Nowty Towers DIY Battery Project

[MrPablo]

Out of curiosity, what forklift fuse have you used, Nowty?
The ones I'm looking at appear to have a 11mm ID, whereas my battery terminals are 6mm.

[nowty]

Out of curiosity, what forklift fuse have you used, Nowty?
The ones I'm looking at appear to have a 11mm ID, whereas my battery terminals are 6mm.

My forklift fuse is sandwiched between two bus bars for max surface contact area so it does not matter that the battery terminals are a smaller size. Alternatively you could use a large brass washer.

[MrPablo]

Thanks for the picture, I wasn't sure how you had handled the terminal interface.
Very neat solution!

[nowty]

I've tweaked a few of my parameters on my Victron SmartShunt after reading up on it in more detail. Some of the default settings are for Lead Acid so I have specifically changed some for my Lithium batteries. Also I want the SOC in % to be my actual useable % from 100% to 0% and NOT the actual gross SOC from 100% to 0%.

Official Victron info here,
https://www.victronenergy.com/media/pg/ ... ex-en.html

An easier to read guide on the parameters here,
https://offgridpower.solutions/van-buil ... -in-detail

Specifically for Lithium the Peukert Exponent needs to be changed from the default value of 1.25 to 1.05, effectively this is the reduction in battery capacity under very high C rates. And the Charge Efficiency Factor needs to be increased from the default value of 95% to 98% or 99% as the internal resistances are much lower and there is no gassing as you get in Lead Acid.

[nowty]

Do you fancy decoupling your new LFP cells and finding out the capacity of the old cells?

Yes in due course I am going to perform a similar test separately on,

Banks 1 + 2 (LMC cells)
Bank 3 (280Ah LFP cells)
Bank 4 (315Ah LFP cells)

As promised I'm now re-running the battery capacity tests with each battery bank.

To recap from a few posts back,

The whole useable capacity in ALL my battery banks (from 55.7V down to 47V) measured 1837Ah which at the nominal cell 3.2V (51.2V for 16 cells) is about 94kWh.
Banks 1 + 2 are LMC cells and have a nominal cell 3.65V, but are only 14 cell banks so that also works out at a very similar nominal system voltage of 51.2V as the newer 16 cell LFP battery banks.

Yesterday I ran the test on both banks (1+2 together) of LMC cells. They are around a decade old now and I bought them all second hand so I was expecting some degradation.

Some started life at Nowty Towers outside on an external wall,



Then they lived in an unheated outside shed,



Now they live in the more temperate Nowty Towers battery cave,



Theoretical max capacity of those LMC banks based on nominal capacity,
10 x 4.8kWh Growatt GBLI5001 Units = 48kWh

But I run them from approx 90% SOC at 55.7V down to 20% SOC at 47V so that should be a theoretical maximum of 0.7 x 48 = 33.6kWh.

Actual test result from yesterday using my Victron SmartShunt,
632Ah or 32.4kWh
So hardly any measurable degradation from nominal capacity at all yet.
Now that I have added extra LFP battery banks, the LMC banks should have an easier life, maybe they will last me another decade or three.

[nowty]

Todays battery capacity test is Battery Bank 3 or the new LFP Envision 280Ah Grade B cells from Fogstar.


From 55.7V down to 47.0V I am using almost ALL the capacity, maybe from 99% SOC down to 4% SOC so theoretical max usable capacity will be,

Based on nominal capacity
0.95 x 280Ah x 2cells x 51.2V = 27.2kWh

But the cells are new with actual tested capacity marked typically as 299Ah
0.95 x 299Ah x 2cells x 51.2V = 29.1kWh

Actual measured results, well I fecked up and let them drop slightly too low as I was getting dinner ready , LFP voltage drops off a cliff under 49V.
That's going to over estimate my useable capacity by 1%.

570Ah x 51.2V = 29.2kWh

Take off the 1% overshoot,
0.99 x 29.2 kWh = 28.9kWh
This tells me the measured test capacity stickers on the grade B's are the real deal and not made by a random number generator.



