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Old 02-16-2016, 08:30 PM   #29
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I think you have seen Al and his strong opinions. Mostly correct, he was trying to convince me that the BMS he sold me would balance all 12 cells and at the same time saying it wasn't necessary. After all it is only 3 cells in series not 50 as in the volt.
From what I can see the BMS will monitor three cells, the ones counting up from the negative post. That is the way the four pin are connected on the 17 pin connector. All the cells are connected to the 17 pin connector so it is very likely that every cell in the volt is at least monitored. They do control the battery temperature with heated water. But that really isn't what we should care about.

To give you my guess as to how the BMS works on this design: the BMS monitored the first three cells and therefore can ensure that the cell voltage is always controlled in both charging and also driving a load. The other 9 cells are just a "me to" environment, assumed to be good. As others have pointed out these Chevy Volt batteries appear to be very rugged. I will post the function of the BMS from the manufactures information. The Main Objective is to control charging and limit the amount of discharge. A lead acid battery charger will not work without this BMS board.
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Old 02-16-2016, 08:43 PM   #30
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I am now charging the battery. It is at 12.50V right now. I expect it to switch off the charge in the next .1V.

Here is a better wiring picture, the method for marking the battery ensures that the wiring is done correctly. Note the working range for this layout is going to be 12.6-11.4V.

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Old 02-16-2016, 08:47 PM   #31
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I'd be interested if the BMS he's providing will balance and not only monitor each series connected cell pack? Please let us know.

In reviewing the 48V in a bit more detail and correcting my previous statements, the 48V battery is comprised of 3 cells in parallel and then strung together 12 times to create 48 V at 45 AH. This would be described as 3P12S. I'll refer to a pouch as three cells in parallel. In your configuration, you're cutting the battery to form 4 pouches in parallel, and then adding them in series, so the correct configuration would be 12P3S.

I found another reference for the Chevy Volt cell that says that the minimum voltage per cell should be 3.5V and the max is 4.0V. So shouldn't your range be 10.5V to 12V?
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Old 02-16-2016, 09:26 PM   #32
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The range is set by the BMS, I am still verifying.
I think it was earlier state by another contributor that the bottom range doesn't have much power. The value is set at some power level. I do see that the upper range seems to be common knowledge but the bottom limit is somewhat objective.

I would agree that you could do this same process on 18 cells too. All of the tools, BMS and methods would apply.
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Old 02-16-2016, 09:44 PM   #33
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It will be interesting when you power your Airstream with this thing.
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Old 02-16-2016, 10:22 PM   #34
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Amen to that Bold Adventure,,,
I have been looking at you posting for the protection system. I need to add a second relay to take advantage of the under-voltage part. I have been spending all the play time on the charging side.

And as to charging. The BMS switched off the charging circuit at 12.6 volts, the battery settled back to 12.55 volts.

Now to discharge.....
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Old 02-17-2016, 01:16 PM   #35
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Here's a government report of the 2013 Chevy Volt batteries. The report says min/max cell voltage: 3.00/4.15V, so perhaps your BMS should stop somewhat south of 12.45V?
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Old 02-17-2016, 08:21 PM   #36
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Well the first test seems to show that I have a inexpensive PCB board, I have also contacted one of the Hong Kong suppliers of this board and find they don't support the Li-NiMnCo batteries.

I will need to do additional testing but at this point I would hold off on making any purchases until I can find and test out parts that will get the job done. As you know I purchase the boards from one seller, and I don't want to make a false statement, enough said.

The WWW points out that the setting of the BMS must meet the batteries safety needs and most of the current crop of imported BMS don't do this, they are rather a backup board, a silly idea, the reliability of the current electronics technologies don't need backup boards.
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Old 02-17-2016, 08:49 PM   #37
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Quote:
Originally Posted by alano View Post
Here's a government report of the 2013 Chevy Volt batteries. The report says min/max cell voltage: 3.00/4.15V, so perhaps your BMS should stop somewhat south of 12.45V?
I agree - 12.6V looks high for the BMS switch off point.

