Changing the battery

[Gunn]

Hi i bought some time a go an hp laptop, i been ussing it for some time, then i realize the battery is trash (2700 mah in total divided in 4 cell) , i wonder if open the battery apart seem preatty easy it just closed with plastic clasps and some glue , and change the 18650 battery for some battery with better capacity. Will it influence the amount of amp that outputs the battery? 

[KarathKasun]

You will not get much higher capacity with other 18650 cells.  You would need a higher cell count to get an appreciably longer run time.

[Gunn]

1 minute ago, KarathKasun said:

You will not get much higher capacity with other 18650 cells.  You would need a higher cell count to get an appreciably longer run time.

That low of capacity are the 18650 batteries are?, i mean the laptop has 675 mah in every batter, my external powerbank that uses the same type of batteries has 10000ish mah in 4 cells

[KarathKasun]

2 minutes ago, Gunn said:

That low of capacity are the 18650 batteries are?, i mean the laptop has 675 mah in every batter, my external powerbank that uses the same type of batteries has 10000ish mah in 4 cells

MAh is not added across all cells, that would be WH.  The battery is putting out at least 7.4v, which means its a 2s2p pack, putting the individual capacity of the batteries at 1350 per cell.

 

Your external power bank is not actually 10k mah either.  Probably closer to ~7500 at only 3.7v if you actually measure it.  The harder you drain the batteries the lower their mah rating as well.

[Gunn]

1 minute ago, KarathKasun said:

MAh is not added across all cells, that would be WH.  The battery is putting out at least 7.4v, which means its a 2s2p pack, putting the individual capacity of the batteries at 1350 per cell.

my battery has 41wh

 

[Mira Yurizaki]

12 minutes ago, Gunn said:

Will it influence the amount of amp that outputs the battery? 

Increasing the capacity of the battery allows you to safely output more amps since the general rule of thumb for safe discharge rates is it should be around 1C on average (C being the amount of amps the battery can discharge for 1 hour before being depleted), but that wouldn't really matter. The parts will consume as much as they need.

[KarathKasun]

1 minute ago, Gunn said:

my battery has 41wh

 

That's 5540mah total.

[KarathKasun]

5 minutes ago, Mira Yurizaki said:

Increasing the capacity of the battery allows you to safely output more amps since the general rule of thumb for safe discharge rates is it should be around 1C on average (C being the amount of amps the battery can discharge for 1 hour before being depleted), but that wouldn't really matter. The parts will consume as much as they need.

Not really true.  Higher capacity batteries have a lower optimal discharge rate as a function of C.

 

18650 cells really top out at around 2000mah.  Any 18650 rated above 2000mah is lying about its specs.  You only get above ~2000mah by using a 0.25c or lower discharge rate.

[HanZie82]

9 minutes ago, KarathKasun said:

Not really true.  Higher capacity batteries have a lower optimal discharge rate as a function of C.

 

18650 cells really top out at around 2000mah.  Any 18650 rated above 2000mah is lying about its specs.  You only get above ~2000mah by using a 0.25c or lower discharge rate.

Well thats kinda true. If it was 3 years ago. 
Now there are 18650 cells that easily do 2500mAh (tested). 

Battery tech improves on a daily basis.

[KarathKasun]

1 minute ago, HanZie82 said:

Well thats kinda true. If it was 3 years ago. 
Now there are 18650 cells that easily do 2500mAh (tested). But those do cost about 10 bucks a piece...

Do discharge tests at 2.5A (1C), they are not 2500mah batteries.  You might get ~2200mah out of them.  Its better than the 18650's of the past, but the form factor is the capacity limiting factor.

 

Building a new pack with a 3D printer and using soft case LiPo cells can get you up to whatever capacity you want.  I can source 7500mah+ single cells in soft cell form, primarily because of the larger cell sizes available.  Another option would be to use 26650 cells.

[HanZie82]

1 minute ago, KarathKasun said:

Do discharge tests at 2.5A (1C), they are not 2500mah batteries.  You might get ~2200mah out of them.

Yeah thats possible, higher Amp draw will result in less Watthours.

But the SAMSUNG INR18650-25R  does really get to 2500mAh tho.

Google it to find your own favorite tester or watch this: 

But this was done at 1A so not exactly the same.

[KarathKasun]

1 minute ago, HanZie82 said:

Yeah thats possible, higher Amp draw will result in less Watthours.

But the SAMSUNG INR18650-25R  does really get to 2500mAh tho.

Google it to find your own favorite tester or watch this: 

But this was done at 1A so not exactly the same.

Capacity tests are supposed to be done at 1C.  That is at 0.2C, which falls in line with the spec.  At 1C it should be ~2200.

 

In a laptop, you are pulling ~30w.  Assuming 7.4v output, that is ~4A total.  Each bank of cells only sees 1/2 of that, so 0.8C.  You might get 2250mah from the cells in this scenario.

 

Regardless, you would see ~50% improved run time (1hr to 1.5hr).  Nothing like double or triple the run time.  To really make a large difference you need to go to 2s4p with ~2000mah cells.  Which would double capacity in addition to reducing per cell load into the more efficient sub 0.5C area.  Which means that you would end up with ~3.2x runtime (1hr to 3.2hr)