Tomorrow its the turn of Battery Bank 4, the LFP Envision 315Ah Grade B cells from Fogstar, although they are typically marked as 335Ah.

So based on nominal capacity
0.95 x 315Ah x 2cells x 51.2V = 30.6kWh

Based on the marked capacity as 335Ah
0.95 x 335Ah x 2cells x 51.2V = 32.6kWh

[Countrypaul]

Todays battery capacity test is Battery Bank 3 or the new LFP Envision 280Ah Grade B cells from Fogstar.


From 55.7V down to 47.0V I am using almost ALL the capacity, maybe from 99% SOC down to 4% SOC so theoretical max usable capacity will be,

Based on nominal capacity
0.95 x 280Ah x 2cells x 51.2V = 27.2kWh

But the cells are new with actual tested capacity marked typically as 299Ah
0.95 x 299Ah x 2cells x 51.2V = 29.1kWh

Actual measured results, well I fecked up and let them drop slightly too low as I was getting dinner ready , LFP voltage drops off a cliff under 49V.
That's going to over estimate my useable capacity by 1%.

570Ah x 51.2V = 29.2kWh

Take off the 1% overshoot,
0.99 x 29.2 kWh = 28.9kWh
This tells me the measured test capacity stickers on the grade B's are the real deal and not made by a random number generator.



Tomorrow its the turn of Battery Bank 4, the LFP Envision 315Ah Grade B cells from Fogstar, although they are typically marked as 335Ah.

So based on nominal capacity
0.95 x 315Ah x 2cells x 51.2V = 30.6kWh

Based on the marked capacity as 335Ah
0.95 x 335Ah x 2cells x 51.2V = 32.6kWh

Sounds ideal for a competition, who can guess the actual value measured?
I'll suggest 31.8

[Richard77]

How many kWh do we win if guess correctly?

31.5

[nowty]

My final capacity test is now complete on Battery Bank 4 which are the LFP Envision 315Ah Grade B cells from Fogstar, although they are typically marked as 335Ah.


So based on nominal capacity
0.95 x 315Ah x 2cells x 51.2V = 30.6kWh

Based on the marked capacity as 335Ah
0.95 x 335Ah x 2cells x 51.2V = 32.6kWh

The measured result is in and I managed to catch the voltage falling off the cliff this time,

To be fair the voltage was almost as low as the test on Battery Bank 3 yesterday before I reduced the load so I feel I need to take off 1% off this result to be comparable to yesterdays result.

So that's 0.99 x 643Ah x 51.2V = 32.6kWh
Again the printed capacities on the cells appear to be genuine.


As a sanity check the total capacity of Test 1 - ALL battery banks was about 94 kWh.
Test 2 - Battery Bank 1 + 2 (LMC cells) was 32.4 kWh
Test 3 - Battery Bank 3 (LFP cells) was 28.9 kWh
Test 4 - Battery Bank 4 (LFP cells) was 32.6 kWh
Total of individual tests adds up to 93.9 kWh

Near enough for government work.

[Stinsy]

Let’s compare what you thought prior to this test:

Battery Bank 1 is 24.37 kWh of LMC but is a decade old.
Battery Bank 2 is 24.37 kWh of LMC but is a decade old.
Battery Bank 3 is 28.67 kWh of LFP and is only a month old.
Battery Bank 4 is 32.26 kWh of LFP and is only a couple of days old.
-----------------------------------------
Total Gross Capacity 109.67 kWh

With what you’ve measured:

Test 2 - Battery Bank 1 + 2 (LMC cells) was 32.4 kWh
Test 3 - Battery Bank 3 (LFP cells) was 28.9 kWh
Test 4 - Battery Bank 4 (LFP cells) was 32.6 kWh
Total of individual tests adds up to 93.9 kWh

So the old cells are showing their age, but assuming their efficiency is still good, then seeing as they were “scrap” when you bought them they’re doing a great job. What is your plan to figure out if a dud cell is pulling the pack down?

And the new cells are in-fact better than you thought!