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Beginner questions on storage system
#1
Hi all,

I've been lurking around the forum a few months, reading up as much as i can in regards to how to build a smallish battery pack to feed into a inverter/charger to increase self consumption.

Given there's so much information scatted across so many different threads, i thought it be a good sanity check to write up my first post to confirm some of the ideologies contained in this forum as part of my research.

I am looking at Jehu's PCB system in a 48v configuration (2PCB's in series) , combined with LGDAS31865 which average around 2200 - 2300mah.

The inverter/charger i am currently looking at is a Victron Multiplus II - GX which has a maximum charge of 32A.


To make a ~2.5kw/h pack suitable to say 50A  in a 14S configuration the maths would be

Voltage:
7*4.2v=29.4v (Single PCB) and  58.8v (2 PCB in series) 

Capacity:
2200mah * 20 = 44000mah ~ 2.5kwh 

I'm a little confused on the parallel side of the equation as although a max discharge exists at 3A , you typically don't want to be anywhere near that correct? 

Nominally around 0.5a would mean a hell of alot of cells (pcb's).
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#2
(08-14-2019, 05:44 AM)intra Wrote: Hi all,

I've been lurking around the forum a few months, reading up as much as i can in regards to how to build a smallish battery pack to feed into a inverter/charger to increase self consumption.
Welcome! Glad you decided to come outta the corner and join the chitchat in the middle of the room  Smile

(08-14-2019, 05:44 AM)intra Wrote: Given there's so much information scatted across so many different threads, i thought it be a good sanity check to write up my first post to confirm some of the ideologies contained in this forum as part of my research.

Sanity checks are always needed. Sometimes we are blind to certain things as we all either have some sort of bias, or are just overwhelmed with the info.

(08-14-2019, 05:44 AM)intra Wrote: I am looking at Jehu's PCB system in a 48v configuration (2PCB's in series) , combined with LGDAS31865 which average around 2200 - 2300mah.

Don't. This is a baaaaad idea. It's fine for testing purposes only. Not at all for production systems. People who use these on large scales are asking for troubles.
Btw, I'm not saying this from a person who is against it, but as someone who regularly chats with and discusses the testing of these boards "with the designer" of these boards. He is on here as rev0, or jkenny on our discord* chat. He is shocked that people are trying to use these are mass scale and will regularly post warnings on any threads he sees these being used, even on Jehu's videos.

So, these are fine for testing, that's it.

(08-14-2019, 05:44 AM)intra Wrote: The inverter/charger i am currently looking at is a Victron Multiplus II - GX which has a maximum charge of 32A.

To make a ~2.5kw/h pack suitable to say 50A  in a 14S configuration the maths would be

Voltage:
7*4.2v=29.4v (Single PCB) and  58.8v (2 PCB in series) 

Capacity:
2200mah * 20 = 44000mah ~ 2.5kwh 

I'm a little confused on the parallel side of the equation as although a max discharge exists at 3A , you typically don't want to be anywhere near that correct? 

Nominally around 0.5a would mean a hell of alot of cells (pcb's).

Brand:LG
Model:LGDAS31865 (ICR18650S3)
Capacity:2200mAh Rated
Voltage:3.60V Nominal
Charging:4.20V Maximum
1075mA Standard
2150mA Maximum
Discharging:3.00V Cutoff
430mA Standard
3225mA Maximum
Description:Blue Cell Wrapper
White Insulator Ring
18650 Form Factor

You don't calculate kWh of capacity with the top voltage (4.2V), you use the nominal voltage (3.6V). This is because the cells will not stay at 4.2V for very long, and in fact will drop to 4.1 or 4.05V "very" quickly.

So, 7 * 3.6 = 25.2V, or 14s * 3.7 = 50.4V
2200 mAh * 20 = 44000 mAh
44000 mAh * 25.2V / 1000 = 1108.8 1.1088 kWh
44000 mAh * 50.4V / 1000 = 2217.6 2.2176 kWh

Considering you stated "2200mAh * 20", this would be 20p, or 20 cells in parallel. 20 cells * 7s = 140 cells or a 24V string, or 20 cells for 14s would be 240 cells in a 48V string.

