## 4 Practical Implementation  
In this section, the practical implementation of the HVED circuit will be discussed. In our first design, the LEDs were chosen in such a way that the threshold voltage for equalization was very close to 3.65 V. This was done by selecting gallium-arsenide-phosphate LEDs. It seems to be an ideal situation since the maximum charging voltage of the lithium-iron-phosphate (LiFePO\({}_{4}\)) cells, as declared by the manufacturer, is 3.65 V (+/ -0.05). However, in LiFePO\({}_{4}\) technology batteries, this limit is less strict than in li-ion batteries since the electrolyte does not decompose quickly and there is much less risk of thermal runaway. This makes a trade-off between a slight overcharge and a resulting decrease in cycle-life possible. There are some reasons to select a higher threshold voltage than 3.65 V. First of all, assume the batteries are to be charged to their full charge. This means all cells should reach 3.65 V. Equalization at this threshold is rather hard to obtain. Anyhow, there is a variety of LED types that are able to get the right effect. Assume that normal charging stops at one cell high. A trickle charging can give a reduced current that HVED circuits can manage. One way is to switch to a trickle charge mode, where a limited current of, for example, 1%, of nominal Ah is given. In this way, the voltage-limiting can fully act without the risk of overcharging. At the end of the trickle charging all HVED units are warm, coming in under the higher-temperature threshold. The LEDs (\(\approx\)1.5 mV/K for Al-GaInP) and BJT base-emitter junction (\(\approx\)2 mV/K) both have a negative temperature coefficient causing the threshold voltage to decrease when the temperature rises. The HVED will equalize earlier than required at higher temperatures. It can be seen as a positive effect since a higher temperature puts stress on the cell. As explained before, it makes it harder to reach full charge, and limits the obtainable range of the vehicle. One could argue that the open-circuit voltage OCV vs. SOC characteristic is also temperature-dependent, but not in the same temperature range. LiFePO\({}_{4}\) batteries have a higher tolerance to overcharge. There is a capacity decrease due to active material loss inside the cell. The type of LED is a choice requiring some thought if some designerswant to use a lower threshold LED, as such types also exist. A rather high threshold voltage is chosen by using aluminum-gallium-indium-phosphide LEDs. Since the purpose of the LEDs is not to emit light, it is not the forward voltage drop mentioned in the data-sheet (1.95 V-2.5 V at 20 mA and 25 \({}^{\circ}\)C) that has to be considered, since the HVED will start working at lower currents and thus lower forward voltages. One could determine the forward voltage drop by making use of the Schottky diode equation and Ebers-Moll model, but this would unnecessarily complicate matters. The estimate for the LED drop is made from the following characteristic as supplied by the manufacturer. The reverse leakage current is also an indication of the forward leakage current. Next to the LED drop, the threshold voltage is also determined by the base-emitter voltage drop of the PNP BIT. The LED supplier specified a reverse LED leakage of 10 \(\upmu\)A. In the assumption of a forward leakage \(<\)10 \(\upmu\)A, the BIT has to be biased sufficiently above that value, so the leakage in no way triggers the equalization. A 10 k\(\Omega\) base-emitter resistor is chosen to be safe in that regard. The current will stay well below 1 mA and the LED drop can thus safely be assumed around 1.65 V according to Figure 4. The transistor works at a limited collector current of \(<\)0.1 mA, the threshold \(V_{\text{BE}}\) can be estimated at 0.55 V according to Figure 5.  
Figure 4: I-V characteristics of the light-emitting diode (LED). Figure 5: Base-emitter voltage as a function of collector current.