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I just purchased a 100 Ah deep cycle group 24 battery at Walmart for $58 plus a $9 core charge, and a $7 plastic enclosure that has extra room. For now I will just use it with a 12V 1000W peak inverter modified to provide 270 VDC for my 230V three phase VFD and motor. However, it does not have regeneration capability, and I came up with an idea to provide that at low cost.

My idea is to obtain a battery charger based on a switching supply that can operate on 200-280 VDC from the DC bus, and feed that to the 12V battery. I know it sounds like a free energy bootstrap scheme, but the idea is that when the DC bus rises above the nominal 270 VDC due to regen, it would activate the charger and transfer the energy into the battery.

This may be practical and cost-effective for a small application like a tractor, but it might be scalable to larger vehicles, and perhaps even adapted to lithium batteries, by adding multiple modules of 270 VDC in parallel to obtain the power and energy needed. One advantage this would have is the inherent safety, where the energy storage is limited to 12V (or perhaps 24, 36, or at most 48), so lower voltage, less expensive fuses and contactors could be used for protection and control. The high voltage is energy limited by the inverters and thus would not need the high interrupting current ratings usually needed. Short term peak current can be provided by 300 VDC capacitors.

Regarding cost, a 1000 watt module would use a $67 battery, $6 enclosure, $50 inverter, and perhaps a $30 360W 30A switching supply/charger. Add maybe $50 in capacitors, fuses, contactors, and assorted other components, and you have about $200 total for 1000W with 500 to 1200 W-h depending on power level. So that's about $300/kWh which compares favorably to just the batteries for LiFePO4.

Efficiency of the inverter can easily be 95%, and the charger would be about the same. Also, the module could be connected to 120/240 VAC power for charging, so you save by not needing that as an extra component. It is simple enough to parallel the inputs to charge multiple modules at up to 0.3C for a reasonable 4 hour charge.

All of these components are readily available from multiple sources and can be easily modified and/or combined with other components for repair or replacement. Failure of any module would only remove 1 kW from the system, so it has inherent redundancy. A complete 10 kWh system would require 20 modules at a cost of $4000 and replacement batteries would be about 1/3 that cost. Weight is about 1000 pounds.

This system could be designed to use 4 100 Ah lithium cells in place of the lead-acid battery, and could be mixed with lead or other chemistries because the output could always be a nominal 270 VDC. Because the inverter and the charger are (or easily could be) totally isolated, the outputs could also be put in series for 540 VDC for 460 VAC motors. And lower voltage inverters (standard 120 VAC automotive types) provide 135 VDC nominal which will work for most DC motor controllers.
 
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