# diagnosing an ailing Li-Ion battery pack

• posted

Hi all, I'm trying to figure out what's wrong with a Li-Ion battery pack. It's a cheap one that I got from eBay for my DSLR camera. After taking a photo with this battery, the camera shuts down suddenly. I presume there's a lot of current drawn during and immediately after the exposure (CCD, stabilizer, shutter, and reflex mirror).

I ordered a new, better battery for about \$10, but for my own edification I am trying to understand what's wrong with this battery. I'm using a Keithley 2400 source-meter in my university lab to test this battery. Can anyone comment on my analysis here and see if it's right?

WHAT'S INSIDE?

I popped the cover, and inside there are two 18500 lithium-ion cells and a small protection circuit. The cells are blue and unlabeled, so I'm not sure what their capacity is, but I'd guess it's 1300 mAh based on simialr cells for sale online.

TEST RESULTS

Protection circuit: Seems to be fine. If I try to apply a charging voltage above 8.5 V, it kicks in and the charging current drops to almost zero. If I try to draw too much current from the cells, the voltage drops below 6 V, it kicks in and I can't draw any current.

Cells: (1) measures 4.14 V drops to 3.70 V if I draw 1 Amp (0.44 Ohm internal resistance) drops to 3.67 V if I draw 1 Amp for about 15 seconds (2) measures 4.06 V drops to 3.5 V if I draw 0.5 Amp (1.12 Ohm IR) drops to 3.3 V if I draw 0.5 Amp for about 15 seconds

Whole battery: Voltage is around 8.2 V fully charged. If I try to draw 1 A from it, voltage falls under 7 V almost immediately.

MY CONCLUSION

Cell #2 is bad. Its internal resistance is too high, so the battery can't provide the current that the camera requires (> 1.8 A spikes).

So, what do you guys think? Is this the correct diagnosis?

What *should* the internal resistance of a Li-Ion cell be? This site

says 320 mOhm is typical for an "aging" Li-Ion battery. So according to that, this battery is totally worn out junk. Anyone know where you can buy good Li- Ion cells?

Thanks for any insight,

Dan

• posted

The appropriate "insight" is that third-party Li-ion batteries often aren't very good.

The one I bought for my Yaesu transceiver -- from a legit, honest battery dealer -- died pretty quickly. The Li-ion battery for my Olympus E-500 -- from a well-known third-party manufacturer -- does not the capacity or charge retention of Olympus's battery.

This is unfortunate, because the manufacturer's batteries seem to be overpriced.

• posted

Yeah, it is unfortunate...

Do you know of any third-party Li-Ion batteries that *are* good? It seems that part of the problem with most of them may be that they don't use very fresh cells.

Dan

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