C rates & Ah capacity

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PlanB
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C rates & Ah capacity

Post by PlanB »

This is not very relevant to EVs but thought I'd share anyway. I bought a random sample of 18650 ebay cells between 4Ah & 2.4Ah capacity. Turns out their internal resistance is nowhere near low enough for EV use. Also a 0.1C discharge suggests the marked Ah capacities are optimistic to say the least compared to what I'm used to with lead acid AGM cells (a 5Ah lead acid will give you 0.5A for 10 hours no sweat)!

Now I've got a few dozen or so cells & I wonder could I use them as replacement for a 12v 5Ah AGM lead acid we normally use with solar powered radios in my day job (when I'm not fantasising about EVs).
So the solar app is really flea power (15mA) & I don't want to wait forever to get a handle on true Ah capacity for the Li Vs Pb in this app, so I decide to discharge at 150mA instead of 15mA to get an answer 10 times quicker. Result Li 4Ah cells are flat after 12 hrs, 12x.15=1.8Ah. Talk about misleading labels!

Interestingly when I hook the litiums to the radio module at 15mA & run it I get 7 days, 7x24x.015=2.5Ah Seems to me in China you can put a wrapper on a cell & label it what you like. Extrapolating I might see 4Ah capacity at 0.0001C?!
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lithbattboss
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C rates & Ah capacity

Post by lithbattboss »

That is quite normal and to be expected if the cell are from a Chinese battery manufacturer/company. Are they a reputable brand or a no name cell?

You have to be very careful when buying cells from China. I received a few 26650 "A123 cells" which I was absolutely, positively guaranteed they were genuine A123 cells. They even had all the proper markings of a genuine cell including the barcode under the shrink wrap etc. I found out the cells aren't even LiFePO4 but lithium cobalt dioxide.
When buying cells from China beware! The attitude in China is "sirry westerners will buy our crap if it looks real because they only rook at cheap price".
ppsssst!Anyone want to buy some genuine Rolex watches and Nike shoes which are going real cheap!!? Image
Where power matters.
XPS Power cells 25C discharge for high performance applicatiopns.
www.lifetechlithium.com
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PlanB
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C rates & Ah capacity

Post by PlanB »

I was testing the 'protection' of the 18650s, 3 in series wired to a 12v 48w auto headlight. Sure enough after a little while the globe would switch off, when it was disconnected & the cells cooled it would light again. I assume some sort of poly fuse in the cap or something?

Any way after a few episodes of this treatment one of the cells just died, voltage across it dropped to 30mV or so but still low impedance internally because I could charge the other 2 in the series string with it still in circuit. So not an open circuit poly fuse.

Any body know if this is a characteristic of Lithium polymer? I'm thinking maybe a cathode to anode short internally? I opened the cell up & unrolled it, no obvious signs of heating or other 'hotspot' type of electrical damage like you'd sometimes see in older paper foil capacitors which the the innards of an 18650 remind me of.
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