New guy (but not new to batteries) builds a portable power box

[jharkin]

I signed up on here mostly to research before putting together my project so I thought I would share.

Background: I'm not doing this for solar, I'm an RC airplane/helicopter hobbyist and use a portable power box to run battery chargers. I used to haul a heavy AGM but wanted to try LFP (in RC we call them "LiFe" or A123) now that my AGM (and my arm ) is worn out. . I do know my way around the chemistry, having used A123 cylindrical 18650 and 26650 cells in my airplane/helicopter power systems since 2008 or so. I use hobby chargers - FMA (Revolectrix) Powerlabs - that have native balanced charging and discharging profiles for these packs. Ive also spoken with the engineers at FMA Direct who did a lot of early research when A123 first came out with this chemistry and introduced the first LFP capable hobby charges around 2006? or so.


Anyway the build-
I got the BLS 90Ah cells from AliX, using the link from Will's website.
Overkill 120A BMS (only for protection, as I am balancing with my charger)
Custom wood enclosure, using mostly bussman and Blue Sea fuses, distribution panel, etc.
I put a panel meter in.. just for fun.

 

[jharkin]

The cells arrived packed very well and visually *look* new. Intact QR codes, etc. They shipped at practically empty SOC as far as I can tell.

I started by top balancing using my CellPro powerlab. It is limited to 20A out in single cell mode so it took over a day. I let it taper down to 2A then cutoff. Overall it took about 380Ah (95Ah per cell) Cell voltage after a 6 hr rest (via Fluke 87):

C1 3.344v
C2 3.343v
C3 3.345v
C4 3.345v


Then I wired in series and did a capacity test to 2.5vpc. (limited to a very slow 50watts on the chargers internal discharge load) The charger monitors each cell and cutoff when one cell hit 2.5.

I got 91.21Ah and the cells ended up a bit out of balance at the bottom. Voltages after a short rest...

C1 2.61v
C2 3.12v
C3 2.90v
C4 2.88v

 

[jharkin]

Then I installed it into the box, clamped. (My smallest torque wrench only goes down to 20in-lb so I just hand tightened the clamps with the cells discharged). I hooked up the BMS and also made a balancing harness for the Powerlab charger. I disabled BMS balance and am using active balance in the charger (it supports 1A per cell active balance and I can set it to balance at any point in the charge. Currently I have it set to balance over 3.3/cell)

I ran a full charge at 30A CC to 3.6vpc CV terminating at 3A (C/30). I set a 1hr timeout for CV. The full charge from dead flat ran about 4hr until it hit the balancing timout. You can see from the charge graphs that the cells stay tight until they get into the knee - over 3.5 one cell runs away and even the active balancing on the Powerlab would have taken a couple hours to pull it inline if I let it.


I did check temps, and everything seems good - the cells all warmed to about 81F and no hotspots or hot cell. You can see the IR of that one runaway cell is a little off.


The full charge took back 95Ah again. Resting cell volts after a 4hr cool down:
C1 3.353
C2 3.395
C3 3.389
C4 3.358

Cell IRs measure by the Powerlab (not sure how accurate it is)
C1 0.40mOhm
C2 0.40mOhm
C3 0.50mOhm
C4 0.40mOhm

 

[jharkin]

And this is my typical use case.... Charge it up with the powerlab before a day of flying, and then I run the hobby chargers off it at the field which can draw up to 20-30A. Goal to leave it between 40 and 70% SOC when I put it away.

The imbalance at the top of the knee.... went away as soon as I burned off the top 1Ah or so of charge... cells all settled back into balance. In practice Im probably going to define a custom charge profile and just terminate at 3.5vpc and limit the CV. I dont really need that last couple % of chage.


For folks who have never seen the hobby chargers, here are screenshots of all the parameters I can define in the profile. This is an FMA Powerlab. FMA was the first company to introduce charges with LiFePO4/A123 charge profiles to the RC hobby community around 2006 or so. Ive spoken to their design engineers (Jamie Marks) a couple times and have used these for years charging 26650 and 18650 cells. Very reliable chargers.

 

[jharkin]

Questions for the group:


1 Does anybody have any idea what manufacturer makes the 90Ah cells on the AliX BLS store? Id like to look at a datasheet if I can.

2 Should I put plastic separators between the cans? I tested them with the fluke and the cans *are* neutral... no continuity to either terminal... But I'm still a little nervous.

3. Thoughts on the balance? Is this typically for grey market cells or should they have top balanced better if I waited longer? I think I can get away with it as I only use active balancing chargers but am open to opinions.

4. Any other feedback on the build.