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Old 02-19-2016, 12:37 AM   #43
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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!
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Old 02-19-2016, 07:17 AM   #44
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Quote:
Originally Posted by sgschwend View Post
My current thinking...........
Well, you are certainly doing a lot of that.
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Old 02-19-2016, 08:51 PM   #45
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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.
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Old 02-20-2016, 08:32 PM   #46
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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.

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Old 02-21-2016, 01:49 PM   #47
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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):

Click image for larger version

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Old 02-21-2016, 09:47 PM   #48
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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.
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Old 02-22-2016, 12:12 PM   #49
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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.
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Old 02-22-2016, 08:04 PM   #50
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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.

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Old 02-23-2016, 11:39 AM   #51
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That's really weird. I have never seen my batteries drop below 13.2v yet even after taking them down 70%.
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Old 02-24-2016, 06:57 PM   #52
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Old 02-24-2016, 07:15 PM   #53
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Hey Mike,
Is that down to 70% or down 70% with 30% remaining?
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Old 02-24-2016, 09:57 PM   #54
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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.
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Old 02-25-2016, 09:24 PM   #55
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Great Thread!

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?

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Old 02-26-2016, 07:32 PM   #56
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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.
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