My Li-ion, 2kW, Battery Project

[sgschwend]

Thanks everybody for the discussions and ideas.
I was successful in finding a US source for a Battery Management System. I was also contacted by the US product manager for Bestechnolgy a Chinese manufacture. The US source does sell parts made in China, but it is nice to talk to someone who is in the business.

Therefore I will be purchasing a new BMS from the Electric Car Parts Company. I will be able, at a minimum to purchase a device with my settings, and perhaps be able to obtain a model that can be connected to a computer and adjusted by me.

My current thinking is a 50A charge and a 50 discharge device a price around $50. The new BMS board will be able to handle more power than the first one I purchase, and match my trailer DC circuit breaker. Now the question becomes using three series cells or four series cells. The energy in the battery is unchanged it will still deliver the programmed power (2kW).

So now here is the idea:
1) set the battery working window to provide improved longevity, this means reducing the maximum voltage down about .1VDC and raising the minimum. That would be a range of 3.5-4.1VDC. This starts to make the four series cells idea more appealing (14-16.4 VDC). This would look more like a motor vehicle voltage range. You should also remember that your trailer wiring will have a voltage drop when you are running those heavy loads. I picked the 3.5V value from researching this specific battery, there are publications that show for the Chevy Volt battery the energy is depleted at 3.5V.

2) The converse would be to use 3 series cells, battery range would be:
10.5-12.3 VDC. I do agree that 10 volts seems low and so I am likely going to switch to four cells in series, unless someone can convince me to stick with 3 series cells.

3) It has been recommend that the re-strapping include all 12 cells. I will look at this. It means a few more jumper. This recommendation would then provide balancing for all twelve cells. The schematic would be each position would be connect to its relation. Or you could visualize the 4 volt cells connected together, the 8 volt cells connected together and so on.

I will bring a data logger home tomorrow and run a power curve versus time graph to prove out these ideas.

Need to see about chargers too.

[alano]

I'd be cautious about stacking four cells to create 12 to 16.4 V. Like I mentioned before, my stock MFCO 1000 W won't run on voltages north of 14.5 V. I've experienced this while trying to use the inverter while my solar charger was driving the battery at 14.5 V. The spec says the useful operating range is 10 to 15 V and includes input under and overvoltage protection. I randomly checked another 1000 W inverter and it too had a input voltage range of 10 to 15 V.

Because these cells are matched at the factory and there are many in parallel, I've come around to believe that each cell group does not need balancing, only monitoring. So why bother with a BMS that's going to balance each cell group? My thought is that you can get away with a BMS that only monitors each cell group and then asserts an undervoltage or overvoltage relay in case any of the cell groups exceed their limits?

[alano]

Another idea would be to stack 4 cell groups in series and then add 2-3 high-current diodes to drop the voltage into a useful range. For example, let's say you put 4 cell groups in series so your voltage range would be Vmin = 4*3 = 12 V, Vmax - 4*4.1 = 16.4 V. But now you add three high current (200 A) diodes that have a voltage drop of approximately 0.7 V each. So your range is now 9.9 V to 14.3 V. These stud mounted diodes cost in the neighborhood of $20-25 each and need to be mounted on a heat-sink. Another caveat is that your converter will no longer charge the battery because the diodes will be reverse biased, so you'll have to rely on a solar charger connected directly to the battery pack. It's starting to feel like I'm pushing a square peg into a round hole!

[Wayne&Sam]

Originally Posted by sgschwend

My current thinking...........

Well, you are certainly doing a lot of that.

[sgschwend]

Alano, things are going well, there will be an easy solution for your application.

I can see two easy solutions:
1) purchase a BMS set for 3.8 VDC. After the battery is charged it settles down to a lower voltage, likely due to cooling down. That would put you at 15.0V, at the battery.
2) Use a regular battery charger or the AS powersupply. That would set the battery to 14.5VDC. I will try this method for fun this weekend and measure the power curve and compare it to the 16 battery.

[sgschwend]

Here is the result of the battery load testing. I setup the load at 12amps. The graph shows the battery voltage over an 8 hours period with the 12 amp continuous load. The data is also provided.

Conclusions: this is a brand new battery, it is developing 2.2 KWh, which is pretty much the value that a Federal Testing laboratory found by in their tests. The 30 amp BMS is an OK part but it really should be setup differently, I also like to have a BMS that matches my AS DC circuit breaker board which is setup for 50 amps. I have stopped the test at 1.2 KWh the voltage is at 11.4 volts which is the low end for a lead acid battery. Since the math has been working out I can pretty much determine that the battery will deliver 2KWh and the final voltage will be down to the 10.5-10.7 Volt range. For my trailer this setup should work just fine. I will run the 3.8VDC cell test next for those who want a higher voltage system.

[ghaynes755]

That doesn't look like any discharge curve I have ever seen from a lithium battery. Lithiums typically drop a bit, stay steady for a long period and then have a decided drop off. Your plot looks more like an AGM or regular car battery. Maybe the Volt batteries work differently?

