DaChesserCat's Journal: Ideal EV Battery

I had a complete mindstorm the other night: a combination voltaic cell and multi-layer supercapacitor, integrated into one, physical unit.

Didn't get that? Some background is probably in order.

I'm thinking about lead-acid batteries. These are usually made of lead electrodes, sitting in some kind of acid-based electrolyte. If you want a starting battery (which needs to dump a great deal of current for a short period of time), you usually use electrodes which have a sponge texture (sponges have tremendous surface area, packed into a small volume). The problem is that, if you discharge most of the current, the lead starts to dissolve into the acid solution. When you recharge the battery, the lead tends to precipitate out of the solution, bonding to the lead electrodes, and fill in the pores in the spongey electrodes. This reduces the surface area, which reduces the amount of current the battery can release. Consequently, a few deep cycles will basically kill a starting battery. Letting them sit, discharged, for a while, will further dissolve the electrodes, sometimes beyond the point which charging can recover them. The result is lead from the electrodes bonding chemically with the sulfur in the sulfuric acid electrolyte, making a lead sufate "mud" which sits in the bottom of the battery. There is no known way to recover a "sulfided battery;" replace it, or drain the electrolyte (and the "mud") and put new electrolyte in it.

Electric Vehicles are usually made with deep-cycle batteries. These batteries still use lead electrodes and acid-based electrolyte, but they use flat plates. If you deep discharge them, the leads starts to dissolve into the electrolyte, but recharging them just puts layers of lead back on the flat plates. This doesn't reduce the surface area (how do you reduce the surface area of a flat plate?) so the batteries keep their capacity for much longer (about 500 cycles to 80% depth of discharge is typical). The newest versions of these batteries don't have the plates sitting in liquid; the electrolyte is either gelled or absorbed into some kind of fiberglass mat (Advanced Glass Mat or AGM batteries). Since their flat-plate electrodes have a lower surface area than the sponge electrodes in starting batteries, they can't provide as much current.

The latest trend is called a spiral cell. Use a long, thin sheet of lead, cover it with a thin glass mat (with the electrolyte absorbed into it), then another lead sheet (the other electrode; always two, there are, a positive and a negative), then a thin layer of some kind of insulator. Then, roll the whole thing up like a jelly roll. The result is something with a very large surface area (able to dump high current), but still able to handle multiple deep-discharge cycles.

Spiral cells are fairly new; I don't of any EV's (yet) which use them.

Batteries still aren't able to comfortably handle very high discharge rates, though. For dumping significant power for, say getting off a stop light or climbing a hill, or for absorbing lots of power from regenerative braking, you need supercapacitors (sometimes known as a supercaps).

I mention supercaps in other journal entries; suffice it to say they don't store as much energy as batteries, but they can very quickly dump what they do store. Consequently, you couldn't do an entire EV with just supercaps, but a few of them added to the system would allow the vehicle to accelerate quickly (high power) for a few seconds, then settle into a more moderate cruising speed. If you think about it, that's what most passenger vehicles need to do. Unfortunately, I don't know of any controllers which can handle both batteries and supercaps. Consequently, building an EV which uses them is still way out in the experimental range.

Supercapacitors are usually made by taking a thin, aluminum plate, covering it with some kind of very porous solid electrolyte (activated carbon is popular), another thin plate (like a battery, a capacitor needs two electrodes), then covering it with some kind of insulator and rolling the whole thing up like a jelly roll.

One good jelly roll deserves another, don't you think?

So here's the idea. Lay down the first electrode, electrolyte and second electrode for a supercap (we'll call this the "layer one supercap"), then some kind of resistive material (not a full insulator; an insulator has extremely high resistance, effectively preventing any energy transfer through it; we WANT energy transfer through this layer, just not too quickly), then lay down all three layers for a second supercapacitor (call it the "layer two supercap"), then another resistance layer, then the three layers for the battery, THEN we lay down an insulator. Now, roll up this entire mess into one cell.

