According to Lego, the Mindstorms battery should be good for over 500 
charging cycles. If you're recharging it once a week, that's about 10 
years. These batteries do wear out from corrosion and chemical changes 
as well, though, so I wouldn't expect one to last that long.

You can check the charge level of the battery from within the NXT 
programming environment. If recharging brings the battery voltage to a 
reasonable level, close to 9V, and the level doesn't drop rapidly as the 
robot is being used, then the battery's probably OK.

If you're going to buy a new battery, be sure to recycle the old one.

Ralph

On 5/16/10 1:26 PM, Sonya Shaver wrote:
> I have no information as I am seriously battery-challenged.  But I 
> wanted to add that I would love to know also.  I think that from what 
> others have said, it seems like after a couple of years the 
> rechargeable battery doesn't work as well anymore.  It kind of wears 
> down and doesn't charge as well over time.  We are entering Year 3 
> with our current robot and battery, so I was thinking about buying a 
> new battery before the start of the season.  I don't honestly notice 
> any difference in the performance of our battery as of yet, however, I 
> don't want to have chaos in the middle of the season!  So I thought 
> maybe starting with a fresh battery would be better.
>
> So, I would love to know also!
>
> best,
> Sonya Shaver in Harriosnburg
>
>
> On Sun, May 16, 2010 at 8:10 AM, Eric Palmer <[log in to unmask] 
> <mailto:[log in to unmask]>> wrote:
>
>     It appears as if Lego has a new recharable battery with 2100mAh of
>     power
>     compared with the original 1400 mAh
>
>
>     At the first site for team registration, products page there is a
>     battery
>     charger and battery listed this way
>
>     --- NEW BATTERY ---
>
>     Rechargeable AC Battery
>        * US 979798
>        * $54.95 USD
>            Description:
>
>     This battery is for robots purchased prior to the Body Forward season.
>
>     And there is a charger listed  the same way.
>     The lego Education Store lists this original batter this way
>     This rechargeable lithium battery is designed for use with the NXT
>     Intelligent Brick. The battery features 1400 mAh of power and an
>     A/C plug
>     for direct power supply. The battery will charge from 0 to full in
>     a maximum
>     of four hours. Use the 979833 AC Adapter to charge the battery (sold
>     separately).
>
>
>     --- ORIGINAL BATTERY ---
>     Then there is another battery and charger listed this way
>
>     Rechargeable DC Battery
>        * US 979693
>        * $54.95 USD
>
>     The above one is a NEW Battery that the Lego Education Store
>     describes this
>     way
>
>     Designed for use with the NXT Intelligent Brick, this lithium ion
>     polymer
>     battery with DC plug gives you an alternative to normal AA
>     batteries. The
>     battery features 2100 mAh of power and uses the Transformer 10V DC
>     (778887)
>     to charge. Estimated recharge time: 4-5 hours. Is not compatible
>     with the AC
>     Charger (979833).
>
>
>
>     The description on the FIRST site leads us to believe that the new
>     battery
>     can't be used with the older NXT bricks. But the Lego Education
>     site just
>     explains that the chargers have to be matched up.
>
>
>     Does anyone have any additional information on these batteries?
>
>     Thanks
>       ---
>     Eric F. Palmer
>     [log in to unmask] <mailto:[log in to unmask]>
>
>     -- To UNSUBSCRIBE or CHANGE your settings, please visit
>     https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join
>     or leave the list".
>
>     -- VADCFLL administrative announcements are sent via
>     VADCFLL-ADMIN-L. Visit
>     https://listserv.jmu.edu/archives/vadcfll-admin-l.html to subscribe.
>
>
> ------------------------------------------------------------------------
> To UNSUBSCRIBE or CHANGE your settings, please visit 
> https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join or 
> leave the list".
> VADCFLL administrative announcements are sent via VADCFLL-ADMIN-L. 
> Visit https://listserv.jmu.edu/archives/vadcfll-admin-l.html to 
> subscribe. 

-- 
Ralph F Grove, PhD
Professor, Graduate Director
James Madison University Department of Computer Science


-- To UNSUBSCRIBE or CHANGE your settings, please visit https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join or leave the list".

-- VADCFLL administrative announcements are sent via VADCFLL-ADMIN-L. Visit https://listserv.jmu.edu/archives/vadcfll-admin-l.html to subscribe.