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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> Common Sense RC Support >> mAh ratings bogus?
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mAh ratings bogus? - 3/1/2008 8:07:03 AM   
daveheart777


 

Posts: 2
Joined: 3/1/2008
From: Chicago, IL, USA
Status: offline 		Hello. I'm new to this forum. I'm travelling overseas and currently in the Philippines. I use a portable dvd screenless player the round one that has an inside lithium battery rated at 7.4v at 950mA. It lasts about 1.5 hours. I built an external battery pack using 7 NIMH aa batteries. (7x 1.2v in series = 8.4v) These batteries are rated at 2700mAh. But this battery pack lasts a shorter amount of time than the 950mAh Lithium battery. I use a "smart" charger so the batteries are getting properly recharged. I am wondering if a lithium battery is rated at a different discharge rate than Nimh battery. Thus the smaller Lithium battery could last longer. But I'm in the Philippines noted for imitation products! It could be that battery ratings have no meaning here! And I can't return the batteries because they all work! Anyway has anyone experienced lithium battery vs Nimh performance? Are Lithium rechargeable batteries rated at a much higher discharge rate---thus last longer, or you think my Nimh batteries are just bogus rated? I tried "Sanyo" and "Sony" batteries. The printing on the batteries and packaging sure looked original to me. Tx. Dave.
       Post #: 1

RE: mAh ratings bogus? - 3/1/2008 9:24:43 AM   
guver


 

Posts: 8962
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		Hi, welcome to rcu.

It's kinda hard to tell what's going on , but capacity is capacity regardless of where it comes from or which battery type.

If we assume the LI batt is correct at 950 mah then perhaps the AA's are not getting full charged. That's my first guess.

It would be nice to test both batts.

Dopes your device list an operating current? voltage?

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(in reply to daveheart777)
       Post #: 2

RE: mAh ratings bogus? - 3/1/2008 5:59:33 PM   
daveheart777


 

Posts: 2
Joined: 3/1/2008
From: Chicago, IL, USA
Status: offline 		Well, the voltage as I said is 7.4 volts with the internal lithium battery. My battery pack is 1.2 volts with 7 batteries in series connected giving 8.4 volts. If I used only 6 1.2v cells, it would be only 7.2 volts. The dvd player turns off when battery discharges to 7.0 volts. I checked battery voltage of the AA batteries after charging. My meter reads 1.48 volts at each of the AA cells, after fully charged, open circuit voltage. ... I wonder if lithium battery drops to 7.0 volts more slowly than the Nimh battery pack. Maybe the discharge rate curves are different? I am using a charger which indicates when full and then turns off when the cells are fully charged.

(in reply to guver)
       Post #: 3

RE: mAh ratings bogus? - 3/2/2008 6:52:57 PM   
guver


 

Posts: 8962
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: offline 		All your numbers seem spot on. It sure would be telling to overlay two discharge graphs. That would sure tell what's going on. The nimh battery should really outlast the lipo (more than double at a 7 volt cutoff)

Do you know what the current useage of the dvd player is?

2 other things come to mind: a bad cell in the nimh pack (falls to 0 volts when discharging) and the charger thinking the batts are full when they are not. Let's throw some numbers at that possibility once.

charge rate times time = capacity.

Does the charger list a current rate and can you verify the amount of time it takes to charge? Are the cells quite warm when charged?

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you hear that silence?

(in reply to daveheart777)
       Post #: 4

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