mWh mAh batteries help!

Hello I'm going battery shopping and confused about battery sizes.

GENUINE Dell Inspiron Mini 10 10v 56Wh 6-Cell Battery - eBay (item 360286691497 end time Aug-09-10 19:00:20 PDT)

that one says 56Wh which is 56000mWh right?

6 cell Battery for Dell Mini 10 Inspiron 10V 1010 K711N - eBay (item 260632657438 end time Aug-07-10 17:28:41 PDT)

this one says mAh which Isn't even converted to watts. From what little I know about energy volts x amps = watts so would 4400mAh be like 48000mWh?

Does anyone have experience with ebay batteries? they come with a 1 year warranty and are 1/3 the price of genuine dells but do they lose charge rapidly or anything else like that?

 

You need to know both the voltage and current capacity to know what is compatible. What are the specs on your current battery?

Edit: So long as the voltage is the same, look for one with the higher Wh, and if it's listed some other way, I'd be hesitant to buy from someone who advertised their battery where people couldn't get a clear jist of what they're getting.

 

It's fairly simple if you keep a few things in mind. Wh (watt-hour, and any derivative unit, e.g. mWh, etc.) is a measure of energy (directly scalable to the standard base unit of energy, the joule), and Ah (ampere-hour) refers to electric charge, similar to the standard unit of the coulomb.

The math is about as simple as it gets, luckily. Since you'll have a unit of hours on both sides of the equation, they will cancel out and the normal electric power equation P=IV works just fine, with P representing power, I current and V voltage.

In terms of units, it's (Wh = Ah * V), so assuming you have the voltage (which is easy enough to obtain), and either the electric charge in terms of Ah or the energy in terms of Wh, you can solve the equation for the unknown variable. Just make sure to keep everything in terms of the base units (i.e. Ah and Wh rather than mAh or mAh) to make sure the math works out neatly.

 

I got one from Amazon, from Battery King to be more precise, although I can't say i they lose charge quickly since I've only had them for about a month. From RMClock, it says that the cells are from Panasonic, and that it only has a capacity of 4500mAh as opposed to the advertised capacity of 5200mAh. But it isn't all bad since my laptop can now run at 3hrs again as opposed to my dead battery which lasts only 15min.

 

Most off brand batteries don't give you much information to go on, so the more information you get, the better the chances are you get quality. Still, there's no guarantee (save the written one, of course). Additionally, off-brand batteries tend to use inferior construction which is why they're cheaper and tend to get hot (and explode? ).

That said, they're not all bad, and some can work just as well as their name brand counterparts at a much reduced price. Typically, the higher the mAh the better.