INTRODUCTION
I’m going to describe my setup and the initial electrical performance results as well as a balancing technique that I think is pretty effective at matching parallel cells (e.g an 8 cell 4S battery or a 16 cell 8S battery).
I’m a semi-retired Electrical Engineer, but this is my first foray into doing a DIY LiFePO4 build. This battery is planned for a van build that is in the process for that will include 350W of solar/40A Solar Charger, 40A DC-DC charging, 1100W MSW/600W PSW inverters.
The process is related to this thread which is discussing a charge capacity balancing process of paralleling cells to get better capacity match across battery arrays. This is essentially the same approach I have tried to formalize and test with hard numbers for a 4S battery using two parallel cells per 1S. This works well for example building a 200 Amp-hour 4S battery using 100Amp-hr cells. The goal is to achieve an improved capacity of used B cells to make them comparable to new A cells.
https://diysolarforum.com/threads/diy-cell-matching-process.16784/
FaFrd describes his possess in this post. He seems to be going through an iterative process to rank strongest to weakest cells. For example, he similarly does a top balance t o3.65V, then configures a 16S 48V battery to discharge and determine the lowest capacity cell. He then dropped that cell and continued down using a 15S battery. He continues down apparently draining each cell 2.5V depletion.
https://diysolarforum.com/threads/diy-cell-matching-process.16784/#post-191353
In my procedure, I think the ranking can be achieved a little simpler, by simply balancing the current draw from all cells using a parallel 4S configuration where I have a total of 8 cells. The trick is to get the current draw balanced which is easies with cell pairing as will become obvious when looking at the Step 2 mirror image battery cell configuration. This can be extended but even numbers of cell per 1S are going to be the only thing manageable. If you are going to be paralleling more than 2 cells then it might be best to do multiple discharges of 4S batteries to obtain the 48V load equipment that faFrd is describing.
Any comments and or advice is welcome. That is why I'm posting.
SYSTEM COMPONENTS
So I just recently received an order from AliExpress for:
VariCore 3.2V 102Ah Battery LiFePO4 Lithium phospha Large capacity DIY 12V 24V 48V Electric car RV Solar Energy storage system
Qty 8 100Amp hour LifePO4 cells $343.36 shipped.
Ordered May 28, 2021
Received July 28, 2021
https://www.aliexpress.com/item/4001253443449.html?spm=a2g0s.9042311.0.0.24b84c4dJ9NxR9
View of my bench test setup.
Closeup of the 4S battery pack with active balancer installed. The pigtails are prewired from the OverKill BMS (not connected).
The AiLi Battery Monitor (with 100 amp shunt)
Description of Components:
Overall the VariCore 100Amp-Hr cells are in good shape except for obvious swelling > 0.15” (both sides) per cell. This will make compressing the cells a bit more challenging. These are supposed to be Grade A cells but are probably refurbished cells that hit their 80% EOL and have been derated 20% , repackaged and resold as Grade A. They are rated for 200 cycles which is over 5.5 year of daily cycle use so for the price I am not going to complain.
Amp hour reading are based on the AiLi battery monitor. The measured capacity is meeting or exceeding 200Amp-Hr but I have yet to do a final test of the balanced cell pair and using active balancing. After doing an initial top balance, I used a Kreiger Modified Sinewave 1100W inverter with a heat gun adjusted for 500W load to do the first discharge capacity test.
https://www.amazon.com/gp/product/B00T564EIY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
AiLi Battery Monitor Voltmeter Ammeter Voltage Current Meter 8-80V 0-100A Auto Car Motor Boat Caravan RV Motorhome
https://www.amazon.com/gp/product/B07CTKYFTG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
The inverter shut down for low battery voltage when the cell voltage was still at 12V (3V cell equivalent). The invertor was probably sensing about <11.5V owing to voltage drops of the 500W heat gun load (48A DC load). So my initial discharge capacity estimate is based on the Ali Battery monitor going from 200 Amp-hr down to 20.75 Amp-hr indicated with 3.0V remaining nominally per cell. This means that 179.25 Amp-hrs is 92% of SOC (assuming 3.0V is 8%SOC). Therefore 179.25/.92=194.8 Amp-Hr total capacity. At this point, I’m not concerned with being below 200Amp hours. The Ali battery monitor is supposed to be accurate to +/- 1% or +/- 2 Amp-Hrs but I take this with a grain of salt. I have compared the current reading from the invertor, the power supplies, a clamp-on meter and the Ali Battery monitor. While doing the 500W load test the Ali-was indicating a significantly lower load current (25-35 amps) so I am surprised it came as close as it did based on above measurements.
I will also be doing some initial evaluation of a 5A active balancer also purchased from AliExpress. I will only use this to top balance the final battery configuration following a 12V/12A recharge.
5A - 8A Active Balancer Equalizer 4S Lipo Lifepo4 Li-ion Battery BMS High Large Blance Current Dynamic Conversion Module Board Paid $40.53 shipped.
https://www.aliexpress.com/item/4001013797039.html?spm=a2g0s.9042311.0.0.24b84c4dJ9NxR9
Although I have not done connected a BMS yet it is worth mentioning that, I originally purchased (Qty 2) Daly smart BMS LiFePO4 4S 60A for a drop-in equivalent 100Amp hour batteries. Later, changed my plan to a combined 200A battery with parallel cells and have the Overkill Solar 4S 120A BMS. At this point I have not connected the BMS, and have only used the Balancer to top balance the (Step 4 configuration) Combined 4S battery at 14.6V (3.65V cell equivalent).
https://overkillsolar.com/product/bms-120a-4s-lifepo4/