Chapter 6. Appendices

6.1 Understanding Li-ion batteries

6.1.1 Charge characteristics

The Power Module uses a battery of rechargeable lithium-ion (Li-ion) cells. These have a good capacity/volume ratio and are commonly used in, for example, electrically-powered vehicles.

As with most cell technologies, a Li-ion cell's terminal voltage depends on its state of charge. The graph below shows the relationship between the cell terminal voltage and the extent to which the cell has been discharged. Note that the graph does not extend down to zero volts: the curve is actually very flat across the majority of charge states - much more so than for a lead-acid battery, for example. The terminal voltage varies by less than 0.3 V between 20% and 80% charge.

Each cell provides a terminal voltage in the range 2.5 V to 4.2 V. Only the useful part of the operating curve is shown above. Once discharged down to 3 V, very little of the available charge is left. If the cell is discharged below this point, internal damage starts to occur because of the chemistry involved. This process accelerates if the cell is discharged to below 2.5 V. If left in this condition for a long period of time, the internal cell damage can make it dangerous to recharge at any more than a very small recovery current because of the internal heating effects.

To protect the cells from this damage, the PPM’s control system will turn off the power output if the cell voltage drops below 3V. The system will then enter a low-power mode to preserve any remaining charge for as long as possible. If charging resumes, the PPM will monitor the cells until a terminal voltage of 3.2 V is reached, at which point the power output will be turned on again.

The Li-ion cells used in the Power Module are connected in “2S” (i.e. “ 2 Series”) configuration which means that two banks of cells are connected in series, as in the simplified diagram below. Because of this, the output voltage is twice that of each individual cell’s terminal voltage.

(There is a third connection, not shown above, to the intermediate contact between the two sets of cells. This is used to monitor and adjust the balance of charge between the two banks.)

6.1.2 Temperature issues

It is important to understand the operational limits of Li-ion cells with regard to the temperature conditions at the intended installation location. Note that the acceptable temperature ranges for charge and discharge differ.

Discharge of the cells (i.e. the PPM's ability to provide power to an instrument) is specified over the range -20°C to +60°C. Charging, however, is specified ONLY over the range 10°C to +45°C.

The PPM monitors its own temperature and controls charging accordingly. Charging will be restricted or even disabled when the temperature approaches or exceeds the operational limits. The maximum charging rate is only available when the temperature is in the range +10°C to +40°C.

The PPM simultaneously limits the charge input current to approximately 8 Amps and the input power to around 80 Watts. As the cell voltage attains it’s maximum voltage (4.2V) the charging rate is reduced accordingly. Even if available power is at a maximum, it cannot allow further charging of the cell for safety reasons.

6.2 Connector pin-outs

6.2.1 Charge input

6.2.2 Power output

6.2.3 Charge cable connectors

The Güralp PPM uses MC4 connectors (manufactured by Stäubli Electrical Connectors) for connection to the solar panels. See section 4.2.1 for polarity details. Suitable mating connectors include MC4, Amphenol Helios H4 and SMK PV-03.