Re: [electricboats] Paralleling LiFePO4 battery packs

My friend is successfully using 6 salvaged BYD  24 volt packs in parallel for his off grid solar energy system. I would not recommend them for a EV boat or any high amperage installation except for temporary or testing use. At low off grid solar loads he can safely operate in the middle voltage range with a huge battery and not go near the ragged edges off the battery. But what your propose doing with BYD packs in 15s configuration should work for a temporary measure. But I would watch it like a hawk. You might only have a few KWh to play with. I estimate my friend has about 10-12 KWh which is a lot for solar. Four of his packs came with internal balancing circuitry and 2 came without. They are staying nicely balanced so far and he hopes to get many years of service for a total cost of about $2500 after expensive shipping. 


Sent from Yahoo Mail for iPad

On Thursday, January 7, 2021, 9:24 AM, Jeff LaCoursiere <jeff@stratustalk.com> wrote:

Hi,

I'd like to interject a question into this thread since BYD came up - I am another opportunist (victim?) of the BYD selloff, and have two of these monsters on my lab bench right now.  Mine came without the case and BMS - just bare cells and straps.   I connected an ANT BMS to the first one and determined that cell 8 is pretty much toast, whereas the rest seem reasonably stable and balanced after a few cycles.  The other BYD seems fine, although ten years old and probably at only half capacity.

My question is... if I simply rewire the first pack to remove the 8th cell, making it a seven cell pack, then put that in series with the other one (the 8 cell pack), I will end up with a *nearly* 48V pack, yes?

I'm guessing my motor (ElectricYacht 20K) doesn't care, and I am about to run some tests with off the shelf step-downs to run my 12v system which I don't think will care, and a test with another off-the-shelf inverter that I also don't think will care.

I don't really mean for this frankenstein pack to be the final solution, but any reason anyone can think of not to use it to get things going?  I'm probably EY's longest installation - I bought the motor almost six years ago and it is still sitting on my bench next to me lol.

Cheers,

j

On 1/3/21 11:56 AM, THOMAS VANDERMEULEN wrote:

Best practice calls for paralleling individual cells for a period of time before building them into a battery pack so that they can equalize in voltage.  As long as they are connected in parallel they will remain equalized in voltage.  In a system that parallels multiple cells to achieve higher capacities at a given voltage, the cells would ideally be paralleled BEFORE the series connection.  In such an arrangement, the individual paralleled cells remain equalized in voltage, and it's necessary only to monitor one of them.
In the arrangement you describe, however, while the pre-built 48v packs will remain relatively equalized in voltage, it will still be possible for cells within each pack to vary in voltage by quite a bit.  An advantage of your approach in such a case would be that if one cell in one of your paralleled systems becomes unstable or drags down that pack, you can simply change your connections to take that pack out of the system until you can address the questionable cell.
On the whole, I agree with Ryan that the best answer to your question depends on the capabilities of the integrated BMS.  In some cases, the pre-built units may only have battery protection systems that are designed to respond to over or under voltage conditions, but not offer monitoring or programming.
One other consideration in your arrangement would be somewhat more complicated temperature monitoring.  This becomes an increasingly important consideration as discharge and charging rates go up.  And if you're planning what amounts to a single, large capacity traction battery pack [600Ah], presumably you'll be specifying a high capacity charger [75 to 80 amps].
Just a few thoughts for your consideration ... 
[-tv]

--   Jeff LaCoursiere  StratusTalk, Inc.  703 496 4990 x108  815 546 6599 cell

--
Jeff LaCoursiere
s/v Angels Quest
St Thomas USVI