Preferred settings for 24 volt Chevy battery bank

rachelsdad · May 2020

Good morning all.

I was referred here by Nobadays on another forum. He very enthusiastically recommended it. I see why, great forums with a nice "tone" as it were.

My existing system consists of 9 x 260 watt Solarworld Sunmodules, a MidNite Solar MS4024 which used to power 8 - 6 volt x 330 amp hour batteries. A weekend cabin located up in the NW Adirondacks, off grid, on water, and had been using the cabin with propane lights, fridge and gravity fed water system. I wanted to go wind or solar for a few years but finding qualified folks up there who are not booked years ahead is hard. I chanced to meet the owner of a solar install company from Rochester, NY while canoeing one morning and he paddled over with me, took a look at what was needed, we discussed my way overblown estimate for usage and he literally installed 7 weeks later, (he had a big job get delayed and I bribed him with having him stay on site to work, eat, paddle, eat...etc) I now have over 30 lights in the cabin, 6 ceiling fans w/DC motors and really enjoying it. Mostly weekends, never had an issue w/capacity.

So after reading up on options I drank the Kool-Aid and purchased 4kwh worth of 24 volt used Chevy Volts, (4) to replace the 1,000 lb battery bank I had to remove from the island every year. I'm heading up this weekend for the first time this season to open up and frankly I am so excited to not have to haul 8 of those monster batteries off the truck, down to the dock, on to a 12' aluminum boat, (4 at a time), over to the other dock, off the boat, and an 83 step, (guess how I know that number) trip...one way... hauling each of those !!! Can't wait to just turn the CC off in the fall and leave all in place! I do have a BMS btw.

MidNite Solar was kind enough to get me the keystrokes needed to adjust my charge settings as the Owners manual was on a flash drive but missing a key chapter...yup...setting charge rates! I, nor MidNite Solar, cannot tell what model Classic the integrated Charge controller is, but he gave me the keystroks as well so that the unit can ID itself to me.

So I'm looking for the generally accepted settings for absorb, float and float time for a 4kw 24 volt bank of Chevy Volt batteries.

As I stated earlier, I had this professionally installed so my knowledge is marginal at best technically so based on the below response from MidNite Solar where should I be? Not looking to push upper limits, prefer to protect against deep discharge...

Also: Why would I not desire a float per below? Does that Absorb time seem reasonable/accurate?

Set the Absorb charge voltage (Charge/Volts).
Set the Float charge voltage (Charge/Volts):

If no Float is desired, then set a low Float value, such as 20V, and the Classic will rest after the Absorb charge cycle ends.

Set the Absorb charge time, usually very quick, like 4-6 mins (Charge/ChgTime).
Turn the temperature compensation off by setting the temp coefficient factor from a common -5mV to “Disabled” (Charge/T-Comp).

Other Matters:

Cables: I had very thick gauge cables for the lead batteries which won't fit the terminals of the Volts. I can't find the specs on line anywhere for the nut size and I'd hate to walk into a hardware store with a mask on, (NYS COVID-19 law, all must wear masks in public), carrying an unusual large suspicious object .

Recommendation for gauge size to connect the 4?

Positive/Neg line: All from end battery or one on first and one on last in line? My current neg is much longer so if there is an advantage to doing it that way I'll stick with it.

Anything I'm missing?

Apologize for the incredibly long winded first post!

If any of you are ever up in the area....stop on by!

Ralph Day · May 2020

Those look like L16's, about 130 pounds each? Brutes. But leaving them fully charged with the controller on but no inverter or loads would be a safe bet...a fully charged FLA battery will not freeze until -75F or so. Lithium batteries have limitations regarding charging and discharging below freezing. I'm not sure how the car application gets around that...heaters probably?

I guess if you leave the Chevy batteries fully charged in the fall without any charge or discharge happening they should not self-discharge to any dangerous level.

Sounds like a nice place to chill, except for humping groceries up the 83 steps.

rachelsdad · May 2020

Well thankfully the FLA's are moot at this point. I've recycled them and plan on setting up the Volts this weekend. Heck, I can throw one of each in my pockets compared to the FLA's

Thanks for the compliment, I'll post some pix in a few. But frankly, I don't mind hauling groceries as much as I did the batteries. it is fun...much more livable too, not as rustic but at least I'm still off grid for a long weekend here and there!

mcgivor · May 2020

The Volt cells fall under the umbrella of LMC chemistry, what's needed is the charge/discharge curve to calculate the charging profile and equally important the discharge cutoff point. There are what is known as knees, where the voltage either spikes or collapses, it is important to avoid these values keeping within the safe zone. The importance of knowing the exact chemistry would determine the setpoints best suited to the requirements of said chemistry, but generally they would fall into the 4.100 to 4.300 volts per cell maximum voltage range, which is very similar to LIPO.