Tie the layer-one supercap to the terminals on the battery.

If you put energy into layer one, it will build up the voltage in that layer. When the layer one voltage goes higher than the voltage in layer two, energy will move through the resistance layer until they equalize. The same is true for layer two and the battery layer.

If the whole thing is charged up, then left sitting, you'll end up with the battery layer being charged, and both of the supercap layers being charged to the same voltage. When you draw current from the cell, layer one will discharge first. As its voltage drops, layer two will feed energy to layer one (but not too quickly). As layer two's voltage drops (slower than layer one), the battery layer will feed layer two (even slower), which will feed layer one. The result is a three-stage punch: very high power for a few seconds, fading into high power for a few seconds more, fading into moderate power from then on. The supercap layers will discharge first, then the battery.

If you feed energy back in from regenerative braking, layer one will charge first. When its voltage goes above layer two, energy will flow from layer one into layer two (not too quickly, though). When layer two's voltage goes above the battery layer, energy will flow from layer two to the battery (again, not too quickly). Consequently, the supercap layers will charge first, the battery last.

Either way, the two layers of supercap will buffer all the major surges in and out, so the battery gets relatively low current flow in either direction.

The resulting battery would have less total capacity than a regular battery of the same weight (supercaps pack less storage per pound than batteries, and this battery is part supercap), but it would be able to handle larger power surges (in either direction) better than a starting battery and be able to handle at least as many deep discharges as a deep-cycle battery. And, since the battery handles all of this internally, you wouldn't need a fancy, new controller and separate supercaps.

You would, however, need direct taps into the layer two supercap and battery layers, if only so you could accurately gauge the state of charge in each layer. I mean, a traditional EV uses a voltmeter as a kind of "fuel gauge," because batteries tend to lose voltage (slowly) as they discharge. Such a meter, tied to the main terminals (and therefore, the layer one supercap), would be swinging around all over the place. A voltmeter tied directly to the battery layer (low current; we want just enough flow to determine how much is there, so we're talking a tiny fraction of an amp) would provide a fairly accurate "fuel gauge" reading, while a volt tied to all the supercaps would provide a kind of "boost gauge," indicating how long you can hold the pedal down before the car turns into a gutless wonder.

An improvement on this idea would be to put a Zener diode between the layer two supercap and the battery. A diode generally allows electric current to flow in only one direction. More accurately, it provides very low resistance to current flow in one direction (called the "forward bias"), but very high resistance to current flow in the other direction (called the "reverse bias"). A Zener diode is slightly different; it provides very low resistance on the forward bias, and high resistance on the reverse bias (like a regular diode), until the voltage goes above a certain level. Then, it allows reverse bias current flow with very low resistance. If you put a Zener diode between the layer two supercap and the battery layer, and set it so that it wouldn't allow current back into the battery layer until, say, the supercap layer went above 2.35 volts, about the ONLY time the battery layer would get current inbound would be when we eat a SERIOUS amount of inbound current from the regen braking (riding the brakes down the side of a mountain, for example), or when we actually want to charge the battery (most battery chargers put a lead-acid battery at about 14.25 volts for charging; since there are six cells in a 12-volt battery, 2.35 volts x 6 = 14.1 volts, so the 14.25 from a charger would get past the diode and charge the battery layer). The rest of the time, the supercaps would eat ALL of the inbound current. This would be a good thing, since you don't typically want to partially discharge, then partially charge, then partially discharge batteries.

In the absence of such batteries, though, you could probably build a "power module" from an existing battery, some supercaps and resistors or Zener diodes, then tie that into the system in place of the battery. That would probably be a good starting point.

And yes, this idea could be applied to other battery technologies, so long as the battery involves a spiral wrap of some kind. The Zener diode would be extremely helpful if you wanted to a combo supercap/NiCad battery, since NiCad batteries are well known for having a "memory effect," meaning that partial charging and discharging will prevent the battery from fully charging.