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Old 03-18-2012, 01:57 PM   #1 (permalink)
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Turtle "Buddy Battery" BMS Idea

Here is an idea (possibly a stupid one) for a BMS that uses a DC-DC converter and an AUX battery to support the weakest battery at any given time. I call it the "Buddy Battery" method.

Like a normal 12 volt aux system, a DC-DC converter thieves energy away from the entire pack, at say 144 volts, and converts it into 12 volts with minimal losses. That part is no different from any other car out there, but I noticed a resemblance to the "Switched Transformer, Energy Converter" BMS system mentioned in an article on EVDL's website

The buddy battery system intelligently picks out the battery with the lowest charge and, through a series of high amp mosfets, places the aux battery in parallel with that one battery. This helps the low battery charge, or at least lets it not work as hard, so that it becomes even with the remainder of the pack. As soon as that battery is no longer the weakest in the bunch, the switches rearrange and place the buddy battery in parallel with the new lowest battery.

The system needs only to operate during charging and driving, and can be switched off easily when the car is idle. Throw in a fuse to stop any dangerous current, and possibly use a resistor to slow down the current draining from the buddy battery.

I could possibly foresee an issue during charging. If the individual battery voltages of the traction pack are higher than the auxiliary system, then current will want to fly the opposite direction. This could in theory be remedied by a beefy diode.

It also seems like you could modify this system to balance LiFePo4 cells as well, if you used an independent DC-DC converter and aux cell that put out 3.2+ volts instead of 12.

What do you guys think? Should I try to build it? See any gaping holes? Please let me know what you think!


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Old 03-18-2012, 01:59 PM   #2 (permalink)
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A quick sketch of the idea is attached to the first post
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Old 03-18-2012, 02:48 PM   #3 (permalink)
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The extra battery seems redundant - the converter could just help the weakest cell directly.

2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
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Old 03-19-2012, 12:57 AM   #4 (permalink)
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That's true jacobnev. It doesn't need the aux battery. It may be more efficient without it. I just thought it could help with the peak loads that may be seen by the system. I don't know what to expect at this point.

Of course, if the DC-DC can't keep up with the system demand, then the system wont work anyways. Maybe it is best to nix the aux battery in this application.

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bms, buddy battery

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