Some things that need to be observed, there should be no temperature compensation or equalization, a float voltage would be approximately 80-% of the fully charged voltage, but not knowing the exact chemistry it's impossible to state a value. This link may provide some information that could be useful https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
One additional point is, when dealing with lithium batteries, one needs to think on a cellular level not overall voltage, this is very important because when approaching the knees cell voltage, or cell block voltage can drift significantly depending on where the cells or blocks were initially ballanced.

Try establishing the exact chemistry along with the charging aand dischargeing curves, this would allow some reasonable predictions on where the settings should be.

solar_dave · May 2020

How are you managing the Volt batteries? You do know that Chevy has liquid cooling for those batteries, especially when charging.

mike95490 · May 2020

You really need to QUICKLY educate yourself about your Li batteries.
You NEVER want to charge them to 100% and never discharge below 10%

They cannot be recharged below 32F and need cautious charging below 40F

Li batteries burn quite well with lots of toxic (very toxic hydroflouric acid) fumes that you learn about while slowly dying in the hospital

You do not want to get this wrong.

Li batteries require no Float or Absorb cycle. Most adjustable charge controllers can emulate this with a 1 or 2 minute Absorb, and setting Float voltage much lower than normal float would be.

rachelsdad · May 2020

solar_dave said:

How are you managing the Volt batteries? You do know that Chevy has liquid cooling for those batteries, especially when charging.

I have a Bluebird BMS from Bobolink Solar: https://bobolinksolar.com/product/bluebird-bms/

mike95490 said:

You really need to QUICKLY educate yourself about your Li batteries.

Thanks. That is why I'm here Mike.

You NEVER want to charge them to 100% and never discharge below 10%

Hence the questions.

They cannot be recharged below 32F and need cautious charging below 40F

Understood, thank you.

Li batteries burn quite well with lots of toxic (very toxic hydroflouric acid) fumes that you learn about while slowly dying in the hospital

You do not want to get this wrong.

Please see my response to your first statement

Li batteries require no Float or Absorb cycle. Most adjustable charge controllers can emulate this with a 1 or 2 minute Absorb, and setting Float voltage much lower than normal float would be.

McGivor: Thank you for the helpful link

mcgivor · May 2020

There are numerous types of lithium cells, each having differing characteristics, some are more stable than others with a tradeoff for energy density. This link https://batteryuniversity.com/learn/article/types_of_lithium_ion provides detailed information which will be helpful in understanding a particular chemistry.
@solar_dave mentioned cooling, my thought is cooling is required in automotive applications due to extremely high charge and discharge rates, not to mention they are not exposed to free air circulation. In an offgrid application it's generally no an issue as the rates are lower thus not generating heat.

@rachelsdad The BMS linked looks to be a Chargery product which are predominantly designed for automotive applications, usually powering external relays or contactors, not ideal but can be used in off grid, I actually use one for it's graphic display and secondary control but not primary control, primary is done by a BMS ordered to specifications provided to the manufacturer. The reasoning to do it this way was to allow use of the solar without solar disconnect, which the Chargery would do if used as designed to cut charging once the battery is fully charged, this. This can however be overcome with careful programming of the controller by limiting the voltage to stay within the plateau, the downside is the relays or contactors need to be continuously powered to provide a fail safe condition, resulting in wasted energy.

Proceed slowly, gathering information will lead to success, the first step is to establish a maximum/minimum voltage setting, usually around 90% and 20% of maximum/minimum allowable, personally I'm reluctant to offer a value because I'm not using the cells you have, but the rules are generally the same, keep between the knees, the safe zone.

rachelsdad · May 2020

All good info and I appreciate the reluctance to not want to specify values.

I did get info from the company I purchased from as well as an installer who uses them frequently.

But yes, I am proceeding slowly.

Thank you

mike95490 · May 2020

Regarding BMS interaction with Charge Controller.

1) the BMS must not disconnect the battery from the CC. It must disconnect the PV from the Controller, or use a logic signal to shut down the controller. If the battery is disconnected in daylight, from the CC, the high voltage PV will likely damage/destroy your controller.

2) capability of BMS. If your solar CC is producing 30A, and your BMS has 2A bypass circuits, you will quickly have a 28A overcharge on one cell. Be sure this is addressed.

Preferred settings for 24 volt Chevy battery bank

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