Simple design for using an inverter to power your 50 amp service RV using a $30 Change over switch
- Thread starter Scott Helmann
- Start date
I agree with Joey here, you have not presented any evidence that 2.0V is the minimum cutoff voltage.
Regardless of whether you choose to continue believing 2.0V is a safe minimum, please don't spread that around if you can't be bothered to show any evidence that recommends going below the minimum (2.5V) as currently defined by every major cell manufacturer that I have come across.
Back to the topic at hand, Inverter cutoff:
Technically speaking 10V, is not below the minimum of 2.5VPC, however it leaves no safety margin whatsoever, and no room for the BMS to have a wider cutoff. Because an inverter cutoff is battery bank level, not cell level, a 10V cutoff would allow individual cells to dip well below 2.5VPC, if your cells are imbalanced or mismatched (which is likely considering the level at which grey market cells get matched, and the fact that BMSes are top balancing). Of course your BMS could step in to protect the battery in this situation, but it would have to be set at the same level as the inverter, which may or may not be a nuisance or problem depending on your usage, your BMS, and your system.
Further, while cell manufacturers define 2.5V as the low voltage floor, virtually all recommend 10-90% SOC bandwidth, ~10% is roughly 12V (3 VPC).
If your inverter has configurable LVD (I think you said LVD is 20v-21V but not sure if this is configurable or not), 21V / 2.625 VPC would be much better (still low, but it would give you a bit of margin), and is actually fairly close to Will's recommendation for inverter LVD (which I, and I think many here consider pretty low) of 21.4V / 2.675 VPC
My $.02 ($CD .03)...
It doesn't take much looking to find stuff on the web that indicates LFP can go to 2.0V/cell. Here are a few:
https://www.electriccarpartscompany.com/Bestgo-20Ah-LiFePO4-Lithium-Pouch-Cell (check one of the two discharge curve pictures and cell specs table farther down the page)
This can easily allow someone to make that conclusion broadly.
It's also helpful to remember that LFP chemistry is absurdly robust.
I have 57 CALB cells that were stored at 0.6V for months after their BMS boards flattened them. They all charged up and are indistinguishable from the 5 that were stored in the charged state (same capacity and IR). They've also held their charges for several months since. This deep discharge happened AFTER they were ABUSED in a plug-in-hybrid conversion kit for the Gen2 Prius in PHOENIX with NO cooling system for a couple years.
One must understand that pushing the chemistry to the extreme limits has rapidly diminishing returns... so much goody is had in that flat part of the curve. If exceeding upper and lower voltage limits is done every cycle, you'll have reduced cycle life. If it's done occasionally and not excessively (DON'T go over 4.2V ever), you'll never see the difference.
It doesn't take much looking to find stuff on the web that indicates LFP can go to 2.0V/cell. Here are a few:
Measuring State-of-charge - Battery University
Explore SoC measurements and why they are not accurate
batteryuniversity.com
3.2V 30Ah Lightweight With Good Charging / Discharging Performance
High quality 3.2V 30Ah Lightweight With Good Charging / Discharging Performance from China, China's leading LFP Pouch Cells product market, With strict quality control LFP Pouch Cells factories, Producing high quality 3.2V 30Ah Lightweight With Good Charging / Discharging Performance products.
www.hecobattery.com
https://www.electriccarpartscompany.com/Bestgo-20Ah-LiFePO4-Lithium-Pouch-Cell (check one of the two discharge curve pictures and cell specs table farther down the page)
This can easily allow someone to make that conclusion broadly.
It's also helpful to remember that LFP chemistry is absurdly robust.
I have 57 CALB cells that were stored at 0.6V for months after their BMS boards flattened them. They all charged up and are indistinguishable from the 5 that were stored in the charged state (same capacity and IR). They've also held their charges for several months since. This deep discharge happened AFTER they were ABUSED in a plug-in-hybrid conversion kit for the Gen2 Prius in PHOENIX with NO cooling system for a couple years.
