Monday, February 10, 2020

Carbon Foam Batteries 

Recently a friend of mine asked me to opine on carbon foam batteries.  This is what I told him:
First, how a conventional lead acid battery works. There are two chemical reactions going on in the battery as it discharges... at the positive plates, PbO2 > PbSO4, and at the negative plate, Pb > PbSO4. (that's a very simplified version). But here's the rub... PbO2 and PbSO4 are both non-metallic powdery solids. How to keep them in contact with the plates (which, BTW are also made of Pb)? The solution which has evolved over the century or so that lead acid batteries have existed is to form the Pb plate into a kind of grid of fine Pb 'wires' supported by a Pb framework, one which will hopefully provide pockets where the solids can be kept in contact with the plate metal.

The carbon foam battery is a departure from this. In this battery, the 'plate' is actually a sheet of carbon foam. Carbon, because it is a (pretty good) conductor - perhaps even better than the Pb of conventional plates - and because (unlike the Pb plates) it is completely inert in sulphuric acid. But the primary benefit is that each of those millions of little tiny pockets in the foam serves to trap and contain the PbO2 and/or PbSO4 powders, keeping them in intimate contact with the plate.

How is this an advantage?
  • First, imagine a standard car battery - it is subjected to vibration all the time the car is moving. Vibration loosens the powders, allowing them to fall out of the Pb grid, to the bottom of the battery, where they are lost forever from participating in the charge/discharge chemistry, thereby reducing the battery's capacity. In fact, if enough falls to the bottom of the battery, it will create a shorted cell. Consequently, batteries are taller than they strictly need to be in order to give a little room at the bottom for lost reactants. Because the powder reactants in a carbon foam battery are more intimately contained and therefore less likely to be shaken loose, I would think therefore that its plates could be taller while still fitting inside the standard battery form factors, creating a slightly greater amp-hour capacity in the same form factor.
  •  Charge and discharge rates are determined by surface area of the plates. Not the gross size of the plates, but the micro surface area. Carbon foam has orders of magnitude more surface area per unit volume than even the best lead screen design of a conventional plate. Therefore, the discharge rates achievable by carbon foam batteries should be much higher (perhaps only temperature limited? I don't know).
  •  Battery capacity (amp-hours) is determined by the quantity of reactants available. The more reactants, the more capacity. I don't know how the reactant storage capacity of carbon foam compares to the capacity of the lead screen plates.  
  • Never forget that the sulphuric acid is also one of the reactants (not shown above). The acid needs to get to and circulate around the plates for the energy producing reactions to happen.  The carbon foam battery needs to make provision for sulphuric acid circulation in depth in the plates. I don't know how they address this issue. If acid flow channels have to be made in the foam, this will reduce the potential storage capacity of the plate.

So, carbon foam batteries should *potentially* have higher capacity and greater discharge rates, but whether this can be realized in practice will be dependent on the specific mechanical design of the carbon foam plates. This is an interesting technology to watch as it develops...  
Maybe someone out there with knowledge in the industry could comment?


Friday, February 7, 2020

Otter Exclusion Technology, Mk II

Well, so far, so good. I hope I'm not jinxing things by reporting this...

The Mark II version of the otter exclusion technology seems to be working.

First, one or two mothballs tucked under the lines where they are wrapped on a cleat seems to be working to discourage the otters from viewing these as their personal toilets.  This is a HUGE gain!

Our lines - see the moth balls?

The boat we share the finger pier with needs them too
...All the cleats on our finger need to be provided with moth balls if we are to discourage the otters from enjoying this finger pier.

But this seems to be the piece d' resistance... 

The first time we checked after installing these, the two big rat traps had been tripped.  No otters were apparently harmed, since there was no blood.  But those traps make plenty of noise when they go off - perhaps that was enough.

Now, a couple of weeks later, the traps and the boat are unmolested!

As I said...  So far, so good...

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