Rechargable Battery Questions And The Gp2x

[DrPibb]

Hi, I have some questions about rechargeable batteries.

1. I just bought a charger with 4 batteries at a RadioShack. I didn’t notice until I got home that it says on the batteries that they're only 1.2volts. Regular batteries say they're 1.5 volts. I have another set of rechargeable batteries that I got with my digital camera a few months ago and they say they're only 1.2v too. I tried some charged up old ones with my GP2X and they don’t work. So my question is why do rechargeable batteries say they're only 1.2volts and regular batteries say they're 1.5volts? Has anyone gotten 1.2volt rechargeable batteries to work with they're GP2X?

2. Is the higher the mAh number on the batteries the better they are? What’s a good number to get? What does the mAh number really mean?

I haven’t opened the batteries and the charger I bought from RadioShack yet. So I could return them anytime.

Thanks

 

[BenScar]

1. Most (if not all) rechargeable batteries are 1.2v, something to do with their make up I believe (not sure why!). It's what I run my 2x on so I know they work

2. Yes, the higher the mAh the better, guessing 2500mAh is a good starting point, mine last about 2/3 hours on a full (slow) charge.

Hope that helps

 

[Vimacs]

mAH means how many (m)milli (a)ampere they can delier over a (h)our.
The more the better, for gp2x at least 2500 is sugested, higher is better but dont fall un unbranded ones they often have "fake" capacity reating.
Good brands i tried are sanyo, gp, ansmann.
And dont forget that the batterys are only half the bill, a good charger realy matters in how much capacity you can pump into them.

I get 4-6h on my 2500mah gp's, havent stopped the others yet but its about the same.

 

[James_G]

mAh is a measure of the storage capacity of a battery. The number itself indicates for how long the battery would last, in hours, if delivering a current of 1mA. Thus if you knew what current a device drew, you would know how long the batteries would last.

So for example, if we were to assume the GP2X draws 1 Amp. (Its not too far off from this, but in practice it will depend what you are using it for)

1 Amp = 1000mA

So we divide the battery's capacity by 1000. 2600/1000=2.6h
But you have two batteries, not just one: 2.6 x 2 = 5.2h

In practice it wont quite work out this way, as the current drawn by the GP2X varies, and other factors play a role. (I believe that the resistance across the battery changes as it runs down, but I may be wrong on that account.) But you may generaly consider that a 2600mAh battery will last twice as long as a 1300mAh one.

Edit: Seems Vimacs got there before me. Teach me to write unecessarily long replies.

 

[DrPibb]

Awesome. I guess I'll be returning what I bought at RadioShack. The batteries I bought there are 2000mAh.

I looked at www.newegg.com and found some 2700mAh. The brands there are "Targus" and "Powerex". Are those safe? Are there any other good web-sites to buy good batteries and chargers with a credit card? How do I know what a good charger is? What should I look for? How many batteries do you have? I think 8 should be enough.

Thanks for your help.

 

[Shikaku]

*Currently has 2700 mAh Powerex batteries...

4 should be enough for most people...

 

[jmetal88]

Awesome. I guess I'll be returning what I bought at RadioShack. The batteries I bought there are 2000mAh.

I looked at www.newegg.com and found some 2700mAh. The brands there are "Targus" and "Powerex". Are those safe? Are there any other good web-sites to buy good batteries and chargers with a credit card? How do I know what a good charger is? What should I look for? How many batteries do you have? I think 8 should be enough.

Thanks for your help.

You should still be able to get 2-3 hours on 2000mAH batteries. That's what I got with my RadioShack I-C3s.

 

[gokalp]

MrPepper: I am using those Powerex batteries, and they are pretty good. I bought 8 of them (2 of 4-packs), but actually never got to open the second pack.

James_G: Afaik, the resistance change in NiMH's are not significant. They inherently have low internal resistance (that's why they are preferred for high current applications), and they have a rather flat discharge curve.

 

[Squidge]

With the energizer batteries I have (2300mAh), the internal resistance is 30 milliohms which increases as the batteries discharge (40 milliohms when half-charged)

 

[paulobecker]

1. Most (if not all) rechargeable batteries are 1.2v, something to do with their make up I believe (not sure why!).

The battery's voltage (measured in volts, V) depends on their chemistry, i.e., what elements it uses for the reaction that provides energy, whereas their capacity (which is measured in milliampere hours, mAh) usually depends on the size of the battery.

 

[BenScar]

The battery's voltage (measured in volts, V) depends on their chemistry, i.e., what elements it uses for the reaction that provides energy, whereas their capacity (which is measured in milliampere hours, mAh) usually depends on the size of the battery.

Surely the make up of the battery has some effect on the mAh, changing from Ni-Cd to NiMh allows more mAh to be available as well. Or am I thinking along the wrong lines there?

 

[gokalp]

With the energizer batteries I have (2300mAh), the internal resistance is 30 milliohms which increases as the batteries discharge (40 milliohms when half-charged)

Exactly. When GP2X draws 1A (which is a value I have read in this forum), the voltage drop is initally 30mV, eventually reaching beyond 40mV. For comparison, alkaline batteries, whose capacities are much higher, have several times more internal resistance. Thus, even though they have higher capacities, they cannot power high drain applications for a long time due to the quick internal voltage drop (>100-200mV @ 70% capacity).

I found a nice figure illustraing the discharge curves for a NiMH and alkaline battery, you can check the rest of the pages as the author explains stuff pretty well:
http://home.att.net/~mikemelni1/battery3.html

 

[mrsnature]

But you have two batteries, not just one: 2.6 x 2 = 5.2h

but the gp2x uses around 3v (say, two AA batts) so it still only delivers the 2.6Ahrs at that voltage....

 

[Gajit]

But you have two batteries, not just one: 2.6 x 2 = 5.2h

but the gp2x uses around 3v (say, two AA batts) so it still only delivers the 2.6Ahrs at that voltage....

Yup, batteries connected in series (like in the GP2X) will deliver twice the voltage at one, but the same capacity (or rather, same running time at a given current). Batteries connected in parallel will deliver the same voltage as one, but twice the capacity/running time.

Series:

	|---------|   |---------|
----| -	  + |--| -	  + |----
	|---------|   |---------|


Series in a battery compartment (such as the GP2x):

	|---------|
----| -	  + |--\
	|---------|   |
				  |
	 |---------|  |
----| +	  - |--/
	 |---------|


Parallel:

	|---------|
  /-| -	  + |-\
  | |---------|  |
-<				>-
  | |---------|  |
  \-| -	  + |-/
	|---------|