Medical Converters for Healthy Batteries

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

Rechargeable batteries have been used for decades to store electrical energy. Many applications are equipped with lead-acid batteries. They are heavy but simple to use. Maintenance-free, AGM or EFP batteries not only made usage easier and safer but also increased current and storage performance and lifetime. Charging is relatively simple, and modern devices use microprocessors to check the type and status of a battery before starting the charging process. The lifetime of the battery is maximized by using multiphase, temperature-controlled charging algorithms (Figure 1).

 

Figure 1. Multiphase charging algorithms avoid overloading or overheating the batteries. The charger can always be connected to the battery. Image used courtesy of Bodo’s Power Systems [PDF]

 

Lithium-Ion Batteries

Dry cells are used for many mobile and handheld devices, but applications like tools need more energy and lighter solutions. NiCd batteries offered higher power density and current capabilities and had been available in the existing dry-cell battery formats. However, the charging current was limited to around C/5 or C/10 (C = nominal capacity in Ah) and took 10 hours or more. Theoretically, they offered more charging cycles, but the memory effect quite often reduced lifetime. Self-discharge had been 10 – 20% per month. Cadmium is toxic, was banned in many countries years ago, and was replaced by NiMH batteries. They are robust and more resistant to overcharging or deep discharge. Memory effect and self-discharge are significantly lower but still not ideal for mobile devices. Fast charging within a few hours requires more sophisticated chargers.

Li-Ion batteries offer higher energy density, a self-discharge rate of only 1-2% per month, and have no memory effect. They also withstand wider ambient temperature ranges and have been the ideal solution for mobile phones and laptops. Today they are the first choice for many applications. Fast charging at rates of 0.5C up to 1C is possible, but the phases are different (Figure 2).

 

Figure 2. Typical charging phases of a Li-Ion battery. Image used courtesy of Bodo’s Power Systems [PDF]

 

Lithium can catch fire and is problematic to mine. Battery topologies based on other, less dangerous, and easier-to-mine materials are investigated and offer different energy densities, faster charging, or lower cost. They all have in common, that managing the charging process and monitoring the status of each cell is essential for safety and a long lifetime. Measuring the state of charge (SoC) of a battery was a major challenge for decades. Using the voltage vs charge curve was somewhat accurate for lead-acid batteries, but for other cell materials, the discharge curves are quite flat (Figure 3). Values also depend on technologies, the number of charging cycles, and the age of a battery.

 

Figure 3. Typical voltage versus discharge curves of different battery topologies. Image used courtesy of Bodo’s Power Systems [PDF]

 

Measuring the current flows during the charging and discharging phases was complicated with analog circuits but is easy with microcontrollers and current sensors. With this Coulomb counting called process, the state of charge can be calculated, and the equation can include losses in the battery, aging, self-discharge, and temperature.

 

Figure 4. Each cell or block of cells in an array is monitored and controlled and data are sent to a central battery management system. Image used courtesy of Bodo’s Power Systems [PDF]

 

Battery Management Systems and Battery Monitoring

Battery management systems (BMS) monitor the battery status and avoid any operation outside the safe operating areas. They control the charging process, balance the energy stored in each cell, monitor critical data like state of charge and temperatures, and report any abnormal condition. Figure 4 shows the simplified block diagram of a BMS system.

Monitoring a few cells in a low-power application is easy but a real challenge in large arrays with hundreds of kWhs or several MWhs and a large number of cells connected in series and parallel. These installations (Figure 5) store the surplus of renewable energy parks, balance AC grids, or act as UPS (uninterruptable power supply) for critical installations.

 

Figure 5. Large-scale energy storage system with battery management system. Image used courtesy of Bodo’s Power Systems [PDF]

 

As shown in Figure 4, each cell must be monitored, and the circuits need individual, isolated supply voltages generated from a 12 V or 24 V bus. Sounds like a simple design challenge that can be solved by low-power, standard converter modules.

But for high-power applications with hundreds of cells connected in series, the voltage of the complete battery pack can reach 600 - 800 V, and reinforced isolation is needed. Most 24 Vin DC/DC converters are designed for typical industrial applications requiring isolation voltages of 500 V or 1600 V.

To avoid degrading and failures of the isolation material, the allowed continuously applied working voltages are much lower than the isolation voltages and are defined by safety standards (Figure 6). These values also depend on the type of application and the environmental conditions.

 

Table 1. Typical isolation voltages for industrial applications.

Isolation Test Voltage (AC)	Rated Working Voltage (AC)
1000	130
1500	230
3000	1100
6100	3050

 

A battery pack with 600 - 800 Vdc requires reinforced isolation barriers of 3000 Vac (or 4243 Vdc) within the DC/DC converters, far too high for many standard converters with only 1600 V isolation. But are converters with 3 kVac isolation really meeting the requirements?

These large battery systems are connected to the AC grid, and close to wind turbines, solar parks, or transformer stations. They are exposed to high transients and should be compliant with OVC III (overvoltage category III), requiring 4 kV isolation voltages for 400 Vac three-phase mains. Although specified for AC applications, these OVC categories help find a solution for DC batteries. Unfortunately, there are normally no matches when searching on a manufacturer’s website for industrial DC/DC converters with 4 kVac isolation.

 

Medical Equipment Standards

But there is a market where the highest isolation barriers are vital. Patients connected to medical equipment must be protected against any electrical shock, and therefore, this market has very stringent isolation and leakage current requirements. Medical standards define different means of protection (MOP) for patients (MOPP) and operators (MOOP). Equipment connected to patients and invasive systems must meet 2 MOPP (two means of patient protection) with isolation voltages of 4kVac. Converters specified for this standard can be used in the above-mentioned BMS systems.

P-Duke has a wide range of DC/DC converters meeting these 2 MOPP requirements and offering an even higher isolation voltage of 5 kVac. The comprehensive set of product families with power levels from 1 W up to 60 W is available with single and dual outputs. With various input voltage ranges from 5 V up to 75 V, they can be deployed in all different applications. A suitable solution can easily be found by using the product search function on P-Duke’s website.

As an example, the MPD30-24S12W, a 30 W converter with 9 -36 input and 12 V output can create from a 12 V or a 24 V bus the individual, isolated supply voltages for the BMS circuitry connected to the cells or cell arrays.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.