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Help with DIY solar charge emergency power bank
#1
Hi Everyone,

I hope someone can give me some advice. So I have want to build a the largest power bank I can make from salvaged 18650 from laptops. I have about over 130 of them with a mix of some CGR18650 and Samsung ICR. Would be ok to mix them?

I bought some 3S and 4S BMS a while ago but sure if would suite this project. It seem like a lot of people are build 7S modules. Would the BMS i have be ok for the job or I need to buy a new 7S?

I also bought a Renogy 100W solar panel starters kit with a Wanderer Li 30A PWM Charge Controller that can charge Lithium batteries. I was hoping I can use that to charge my power bank when the power goes out.

I was hoping this can power a freezer (0.58 kW) or water tank ac blower(85 watts) when the power is out

Some advice would be great .

Thanks
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#2
You "can" put a 3s and 4s together, but not advisable and might kill the units.
[Edit] This is referring to the bms units, not cell packs.

you will also need a lot more than 130 cells. The current listed on the fridge/blower is the running current, not the start current. Start current is at least twice the running current.
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#3
yes you can mix different cells, and different mah. Myself I would get a internal resistance meter to weed out the cells that test good but have high resistance. I found a few that I would have used in a pack. Last thing you want is build a pack and have balancing problems. By high resistance I mean cells that are in the 100's or 200's milliohm range while all your other cells are in the 40's and 50's milliohm range. The high cells are probably still good but why risk putting them in a pack. I learn the hard way and now test everything.

For your situation I would go with the 3s, that will charge the quickest with a solar panel (the bigger the differential between charge voltage and battery voltage the more amps your pack gets), a 4s (full charge 16.8 volts) is too close to the max voltage (about 16 volts) of most controllers. A 100 watt panel (usually 21 volt panel) might not be that efficient charging a 7s battery. With 130 cells, if each cell is around 2000 mah you might get a 3s43p about 86ah, it won't run a freezer that long and if you use an inverter it would be less efficient. If you go with a 7s pack, I would get a larger 36 volt panel and mppt controller.

Most controllers manuals will tell you to never disconnect the battery from the controller while the panel is producing power, which is what a regular bms does, thats the problem I see with solar, so you need a bms that can actually disconnect the panel from the controller when the battery is full. A normal mosfet bms will stop the charging by disconnecting the controller from the battery with electronic relays (which leak voltage), this produces voltage surges. It will destroy anything connected to the battery. I encounter voltage surges many times with mppt/pwm controllers. Now the only bms I recommend for lithium/solar is the chargery bms8 (about 95 dollars) Its the only controller I trust not to destroy devices and give my pack a full charge everytime. It uses contactors/relays (i use a 4 dollar 30 amp automotive relay) to disconnect the panel from the controller.

If you will be using a regular bms or a chargery, also get an overvoltage protection relay (about 5 dollars) this will be a deadman switch in case the bms fails, and also get a 12 volt voltage stabilizer (one that can handle the amps you require), then connect all your devices to the stabilizer, it will protect from voltage surges. With lithium you need to have a backup for your backup, you can never risk overcharging the pack.

internal resistance meter I bought (57 dollars)


chargery bms8 been using for the past year and its been rock solid, it just works.
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#4
It would not be advisable to use 3s. The voltage drop under heavy load would be too great. Also, the working voltage range is too narrow.

Also, a 3s200p will charge just as quickly as a 4s150p, which will be the same as 7s86p. It's still the same amount of energy stored.

.58kW running would equate to about 2x that on start (conservative), so 1.16kW.
If running 3s, nominal voltage would be about 10.8V. For 1.7kW load, you would need at least 55p "IF" all the cells in parallel were 2000mAh.
Now, going with this type of load, you would also need 1700W / 10.8V = 157A surge

If you run 4s, then 1700W / 14.8V = 115A surge
Run at 7s then 1700W / 25.9V = 66A surge
Run at 14s then 1700W / 51.8V = 33A surge

Take 115A / 55P = 2A/cell
66A / 55P = 1.2A/cell
33A / 55P = .6A/cell

Obviously I'm sticking with 55P minimum. But overall cell count would go up with each increase in string count (3s, 4s, 7s, 14s).

Basically, overall 130 cells won't cut it currently. You need a LOT more cells to run a freezer. Plus, you need to account for the inefficiencies of an inverter (which I did not include here) and the length of time the devices need to run.

Overall, if you are running 12VDC appliances, go with 4s if you have to, otherwise go 7s and use a buck converter. If you are running 110+VAC, go with 14s most recommended and 7s minimum. If you go smaller than this you will not incur much saving due to the extra expense of higher amp rating on the inverter and the much heavier gauge wire you need to handle the extra amps.

