diy solar

diy solar

DIY battery waiting for BMS

daikerjohn

New Member
Joined
Jan 31, 2023
Messages
7
Location
Oregon
Hey y'all. I've jumped into this DIY solar thing quite quickly (although I did have a 17kwh grid-tie system installed on top our riding arena four years ago and have been happy overall) but have a question about building my own battery.

For sake of completeness and future posts/queries, here's my end goal:
I have a ~105watt DC load (could be up to 220w DC when I decide in the future) that I want to run 24x7x365 using this homemade battery for overnight juice.
I currently have a Renogy Wander 10amp PWM charge controller (https://www.amazon.com/Renogy-Wanderer-Amp-12V-24V/dp/B07NPDWZJ7) that's being driven by a single 250w Trina Solar panel (https://www.ecodirect.com/Trina-Solar-TSM-250PA05-250W-30V-PV-Panel-p/trina-tsm-250pa05.htm).
This current controller/panel setup is woefully undersized but that's a temporary situation until I get my battery sorted.
Quick math says I need 6x 250watt panels. I was thinking of a Renogy 40amp MQTT for the charger but the series/parallel setup of the panels (and subsequent VoC, amps, etc) confuses my brain because I haven't written it down and done the math yet.

I ordered 4x 280ah Grade B REPT cells (https://www.18650batterystore.com/p...ells-3-2v-lifepo4-prismatic-battery-pack-of-4) that arrived yesterday.
My DC bench supply (https://www.amazon.com/dp/B092DH6GM8?psc=1&ref=ppx_yo2ov_dt_b_product_details) also arrived yesterday.
The 4s JBD BMS won't be here for another couple of weeks, but that's not a huge concern at the moment.

Back to the battery itself:
My intent was to top-balance the cells, but now I'm wondering if that's even necessary?
I unpacked the cells and visually inspected them. They look nearly new (save for the scratched-out QR code) and don't have any noticeable bulging or swelling at all. My multimeter says they're all at 3.35 volts. (My multimeter only has two decimal places precision... is that going to be good enough?)
The cells each had little test stickers on them: 282ah, 292ah, 292ah, 293ah all tested October 22, 2022. Bummer about that 282ah cell... but I'll work with what I have.

Here's my question:
If all the cells are sitting at 3.35 volts... is there even a need to top-balance each individual cell using the DC bench supply? Can I wait on the BMS, assemble the battery and then top-balance the assembled unit?

Any/all help and comments are greatly appreciated!
 
LFP voltage curve is pretty flat. That 3.35 volts could be 30%, or 40%, or 50%, etc. There really is no way to determine the SoC by voltage alone.
 
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