NiMH battery questions

[luckie]

Newbie questions: For those of you with a good amount of experience with battery technologies.

1. (Just thinking theoretically, very interested in the answer to this question)
If my commute back and forth to work each day only consumes 40% of the Segway's full charge, to maximize the longevity of the batteries I should:
a. Charge every other night allowing the discharge to get to 80% before recharging.
b. Charge every night even though it's only used 40% of the charge.


2. (Still thinking theoretically, only mildly interested in the answer to this question)
Given the Toyota Prius uses NiMH batteries just like the Segway (likely good for 300-500 charges?), and it's charge-recharge cycling must be even more erratic (ie shallow discharge for 15 minutes, followed by brief charge 5 minutes, then discharge in town for 45 minutes, then charge on highway for 30 minutes etc), why would Toyota predict the batteries have a 8-10 year life?

----
Douglas B. Luckie, Ph.D.
Cystic Fibrosis Research Lab
Department of Physiology and Lyman Briggs School of Science
Michigan State University
http://surf.to/msucf

[SegwayUtah]

I don't know a whole lot about the magic of batteries, but I read an article that said that discharging batteries only 10% before recharging would lead to a cycle life multitudes of times higher than running the batteries down a lot before discharging.

This makes me think that plugging in your HT as often as possible actually leads to better battery life. But then again, not letting your HT batteries cool down before recharging might (or might not) shorten their life. I'm really not sure.

What I do know is that Segway is planning on releasing some advanced battery care guidelines. They're not out on the web yet, but when they are you may be able to find them at:
http://www.segway.com/support/docs/

Chris

[citivolus]

quote:why would Toyota predict the batteries have a 8-10 year life?Toyota uses Panasonic batteries, as does Honda who also claim an 8 year life. Coincidence? not likely.

--
swiftly flying

[Deviant]

Simple: You should charge every night, but allow the batteries time to cool first. I actually recharge on each end of my commute, even though the indicator claims I have over half left. (returning without recharging requires very slow travel. i.e. don't trust the indicator)

Your batteries will live longer if they are not discharged as deeply. Deep discharging shortens their life. The Prius batteries live long because they are not allowed to get very low.

(I am not a battery engineer, nor am I to be held responsible for anyone else's actions)

[luckie]

(This isn't a big deal or urgent but I'm still curious)
I emailed Segway LLC, and the response email from them echo'ed your opinions that battery longevity is increased with shallow discharges followed by frequent recharging.

Some internet battery sites said something similar to that: "The NiMH is rated for only 500 charge/discharge cycles. Shallow rather than deep discharge cycles are preferred. The battery’s longevity is directly related to the depth of discharge."

And as soon as Segway LLC releases their advanced battery guide the question may become moot, but I'm curious about how longevity is defined. If it's just the number of charge/discharge cycles, I would still have a dilemma.

For example:
a. If I discharge 40% each day on my commute to/fro work and charge every night, perhaps that allows the battery to last through 500 charge cycles = 500 days.
b. If I discharge 40% each day on my commute to/fro work and charge every *other* night, that might decrease longevity to last through only 300 charge cycles BUT = 600 days.

Thanks in advance to anyone who has any more thoughts concerning my questions, as well as the upcoming information from Segway LLC.



----
Douglas B. Luckie, Ph.D.
Cystic Fibrosis Research Lab
Department of Physiology and Lyman Briggs School of Science
Michigan State University
http://surf.to/msucf

[Florida Ever-Glides]

The bes thing that I do for my batteries now is to let them cool down for a couple of hours prior to recharging. Also, now I have the HT's on timers that allow the chargers to turn off 2x per 24hour period for a couple of hours each time to have the batteries cool down. They seem to now have a 'renewed life'...

Tom Jacobson

[SegwayUtah]

Luckie,

I may be wrong, but I'm pretty sure that a "charge cycle" is a full discharge and recharge of the battery. If you ran your HT down half-way, it would take 2 recharges to equal a "full charge cycle."

So:

Run down your HT 100% every day:
300 - 500 days

Run down your HT 50% every day:
600 - 1000 days

The question is: if you charge your HT twice a day instead of once a day, and only run it down 25% each time, is it possible to achieve 600 full charge cycles?

I read a website about NiMH batteries and it indicated that a certain NiMH battery could go, say, 300 full charge cycles when fully discharged between charged. Or it could go for like 3,000 full charge cycles if it was only discharged 10%. In other words, one could use a battery for ten times as long by charging them more frequently.

I should track that down so I have accurate numbers.

Chris

[W9GFO]

It would be good to know how longevity is defined.

It has been my experience that full discharges result in batteries that can deliver full capacity. Shallow discharges seem to adversely affect capacity. Hence the reason for periodic "conditioning". BTW, I think we should call it "cycling" since "conditioning" is what you do to your batteries when they are new.

Also, two half cycles are not equal to one full cycle. You will lose a little.

My recommendation;

On Mon, Tue, and Wed nite, plug in. Skip Thursday and plug in Friday nite for the weekend.

Rich H
Everett, WA

[SegwayUtah]

quote:Originally posted by W9GFO

Also, two half cycles are not equal to one full cycle. You will lose a little.

If discharging only half-way let us get 7,000 miles out a pair of batteries instead of 5,000 miles, wouldn't that be a better way to go?

I'm eagerly awaiting Segway's "advanced battery care" guidelines, for sure.

Chris

[Ground Loop]

The Toyota Prius batteries are prismatic NiMH cells designed by Panasonic. They're impressive.. 7.2v 6.5Ah each, and pretty large at 1040g. That's 45Wh/kg, but our SAFT cells still best it at around 62Wh/kg.

So you'd add about 15 kilos more battery weight for the same range, but get a longer lifetime out of them.

The Toyota also has very careful thermal management (including thermal taps INSIDE each cell) during operation to keep the battery packs in their optimum temperature band. Our HT does not.

[W9GFO]

quote:Originally posted by SegwayUtah

If discharging only half-way let us get 7,000 miles out a pair of batteries instead of 5,000 miles, wouldn't that be a better way to go?

Yes, probably.

I just wonder if after all those half charges if it impacts the total capacity. I guess if you're only using half the capacity it doesn't really matter.

Rich H
Everett, WA

[brianh]

NiMH battery tech is a crazy world and it's hard to say with any reliability that doing X will increase your usage (of any NiMH device) by Y times. I would think especially the HT because of the unique demands it places on battery needs -- lots of power instantaneously and reliably over the entire course of the battery's charge cycle.

[luckie]

Hmmmm....

After reading the new Segway "Battery Care Booklet" I think we have to re-examine this question. Previously conventional wisdom chose answer "b" but now?

1. If my commute back and forth to work each day only consumes 40% of the Segway's full charge, to maximize the *longevity* of the batteries I should:
a. Charge every other night allowing the discharge to get to 80% before recharging.
b. Charge every night even though it has only used 40% of its charge.


----
Douglas B. Luckie, Ph.D.
Cystic Fibrosis Research Lab
Department of Physiology and Lyman Briggs School of Science
Michigan State University
http://surf.to/msucf
http://35.9.122.184/blinky.gif

[W9GFO]

I think a compromise is best, as I suggested above.

Rich H
Ellensburg, WA

[luckie]

I tend to agree with you Rich and like your suggestion.



----
Douglas B. Luckie, Ph.D.
Cystic Fibrosis Research Lab
Department of Physiology and Lyman Briggs School of Science
Michigan State University
http://surf.to/msucf
http://35.9.122.184/blinky.gif