Having seen the actual cell voltage values I'm also thinking that this Li Ion chemistry may not be the best for a 12V system since its range of voltages are on the low side compared to a lead acid battery.

Min V (3V x 3) = 9 V - very low for a 12V system, typical lead acid fully discharged is 11.6V
Nominal (3.7V x 3) = 11.1 V, still below fully discharged lead acid
Max (4.15V x 3) = 12.45 V, about 75% charged lead acid

May have some issues with appliances, consistent furnace ignition, inverter performance etc at the low end of these voltages.

In comparison, typical LiFePO4

Min V = 11.3
Nominal V = 12.8
Max V = 14.4 to 14.8

High end here is obviously much higher than lead acid but consistent with the higher voltages seen when bulk charging a lead acid battery or charging at lower ambients with temp compensation. Nominal is right at 100% charged for a lead acid.
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Old 02-17-2016, 09:45 PM   #38
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The fact that there aren't many (or even possibly any) reasonably priced BMS systems out there that currently support this chemistry doesn't dissuade me from considering this highly original and inexpensive lithium solution.

But, as ohiobrits points out, the operating voltages are concerning. In actual practice, you could say that you should never deplete the cells more than 80%. Since the discharge curve decreased linearly, the Min voltage at 80% discharge should be around 9.7 V giving a practical range of 9.7 V to 12.45 V. I know there used to be an issue with the Zip Dee Relax awnings regarding motor burn-out when operating at low voltages - less than 11 V. This might have been addressed with a firmware update but I would need to check. Not sure about the other systems though.

It's also possible to connect 4 cell groups in series. This would yield a Min of 12 V and a Max of 16.6 V. I'm pretty sure my 1000 W inverter won't run with an input of 16 V. Not sure my solar charger can even be set this high. If the voltage is arbitrarily limited to say 14.5 V, then you're walking away from about 45% of the capacity. So this solution seems limited.

Perhaps it's worthwhile to focus on how well the existing systems operate with a voltage range of 9 V to 12.45 V? One would need to verify lighting, radio, inverter, water pump, heating (furnace and hot water), awning. And of course you're mileage may vary depending on the age and equipment choices that have been made over the years. Seems like an adjustable power supply could give an idea of the minimum operating voltage including margin for these common systems in a relatively quick fashion.
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Old 02-17-2016, 09:51 PM   #39
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This is all good stuff/discussion. I have had similar thoughts trying to figure out how many cells to put in series. My seller has been using three cells so that is the way I have started this development.

A couple of things, yes the battery will be lower.
Li-NMC (charge value is 4.2 Volts) is a quite a bit higher than Li-FePo4 (charge value is 3.6 Volts) 0.6VDC difference; one battery would be 3 cells (12.3 VDC, nominal, 4.1V), the other is 4 cells, 12.6V, 3.2V nominal). I know we are coming up with different values. Also note that Li batteries provide most of their power with little droop in output voltage; this is one of the problems for a battery management system (determining what is the state of charge SOC).

My plan was to install a BMS that allowed me to set the working battery range and protection values. There are BMS solutions that will allow this. Lastly a 2kW battery or a 3kW or larger is going to be lightly loaded and therefore the trailer's distribution system will have minimum voltage drop. Many users are still running a group 24/27 lead acid battery, this move should be a big step to increasing stored electrical power.

The minimum of 3V is too low to protect the Li-NMC battery.

I will run a power curve and share it, should have it by the end of Saturday.
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Old 02-17-2016, 10:18 PM   #40
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Best wishes for a successful project! One other simple system that draws extra amps at low voltage and tends to blow fuses: electric tongue jack.

We only know this because when we first bought our trailer, the batteries were not in good shape. At the dealer while we were trying to hitch up for the first time, they ran through three fuses before they realized what was causing it.
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Old 02-18-2016, 08:59 PM   #41
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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.
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Old 02-18-2016, 09:33 PM   #42
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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?
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