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Old 07-13-2012, 02:33 AM   #13 (permalink)
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Ideas are cheap ...

Originally Posted by Daox View Post
Mainly, I have to move the ground reference every time I jump to the next cell. Currently, I'm not even grounded to the high voltage pack, just the 12V system. So, if anyone has any ideas I'm all ears.
My experience in hardware design is from many many years ago, so take this information with a grain of salt and check it out before doing any circuit work ....

The BMS topic does interest me, so I did some digging and this is what I found ...

Most of the circuits that are out there for analog isolation use a voltage to frequency converter, then go through an optoisolator, and back through a frequency to voltage converter. Some use a transformer for isolation, but those circuits are pretty old.

The isolated side, the voltage to frequency converter, needs it's own isolated power supply. That also powers one half (the input side) of the optoisolator.

The other side of the optoisolator is powered from the microprocessor side as well as the frequency to voltage converter (if you use one). The frequency to voltage converter makes the frequency signal back into an analog signal 0 - 5VDC.

An elegant solution would be to power the (small and pretty trivial) V to F from the battery that it is measuring, then have the frequency put through the optoisolator. The resulting frequency, or train of pulses, comes out of the output of the optoisolator and is powered from the microprocessor. From there, the frequency can be read directly into the micro, or put through a mux so that fewer micrprocessor pins are used up as frequency counters, or converted back to analog by a frequency to voltage converter, and put through an analog mux ... you get the idea.

Each battery would have it's own small circuit with just a V to F and an optoisolator ... about 10 ma, depending on which chips you choose and how good you are at soldering surface-mount.

However ... I can locate no Voltage to Frequency converter that will accept the voltage drop that each cell can experience during acceleration ... I'm assuming a low of about 2.0V for acceleration when the batteries are low and someone stomps on the accelerator to a high of about 4.2V for charging. I can't locate such a thing for sale ... so the next idea makes things a bit more complicated ...

Use another chip to boost the power supply from your battery voltage of 2V - 4.2V to a solid 3.3V, which can be used to power the V to F and the optoisolator.

- TPS60240 boost power supply. This will take the 2 - 4.2V (in fact, it will go as low as 1.8V and as high as 5.5V) from your cell and put out a regulated 3.3V for the rest of the circuit. 50 microamps standby, so it won't unbalance your pack.
- AD7740 voltage to frequency converter has a supply voltage of 3 - 5.25V, so 3.3V is fine. 1.5mA supply current is pretty low. under a dollar for an 8-lead msop (I hate soldering surface-mount stuff). The input voltage from the battery will have to be divided since the max input voltage is 2.5V - maybe 2 10K resistors in series would keep the current down under 1 mA. You also need a crystal for a clock and to set the max output frequency of the output, which is up to 1 Mhz.
- and you need an optoisolator, say a NTE3086 - no pricing on that one.

As with any circuit, component selection is very important. The minimal research that I did gave me only one boost power supply that would deal with a voltage dropping to 2.0V - that's why it's surface mount. It has one fixed output voltage of 3.3V. If the output voltage could be 5V there are dozens of Voltage to Frequency converters available. The AD7740 is one of 2 that I found that would work down to 3V.

The crystal at under 1 Mhz should not be a big problem. The optoisolator is only rated for about 100 Khz. Keeping the frequency as high as possible will give you a low time to sample (the micro can tell what the input voltage is within a few pulses) each channel. You'll need to put in a current limiting resistor to avoid saturating the optocoupler input.

Last edited by thingstodo; 07-13-2012 at 02:34 AM.. Reason: spelling mistakes
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