# HP 8-Cell Li-ion replacement

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#### avenger09123

##### New Member
Alrighty so I'm starting to get tired of my 2.5 hours of battery life out of my desktop replacement and am looking to open up the battery and redo the cells myself. (yes I know its a desktop replacement, i'm just looking for something to do really).

I haven't opened it up yet but I'm going to I just wanted to do the math first see if it will be with buying higher capacity batteries and need my math checked

Battery info on the HP label is sit at 14.4v for 73,440 mWh (8 cells total)

Seeing as laptops today monitor more pairs of cells than individual or groups of 3 or more I can figure out that:

If my math is right then 73440mWh/14.4v gives me 5100mA for the entire 8 cell battery

2 cells in parallel give me 3.6v
x4 in series my bad I forgot to say in series.... for 14.4v @ 5100mA

so can I divide the 5100mA by 4 sets to get the mA of a set of 2 and then divide by 2 to get the capacity of one cell...right?

Is my math right? can anyone check this?
OK edited....should be correct now....

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##### Banned
No your math is definitely not right.
4 X 3.6 = 14.4
Since you have 8 cells the other 4 are a seperate series pack in parallel with the first. This is common to allow higher discharge rates rather than simply using higher mah rated cells because in parallel the equivalent resistance of the two cells decreases which isn't true of a cell with a higher capacity.

The capacity adds in parallel it doesn't in series. So it's 5100ma \ 2 or 2550ma per cell. You're not just going to be able to use higher capacity cells in the same form factor as your existing battery, it's simply not going to fit. You could use an external battery pack or use an open expansion bay for a second pack though. The external battery pack if you use Lithiums is going to be VERY expensive, and pretty ugly looking unless you're damn good at making external cases. Even then you're going to have to lug around that extra pack with you. www.fmadirect.com sells customizable lithium packs some with very high capacities which you could use if you were really set on it.

If your laptop is a desktop replacement then that means there should be an alternate source of power nearby.

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#### avenger09123

##### New Member
So it's 5100ma \ 2 or 2550ma per cell.
maybe you misread what I typed...on the 8 cell battery label by HP it says 14.4v @ 73,440mWh, thats the entire battery's capacity, not one set of two in a series.....,

#### avenger09123

##### New Member
No your math is definitely not right.
4 X 3.6 = 14.4. So it's 5100ma \ 2 or 2550ma per cell.
OK maybe you misread what I said....and I forgot to say one thing too....
HP LABEL FOR THE ENTIRE BATTERY is [email protected] or 5100mA

Break it down to basics
2 cells in parallel putting out 3.6v
x4 sets in series = 14.4v

to do the amperage of each cell i'm assuming i can take the TOTAL BATTERY mA as calculated above and divide by 8 cells.....

thats I guess what I meant to say.....

##### Banned
I take that back about FMA direct, I just visited their site again and their pack selection has dropped dramatically from what I remember it.

Check out this bad boy though, and the price... ~220$Rechargeable Batteries & Battery Chargers - Li-Ion 18650 14.8V 13000 mAh Rechargeable Battery Pack PCB Protection with 18AWG Bare Leads(Customize) These seem a little more reasonable at ~74$
Rechargeable Batteries & Battery Chargers - Li-Ion 18650 14.8V 4400 mAh PCB Preotected Rechargeable Battery with 2-Hole Connector

A good separate charger is going to set you back 40-100\$

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