One must understand that pushing the chemistry to the extreme limits has rapidly diminishing returns... so much goody is had in that flat part of the curve. If exceeding upper and lower voltage limits is done every cycle, you'll have reduced cycle life. If it's done occasionally and not excessively (DON'T go over 4.2V ever), you'll never see the difference.
John Frum
Tell me your problems
- Joined
- Nov 30, 2019
- Messages
- 15,233
While I recognize your point, and I acknowledge that I tend to err on the side of caution when faced with limited info, and I thank you for providing these links, I don't think they necessarily contradict the advice about 2.5V being the current industry standard for minimum voltage.
As mentioned, it is my understanding that 2.0V was oft cited some years back but since than for reasons I'm not fully aware of, the industry has refined and tightened allowable cycle bandwidth towards 2.5VPC as the minimum discharge voltage and similarly has adopted a lower maximum charge voltage. The fact that immediate or severe harm will not occur below 2.5V is not a great reason to push that limit against the advice of virtually all major cell manufacturers that I am aware of (in my marginally informed opinion).
Of the 3 links provided
- the third states 2.0V to be the minimum voltage spec for 20Ah Bestgo pouch cells
- the second states 2.3V is minimum voltage for 20Ah Heco Green pouch cells
- the first gives no recommendation, the referenced voltage curve shows from 3.15ish to 1.9ish, but isn't meant to convey safe or recommended voltage range, and isn't on the topic of min or max voltage (the article is about how to measure SOC, and measures a single amp-hour cell at C rates of 4C to 12C--so a very unique model).
I'm not trying to nitpick each link, but I do think that a few random links are not a sufficient evidence to contradict the current industry standard, or your specific cell manufacturers recommendations.
I accept your point that no immediate/severe harm may come from dipping below 2.5VPC, and that 2.0V was not uncommon in the past, but I don't think it follows that manufacturer limits be intentionally pushed or violated, particularly with an inverter which is only aware of total battery bank voltage and is ignorant of true cell level voltage.
Agreed
Agreed
Not prepared to agree with this, but also not prepared to disagree.
Overall I think you raise some good and valid points about the robustness of the chemistry, and the ability to tolerate or recover from some abuse. Still I think its inadvisable to design around that, much like it would be inadvisable to design around using the safety margin of any other component in your system. I personally feel it prudent to have the BMS cutoff no lower than 2.5VPC at the lowest, and pack/bank level limits set higher.
@Dzl
I was only providing links to establish evidence.
One should design around one's needs and the limitations of the components. If someone is willing to sacrifice cycle life to squeeze out a little more performance on a per-cycle basis, that's their prerogative.
The key is to understand the limitations of the components when formulating the design. Those limitations are obtained from the component manufacturer - a point I had intended to make in my post, where I believe you and I completely agree...
I was only providing links to establish evidence.
One should design around one's needs and the limitations of the components. If someone is willing to sacrifice cycle life to squeeze out a little more performance on a per-cycle basis, that's their prerogative.
The key is to understand the limitations of the components when formulating the design. Those limitations are obtained from the component manufacturer - a point I had intended to make in my post, where I believe you and I completely agree...
We are definitely in agreement here!
Those limitations are obtained from the component manufacturer - a point I had intended to make in my post, where I believe you and I completely agree...
I think we agree on most things, including most things on this topic to an extent, I'm just bad at making my comments come off that way
Uncle! UNCLE!!!
Ok I have a 50 amp service panel and a 30 amp inverter. I'm ok with only running power from the inverter to one side of the service panel. I understand in your diagram how to use the manual switch for the shore power. I'm a little lost of the inverter section. I can understand connecting the inverter to the switch but I don't understand how to connect that part of the switch to my service panel. On my service panel I have the receptacle, the GFI receptacles and the microwave. How do I power these from my inverter please?
Thanks in advance,
