Question on batteries and minimum solar requirements.

Sparkynthouse · Jul 23, 2020

What are the performance differences between LiFePO4 and Lithuim-Ion? How safe and stable are Lithium-Ion? As I've read alot about LiFePO4 but not much on lithium-ion. I've found a deal on a BMW Lithium-Ion 8s 28.8v 70Ah 2kWh battery and was looking into using it in a shed for lights primarily. I have 1 panel @100watts, a 20amp controller, DC panel, primary fuse with lightning protection. Looking for a DC relay to kill battery from outside and primary DC fuse/breaker with a future ac/DC inverter. Is the above battery to big for what I have? The controller I have will do 12v or 24v and various battery types. Any help is appreciated. Thanks.

ianganderton · Jul 23, 2020

The big problem with some lithium chemistries is if a cell gets out of whack it can cause a fire with very nasty fumes. Lifepo4 doesn’t.

Lifepo4 cells also come in nominal voltages that build neatly into packs suitable for use with existing systems (multiples of 12V) because they are similar enough to lead acid voltages (precious technology) to work well.

Archetype-IS · Jul 23, 2020

Sparkynthouse said:
What are the performance differences between LiFePO4 and Lithuim-Ion? How safe and stable are Lithium-Ion? As I've read alot about LiFePO4 but not much on lithium-ion. I've found a deal on a BMW Lithium-Ion 8s 28.8v 70Ah 2kWh battery and was looking into using it in a shed for lights primarily. I have 1 panel @100watts, a 20amp controller, DC panel, primary fuse with lightning protection. Looking for a DC relay to kill battery from outside and primary DC fuse/breaker with a future ac/DC inverter. Is the above battery to big for what I have? The controller I have will do 12v or 24v and various battery types. Any help is appreciated. Thanks.

The battery 24v is too big for your 12v 100w panel setup.

The BMS for 8S Li-Ion battery doesn't exist[correction, looks like they do exist after @snoobler pointed out]. Daly makes them too. Or You can get the pack and cut one cell out and make a 7S and that voltage would be the ideal 24V nominal voltages for the 7S Daly BMS for Li-Ion and for most 24v inverters. That's why the BatteryHookup still have them because it is non-standard string. I know they said to use a 8S BMS, I wish they said which one brand. I searched and asked the question about it and given the same answer above. [Corrected above on BMS]

Either 8S or 7S would work. Just 7S is more ideal for most 24v inverters.

The 100W panel is too low to charge 24v battery pack, you need extra 100w panel to do 200Watts to increase the 12v to 24v for the SCC for the BMS.

snoobler · Jul 23, 2020

My Avatar is 84 26Ah Lithium-ion cells. These cells will explode and/or catch fire if they are operated too far outside their operating range or mechanically damaged. The electrolyte is flammable.

The best practice of adapting explody Lithium ion is to fuse every single cell to a common bus bar so that if there is a short in a cell, the fuse isolates the cell from a pack. A BMS and balancing is best practice.

At a minimum, you need a BMS to manage the battery and use charge/discharge cut offs. Archetype-IS is not correct. 8S BMS are readily available, but they are more expensive. Do not deploy the battery without a BMS.

Archetype-IS · Jul 23, 2020

snoobler said:
My Avatar is 84 26Ah Lithium-ion cells. These cells will explode and/or catch fire if they are operated too far outside their operating range or mechanically damaged. The electrolyte is flammable.

The best practice of adapting explody Lithium ion is to fuse every single cell to a common bus bar so that if there is a short in a cell, the fuse isolates the cell from a pack. A BMS and balancing is best practice.

At a minimum, you need a BMS to manage the battery and use charge/discharge cut offs. Archetype-IS is not correct. 8S BMS are readily available, but they are more expensive. Do not deploy the battery without a BMS.

Could you please share the brand and model? I would like to read some specs on them. Thanks.

snoobler · Jul 23, 2020

Archetype-IS said:
Could you please share the brand and model? I would like to read some specs on them. Thanks.

Zero Emission Vehicles Australia

Archetype-IS · Jul 23, 2020

snoobler said:
Zero Emission Vehicles Australia

Thanks for pointing that out. I corrected the post. Looks like Daly do make for one for 8S Li-Ion type too.

Sparkynthouse · Jul 24, 2020

Archetype-IS said:
The battery 24v is too big for your 12v 100w panel setup.

The 100W panel is too low to charge 24v battery pack, you need extra 100w panel to do 200Watts to increase the 12v to 24v for the SCC for the BMS.

Thank you. I got the first panel for free but, I do plan to add 3 more panels as 2 will get morning sun and 2 panels on the back get afternoon sun due to position of shed. My thoughts were to wire the front in series and the back in series, with the front and back wired in parallel. Would this be an issue? So it sounds like I need to be 12v. Is this correct?

