Hi from south west England.

Ken140tdi

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May 16, 2017
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Hi everyone. My name is Ken.
I'm a 49 year old self employed motor engineer. I specialise in engine reconditioning.
I'm looking to start building my own powerwall and once you know my story I think you will understand why.
Two years ago I had a 4kw solar edge solar system installed at my home. I very quickly found that as I have my workshop at my home and a self contained flat which I also rent out that a 4 jw solar system simply was not big enough to make any noticeable dent in my carbon foot print or my bills for that matter either lol.
So 6 months later I upped my mortgage and installed a second 6 kw solar edge system bringing me up to 10 kw of solar power. This combined with anew gas boiler and 250ltrpressurised hot water tank with imersuon and a solar diverter has cut my gas and electric bills by about half.
The thing is now on a nice day in spring I can produce 60 to 70 kw and half of it goes out to the grid.
I've even designed and built my own solar tracker with 6 panels on it. Lucky I have a massive garden.
Any way after watchingloads of you tube videos from guys building they own power walls I decided I want to do the same. Rightly or may be wrongly as a basis to get me started I've ended up buying a victron 5 kw multiplus and 5 kw of byd lipo4 batteries. Thinking I can just build my own 48 volt battery packs from 18650 cells.

I think I have my sums right and if I built 14 packs that in series that would give me 50.4 volts nominal which is the same voltage as the byd batteries. As I'm a very busy man I was hoping to buy new cells to save time and give me batteries with the maximum life span. About the best price I have found per mah is the lg 2850 mah cell with 10 amp draw rating so I thought 36 cells in each pack x 14 packs should be 10 kWh if I've learnt the calculations correctly.

The problem is everyone I've spoken to says u can't use 18650 cells with lifepo4 batteries. I don't see what the problem is as I thought they both have very similar characteristics. The other problem is managing the batteries I build as the byd ones have their own bmu which communicated with the victron colour control.
I was wondering if there was a battery manager that I could use and fit just to my 18650 packs and leave the byd ones controlled by the byd bmu.


Forgive me for posting such a long post on my first post and if I'm being a bit of an inexperienced idiot with my way of thinking here.
Sujestions and comments welcome
 
Welcome.

U could use normal laptop cells with LiFePo4 BUT nominal of laptop cells are 3.7v compare to 3.2v on life cells.... so you need to mix 14s with 16s.

Im doing that and will be posting more info on that systen in some weeks. But its important to understand the differences and even more the flat curve that life have compare to liion. This will cause the liion to not being used unless u cycle them 10%

Its kind of the same scenario with mixing lead acid in.
 
I think I understand u there. Are u saying that the lifepo cells have a smaller change in voltage between full and flat than the normal 18650 ?? So if I build packs of 18650 cells they would never quite reach fully charged by 10%.
The nominal voltage of the 2.5 kWh byd box batteries is 51.2 volts. Sorry if this seems a dumb question. But when we say nominal voltage is this the fully charged voltage or the 50% or flat. I thought it was the 50 % charge.
This is the spec of the brand new cells I was thinking of using
Batteries Brand: LG
Batteries coding: 18650 MG1
Nominal capacity: 2850 MAH
Batteries Specifications: 65 * 18.4 mm
Batteries resistance: 50 milliohms or less
Batteries Voltage: 3.6V
Test current: 0.2C
Charging cut-off voltage: 4.2V
Discharge cut-off voltage: 2.75V
Maximum discharge current: 10A

So my thinking was 14 packs =50.4 volts
Fully charged at 4.2 volts would be 58.8. But I think I read that always charging to max capacity shortens the life so may be only charge to 4.1 v.
I've not got it all worked out yet and am still a little unsure my calculations are correct.

I still don't know what the byd batteries voltage is when fully charged as the system is not working properly yet because the bmu unit that came with the batteries needs a different firm ware to make it work with the victron colour control but it would appear that flat battery switch off is 47 volts so I think if nominal is 50.8 fully charged is going to be around 52 but will have to wait and see when the bmu controller is running properly.
I'll post more info on what I'm hoping to do later tonight perhaps I'll start a new topic in the correct section as I think this is not going to be a quick build project.
Thanks for your help.
 
Yeap i think you are on track


Lets take example:
LifePo4: nominal:3.2-3.3
Full 3.6
Empty 2.5

Liion (Generic)
Nomonial: 3.6-3.7
full 4.2
empty: 3 (or 2.8)

So you can combine them with 16s vs 14s like I do but!...
The problem if you look at the charge and discharge charts is that LiFepo4 only varies between 3.1-3.35V in general. Thats between 49,6-53,6 meanwhile same capacity range of Liion is between 3.5-4V => 49V - 56V

So you see that when using like 80% of capacity of LiFePo4 type cells you will only use few % of the Liion type cells.
My LifePo4 doesnt move many % of Voltage between 80% SOC and 20%SOC.

We are talking about mV meanwhile LiIon is more of several 100mV.

LiFePo4:

image_bvxjwt.jpg


18650 generic:
18650-2200mAh-discharge-curve.jpg


Comparision between 18650:

image_sdmjwe.jpg


This is why its a bit tricky and you need to be aware that you will stress LiFepo4 if you chose voltages that the Liion like or on the other hand if you use LiFe numbers you may end up not using many 5 of the LiIon cells :)

Im yet to set the numbers im using since i have not done enough testing on optimal numbers that I think
 
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