Capacitors have a number of great properties. They store power as electrical charge rather than chemical energy, for one. This typically allows near instant charge times and very high peak output currents. They can survive hundreds of thousands of charge-discharge cycles, rather than the hundreds of of cycles for full-cycled batteries. So what’s the problem?
A battery provides a fairly constant voltage over a long useful life. Depending on the device, you may have performance issues near depletion. Smartphones, for example, go into power savings mode. That’s not just to keep them running a bit longer, but to prevent instant shutdowns without warning.
As you can see, the voltage drops as the battery gets near exhaustion. In your phone, there’s a power conversion circuit, part of the overall power management, that works to convert not terribly constant battery power into a very tightly regulated system power (probably a bunch of different voltages). Note there’s an important relationship here: power=current∗voltage. So to keep the same power, as the voltage drops, my circuit has to draw more current.
Every battery has a little internal resistance, and because of another relationship, called Ohm’s Law, you know that there will be some voltage dropped in the battery. In the drawing, the Vout=V0−r∗I, where I is the current. Thus, as my V0 drops and my power management circuit has to draw more current to deliver the same power, the battery output voltage drops even faster. This limited the maximum current output of a battery, and it also means they drop out pretty quickly when near exhaustion.
But the output voltage, peak current, and total power in a capacitor drop exponentially over time. The capacitor has one advantage: it’s storing electrical charge, rather than converting electrical charge to chemical charge as in a battery, so while there is an internal resistance, it’s tiny and can usually be ignored. Capacitors can provide very, very high currents for a short time.
But for powering a thing, they’re problematic. Recall my desire to keep a constant power going into my power management system, and that power=current∗voltage. As our voltage rapidly drops, we have to make it up with rapidly rising current to deliver the same power. Very high currents make for a much more expensive circuit, larger power conversion components, more power loss in circuit boards, etc… same basic problem the battery has near the end, only this is starting to happen very early in the capacitor’s useful power storage life. And as the capacitor depletes, the peak current, while still relatively high, drops as well.
The other problem is that modern ultracapacitors have much lower specific energy than batteries. The best ultracaps on the market manage 8-10 Wh/kg, most are more like 5 Wh/kg. The best Li-ion batteries deliver close to 200 Wh/kg, many formulations can reach over 100 Wh/kg. So you need about 20x the weight to use ultracaps. But possibly more, since at some point during discharge, depending on application, the voltage will drop too low to be usable, leaving power unused. As well, unlike more traditional capacitors, ultracapacitors also have a relatively high internal resistance. So they can’t necessary support much trading of voltage for current.
Then there’s self-discharge: how quickly does power “leak” from a storage device. The only NiMh cells are rugged, but self discharge as high as 20–30% per month. Li-ion cells reduce this to more like <2% per month depending on the specific Li-ion technology, maybe 3% in some systems depending on battery monitoring overhead. Today’s Ultracapacitors drop as much as 50% of charge in the first month. That may not matter in a device that’s recharged daily, but it absolutely limits the use cases for caps vs batteries, at least until better designs are created.
And because you need so many, the current cost of ultracapacitors can be 6x-20x the cost of batteries. If your application needs a very small power output, particularly with very short high current surges, the ultracap may be an option. Otherwise, it’s not going to be a battery replacement in the near future.
For high current applications like electric cars, not really a useful consideration yet, as a standalone. Though systems using both ultracaps and batteries can be compelling, since their differences are very complementary, the high current transfer and long life of the cap versus the high specific energy/energy density of the battery. And there is a ton of work being done to deliver much better ultracapacitors, as well as much better batteries. So maybe some day the ultracap takes on more of the typical battery duties.
article from:https://qr.ae/pCacU0
Post time: Jan-06-2026