Traction battery - memory effect

There are plenty of folks around with more technical knowledge but my best guess would be it's because the "battery" is not really one battery but rather a collection of smaller cells. Thus individual cells are more completely drained and charged. Even so, they must be engineered for rapid cycling and to withstand a tremendous number of cycles.

 

"Memory Effect" has become a catch phrase for all sorts of things that have nothing to do with "memory" - I've read that the true 'memory effect' was only observed in computer-controlled charge-discharge cycles in satellite applications, and only in NiCd batteries. My own experience with NiCds was that battery failure, low voltage and decreased capacity were most often due to improper charging or just 'old age.'

Have a look here: "Battery Memory Effect - What it is and what you can do about it"
The author claims that 'memory effect' almost never occurs in NiMH. The Prius computers work hard to maximize the lifetime and capacity of the traction battery, so I think they qualify as what the author calls 'specifically made' for the NiMH battery chemistry.

 

I believe the very fact that the Prius doesn't use all the battery would shield it from the memory effect. Who cares if the battery capacity gets depleted to 80% of nominal if you only want to use 40% of nominal?

And as an aside - the Prius does have a battery conditioning function. It will very occasionally perform a discharge/charge cycle on the battery (over the limited range it uses).

 

NiMH batteries are much more robust than NiCd, and are nearly immune from the "memory" effects that plague Pb-Acid and NiCd. LiIon technology is actually much more delicate, potentially dangerous, and expensive than NiMH. Really, the only advantages of LiIon over NiMH are charge density and a greatly lessened self-discharge rate.

Self-discharge is the rate at which a battery loses charge while in storage. NiMH are the fastest self-dischargers of all of the common battery types, which is one of the reasons why Toyota recommends that the Prius be driven at least once every three weeks.

The important concept about Prius battery management, however, is that the onboard computers are very smart about how they allow the battery to be used. If properly cared for, NiMH batteries can last a very, very long time - in our case, Toyota expects 150,000 miles of typical use.

 

One other bit after reading the link supplied above - the information given about voltage depression ("memory" effect) is only half of the story. The other half (or 80-100% in the case of NiMH and LiIon batteries) is mechanical failure. The storage of energy in a chemical battery is a mechanical process that involves chemical and conformational changes to a substrate. In the case of the good ol' Pb-acid battery, the substrate is big plates of lead. (Unless you're using a Pb-Gelly battery.)

Eventually, given enough charge/discharge cycles, heat extremes, and mechanical trauma (bumps, vibrations, etc.), the substrate in any battery will wear out. This is why your laptop batteries, which these days are all LiIon, slowly loose their potency and must be replaced every year or so.

Deep discharge and overcharge are especially injurious to NiMH batteries, which is why the Prius uses only about 40% of the battery's full charge range. Prius also closely monitors the temperature of the battery, and will stop charging in the event of battery overheat.

More than you needed to know. Again, the take-away is... "just drive it!"

 

No need to take the battery out of a modern laptop, as they manage the charge intelligently as well.

Laptop batteries just see a lot more use and complete charge/discharge cycles than does the Prius traction battery. This is what laptop batteries are designed for, though - replacing them every year or two is just part of the maintenance cost of owning a laptop.

Happy driving!

 

I still have the first laptop that a company purchased for me: a 1994 Toshiba Portege with two Li batteries. They still work, and I even play with it once in awhile.

I've never had to replace a Li battery in a laptop.

 

My LI Motorola StarTac cell phone battery is 8 years old, still charges fast. I don't usually let it run down close to empty, which may explain my good luck.

 

I wish I could say the same. The capacity of my laptop batteries has always tracked the manufacturer's predicted downslide fairly accurately. Brand-new 65Wh batteries turn into <20Wh paperweights in about 14-18 months. Very noticable when you're trying to watch DVDs - er, I mean finish that spreadsheet - on a five hour flight.

The newer LiIon polymer batteries last noticeably longer for me, though. I'm finally replacing my first set after over two years of use. Too bad I can't replace the laptop as well!

 

I think my favorite article explaining memory effect to laypeople pretty well is by Dan of Dans Data (dot com).

http://www.dansdata.com/gz011.htm

"Dan's Quick Guide to Memory Effect, You Idiots"
"(with apologies to Stephen Notley)"

(not actually calling anyone an idiot -- it was an obscure reference to Bob the Angry Flower's "Bob's Quick Guide to the Apostrophe, You Idiots")