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digreto

New member
Joined
Jun 20, 2021
Messages
4
I've been reading a lot about building power packs and then I bumped into this forum I am looking to build a 3kWH battery pack to power my Dell Inspiron 15 5570. Here in Zimbabwe, our electricity grid is old and archaic so there are lots of blackouts. So I want something that can power the laptop for at least 5 days using it for 10hours a day which is roughly 50 hours. The laptop comes with a 45Wh 4 Cell battery from Samsung that lasts about 4 hours at best.

I have access to about 80 18650 (2300mAH 3.7V) cells from smartphone power banks which I am looking to use for this project. The challenge I am having at the moment are:
  • Choosing the right configuration for the batteries (xSxP)
  • Choosing the right BMS to charge the batteries without them becoming a fire hazard.
  • If it's necessary to use an AC inverter or a DC to DC would do just fine.
Looking forward to learning from you.
 

floydR

Well-known member
Joined
Aug 23, 2017
Messages
1,224
Welcome to the best lithium support group around.
Get a small solar panel (if you get 5 hours of sunlight a day could manage with a 20watt solar panel) ->charge controller--> 7s bms--> power bank ---> 60W dc-dc usb type c converter---> laptop
most any 7s bms even the least expensive ones 10A would work.
DC to DC is fine.

later floyd
 

italianuser

Active member
Joined
Feb 25, 2020
Messages
294
Hi Digreto and welcome :giggle:
I suppose your notebook uses, like mine, about 15-20Wh. If you do CPU intensive stuff it could use more. To keep it on 50 hours at 20Wh you need 1000Wh of batteries. With 18650 it's good because you use full capacity while acid lead (AGM deep cycle like mine) you can consider 50% or less of their nominal capacity.

So lets say, to keep things dimensioned well, you could be OK with 2000Wh of battery storage. Your 80 cells (2300mAh) can give you about 3.7V * 2.3Ah = 8.51Wh, so to make 2000Wh of battery you'd need 235 cells. With 80 cells you can make a 8.51Wh * 80 = 680Wh battery.

It's surely better, like @floydR says, to use DC-DC current without passing throught a DC-AC-DC conversion because you loose something at each conversion. Plus, when using an AC-DC inverter, expecially on small systems, you must consider an extra loss caused by the inverter. I have a small solar system for my 20Wh notebook with a 1500W 12V inverter and I measure a 40Wh overhead (between conversion losses and inverter efficiency/self usage), so I must produce at least 60Wh from solar panels to keep notebook running.

Now, using the notebook directly from your 18650 cells sounds good, although I never did it. Your notebook wants a 19V input, uhm yes, @floydR suggests a 7S, seems good enough, so I'd go for something like this (never tried, so experiment is all for you LOL):

7S battery -> DC-DC buck converter set at 19V output (they cost nothing!) -> notebook.

With your 80 cells put in 7 series you have 80 / 7 = round to 11 -> 7s11p.

To charge the battery you choose the source, consider you charge a 7s at 29.4V (7 x 4.2V) and that 11p means 25.3Ah (11 x 2.3Ah), so you could easily charge them at 10A:

Source -> BMS -> 7s11p battery.

I was thinking about a nice 280W solar panel, which produces about 8.5-9A current with full sun. Not sure... I would connect the panel to BMS directly LOL but DO use necessary protections (fuses, MCB and BMS!).
 

digreto

New member
Joined
Jun 20, 2021
Messages
4
Hi Digreto and welcome :giggle:
I suppose your notebook uses, like mine, about 15-20Wh. If you do CPU intensive stuff it could use more. To keep it on 50 hours at 20Wh you need 1000Wh of batteries. With 18650 it's good because you use full capacity while acid lead (AGM deep cycle like mine) you can consider 50% or less of their nominal capacity.

So lets say, to keep things dimensioned well, you could be OK with 2000Wh of battery storage. Your 80 cells (2300mAh) can give you about 3.7V * 2.3Ah = 8.51Wh, so to make 2000Wh of battery you'd need 235 cells. With 80 cells you can make a 8.51Wh * 80 = 680Wh battery.

