Electronic – POE Battery Survey Toolkit

For measuring remaining capacity of a LiPo battery pack, search for Battery Fuel Gauge ICs.

For instance, the Texas Instruments bq34z100 Wide Range Fuel Gauge supports LiFePo4 batteries from single cell through to 18 cells (3V to 65 V range), monitors battery health, charge and discharge, battery aging and self-discharge. It interfaces using I2C 2-wire, and also has direct indication support via an LED direct output pin.

Fuel Gauge or Gas Gauge ICs like this can both estimate remaining capacity, and learn the battery parameters over time, thus improving precision of estimate with use.

To take battery management further, LiPo battery pack manager ICs such as bq3055 not only provide fuel gauging, they also incorporate over and under-voltage protection, overcurrent and over-temperature protection, all in one package.

For measuring current in the 50 Ampere range mentioned in the question, an integrated Hall Effect Current Sensor IC such as Allegro's ACS756 Hall Effect based Linear 50-100A Current Sensor can be used. While sub-50 Ampere current sensors are also available, at lower prices, it would be better to use a sensor rated for higher than the maximum current envisaged.

The down-sides of measuring current flow and thereby attempting to estimate remaining capacity are:

1. Current has to be monitored constantly, as any missed readings due to interrupts on the microcontroller, for instance, would lead to imprecision in readings.
2. Reactive loads like motors do not have a linear current draw: Besides ripple, they can also back-feed current. With the proper battery protection in place, this back-feed will not harm the batteries, but may be partially used to charge the batteries. This leads to further imprecision.
3. A low current draw when the motors are off but the rest of the circuit is operating may not register at all in a high current sensor, and self-discharge certainly will not register. More imprecision.

The current sensor mentioned has an analog voltage output, 20 mV per Ampere, and requires an ADC pin at the microcontroller to constantly poll the readings. Other current sensors exist with direct serial (I2C or SPI) output, saving the ADC pin, and for buffered output devices, the requirement for constant sampling by the microcontroller.

So basically, you're just going to do power over UTP, not power over ethernet. PoE is a specific set of standards designed to safely supply a certain maximum amount of power from a PSE to a PD, whereas you are going to omit the protocol entirely and just running power. Correct?

In that case you are only limited by three things:

• Voltage rating of the jacks and cable (CAT6 cable is usually already extremely good dielectric stuff, often PTFE, so you can run hundreds of volts over that)
• Current rating of the cable and isolation transformers
• Noise of your PSE

You won't need to worry about ratings of the signalling electronics because they are fairly aggressively clamped to counteract insertion transients and of course they are AC coupled through a transformer. The only thing to worry about on the receiving end is that you have a dc/dc converter that converts the power down to a voltage suitable for your application.

You are injecting common mode current into the wire, and this has to go through the common terminal of the isolation transformers, through the transformers themselves and into the UTP cable. This means that your average magjack won't work - you need pretty beefy isolation transformers. This is easy to find in 100Mbit form, bit harder if you want gigabit communications (try shopping at Würth for those, but avoid them for as much as possible as they have the worst 'active marketing' department of any electronics company). I've designed a very big PSE in the past using these transformers.

Another, probably cheaper, way of doing things is just to use standard 100Mbps comms over two wire pairs and throw the power over the unused pairs. This saves the ~7 dollar expense on harder-to-get magnetics. You won't be able to reduce the wire diameter by removing those two pairs anyway, because you're bound to UTP anyway. the outside shell will stay the same no matter what you remove from the inside. You still can't use magjacks for obvious reasons.

Last thing to make sure is that you are not throwing any excessive noise around in the frequency domain of Ethernet. Ethernet has significant signal bands between 2 and 125MHz, so you need to make sure that you very much attenuate anything above say a MHz. You should be able to verify this with even the simplest oscilloscopes.