Medical Converters for Healthy Batteries

Rechargeable batteries had been used for decades to store electrical energy. Many applications are equipped with lead acid batteries, according to Wikipedia invented in 1859 by Gaston Planté. 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. 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

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. But 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 over-charging 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

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 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

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 SoC can be calculated, and the equation can include losses in the battery, aging, self-discharge, and temperature.

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:

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

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

As shown in figure 4, each cell must be monitored, and the circuits need individual, isolated supply voltages generated from a 12V or 24V bus. Sounds like a simple design challenge which 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 Volts and reinforced isolation is needed. Most 24Vin DC/DC converters are designed for typical industrial applications requiring isolation voltages of 500V or 1600V.

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.

Figure 6: typical isolation voltages for industrial applications

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

These large battery systems are connected to the AC grid, close to wind turbines, solar parks or transformer stations. They are exposed to high transients and should be compliant to OVC III (overvoltage category III) requiring 4kV isolation voltages for 400Vac three phase mains.

Although specified for AC applications these OVC categories help finding a solution for DC batteries. Unfortunately, there are normally no matches when searching on a manufacturer’s website for industrial DC/DC converters with 4kVac isolation,
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 2MOPP requirements and offering an even higher isolation voltage of 5kVac. The comprehensive set of product families with power levels from 1W up to 60W is available with single and dual outputs. With various input voltage ranges from 5V 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.

Figure 7: P-DUKE has a very comprehensive set of medical grade DC/DC converters with single and dual outputs and power levels from 1W up to 60W

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

The medical standards also require leakage currents in the range of microamps and creepage distances of 8mm. This is the shortest distance along the isolation material between input and output inside the converter. Similar to a capacitor, material (ε0) and thickness (d) of the isolation inside a converter defines the input to output capacitance:

The thicker the material and the wider the distances, the lower the capacitance between input and output. Whilst not important when using one or two converters in a system, it becomes an important factor when combining hundreds of converters on different voltage levels in a large BMS system. AC voltages, transients, and noise can be coupled across these isolation barriers. It can not only disturb highly sensitive measurement or communication equipment but also lead to high and dangerous leakage currents from high voltage AC mains.

Let’s compare a practical example. P-DUKE’s medical converter MPD30 has an isolation capacitance of only 20pF. Even for a 1MHz sensor signal this represents a high impedance of 8kΩ. A standard industrial converter can have an isolation capacitance of more than 1500pf. This is 75 times higher and impedance for a 1Mhz signal goes down to just 107Ω. When many converters are connected in parallel, the total capacitance and impedance can reach critical values for noise coupling but also for AC leakage currents across these barriers.

With 100 medical grade converters in an application, the total capacitance is only 2nF and according to the formula I = U*2*π*f*C the leakage current in a 400V/50Hz application will be just 0,25µA.

When using industrial grade converters total capacitance would be 150nF and leakage current would increase to around 19mA. Although not yet fatal, it will cause a significant electrical shock. Together with the leakage currents of other devices in the system a 35mA RCD breaker (residual current circuit breaker) can be tripped.

As in medical applications, reliability is another important factor for battery storage systems. The MPD30 converter family from P-DUKE was designed for the highest reliability, with MTBF values greater than 1 million hours (MIL-HDBK-217F at full load). P-DUKE also offers 5 years product warranty for these medical devices, much longer than the 2 years of many other converter manufacturers.

Did you wonder why we said: medical converters for healthy batteries? Highly sophisticated BMS systems are required to guarantee the safe and long-lasting operation of such large battery packs. These applications are challenging but with P-DUKE’s medical converters and strong technical support, they can be solved to secure healthy operations with long lifetimes.