Li-ion\li-poly cells - are they easy to source safely?

[knotlogic]

I was wanting to work on a project that would run off a pair of 9 V batteries.  The capacity of alkaline 9 Vs and the corresponding run time was nothing fantastic, then I discovered that there there are rechargable lithium 9 V batteries out there.  Great!  The only problem is, the places I asked quoted me really high shipping costs.  One quotation had the shipping cost at nearly three times the cost of the batteries and charger.  I'm guessing there's a bunch of checks and special shipping arrangements for shipping lithium cells, but I'm not familiar with them.

Then I figured those 9 V rechargable lithium cells are just 2 li-ion or li-poly cells in series, with appropriate protection circuitry.  They're all listeb as actually being 7.4 V nominal.  So I took a look at battery packs for R/C use.  Places like Hobbyking / Aliexpress / Taobao have all sorts of capacities and sizes, plus they're cheaper and I don't need a same-brand charger.  Or, going a step further, I could pick up bare cells and add protection circuits to them.

The question is, what kind of quality could one reasonably expect from cells sourced from places like Aliexpress or Taobao? (I'm not even going to consider eBay.)

Some of the stores seem to be set up by manufacturers themselves, selling small quantities in hopes of landing production orders.  But a quick read through of R/C forums has lots of cautionary tales of R/C packs swelling up or catching fire.  Heck, they actually sell special bags for charging cells in that claim to help contain any fire or minor explosion.  And those are R/C packs which I would have thought have slightly higher Q/C standards applied to than loose cells.

I'm even planning to adopt a rather conservative power cycle - charge voltage of 4.1 V and cut-off of 3.1 V.  Assuming I can set those.  But that wouldn't protect me from a cell that was defective in the first place.

So what do you guys think?  Any first hand experiences working with / sourcing these cells for home projects?  Am I overly paranoid or would this be a bad idea?

[mikerj]

RC Li-Ion packs are much much greater risk of failure because they are regularly abused, both physically by crashing models and electrically by demanding very high currents and using very fast charge profiles with no inherent protection circuitry other than that provided by the charger itself.

Provided you use a conservative charge and discharge profile you are unlikely to see failures, but note that cheap chinese cells tend to exaggerate the capacity ratings, sometimes significantly, and if you are putting two in series you might want to consider cell balancing to maximise life.  You don't need cell balancing provided there is a protection circuit, but it means when the cells become imbalanced the amount of energy stored (and hence your runtime) decreases.

[katzohki]

Hobby King is fine. I think they're regarded as reputable in the RC market. I've bought a few LiPo packs for my helicopter. Don't believe what's too good to be true is one rule of thumb for buying these. I would recommend a charger specifically for LiPo batteries as they do not charge in the same way (and a good one has protections built in). You can also build an auto shutoff into your application circuit (or they sell little circuits that do this).

If you set your country location in your profile to display, it can help people give you more local sources.

[knotlogic]

If you set your country location in your profile to display, it can help people give you more local sources.

uh... Done.  I'm in Singapore, and it's reasonably easy to purchase stuff from the States for me.

I definitely wouldn't be pushing the cells anywhere near what the R/C folk do.  At most a draw of 50 mA?  The question would be if the cells themselves were safe to begin with, or the product of shoddy manufacturing.

What I would need is two packs of two series cells each.  Which is essentially what the lithium '9 V' cells are.

I came across this charger while looking for info, which came recommended on one site.  Not the cheapest, but I wouldn't have to worry about also designing a safe and effective charger.

[Chalcogenide]

Have you considered running a single lithium cell with a DC/DC boost circuit? Single lithium cells can be found from reliable suppliers at reasonable cost, can be charged easily from a USB port with a TP4056 chip and many have a protection circuit built-in. You don't have to deal with balancing and don't need an expensive charger at all.

[Audioguru]

I buy Name Brand lithium batteries for my RC airplanes. My friend gave me a cheap battery from Hobby King that failed soon. Most people on the RC Groups forum say that cheap Hobby King batteries are poor quality. Lithium batteries are dangerous and explode or catch on fire (especially the poorly made ones) so they are expensive to ship.
Aliexpress is a Chinese reseller the same as ebay and their quality (and fake junk) is just as poor. Their "detailed spec's" list only the size and weight in horrible Engrish.

An ordinary Lithium-Cobalt battery cell averages 3.7V during a discharge (two cells are 7.4V). The voltage of a cell drops to about 3V when its load must be disconnected or the battery will become ruined and is 4.20V when fully charged and when its charging current has dropped to a low amount when it must be disconnected. Then 2 cells make 6.0V to 8.4V.