P.S. It's late in the evening and time for bed, but I think I got my math right, or at least close.
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#5
(03-23-2020, 03:00 AM)Korishan Wrote: It would not be advisable to use 3s. The voltage drop under heavy load would be too great. Also, the working voltage range is too narrow.

Also, a 3s200p will charge just as quickly as a 4s150p, which will be the same as 7s86p. It's still the same amount of energy stored.

.58kW running would equate to about 2x that on start (conservative), so 1.16kW.
If running 3s, nominal voltage would be about 10.8V. For 1.7kW load, you would need at least 55p "IF" all the cells in parallel were 2000mAh.
Now, going with this type of load, you would also need 1700W / 10.8V = 157A surge

If you run 4s, then 1700W / 14.8V = 115A surge
Run at 7s then 1700W / 25.9V = 66A surge
Run at 14s then 1700W / 51.8V = 33A surge

Take 115A / 55P = 2A/cell
66A / 55P = 1.2A/cell
33A / 55P = .6A/cell

Obviously I'm sticking with 55P minimum. But overall cell count would go up with each increase in string count (3s, 4s, 7s, 14s).

Basically, overall 130 cells won't cut it currently. You need a LOT more cells to run a freezer. Plus, you need to account for the inefficiencies of an inverter (which I did not include here) and the length of time the devices need to run.

Overall, if you are running 12VDC appliances, go with 4s if you have to, otherwise go 7s and use a buck converter. If you are running 110+VAC, go with 14s most recommended and 7s minimum. If you go smaller than this you will not incur much saving due to the extra expense of higher amp rating on the inverter and the much heavier gauge wire you need to handle the extra amps.

P.S. It's late in the evening and time for bed, but I think I got my math right, or at least close.

Thanks everyone for the quick reply . So my dream to run the freezer seem very far away and might need to get a lot more battery to do so.

With my 130 cells is it still possible to something to run the water tank blower which is 115V AC 85 Watt blower? I guess I would also need to get a pure sine wave inverter since it's an AC motor. Would 1000 Watt pure sine wave inverter be enough?

Thanks
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#6
85W /115VAC = .74A
That's on the AC side of the inverter.
Depending on which inverter you go with (12, 24, 48VDC) will depend on the battery side load

Figure 20% added for inefficiencies and inverter draw, so about 102W total load on the battery bank.

102W / 14.8V = 6.9A
102W / 25.9V = 4A
102W / 51.8V = 1.97A
That's on nominal voltage of 3.7V

On the worse case, figure 3V, then the numbers change a bit:
102W / 12.0V = 8.5A
102W / 21.0V = 4.85A
102W / 42.0V = 2.43A

If you go with 10P, you could do it, yeah. Even on a 4s, though it would be heavy load on the cells. Also, the more cells in parallel, the "longer" the run time. So 20p will run twice as long as 10p under the same load conditions.
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#7
(03-23-2020, 08:24 PM)Korishan Wrote: 85W /115VAC = .74A
That's on the AC side of the inverter.
Depending on which inverter you go with (12, 24, 48VDC) will depend on the battery side load

Figure 20% added for inefficiencies and inverter draw, so about 102W total load on the battery bank.

102W / 14.8V = 6.9A
102W / 25.9V = 4A
102W / 51.8V = 1.97A
That's on nominal voltage of 3.7V

On the worse case, figure 3V, then the numbers change a bit:
102W / 12.0V = 8.5A
102W / 21.0V = 4.85A
102W / 42.0V = 2.43A

If you go with 10P, you could do it, yeah. Even on a 4s, though it would be heavy load on the cells. Also, the more cells in parallel, the "longer" the run time. So 20p will run twice as long as 10p under the same load conditions.


I think I'll try to make a 4s 30p battery. I bought a spot welder so I was going to spot weld them together but I see everyone here seem to solder with fuse to a bus bar. Do I need to use a fuse?

Also would be ok to use a mix of battery brand to make my battery pack? I guess I'll be using repacker to help me balance them. My 120 are made of the battery brand such as
Samsung ICR18650-26C
Samsung ICR18650-28A
Panasonic CGR18650D
Panasonic CGR18650F
Panasonic CGR18650EA
LG LGAABB4165

Thanks
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#8
Cell brands aren't that big of a deal in mixing as long as they have similar properties.

30 x 2000 = 60,000mAH (60Ah). Under full load of about 7A, you'll get about 8.5hrs of run time. That is under full duty cycle and assuming 2000mAh average per cell
Proceed with caution. Knowledge is Power! Literally! Cool 
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