In this new TechLetter, we will focus on the case study of a battery in a fuel cell system. In an hydrogen vehicle, the fuel cell produces electricity to power the electric motor by consuming hydrogen as a fuel. There are numerous fuel cell technologies but we will here only work on the case of low temperature hydrogen fuel cell of PEMFC type.

If autonomies obtained with battery electric vehicles and fuel cell vehicles are quite similar, the hydrogen vehicle is refueled or recharged much quicker (in a few minutes). As opposed to the diesel or gasoline vehicles, the hydrogen vehicle only rejects water when used. The pollution linked to hydrogen is only generated during its production phase and is dependent on the used process. You can find more information on this topic by reading the report from ADEME .

In the fuel cell, an oxidoreduction reaction occurs and permits to create electricity and heat. There are 2 electrodes : an oxidizing anode and a reducing cathode separated by a central electrolyte. A specific tank continuously supplies hydrogen to the cathode. The cathode is supplied in oxygen thanks to an air compressor linked to the ambient air. The reaction oxidizes hydrogen (creating water) and frees electrons. These electrons supply the electrical circuit in continuous current at a voltage depending on the number of elementary hydrogen cells connected in series. You will be able to find more information on the websites of H2SYS or the CEA.


Fuel Cell principles

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The fuel cell is generally associated to a power or « buffer » battery (as illustrated for the Audi h-tron here under). The main functions of this battery are the following ones :

  • Supply power peaks, which are commonly seen in mobile application, to level the power demand on the fuel cell. Indeed, the dynamic of the air compressor is often too slow to answer to the strong power peaks of mobile applications. This also permits to size the fuel cell on average power and not on maximum power, thus reducing the overall system cost.
  • Working at low temperature (-20°C) : The functioning of a fuel cell at low temperature is complex. Water formation at power off has to be limited to avoid freezing (and then damage to the fuel cell). Besides, there must be some moisture on the membranes at start-up to help current circulation. The battery supports the heat-up of the fuel cell by avoiding internal freeze.
  • Be as compact as possible while delivering maximum power at high voltage. As it can be seen on the Audi h-tron concept shown here under, numerous components are integrated to make a full powertrain and the volume dedicated to the battery is small.
  • Enable energy recovery during braking phases. Indeed, PEM fuel cells are not reversible and do not permit to regenerate kinetic energy during any braking. Adding a battery enables to significantly reduce the energy consumption of the vehicle by such a mean.

Audi H Tron fuel cell

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There are several battery chemistries available to make this “buffer” battery function. You will find more information on the various lithium-ion chemistries in our TechLetter : Lithium-ion positive electrode technologies. WATTALPS can offer 3 types of chemistries for fuel cell applications, the power LFP, the power NCA and the long life NCA:

  • The Power LFP Module has a long life and higher available charging peaks than the NCA modules, but outputs a lot less energy. This chemistry is also without Nickel, nor Cobalt, which can be a major advantage in some circumstances. This module is well adapted for hybrid applications requiring high peak power in charge and discharge but low autonomy with the battery only.
      • ⇒ Perfectly adapted for full power hybrid fuel cell system => design with most power on the fuel cell
  • The Power NCA Module can deliver high discharge peaks and be charged quickly, while offering a higher energy density and a competitive price compared to chemistries like LTO for example.
      • ⇒ Perfectly adapted for a mid-power fuel cell system (50/50) and for a range extender system (more requirement on the battery, the fuel cell is only early to extend autonomy)
  • The Long life NCA module is well adapted for plug-in hybrid applications for which the battery must provide sufficient autonomy without using hydrogen.
      • ⇒ Perfectly adapted for a mid-power fuel cell system (50/50) and for a range extender system (more requirement on the battery, the fuel cell is only early to extend autonomy)

To answer the specifc needs of fuel cell application, batteries must also be equipped with a dedicated BMS. The batteries for the fuel cell are often maintained at an average state of charge et may never be fully charged. The majority of lithium-ion batteries need a full charge to properly implement two important functions: cell balancing and state of charge calibration. If these two functions are not activated regularly, the state of charge evaluation will drift and the battery performance can be significantly lowered. The battery BMS must therefore enable cell balancing and state of charge recalibration even if the battery is not fully charged, which is rarely the case for batteries available on the market.

You will find more information on a BMS matching these requirements in our TechLetter : The Full-BMS and WATTALPS’ products.


Battery balancing


WATTALPS BMS can balance cells at partial charge

Source : Review of battery cell balancing techniques

As we have seen earlier, the fuel cell functioning has challenges at low temperature dure to freezing and/or condensation issues. It requires an external energy source to heat up some of its components. Once started, the fuel cell generates a lot of heat. This heat can be advantageously used to maintain battery temperature, improving the battery performance and life. WATTALPS’ batteries equipped with a liquid thermalization system can be easily interface with a fuel cell for such a function.

Besides, the battery is submitted to high current peaks which will heat it up. At higher temperature, the battery must be properly cooled to keep its performance and life. A full and effective thermalization system is then necessary. For more information, you can read our TechLetter : Lithium-ion battery aging.

WATTALPS immersion cooling technology

WATTALPS 400V battery equipped with a heater and a cooler

Finally, in a fuel cell system, the choice of the voltage level is dependent on the power need, on the bipolar plate surface and on available DCDC converters. The battery must adapt to the voltage level selected for the powertrain. The small WATTALPS’ module combined with battery immersion cooling technology can easily adapt to multiple requirements without redesigning a new battery.

* For large volume orders, the power can be increased.