Battery Conditioning

The 120 Lithium-ion iron phosphate batteries that I am going to use need to be conditioned first - a process known as bottom balancing.  During this process the batteries are all discharged to exactly the same open circuit voltage.  Having the batteries all at the same discharge voltage is a preventative measure to protect the batteries in case of un-intentioned complete discharge when the batteries are in the car.  If all the batteries are not at the same bottom level during a complete discharge then the lowest battery will be drained first, probably irreparably damaging the battery.  When all the batteries discharge at the same rate there is a very good chance to recover all the batteries.  Part of the process for new batteries to condition them for bottom balancing is to fullly charge them first.  Once they are fully charged then all that energy in the battery has to be removed.  Usually that is done by running the car or some accessory in the car like the heater.  But my batteries are not in the car yet.  I could use some large wattage resistors.  I have a set I plan to use to do the final voltage tweaking.  But using them to dump all that energy seems wasteful to me.  So I came up with the idea to use a grid tie power inverter.  These modules are used to convert PV solar DC energy into 120VAC that is coupled into a house power system.  The inverters come in all shapes and sizes of wattage.  The kind of inverter that I would use if I ever put solar panels up costs about $2 a watt.  I found on Ebay there are these Chinese built inverters that cost $0.20 a watt.  Of course these inverters are not UL certified but for my purposes that does not really matter.  I don't plan to use these like they would be used on a PV solar system.  The inverter I got is 2500 watts and can accept up to 88V DC.  I had to change the strapping on the battery boxes to reduce it to 80 volts.  The first set of batteries I discharged into the inverter worked great.  The inverter drew about 29Amp from the battery pack and produce near 2.5KW of AC voltage.  I have a whole house power meter and I could see the wattage reduction of my power usage when the batteries were being discharged.  I connected a second set of batteries, fully charged them and then started the discharge process with the inverter.  Nearly the exact same DC starting voltage.  The inverter started up fine and I could see the power reduction on the whole house meter.  The central air conditioning was running at the time so the house was pulling 8KW.  The discharge process was working fine for about 2 minutes.  I was video recording the process for my video record and suddenly there was a large electrical discharge sound.  Now that is kind of scary because I had over 4KW of battery capacity connected to the inverter and if all the energy was released quickly that would be dangerous (the battery pack was protected by a fuse).  The sound came from the inverter and it was obvious that was the problem because it had stopped converting power.  I disconnected everything and took the inverter apart.  This inverter is basically two 1200W inverters in one package, one board on top and one on the bottom.  I found on the top board one of the power DC mosfets was totally burned.  I checked the other mosfets and found about half of them were shorted.  The DC fuses on both circuit boards were blown, but the AC fuses were still okay.  From looking at the inverter circuitry I think this is a very marginal design, for instance the DC fuses were not rated for the voltage or current being used.  People are probably using these cheap inverters to convert energy from their solar cells and putting their systems in danger of catching on fire if these inverters have a propensity to fail.  After I experienced this failure I found on YouTube many videos for repairing these inverters.  Along with the inverters on EBay there are also mosfet repair kits, so I think this is a common problem.  I probably will repair this inverter because I still have a lot of batteries to condition.  I don't know why the inverter failed, it worked great the first time.  The main difference was that the central air conditioning was running when the inverter failed.  These inverters work by phase matching the 60Hz 120VAC from the house.  It is possible the reactive load of the air conditioner skewed the AC voltage so the inverter freaked out and drew too much DC current.  I did notice a spike in the DC current on the battery monitor I was running right before the inverter failed, but who knows, could be anything with a marginal design.
A video of the discharge process can be found here.  The video also shows the battery cable layout in the car.

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