There's more to a good rechargeable battery than energy density by volume and weight.
Many of our common devices use rechargeable batteries (also called “secondary” batteries) rather than single-use non-rechargeable ones (primary batteries). We expect that sooner or later we’ll need to replace the non-rechargeable batteries at some point, assuming the entire unit is not a “throwaway” one-time use product (I know that’s a bad thing in most cases, but sometimes it is the only viable and effective way to power the product). But it’s also easy to get so comfortable using products with rechargeable devices that we easily forget that they, too, have a limited lifetime based on the number of charge/discharge cycles they can support while also delivering most of their original capacity.
How many cycles can we expect? As in most situations in the real world, the answer is simple: “it depends.” What it depends on is a wide range of factors such as cell chemistry, quality, battery pack construction, algorithms (if any) of the battery management system (BMS), user discharge/recharge pattens and depth, operating temperature, duty cycle, and more. There’s no single answer, but a good first estimate is there are somewhere between 500 and 1000 cycles before the ability to recharge and use the battery is seriously diminished (and the standard for that differs, as well).
You’ve undoubtedly experienced the “dead” (or nearly so) rechargeable battery scenario. Fixing it can range from simple to frustrating and almost impossible to overcome. Some devices make it easy by using standard rechargeable cells such as AAs or 18650 size with a convenient access arrangement.
At the other end are custom-size and/or hard-to-access rechargeable batteries. A customer-friendly design should make every effort to use standard cells if possible (pacemakers and similar are exceptions) and make them easily replaceable (again there are very legitimate exceptions).
For these and other reasons, I try to follow R&D updates on progress in battery technology in general and rechargeable cells in particular, and across all capacities from portable to grid-size. That’s why I was fascinated, intrigued, and somewhat mystified by a research project at Chalmers University of Technology (Sweden).
There, a team is investigating using concrete in rechargeable batteries. In highly simplified terms, their development uses a cement-based mixture with small amounts of short carbon fibers added to increase the conductivity and flexural toughness. Then, a metal-coated carbon fiber mesh – iron for the anode, and nickel for the cathode – is embedded. The researchers investigated two different arrangements for their concrete electrolyte, anode, and cathode, with a layered structure and an immersed structure, Figure 1, as well as different ways to combine the constituent materials and mixes.
Figure 1 Two common cell arrangements of the concrete batteries: (a) the layered structure and (b) the immersed structure.
The result is a rechargeable battery which makes it case for viability based on the large volume in a whole-building structure rather than customary virtues of high energy density by weight or volume. They experimented with various formulations and found their best version had a volume-based energy density of 0.8 watt-hours/liter, Figure 2.
Figure 2 The single-cell cement-based structural battery with a multimeter showing the cell potential of 1.24 V after a 3-h discharge.
In comparison, lithium-based cells have a density between around 250-70 W-hr/L while lead-acid batteries register at around 80-100 W-hr/L. Clearly, the energy density of these concrete batteries is orders of magnitude less than those other leading battery-chemistry options.
The researchers counter these low numbers by noting that concrete-based rechargeables are, in effect, a “something for almost nothing” source if they are used in the construction of buildings with the huge volume of concrete used. I can sort-of understand that logic, but maybe not: it’s a variation on the quip, “we lose money on each one, but we make it up in volume” except that here, we’re looking at energy density rather than money.
Finally, their research comes with a big caveat: thus far, the concrete-based cells can endure only a modest number of cycles. In many ways, that number is as important as the energy-density specifications. Their academic paper is a little vague on this cycle number, but implies it is in the low double digits. Using this rechargeable concrete as a large-scale building material means the building’s structure would have to be modular so concrete portions could be removed and replaced, and you then have to deal with the removed material as well. And what about the implications of this new formulations on the strength of the concrete itself?
Frankly, this doesn’t seem like a promising approach, at least not yet. Even if it improves the rating to hundreds or even thousands of cycles, concrete buildings are typically intended to last for 50 to 100 years. That concrete rechargeable battery will need replacement many times over that lifetime. Even if the concrete battery is used only for emergency-power backup, the numbers still don’t look good. Still, I could be very wrong; the long-term future is unpredictable and unforeseeable despite the best efforts of experts and musings of pundits. Perhaps such batteries will find a role as roadside booster-charging stations for EVs that have run out of charge: due to their weight and low residual-materials value, they are unlikely to be stolen!
You can see more details via the press release on their work “World first rechargeable cement-based batteries” as well as their full paper with the very short and direct title “Rechargeable Concrete Battery.” What’s your view of concrete as a potential rechargeable battery? It is worth further R&D effort, or is it a highly limited solution even if the density and cycle numbers improve significantly?
This article was originally published on EDN.
Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.