It's surely better, like @floydR says, to use DC-DC current without passing throught a DC-AC-DC conversion because you loose something at each conversion. Plus, when using an AC-DC inverter, expecially on small systems, you must consider an extra loss caused by the inverter. I have a small solar system for my 20Wh notebook with a 1500W 12V inverter and I measure a 40Wh overhead (between conversion losses and inverter efficiency/self usage), so I must produce at least 60Wh from solar panels to keep notebook running.

Now, using the notebook directly from your 18650 cells sounds good, although I never did it. Your notebook wants a 19V input, uhm yes, @floydR suggests a 7S, seems good enough, so I'd go for something like this (never tried, so experiment is all for you LOL):

7S battery -> DC-DC buck converter set at 19V output (they cost nothing!) -> notebook.

With your 80 cells put in 7 series you have 80 / 7 = round to 11 -> 7s11p.

To charge the battery you choose the source, consider you charge a 7s at 29.4V (7 x 4.2V) and that 11p means 25.3Ah (11 x 2.3Ah), so you could easily charge them at 10A:

Source -> BMS -> 7s11p battery.

I was thinking about a nice 280W solar panel, which produces about 8.5-9A current with full sun. Not sure... I would connect the panel to BMS directly LOL but DO use necessary protections (fuses, MCB and BMS!).
How about using say a 3S at 12.6V (3 x 4.2V) and get 12V then use those USB C Car Charger 95W. like the one in the frame, that is capable of delivering the 65W using 12V/24V will the buck converter be necessarry in such a scenario? ANd what if I don't want to charge the power bank with a solar panel, but plugin it into an AC output. What charge controller will work?
1624257376965.png
 

italianuser

Active member
Joined
Feb 25, 2020
Messages
294
How about using say a 3S at 12.6V (3 x 4.2V) and get 12V then use those USB C Car Charger 95W. like the one in the frame, that is capable of delivering the 65W using 12V/24V will the buck converter be necessarry in such a scenario? ANd what if I don't want to charge the power bank with a solar panel, but plugin it into an AC output. What charge controller will work?
Series
3S isn't enough. Check you notebook power supply, it should be a 19V DC output.
For a 3S when cells are at 3.50V or 3.20V doesn't reach 12V. At 4.20V you're considering the cell at full charge, but 99% of the time the cells will be lower because you're using them.
So, first you must choose the correct series; 7S does make sense if you need 19V because when cells are nearly empty, lets say at 3.20V you are still getting 22.4V out of them.

Charger
Uhm, that car charger, I wouldn't use it, don't think it has enough output current to charge a big battery. 95W? Probably it's a peak value, it could melt LOL.
You can charge from AC, yes, using a normal Power Supply Unit of the correct voltage (or, even better, slightly higher and you regulate down to the exact voltage you need).
So, now you've chose your series, lets say 7S you need a charger for that exact voltage. For a 7S you need to charge at 7 x 4.2V = 29.4V. You could get yourself a variable PSU, I bought it for 30$ second hand (max 32V 5A). What's important is to check that your charging voltage is exact using a decent multimeter, sometimes PSU says 30V but it could be 31V or 32V, you could damage the cells.

PSU32V-5A.jpeg
PS305-D, you can find a model like this under different brand names. On AliExpress they sell them new for 30-50$ (but I would have paid customs taxes so I bought it used on Ebay).

Or, any >32V PSU unit would be OK, and if it's not variable you could get a cheap DC-DC step down converter which can give you the correct output voltage and a decent output current.

PSU -> DC-DC step down (set at 29.4V) -> BMS -> Battery

dcdc-buck-step-down.jpg

https://www.aliexpress.com/item/32797957068.html

Remember when they say 95W you should consider that number at 50% or less. That DC-DC buck converter in the photo says 8A, I would consider using it at 3-4A for a long charging session.

Now you're ready to open a new thread so we can leave this Introduction section before @Korishan wakes up 😅
 
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