Batteries and Fuel Cells
Figure 2a. MAX17702 lead-acid charging cycle.
the recharge threshold voltage. This is a nice feature to keep the battery fully charged, if left in the charging cradle for a long period, without using too much power and to comply with Energy Star requirements. The device can detect and precondition deeply discharged batteries, waking them up with the precharge feature. For added protection, the device senses the battery temperature and allows charging only when within the temperature range. There is also an input short-circuit protection feature, which prevents discharging of the battery when the input is accidentally short circuited. Figure 3a illustrates the MAX17703’s charging cycle.
produces good battery performance but shortens service life due to grid corrosion on the positive plate. A low voltage limit is subject to sulfation on the negative plate. Temperature also affects the cell voltage with a typical –5 mV/°C (0.028 V per cell for every 10°F). A good charger must compensate for this temperature coefficient to avoid overcharge of the battery when hot or undercharge when cold.
As an example, the MAX17702 (see Figure 2) is a complete lead-acid battery charger controller designed to operate over an input voltage range of 4.5 V to 60 V. The device offers a high efficiency (over 97 per cent), high voltage, synchronous buck solution to charge 12 V/24 V/48 V lead-acid battery stacks. The lead-acid battery has low energy densities, making it unsuitable for portable devices. This is where a lithium-based battery comes into play.
Li-Ion battery charger
Li-Ion is the universally accepted battery for portable applications, heavy industries, electric powertrains, and satellites due to its light weight and high energy density. Li-Ion is a low maintenance battery. The
battery has no memory and does not need exercising (deliberate full discharge) to keep it in good shape. But it needs protection circuits, both built-in inside the battery pack as well as in the charger to prevent short circuit, overcharge, thermal runaway, and overdischarge. If a Li-Ion battery has dwelled below 1.5 V/cell for a week or longer, dendrites may have developed that could compromise safety.
To prevent overdischarge, the built-in battery protection circuit puts the battery into a sleep condition. This happens when storing the battery in a discharged state in which self-discharge brings the voltage to the cutoff point. A regular charger treats such a battery as unserviceable, and the pack is often discarded. An advanced Li-Ion charger includes a wake-up feature, or “precharge,” to allow recharging if a Li-Ion battery has fallen asleep due to overdischarge. In precharge mode, the charger applies a small charge current to safely raise the voltage to between 2.2 V/cell and 2.9 V/cell to activate the protection circuit, at which point a normal charge commences. During normal charge, the Li-Ion charger operates on constant current constant voltage (CCCV). The charge current is constant, and
Figure 3. Advanced, high voltage Li-Ion battery charger circuit.
the voltage is capped when it reaches a set limit. Reaching the voltage limit, the battery saturates; the current drops until the battery can no longer accept further charge and charging terminates. Each battery has its own low current threshold.
Li-Ion batteries should always stay cool on charge. Li-Ion cannot absorb overcharge. Thus, it is very important to monitor the battery temperature and its charging voltage to assure battery health and safety. A good charger must include these features. Figure 3 shows an example of an advanced Li-Ion battery charger. The MAX17703 is a high efficiency, high voltage, synchronous, step-down charger controller designed to operate over a wide input voltage range of 4.5 V to 60 V. The device offers a complete charging solution for up to 12 Li-Ion cell stacks.
The device offers accurate CCCV charging current/voltage at ±4 per cent and ±1 per cent, respectively. The charger enters a top- up-charge state when the charging current reduces to the taper-current threshold and then exits charging after a taper-timer period elapse. The charger initiates a recharge cycle when the output voltage falls below
Supercapacitor charger Supercapacitors are increasingly finding usage in a variety of applications, thanks to their unique advantages over batteries. Supercapacitors function on electrostatic principles with no chemical reactions, avoiding the lifetime issues associated with chemical storage of batteries. Their high durability allows for millions of charge/discharge cycles with lifetimes up to 20 years, one order of magnitude above batteries. Their low impedance enables fast charge and discharge in a matter of seconds. This, in conjunction with their moderate ability to hold charge over long periods of time, makes supercapacitors ideal for applications requiring short charge and discharge cycles. They are also used in parallel with batteries, in applications where instantaneous peaks of power delivery are necessary during load transitions. Supercapacitor short-charge and discharge cycles require chargers to handle high currents and work smoothly in constant current (CC) mode during a charge, which may start at 0 V, and in constant voltage (CV) mode once the final output value is achieved. In high voltage applications, many supercapacitors are connected in series, requiring chargers to manage high input and output voltage.
Figure 3a. MAX17703 Li-Ion battery charging cycle.
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Components in Electronics
April 2022 23
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