Example plot from Elite Solutions 100ah cell pack (note that the voltages shown are for an individual cell):

[sgschwend]

Thanks for the posting, I would guess your data demonstrate the current delivery (or charging) capability of that cell. Seems to do a great job up to 2C.

Note your graph is voltage versus Ah, while mine is voltage versus elapsed time with nearly a constant load. I did terminate the test at 50% discharge, the battery delivered 1.2kWh.

I would like to see the test setup for a 300 amp load bank, 1000 watts of heat from a 3.1VDC battery is very impressive.

[ghaynes755]

For clarity I was looking at your voltage drops. Seems like your setup has a linear voltage drop. Different chemistry in the GBC cells that Elite sells but the voltage drop with use isn't linear. That's the difference I saw and maybe the VOLT batteries function differently.

LiPo's I use in flying large drones demonstrate the same type of voltage curve. Very steady until about the last 20% of capacity and then voltage starts dropping rapidly.

[sgschwend]

Here is the full Chevy Volt battery test, I believe it was conducted by Idaho National Laboratories. I don't plan on doing a lot of testing just checking the parts I purchased to make sure they are working properly. I used this curve and divided by the number of cells (rumor to be 98 cells in series) to select the values for the protection circuit I will be purchasing.

[BoldAdventure]

That's really weird. I have never seen my batteries drop below 13.2v yet even after taking them down 70%.

[Ramble Can]

Following


Sent from my iPhone using Airstream Forums

[BigAl]

Hey Mike,
Is that down to 70% or down 70% with 30% remaining?
Al

[sgschwend]

You likely have four series cells. I put this one together with three because each of these cell are over 4 volts. The four cell version would be above 16V. This difference in cell voltage has developed questions that I and others have been working on, such as charging, over voltage for the 4 series cell solution. These Li-NMC batteries drop about 2 volts per charge. My graph shows the battery dropped .9 volts after delivering 60% of of it rated charge.

My battery is now installed and running my 25' trailer. I do have LED lights, bathroom fan, exhaust fan, water pump, furnace, radio, hot water circulation pump, and a tongue jack. The tongue jack is the largest load at 25A, then the furnace at 5 amps, the rest too small to list.

I will be installing a power manager to watch the battery performance versus loads, SOC, set alarms, and summarize energy used per battery charge.

Earlier a contributor asked about freezing temperatures. Li batteries can not be charged at those temperatures; I did not find any issue with using the battery just charging. I also live in the North, many years in the Idaho panhandle. Perhaps not as cold as some places but I found that it was more cost effective to keep some heat in the trailer, that was a lot easier than finding and fixing damage, the damage does not always show up right away.

[Star-Man]

Thanks for the very informative thread.

Could you possibly summarize a list of your materials and where you purchased them? Also, any modifications that were not shown?

Star-man

[sgschwend]

S.man you hit the nail on the head. I hope to assemble a complete package so somebody can build one for their use. I just received a system size circuit breaker, and a power meter. I hope to install these this weekend, then I should have a complete system.

A summary list, with images and drawing along with the cost.

[sgschwend]

Here is the block diagram of what I am implementing for my trailer. I hope to have options listed with the material list.

More details to follow:

[BoldAdventure]

Originally Posted by BigAl

Hey Mike,
Is that down to 70% or down 70% with 30% remaining?
Al

Down 70% with 30% remaining. I believe the cells have to drop below 3.0v's before they start dipping into the 12's. I have yet to attempt to really deplete the system.

Originally Posted by sgschwend

You likely have four series cells. I put this one together with three ....

Yup, 4 sets of 3.2V cells wired in series, then in parallel. So a total of 16 cells. Basically I have 4 12V battery's if you want to look at it like that for ease of understanding. I just find the differences interesting. I am planning on buying some of the 18650 cells that are used in Tesla batteries to build a backup for my laptop.

[Newmarketca]

Subscribed - very interesting project!

[sgschwend]

Material List:
Item/Price/Source
Battery, Chevy Volt 12 cell, $550 Ebay plus shipping
Battery cover, $30
BMS 100A, $60, Electric Car Parts Company
Charger 10A, 12.6VDC max, $139, plus shipping
Bussmann 100A Circuit Breaker, $28, Amazon
Bayite 100A power meter w shunt, $20


Notes:

1. A blank source line means same as above
2. The Bayite display is praised and hated. Easy to install, and seller replaces for free. It would have been nice to find a better device that wasn't ten times the cost.
3. I recommend you only purchase the battery and cover for the EBAY seller. However, he will provide a complete battery with BMS for a price. His solution is below my standards in materials and circuitry wiring. My thinking is for the money the buyer should obtain all the capability this system has to offer and not end up with low cost materials and a partially wire system.
4. Remainder of the material is miscellaneous material, such as wire, connectors. I am trying to improve on the battery lug wiring, looking for a crimp connector or a drill fixture.
5. Total for the BMS and charger with shipping $224