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Li-ion pack strangeness ....

I'm no stranger to Li-Ion behaviour, having studied them a fair bit and having designed commercial chargers. I think I know what the issue is here, but would appreciate informed comment from others.

Laptop pack, "Li-Ion 10v8 @ 4500mAh" Haven't opened it up yet, but NiXX chemistry wouldn't achieve the capacity in that size anyway.

Charge in laptop (AC on, machine off), indicator LED goes green after ~ 3 hours. After 1/2 hour "rest", boot to Windoze on battery power, on-screen gauge shows 100% declining to 54% after 2 hours operation. Resume AC supply, turn machine off, battery recharges to green LED status (not timed). Rest 1/2 hour, measure pack terminal volts =

12v51. Leave pack out of machine.

Ten hours later, measure 12v48. Alles ist gut. Insert pack into machine (no AC) and boot. Reach desktop, then machine shuts off. Restore AC, reboot and check on-screen gauge - "2% and charging".

Clealry the pack has usable capacity as it sustained 2 hours' operation without drama. Clearly the pack did *not* self-discharge in the ten hours. Right now it is "recharging", meaning going through the motions to satisfy the electronics - while the pack is already effectively at full SOC.

What I failed to do is measure the pack voltage *after* the premature shutdown. Will do that on a later test.

Possibilities that I see are:

(a) the pack protection module decided prematurely (on some basis) that the pack was exhausted.

(b) the laptop, based on data (or lack of) from the pack's electronics, made that call. The pack is "brand X", and the on-screen SOC panel tags it "standard APM battery" and "Manufacturer: unknown"

Anyone seen this type of behaviour before? Comments? Ideas?

Reply to
who where
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Follow-up: After another premature shutdown (which was prefaced by a warning message "you should shut down immediately to save your work" or similar), the pack was removed and measure 12v52.

Reply to
who where

Clearly??? The initial conclusion from the experimental evidence suggests the "it did".

the pack did *not* self-discharge in

Pack voltage is mostly irrelevant. Bad voltage implies a bad pack. Good voltage implies nothing.

Battery controller IC's are not designed to optimize the life of the battery. They're designed to limit the liability of the vendor. They're designed to protect you from suing the vendor. They're designed to be lowest cost. They sell more high-margin batteries, so there's little incentive to fix it.

Lithium batteries increase in series resistance as they age. I've taken failed packs that won't even run the computer long enough to boot and discharged a significant portion of the capacity from them. Problem is that the resistance is high and lapotp peak currents are high. The voltage dips and the machine shuts off. The electrons are in there, but the system won't let you get them out. You could set the trip voltage lower, but then you'd overdischarge a good pack. There's considerable opportunity to have better gas-gauge performance, but it comes with risk of being sued. Keep it safe, no matter what it costs the customer.

The gas gauges have varying sophistication, but measure charge in/out and voltage on a cell basis. Some have memory that remembers the current capacity of the cell to more realistically represent the available run time left. Bad news is that these can get out of sync. Some can get back in sync, others can't. In many cases, you can replace the cells and the chip still remembers that the pack is bad. This can be reset, but nobody will divulge how...to protect you from hurting yourself.

One limp cell can cause the pack to be bad even though the pack voltage seems to be ok. And a bad cell surely can self-discharge overnight.

Reply to
mike

Indeed.

Would disagree completely. For a healthy - or even reasonably usable

- cell/pack, the OC voltage gives a very good indication of SOC. The voltages measured reflect close to 100% charge.

What that means in terms of degraded cells/packs' capability to operate a load depends on the type of degradation. Loss of usable capacity still validates the "100% SOC" comment, it just applies a different scale to the discharge curve.

Increase in effective series resistance will obviously have an effect on operation under load, and may well result in not just premature shutoff but in failure to accept load.

Having accepted that this pack *can* hold load for 2 hours with some capacity remaining, it is a reasonable assumption that internal cell resistance isn't causing the failure to accept load after a rest period.

A tad cynical.

Having worked with these pack protection modules I understand pretty well the criteria for their intervention. Unless the raw cell behaviour changes dramatically after a ten (vs. 1/2) hour rest period, I can't see an alternative to the module's intervention. But I am open to suggestions that explain - or at least fit in with - that behaviour.

Aware of the above, including how to reset some - but I'm not about to expose myself to risk of litigation by passing it on.

But self-discharge isn't going to present the same OC voltage.

Reply to
who where

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