Sparkynthouse · Jul 24, 2020

ianganderton said:
The big problem with some lithium chemistries is if a cell gets out of whack it can cause a fire with very nasty fumes. Lifepo4 doesn’t.

Thank you. I want it safe but worthwhile. So LiFePO4 sounds like the way I'll head.

Archetype-IS · Jul 24, 2020

Sparkynthouse said:
Thank you. I got the first panel for free but, I do plan to add 3 more panels as 2 will get morning sun and 2 panels on the back get afternoon sun due to position of shed. My thoughts were to wire the front in series and the back in series, with the front and back wired in parallel. Would this be an issue? So it sounds like I need to be 12v. Is this correct?

If you plan to get the 24v battery, you will need to run the system at 24v for the SCC. Not sure which 20amp SCC you have, if you read the spec, I am sure it can do it. Make sure it can do 12v/24v. Could you list the SCC brand or spec to double check?

For example:

Two series(12v+12v=24v nominal) in the front ; assume 6amp
Two series (12v+12v=24v norminal) in the back; assume 6amp
Then parallel the front and back (24v +12amp) together to the SCC. Just be mindful of the total voltage or amperage of the SCC parameters to the battery.

For example of the Victron SmartSolar SCC specifications:

When the solar panel said 12v nominal voltage, the open circuit voltage could be 18v to 23v per panel(see below for the example solar panel spec)
The total of the two in series will be in the range of 36v to 46v.
Parallel the front and back, the voltage stays the same, the amperage increases. In this case, the assumed number will be 12amp.

The charge controller that will fit for the setup with 24v, the wattage (400W) and amp at 12amp would be the MPPT 75/15 or 100/15 or the 100/20.

Specification:
1pcs 100W monocrystalline solar panel（Grade A solar cell )

Model Type:SG-100WM Rate

Maximum Power:100W

Voltage: DC 12V

Output Tolerance:±3%

Voltage at Pmp:18.5V

Current at Pmp:5.41A

Open-Circuit Voltage:22.4V

Short-Circuit Current:5.9A

Maximum System Voltage:1000V

Sparkynthouse · Jul 25, 2020

Archetype-IS said:
If you plan to get the 24v battery, you will need to run the system at 24v for the SCC. Not sure which 20amp SCC you have, if you read the spec, I am sure it can do it. Make sure it can do 12v/24v. Could you list the SCC brand or spec to double check?

Here is the solar charge controller I have.
Q
EPEVER Latest 20A Mppt Charge Controller,TRIRON 2210N Intelligent Regulator Dual USB Port PC Software Moblie APP Support [Updated Version of Tracer A/an Series]

7 type batteries and full protection support: Sealed(AGM), Gel, Flooded, Lead-acid User, LiFePO4 Lithium, Li(NiCoMn)O2 Lithium, Lithium User(9~34V). Four stages charging mode and all-around electronic protection prolong the lifecycle of batteries.

------2210N MPPT Solar Charge Controller
System voltage: 12/24V DC Auto work
Rated charge/discharge current: 20A
USB interface: 5VDC/2.2A(total)
Working environment temperature:-25℃～+55℃（with LCD）/-30℃～+55℃（without LCD）
Storage temperature range: -30℃～+70℃
MPP voltage range: (Vbat+2V)72V
LiFePO4/Li-NiCoMn/8-32V Li Support
Sealed(AGM), Gel, Flooded, 8-32V Lead-acid User Support
Max. input solar power: 260W when battery is 12V.
Max. input solar power: 520W when battery is 24V.
Max. Input PV open circuit voltage:100V
LCD backlight time: 60s(default)

So why are the wattages so different for input based on battery voltage?

Archetype-IS · Jul 25, 2020

You can do 24v, it will give you more power and save you on the wires, because, the wire could be thinner to carry the current.

It's ohm's law Watts(Power) = I(Current) X (Voltage) or P=IV

12/24v

P260W (P max) = 20amp X 12v = 240 Watt
520W (P max) = 20amp X 24v = 480 Watt

Why lower or higher for the Wattage? It has to do with temperature of the solar panels and solar panel voltage range of each panel. As explained earlier, the 12v/24v is a nominal voltage but there is a range to that, so the engineer factor those into the device.

Here is the technical details in this link for the high and low PV calculation.

The Highs and Lows of Photovoltaic System Calculations

Understanding calculations for proper system design and installation of photovoltaic (PV) systems

www.ecmweb.com

Question on batteries and minimum solar requirements.

Sparkynthouse

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ianganderton

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snoobler

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Sparkynthouse

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The Highs and Lows of Photovoltaic System Calculations

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