Advice for NMC batteries in small setup

[whitepine]

Hi all,

Happy to provide complete system details if needed, but it's a small off-grid recreational (lakeside cottage) application. My questions are related to the batteries.

We have (2) x 24V 1kWh NMC Chevy Volt batteries (total of 2kWh). This first season of use we have probably used an average of 600Wh/day. So all good so far and things are charged up by noon most days with our two 255W panels.

Question 1:

What settings are recommended for the CC for the NMC batteries? Based on advice and research I currently have it set to:
Bulk: 24v
Absorption: 24.3v - 3hrs
Float: 24v
EQ: off

Part B: With our relatively light, non-essential use, I'd like to keep it conservative to prolong battery life. Regarding voltages, what would be a safe, i.e. say 20% SOC, low-voltage? I can't seem to get a straight answer on this one. Looks like something around 3.4v per cell for 20.4v. (And it looks like 24v is around 80% SOC, ya?)

Question 2:

I searched but can't find on this forum any recommendations for a plug-in charger for something like the modules we have. (We have a 2800W Yamaha generator to plug in a charger if need be.) Any suggestions of what we should look for in a charger? maybe I'm over thinking it?

Thanks

[Mike90250]

Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC) batteries are not easily supported by DIY chargers, You could read up on the batteries here

http://batteryuniversity.com/learn/article/types_of_lithium_ion

but to charge them properly, you need to know what variant you have and what voltage it wants

Lithium Nickel Manganese Cobalt Oxide: LiNiMnCoO2. cathode, graphite anode
Short form: NMC (NCM, CMN, CNM, MNC, MCN similar with different metal combinations) Since 2008
Voltages 3.60V, 3.70V nominal; typical operating range 3.0

[NEOH]

That battery is from an Electric Car so ...
what about posting your question on a "DIY Electric Car" type forum?

This battery is not commonly used for off-grid solar.

"...
Most Li-manganese batteries (LMO) blend with lithium nickel manganese cobalt oxide (NMC) to improve the specific energy and prolong the life span. This combination brings out the best in each system, and the LMO / NMC is chosen for most electric vehicles, such as the Nissan Leaf, Chevy Volt and BMW i3. The LMO part of the battery, which can be about 30 percent, provides high current boost on acceleration; the NMC part gives the long driving range.
..."

Lithium Manganese Oxide (LiMn2O4) = the LMO part
3.6V nominal
Min 3.0 - Max 4.2V / cell - typical operating range

Lithium Nickel Manganese Cobalt Oxide: LiNiMnCoO2 = the NMC part
3.6V nominal
Min 3.0 - Max 4.2V / cell - typical operating range


How many cells are connected in series to make 24 Volts ?

Six cells in series ?
===============
6 x 4.2v = 25.2v (max charge voltage)
6 x 4.1v = 24.6v (more total cycles)
6 x 3.6v = 21.6v (nominal operating voltage)
6 x 3.0v = 18.0v (min discharge voltage)


Seven cells in series ?
=================
7 x 4.2v = 29.4v (max charge voltage)
7 x 4.1v = 28.7v (more total cycles)
7 x 3.6v = 25.2v (nominal operating voltage)
7 x 3.0v = 21.0v (min discharge voltage)

[whitepine]

Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO2 or NMC) batteries are not easily supported by DIY chargers, You could read up on the batteries here
http://batteryuniversity.com/learn/a...of_lithium_ion...

Thanks very much for the link. I had read that and while it does provide a per-cell voltage range (3.0 to 4.2), there's no SOC curve... and I'm wanting to stay "away from the knees" as is so often written here...

[whitepine]

That battery is from an Electric Car so ...
what about posting your question on a "DIY Electric Car" type forum?

This battery is not commonly used for off-grid solar.

Yes I've gleaned as much as possible from the DIY electric car forums but you're right, I'll post the same questions there.

As for "not commonly used for off-grid", that may be true currently, but I suspect that NMC is up-and-coming (i.e. Tesla powerwall, repurposed EV batteries, etc.)

