18650 battery pack for 65W laptop that powers it for +50hours

Announcement - Help us fight the BOTS! Please report all spam including stuff in your inbox!

digreto

New member
Joined
Jun 20, 2021
Messages
4
Hello. I want to build a power bank/power wall that is able to power a laptop that draws 65W at peak and comes with a 45Wh battery.
1. What battery configuration (xSxP) works best if I need to use the laptop for say 50 to 80 hours (5 days/10 hours/day) plugged into the power bank? I have access to up to 80 2300mAH 18650 cells from new phone portable power banks.
2. Is it necessary to use a DC~AC inverter or will DC~DC voltage boosters work just fine? Is there an advantage of using one over the other?
3. What BMS works best at the lower end of the cost spectrum to handle such setup in (1).
4. What other components do I need apart from the batteries, and BMS?

The output of this power bank will be 12V and some 5V/2A USB ports which I will utilise the USB C Car Charger 95W

Thank you.
1624207860548.png
 

floydR

Well-known member
Joined
Aug 23, 2017
Messages
1,226
dc-dc converter would be best if you can find a 65w one. 60w usb type c is the hightest output I can find.
https://www.ebay.com/sch/i.html?_fr...LH_TitleDesc=0&_odkw=dc+to+dc+C+usb&_osacat=0
a 7s11p would be a 25300mAh bank @24v around .6KWh capacity. I presume the laptop only uses 65w for brief moments. The USB adapter shown above can provide 65W but what does the laptop use I suspect much less so 60W may work. How long does the laptop battery last on a typical day before it needs recharged? find out the Wh of the battery/hrs it lasted =watts needed per hour of use
for 50hr@65w=3.25kWh
Later floyd
 

digreto

New member
Joined
Jun 20, 2021
Messages
4
The laptop usually lasts up to 4 hours on that 45Wh 4 cell battery. I mainly use the laptop for photo editing so yes, I usually use on average 50 to 60W.

So that 7s11p, will it be able to last 50 hours, drawing roughly 50W (25kWh)?, if yes what is the best BMS to manage a 7S11p battery configuration and what's the difference in having say a 4s20p vs the 7s11p?
 

Korishan

Moderator
Joined
Jan 7, 2017
Messages
6,518
Why not just build a battery that is equivalent to the AC power bricks output? I believe it's rated for 19VDC and something like 5A.

So, for Li-Ion cells that would be a 5s
Charged: 4.1V * 5 = 20.5V
Nominal: 3.7V * 5 = 18.5V
DisChrd: 3.2V * 5 = 16.0V

You could go full out 24V with the LiIon cells, which is 7s, and just use a buck converter that can handle 5A to supply a steady 19V/50W output. This would give you the bonus of being able to use the battery pack with a 24VDC inverter if needed.


Just read floyd's comment :p

what's the difference in having say a 4s20p vs the 7s11p?
The difference is voltage output for starters. Secondly you can operate the cells closer to their designed voltage ranges. Li-Ion cells are not designed to run in the 3s/4s voltage ranges. They are either overly discharged, or overcharged. Whereas a 7s allows for a better voltage range, altho they can still be over/under charged if not used with a bms.

20p vs 11p is capacity. So if all cells are 2000mAh, a 20p will yield 40,000mAh (40Ah) whereas an 11p is 22,000 mAh (22Ah). The kWh is about the same, but the amp draw is different. You can pull more amps with 20p than you can with 11p. If you need, for example, 10A, then the 20p will be able to handle that load just fine. Whereas the 11p would struggle and possibly have high voltage drop. 10/20 = 0.5A/cell vs 10/11 = 0.9A/cell. Laptop cells are designed for low current draw, and usually 0.5A is close to their max design limit.
However, if you are only needing to pull 5A, either one will work just fine. In the case of powering the laptop, 5A is probably all you would need max anyways. At 65W / 12VDC = 5.4A. But if your laptop can run at the higher voltage, 19V for example, then 65W / 19V = 3.4A
 

Redpacket

Well-known member
Joined
Feb 28, 2018
Messages
1,283
The more converters you go through, the worse losses will be (less hours laptop use) & step down is more efficient than step up.
Eg battery pack > step down to 12V > car adaptor step up to 20V again.
You'd get longer run time with a 24V nominal ie 7s battery & use a DC-DC step down converter to the usually 20V that laptops take.

A simple BMS would do the job fine.
 

italianuser

Active member
Joined
Feb 25, 2020
Messages
296
The laptop with a 45Wh battery which lasts about 4 hours consumes on average less that 15Wh, which is the same value I measure on an I3 and a I5 15.6" notebook at my place. Otherwise the battery wouldn't last 4 hours!

EDIT: the 65W PSU, when connected charges the battery. When charging a very low voltage battery I measure 35-40W instant current. As the battery reaches 80% and 90% watt usage lowers. So: running on batteries you need an average of about 15Wh. For charging the laptop battery using the PSU you need 35-40Wh. With PSUs it's always better not to stay too long next to the maximum output written on the label.
 
Last edited:
Top