will be almost too warm to touch and slightly puffed.

Actually.  Seems thing have moved on a little since I last looked...

90C constant 6.6Ah = 594Amps.  (Although 2S2P)
https://hobbyking.com/en_us/graphene-6600mah-2s-hardcase-w-5mm-female-bullet-connector.html

[Peabody]

I finished charging the lipo this morning.  I connected it to my 18650 charger, which says it's output is 4.2V, 650mA.  The lipo claims to be 1800mAh, but my guess is the 650mA is really something like 0.5C.

The voltage across the lipo increased slowly, and I finally lost my courage when my cheap digital multimeter said it had reached 4.25V and the red LED on the charger was still on.  The lipo measured 4.19V on my meter after terminating the charging. 

I think what I need to do is charge an 18650 while watching the voltage, and see where it is when the green LED comes on.

There was no heat produced in the lipo that I could detect.  But I don't know what protection is in the lipo, or in the charger.  But if I plug in the charger with no cell in it, or connected to it, the green LED comes on.  So I kinda assumed the charger wouldn't overcharge.  But 4.25V is a bit much.  The 18650s I have are protected.  So I may try to get into the charger and see what's there.

[paulca]

If it holds the 4.19V or close to it overnight it's a winner.  Put it somewhere it won't set fire to stuff though

When you aren't using it the LiPo will thank you for dropping it to around 3.8V

[james_s]

So no, 18650 cells aren’t relegated to low current usage.

RC LiPo batteries are rated in 30C constant 50C burst and some even claim 50C constant.   You can get 8Ah cells that are rated 50C.  So thats 400A without parallel cells.  Pro 700 size 3D heli motor can pull 3kW.  They often use 2x6S in series (44V) to try and lower the amps and prevent melting wires etc.

The pack will be depleted in a single 6 minute display.  It will be almost too warm to touch and slightly puffed.

Actually.  Seems thing have moved on a little since I last looked...

90C constant 6.6Ah = 594Amps.  (Although 2S2P)
https://hobbyking.com/en_us/graphene-6600mah-2s-hardcase-w-5mm-female-bullet-connector.html

A lot of those C ratings are pulled out of thin air. I've seen LiPos that claimed C ratings so high that the discharge leads would burn out almost instantly. Any claims beyond about 40C continuous are highly suspect. A true 40C discharge would deplete the pack in under 2 minutes, 90C would mean the pack goes from full to dead in 30 seconds. If that were even possible it might work for a drag race but not much else. 

[paulca]

A lot of those C ratings are pulled out of thin air. I've seen LiPos that claimed C ratings so high that the discharge leads would burn out almost instantly. Any claims beyond about 40C continuous are highly suspect. A true 40C discharge would deplete the pack in under 2 minutes, 90C would mean the pack goes from full to dead in 30 seconds. If that were even possible it might work for a drag race but not much else.

Yes.  I have seen people doing speed trials with RC cars where they got one run from the battery.  Probably binned it to afterwards.  I think the record is around 200mph.

I'm sure a lot of them are fictional.  I accidentally shorted one through a mosfet load that went fully open when the sense line broke off.  Luckily it was a little small one, 500mAh, but it was rated for 15C.  The multi meter registered 16 Amps while screaming at me as it's 10A rated.  Luckily I got every disconnected fast enough that I didn't blow the fuse or the LiPo.  The LiPo was puffed and warm after only about 10 seconds.

[james_s]

Some guys have done internal resistance tests and found many of the C ratings to be demonstrably false. I typically assume that a LiPo from a reputable brand is good for about half the stated C rating.

[paulca]

I can test a few small ones, but I'm not testing a big one

The 500mAh one for example has been through the wars a little. I can test if it will sustain a load of 15*0.5 = 7.5A.  I recon it will.

I know the 2200mAh 20C packs can sustain 200-300W@12V, so thats 17-25A, not really a challenge to the claimed 44A and I don't really have a load that will test it at 44A.  That is of course the other problem.  Testing them at those loads. 

The 30C sustained, 50C burst 5000mAh LiPo should be able to start my car. 250A burst.  I could probably measure that, but it would require quite an interesting set up, probably involving copper pipe or welders wire as a sense resistor.  How long could it sustain 150As without melting is more the question a 0.1Ohm resistor in a bucket of water can't possibly be sensible, can it?

But Lipos producing enough burst current to start a car is not new, they sell them for that purpose in some devices.

[Don Hills]

... it would require quite an interesting set up, probably involving copper pipe or welders wire as a sense resistor.  ...

Big Clive did exactly that. Quite spectacular.