Here's a cool article: https://www.wired.com/2015/06/gms-us...ower-building/

Thanks again for all the info on this board. It's been a great help with my setup. I'll do my best to pay it forward, so if anyone wants to hear my experiences with NMC just let me know

[whitepine]

That battery is from an Electric Car so ...
what about posting your question on a "DIY Electric Car" type forum?

This battery is not commonly used for off-grid solar.

I also replied to your post to say thanks very much. I linked to a couple of cool examples of NMC uses like Tesla and repurposed EV batteries for powering buildings. I bet that post is awaiting approval, either that or it's lost to the ether...! Good idea to ask the EV crowd. I've combed over their forums with no luck, so will do!

[NEOH]

4.2v = 100%
3.0v = 0%

Saturated vs Quick Cut-off charging makes a difference regarding SOC

SOC is discussed here:
http://batteryuniversity.com/learn/a..._ion_batteries

and here:
http://batteryuniversity.com/learn/a...ased_batteries

A lower final charging voltage = increased # of cycles ( slightly lower capacity )
Shallow discharges = increased # of cycles ( lower effective capacity )

Floating at a high final voltage = decreased life expectancy.

[NEOH]

The guys at "DIY Electric Car" can tell you what Min / Max Voltage they use for the Chevy Volt NMC battery pack.
Once you determine your operating voltage range then you can program the Solar Charge Controller to the proper Absorb Voltage.

The power wall devices have an integrated charger controller, the end user does not need to know the Absorb Voltage.
LiFePO4 are less dangerous and they have a different set of voltages ( lower ).

How many Chevy Volt NMC cells are in SERIES to make 24 Volts? Six? Seven?

[whitepine]

Thanks for the links re: charging Li batts. Feeling better informed now after reading them fully I didn't realize that NMC seems to share the "most common" Lithium cell voltages (3.0 - 4.2, nom 3.7) so I skimmed those articles previously and kept searching. Silly me. I was under the impression that the lower cell voltages of LiFePo, which I've read about on here, were the common Li ones.

...

How many Chevy Volt NMC cells are in SERIES to make 24 Volts? Six? Seven?

There are six cells in series.

As mentioned, I'm using 24v (4.0v per cell) as 100% in order to be conservative with SOC (at 4.2v per cell I could go to 25.2v for 100% with the trade-off of reduced lifespan), which you've kindly corroborated with the info provided. Well, mine rest at 24.15 or so after they hit the "absorb" at 24.3 then settle. So 4.025v...

I'm wondering if I should just turn the "float" off; as well as set the voltages lower as we seem to have lots of capacity for our daily needs... anyway just thinking out loud now.

Thanks again for chiming in with your thoughts. Cheers!

[NEOH]

How to prolong Lithium batteries ...

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

It is a trade-off between capacity and total cycles, but time (aging) is always ticking, too.

Float at the lower 24 Volts is OK, as long as all cells are equal at 4.0 Volts.
Have you verified that the BMS maintains all six (6) cells at 4.0 volts during FLOAT?
What is your cell-to-cell variance?

[whitepine]

How to prolong Lithium batteries ...

http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

It is a trade-off between capacity and total cycles, but time (aging) is always ticking, too.

Float at the lower 24 Volts is OK, as long as all cells are equal at 4.0 Volts.
Have you verified that the BMS maintains all six (6) cells at 4.0 volts during FLOAT?
What is your cell-to-cell variance?

Yes I've been babysitting the batteries quite closely this first season and all cells have stayed within .01v. In fact they have been dead on equal most of the time so that's good.

And yes, the trade-off is why I was considering working with a slightly lower voltage range.

Thanks again

[NEOH]

Wow +/- 0.01 Volts, that is really good.
Do you have the GM Battery Balancing Module connected?

It seems like everything is working perfectly.
In 5 years, let us know how many total cycles, (avg) Depth of Discharge and any remaining capacity.

Will the battery bank ever get near or below freezing?

