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My 2kwh 7s "what to do with these cells?" project
#1
Hi all-
Long time lurker in San Diego, I've been on the fence about building one as I didn't' want to start another project. In the past I've sourced 1000 cells, sold them, given others away, and this past week sat there looking at this box of 280 tested cells I still have. I've tried selling them locally to others on Craigslist, but no bites.

so.... I was sick of moving them around the garage so I built my 1st large pack. I've built small 3s and 4s in the past, and a 7s10p for an electric bike a few years ago, but just completed my 1st 7s40p pack.

Why 7s? I have a 24v UPS, 24v of spare solar panels (180w), and a grid tie inverter that supports 24v.

Took me a few hours of sorting, counting, soldering, and assembling, but it's done. I plan on making additional modules as I harvest/grab cells.

I used 1/4 watt resistor wires and soldered them quickly with a big-fat soldering iron (to keep the battery temps down). The cheap-o solar charge controller (shown) supports 24v setups, and stops charging cells at ~28.4v. The whole pack has a 30a in-line fuse.

I'm storing them in large plastic trays w/ voltage output via terminals in the front. 

I haven't hooked it up to the UPS for any load tests, but did hook it up to a 600w grid tie inverter and it put out 440w back into the grid (I have an inter-tie agreement w/ the local utility for an existing solar setup).

We (in San Diego) are heading into our expensive on-peak period. $0.50/kwh on-peak, and $0.19 off peak. I plan on charging it during the night w/ cheap power (and a 3a 7s charger), and spitting it back in their faces mid-day.

I feel like I'm part of an elite club of lap-top battery hoarding peeps now!

BMS and cell monitor to follow. any cheap-o "made in china" options that you're using?

Crimp Daddy likes this post
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#2
Basic RC style cell monitor - Battgo bg-8s is solid.  I did some accuracy testing in this and thought it was pretty dang good.


For an inexpensive / affordable BMS, I highly recommend the Chinese Bluetooth capable units.  They are more expensive than the ultra cheap units, but still far less expensive than something like a Batrium and totally worth the coin.  They range about $40-50

The native app is android, but a 3rd party has made an iOS version which is fantastic.
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If you ever plan on upgrading the solar charge controller, the Victron Smart MPPT units are absolutely awesome for the money.  I've used a bunch and have yet to find something else I prefer to use.

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philsocal likes this post
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#3
When charging off-peak and discharging on-peak, you need to consider that there are losses during charging (10%?) as well as in the inverter/GTI (10~20%?). Also the cost of using up cell cycles.
I've come to the conclusion that it's not worth it in my situation ($0.10 off and $0.30 on), so I'm just charging with heaps and heaps of PV panels.
philsocal and Ibiza like this post
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#4
(04-15-2020, 11:51 PM)Crimp Daddy Wrote: The native app is android, but a 3rd party has made an iOS version which is fantastic.

Cool! as a 3rd party has developed an IOS app, it's obvious the protocol is known. Do you know if anyone has managed to get a different system to work with it? Like a Raspberry PI w/ web interface?

I know there is a PC version (I'd probably run this), but woudl LOVE something I could use w/ Grafana and mongodb or something else.
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#5
I don't know if it's in your budget, but a good mid-range bms is the SMBS0 from Electrodacus. It's modular, so you can add a solar charge controller to it, too.

I second the efficiency concern ajw22 raised. Depending on your equipment, your round trip could be 95% efficient, or 60% efficient, or somewhere in between.
philsocal likes this post
Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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#6
(04-16-2020, 04:15 AM)philsocal Wrote:
(04-15-2020, 11:51 PM)Crimp Daddy Wrote: The native app is android, but a 3rd party has made an iOS version which is fantastic.

Cool! as a 3rd party has developed an IOS app, it's obvious the protocol is known. Do you know if anyone has managed to get a different system to work with it? Like a Raspberry PI w/ web interface?

I know there is a PC version (I'd probably run this), but woudl LOVE something I could use w/ Grafana and mongodb or something else.

I don't know bud... i'm a simply guy and want easy when it comes to stuff like that... plus I can't program at all... Raspberry PI is something I want to eat, not configure.

The efficiency conversation good one... my loads are primary direct DC, and I did measure my Victron MPPT to be 98.8% efficient on my last input/output power comparison.
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#7
So far, I'm planning on a cheap-o smart BMS, and will run this software-
https://mono.software/2018/11/15/multiple-bms-monitor/
Crimp Daddy likes this post
2kwh 7s40p of 18650's
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#8
10 day on-line status report.
I just installed balance leads today, and all cell banks were within 0.05v of each other. Not bad.
I have 2 kill-a-watt meters. One on the "in" and one on the "out". THe "in" side just has a 48v UPS that I'm using to charge the bank with. I also have a few

The "in" one reads 11.45kwh, the out one reads 14.2kwh. At the night/day rates of $0.20 and $0.57, I've spent $2.29 to charge, and put $8.94 back into the grid. Net cost of $5.80 profit. ~$0.60/day.
It would be greater, but the UPS doesn't charge 'till full, and the Grid tie inverter doesn't drain them 'till empty.
2kwh 7s40p of 18650's
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