Planning on 50A discharge, 50A / 20cells = 2.5A/cell.  This you would not want with these cells on a regular basis. Would be fine for surge (this is for new cells or new old stock, not reclaimed from laptops or such). They can handle 3.2A max current discharge, but they will get warm, and heat buildup = loss of energy.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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#3
Rate of charge & discharge you typically want to be around 0.5A per 18650 cell
# of usable amp hrs is related to the # of cells you have.
You have to think about a daily cycle: how many hours of sun (or cheap off peak power time) do you have to charge the batteries.
Eg with a 2.5Ahr cell, charging at 0.5A/cell means you'd need ~5hrs to recharge - do you have that much sun?
You want to be able to fully recharge the pack every day ideally give or take some cloudy days if solar.
and
Realistically how much use does say 2.5kWhrs of storage get you? Would larger get you through the day on solar? Or a few days with cloud?
Working the batteries hard with DoD 0 - 100% they won;t last well.
The car manufacturers have their packs running approx say 10% to 95% DoD to extend the life.
Similarly the suggested max voltage is 4.1V/cell (Li-Ion) or ~3.4-3.5V/cell (LiFePo4)
Hope that helps!
Korishan likes this post
Running off solar, DIY & electronics fan :-)
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#4
(08-14-2019, 01:17 PM)Korishan Wrote: So, 7 * 3.6 = 25.2V, or 14s * 3.7 = 50.4V
2200 mAh * 20 = 44000 mAh
44000 mAh * 25.2V / 1000 = 1108.8 kWh
44000 mAh * 50.4V / 1000 = 2217.6 kWh

Hi Korishan

Should that be Wh not kWh.

2200 mAh * 20 = 44000 mAh / 1000= 44A
44A * 25.2V = 1108.8 Wh / 1000 = 1.1088 kWh
44A * 50.4V = 2217.6 Wh / 1000 = 2.2176 kWh

Unless I've been getting it wrong.
UK Southwest.

7 kWp Solar Panels (28 x 250Wp Shinetime Mono).
14 X APS YC500i Micro Inverters.
Still sourcing and processing cells for powerwall.
Planning on a Sofar Mass Energy ME3000SP AC coupled grid tied inverter.
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#5
(08-14-2019, 10:49 PM)chuckp Wrote:
(08-14-2019, 01:17 PM)Korishan Wrote: So, 7 * 3.6 = 25.2V, or 14s * 3.7 = 50.4V
2200 mAh * 20 = 44000 mAh
44000 mAh * 25.2V / 1000 = 1108.8 kWh
44000 mAh * 50.4V / 1000 = 2217.6 kWh

Hi Korishan

Should that be Wh not kWh.

2200 mAh * 20 = 44000 mAh / 1000= 44A
44A * 25.2V = 1108.8 Wh / 1000 = 1.1088 kWh
44A * 50.4V = 2217.6 Wh / 1000 = 2.2176 kWh

Unless I've been getting it wrong.

Ahhh yes, Chuck, you are right. I dropped the "." in the wrong spot when I copied from calculator Tongue Thanks for the correction. I'll update the original for future reference
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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#6
(08-14-2019, 01:21 PM)Redpacket Wrote: Realistically how much use does say 2.5kWhrs of storage get you? Would larger get you through the day on solar? Or a few days with cloud?
Working the batteries hard with DoD 0 - 100% they won;t last well.
The car manufacturers have their packs running approx say 10% to 95% DoD to extend the life.
Similarly the suggested max voltage is 4.1V/cell (Li-Ion) or ~3.4-3.5V/cell (LiFePo4)

Also good points. For my house, I use approximately 25kWh/day average over the year. I can reduce that by changing my cooling options (which is a work in progress). So with 2.5kWh, that's not much chooch for your chach. Unless this is just a testing setup and running some small motors or such.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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