Did you purchase the Chevy Volt Battery Pack new or used?
How much did you pay?
For comparison:
A 14 KWHr Power Wall 2 ( with BMS, Charger & AC Inverter ) costs $5,500 + install
At the same ratio a new 2 KWHr Chevy Volt Battery Pack ( no BMS, no charger & no inverter )
should be much less than $800 ( = $5,500 / 7 )

Compared to a 5.4 KWHr Trojan Lead Acid Battery Bank = $600

In message #1, you stated that you used 600 KWHr per day?
That seems way too high for a 2 KWHr battery bank and 500 Watt PV.

[whitepine]

Wow +/- 0.01 Volts, that is really good.
Do you have the GM Battery Balancing Module connected?

It seems like everything is working perfectly.
In 5 years, let us know how many total cycles, (avg) Depth of Discharge and any remaining capacity.

Will the battery bank ever get near or below freezing?

Did you purchase the Chevy Volt Battery Pack new or used?
How much did you pay?
For comparison:
A 14 KWHr Power Wall 2 ( with BMS, Charger & AC Inverter ) costs $5,500 + install
At the same ratio a new 2 KWHr Chevy Volt Battery Pack ( no BMS, no charger & no inverter )
should be much less than $800 ( = $5,500 / 7 )

Compared to a 5.4 KWHr Trojan Lead Acid Battery Bank = $600

In message #1, you stated that you used 600 KWHr per day?
That seems way too high for a 2 KWHr battery bank and 500 Watt PV.

I've edited my first post to correct the error--oops, I meant 600Wh/day not 600kWh/day. Slightly different... glad you're paying attention! Thanks for catching my mistake

I paid $200 US ea for the batteries. No they will not be used below freezing. It is one drawback of NMC, they shouldn't be charged below 25 degrees F. They can be discharged that cold, just not charged. Pluses include they don't spill, don't off-gas and that they are about 1/3 the weight of lead-acid. Weigh even less if you consider "usable" capacity.

Anyway, I know mine is still a somewhat "unproven" approach, and will watch things closely. I will gladly let you know how it's going as things progress. Cheers!

PS I see they are available here and there, for example:

http://hybridautocenter.com/HAC4/index.php?option=com_hikashop&ctrl=product&task=show&cid=83&name=1kwh-lithium-battery-pack-chevrolet-volt-22-5v-45ah-6-cells&Itemid=605

(The 1kw 24v modules are rarer than the 2kw 48v modules. I think there are only two 24v modules in the complete chevy volt battery bank and seven 48v modules.)

[wardneal]

New Member
Can someone tell me if I can use 10 chevy volt 2mwh 48v battery's for peak load reduction on a 17kw grid tie solar system? I need to keep my peak load under 25kw. I peak around 30kw for less than 1 hour.

[Wilbour]

Hi all,

Happy to provide complete system details if needed, but it's a small off-grid recreational (lakeside cottage) application. My questions are related to the batteries.

We have (2) x 24V 1kWh NMC Chevy Volt batteries (total of 2kWh). This first season of use we have probably used an average of 600Wh/day. So all good so far and things are charged up by noon most days with our two 255W panels.

Question 1:

What settings are recommended for the CC for the NMC batteries? Based on advice and research I currently have it set to:
Bulk: 24v
Absorption: 24.3v - 3hrs
Float: 24v
EQ: off

Part B: With our relatively light, non-essential use, I'd like to keep it conservative to prolong battery life. Regarding voltages, what would be a safe, i.e. say 20% SOC, low-voltage? I can't seem to get a straight answer on this one. Looks like something around 3.4v per cell for 20.4v. (And it looks like 24v is around 80% SOC, ya?)

Question 2:

I searched but can't find on this forum any recommendations for a plug-in charger for something like the modules we have. (We have a 2800W Yamaha generator to plug in a charger if need be.) Any suggestions of what we should look for in a charger? maybe I'm over thinking it?

Thanks

I am using one battery pack and one 285w panel. So far so good. I'm using the same values except the float voltage 24.26v Can't remember why that is my float value.