Bob's Battery Box
A battery that's suitable for storing renewable energy isn't a single unit like the battery in your car. Instead, it's an assemblage of 6 volt cells that are wired together in a variety of ways in order to arrive at the voltage and amperage ratings that make sense for the specific application.
The battery set for the dining hall consists of six 6 volt units that are configured in a 2 x 3 array which provides a thousand amp hours of capacity at 12 volts. That means that we could theoretically draw ten amps of power, enough to run the lights in the dining hall, for more than four days straight.
The 2x3 battery array
In theory, that is. In actually practice, you wouldn't want to take your battery pack below about half charge before you cranked up the generator to replace the current used.
Your standard battery battery consists of metal plates suspended in an solution of sulfuric acid. Electricity is generated by the battery as its metal plates are dissolved by the acid, so the more you discharge your battery, the more your metal plates are dissolved away. Recharging the battery drives this process in reverse, depositing metal back on the plates, but there are inefficiencies involved and not all the metal winds up where it's supposed to go.
Deep discharge batteries are designed to cope with these effects to some degree, and you can get away with doing it now and then, but ultimately they too will fall victim to the impact of being discharged more than half way - it will just take longer to happen.
The goal of a properly designed battery system is to match your load and capacity such that you're working between 100% and 75% capacity under ordinary conditions. In the case of the dining hall, we can easily operate the lights and ventilation systems for two days off-grid without dropping below that 75% threshold.
You could theoretically just keep the generator running day and night, and not install a battery at all, but there are a number of reasons why that's not a good idea:
1) Generators aren't efficient. The backup generator is puts out 3,500 watts of power, about ten times the electricity needed to run the lights. Fuel driven gensets aren't very efficient to start with; running one at such a low power load setting is especially inefficient.
2) Generators are slow to respond to changes in load. A genset is a mechanical device in which an internal combustion engine rotates an alternator core - about fifty pounds of gear rotating at 3,600 revolutions per minute. The key point there is that all these moving parts have momentum, and while the genset can respond to a change in loading, it does so in real time measured in seconds whereas electrical load changes happen in one 120th of a second. [note: the electricity in a battery is direct current that flows from the negative terminal to the positive terminal - I know that sounds backwards, but so it goes. The electricity in a wall socket is alternating current switching at sixty cycles per second. First it goes in one direction for 120th of a second, and then reverses and goes in the opposite direction for 120th of a second.]
Because of the momentum involved in the workings of the genset, when a big load switches on, the entire circuit experiences low voltage while the genset struggles to catch up with the load. The reverse is also true; when a big load switches off, the system experiences an over-voltage condition for a few seconds as the genset adjusts. Low voltage conditions are bad and can lead to burnt out motors. High voltage conditions are even worse, since too high a voltage will burn out light bulbs and fry sensitive electronics in way less than a heartbeat.
The more loads you have on a generating system, the less impact you'll notice when any one load cuts in or out, but even grid power is subject to fluctuations. You've probably noticed lights in the kitchen dim for a moment when the refrigerator kicks in. Because a solid state power system has no moving parts, it can respond to a change in the power load in that first 120th of a second - thereby protecting all the equipment that's online.
3) Generators are noisy. A solid state power supply just hums quietly in the corner.
4) Generators don't last. The life expectancy of a small genset is a few thousand hours, so every few months you'd be looking for another motor. One way the operational life expectancy can be greatly extended is if you operate the engine on propane, since propane doesn't foul the oil and interfere with lubrication the way that gasoline does.
Another option is to leave the engine running 24x7. That makes a huge difference in an engine's life expectancy since much of the wear comes while the engine is warming up or cooling down, but it comes at the price of wasted fuel. Some natural gas powered pump engines in municipal water plants are run for more than a decade without ever being shut down.
Once you've made the decision to go with a really big battery, there are a number of considerations which need to be dealt with.
A battery is a heavy container filled with lead and sulfuric acid. When it's being charged it generates enough hydrogen gas to blow itself up, along with anyone unfortunate to be standing nearby. When charged, it is contains enough stored energy to turn itself into a molten nightmare.
So the first consideration is to assembly the battery in a container which will protect it from the world around it, and vice versa. Reviewing the options, Bob came up with the idea of stripping out a dead refrigerator, and using that as a battery box.
The dead frig worked out great. First of all, it has an enameled interior that resists corrosion. Secondly, it has an air tight lid which provides full and easy access to the battery when open, and when closed, it allows us to vent the battery compartment to the outside. This is really important because charging the battery generates gases which include a small amount of sulfuric acid.
A smalll 12 VDC fan ventilates the battery chamber
When you're using a solid state electronics, it's amazing how much damage "a small amount of sulfuric acid" can do. You may be familiar with the fuzzy growth around your car's battery terminals; that's caused by the sulfuric acid fumes. The thought of that happening to the internal circuitry of an expensive, solid-state inverter is the sort of thing nightmares are made of.
To prevent any gasses given off by the battery from getting anywhere near the electronics, Bob rigged a small 12 vdc fan motor to blow air into the sealed battery compartment, and then outside through a venting tube.
When sulfuric acid dissolves metallic lead, the reaction produces lead sulfate and electricity, so the liquid in a partially discharged battery contains lead sulfate. When the battery is connected to a source of higher electrical potential, the reaction is driven in reverse; metallic lead is plated out on the surface of the electrodes, and the sulfate ion is reduced back into sulfuric acid.
In a world of perfect efficiency, that's all that would happen. In the real world, the process is less than perfect, and some of the incoming energy causes the water in the battery to break down into hydrogen and oxygen.
This is a problem because these gases, when mixed, are explosive. While a modern battery will rarely explode, you really don't want to be there when it happens.
Ventilating the battery box is one way to make sure that the charging gases don't accumulate. Another way, a better way, is to eliminate them at the point of generation. Internally, each Trojan L-16 battery consists of three 2 volt cells connected in series. Each of these internal cells has a vent cap, which is also how you check the water level in the cell.
In place of the original caps, we've installed special, mushroom shaped caps which contain a platinum catalyst. This material adsorbs the hydrogen and acts as a catalyst to combine it with oxygen to reproduce water which then drips back down into the cell to maintain the water level.
Over time, this process will cause the concentration of the electrolyte to vary from dilute in the top of the cell to concentrated in the bottom. That's fixed by doing what's called an "equalizing charge" in which you intentionally over charge the battery. As each cell maxes out, it starts to generate substantial amounts of electrolytic gases which churn the solution within the cell. Twenty minutes of that, and the cells are all fully charged, fully mixed and ready to go for another month.
May not be very sexy, but ....
What we're going to do
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