Memory Effect Discovered In Lithium-Ion Batteries 157
rwise2112 writes "Lithium-ion batteries have long been thought to be free of the memory effects of other rechargeable batteries. However, this appears to be not the case. Scientists at the Paul Scherrer Institute PSI, together with colleagues from the Toyota Research Laboratories in Japan have now discovered that a widely-used type of lithium-ion battery has a memory effect."
paywall derp (Score:5, Informative)
And that type of battery is.... (Score:5, Informative)
LiFePO4.
Cliffhangers in the summary now?
Wrong description (Score:5, Informative)
I don't know about Lithium batteries but NiCad cells exhibit a second plateau which gradually gets more difficult for the charge system to punch through. The usual cure is a couple of heavy charge/discharge cycles.
Don't think I want to try that with Lithium though!
Re:No Shit (Score:4, Informative)
Man ... living in your head must be terrible, in a world so full of daemons and enemies constantly conspiring against you. No wonder you are a bitter AC.
To my knowledge, both his assertions are accurate. One of them is supported by TFA.
Re:Laptop batteries, anyone? (Score:4, Informative)
Okay, first up let's define memory effect: Memory effect is NOT a battery dying. Memory effect is the tendency for a battery to stop charging/discharging at a set level if you regularly fail to completely charge/discharge it. It develops a 'memory', and thus falsely acts as though it's fully charged or discharged before it actually is.
While this can ruin a battery, a number of techniques have been developed to rehabilitate such batteries to restore full function.
However, batteries don't just wear from charge/discharge cycles. They age over time as well. Alkaline and Lithium primary cells are especially resistant to this, but until very recently LiIon rechargeable cells were very, very vulnerable to this, losing 10% or more of total capacity just sitting on a shelf in a cool warehouse at 70% charge(the ideal situation for them).
Sitting in a hot laptop being kept at 100% is much worse than ideal, you could be losing 30% or more per year in that scenario.
Re:Small effect big consequences (Score:5, Informative)
Pretty accurate. They see a small deflection on what was commonly believed to be a smooth curve near the previous peak charge.voltage. It does not affect the overall charge capacity of the battery over time (what people commonly think about when "memory effect" is used), just the ability of the charge/lifetime remaining software to make accurate estimates.
For all examples shown, a user would be told they have significantly more charge remaining until near that point, then immediately after it would appear their predicted battery life would drop dramatically, and then it would stabilize again. It makes sense that this would be of keen interest to Toyota and other electric vehicle manufacturers.
If their graphs are as accurate, noise-free and reproducible as the figures lead the reader to believe... then the good news is, this effect can probably be accurately modeled and compensated for now that we know it exists. In that respect, it is a significant step forward for Li charge remaining prediction software.
Re:Small effect big consequences (Score:4, Informative)
Voltage monitoring should be just one aspect of charge determination. Properly done charge monitoring integrates the electrical charge actually delivered to or taken out of the battery -- as in taking the time integral of the current. The cell voltage should only used together with other indicators (cell temperature!) to determine each cell's health and charge/discharge endpoints (fully charged and fully discharged). The cell voltage should not figure in normal battery "% remaining" indications -- those solely base on the charge taken out of the battery, and the estimation of the 100% charge capacity.
Re:Small effect big consequences (Score:5, Informative)
The question is: how big is the effect. Even a small effect will cause significant distortions in battery metering,
True, and this is what is seen.
but if the effect is large enough, it will cause the batteries not to last any where near as many cycles as originally believed.
Rubbish. It doesn't diminish the battery's capacity, just changes the q-v characteristic DURING CHARGING. Even if it were a large effect, it's still not going to effect total capacity, or estimating remaining charge by measuring voltage during discharge.
Notes regarding this:
1. This effect is shown in LiFePO4, which is commonly marketed as a "safe" chemistry Li-ion. Laptop and mobile phone batteries almost universally use LiCoO, which is not, AFAIK, addressed in this research. The Tesla Roadster also uses either LiCoO or LiMnO (I've seen conflicting reports -- probably because these two have similar electrical characteristics -- and both have much better energy density than LiFePO4), so your plea to Roadster owners seems a little odd...
2. This effect is caused by starting with discharged battery, charging partially (greatest effect for charging ~50%), discharging completely, then recharging completely. During the final (complete) charge process, the voltage starts at baseline (i.e. full-discharge/full-charge cycle, at the same % charging), increases slightly faster than baseline, so that the voltage difference over baseline peaks at 50% (or whatever state you partially charged it to), then increases more slowly than baseline to arrive at the same voltage when fully charged -- so capacity measured while charging will be overestimated. On the subsequent discharge, however, the q-v characteristic conforms to the baseline -- so capacity remaining will be measured accurately when a device is in use.
Re:No Shit (Score:5, Informative)
Re:Small effect big consequences (Score:4, Informative)
My '08 Roadster (there are no '07 roadsters) has 33k miles on it, and after 4 1/2 years, its battery capacity has been reduced about 8%. I now get 225 miles on a full charge, down from 244 on day 1. That's even better than Tesla's initial projections, actually.