Monday, July 27, 2009

Charge it! (Roxi)

Batteries! So, we decided last that to finance today's energy lifestyle would require about 835 AH of battery storage.

deep cycle batteries

We've been looking at Trojan, Deka, Lifeline, and probably some others of good repute. Things that matter:

  • battery type
  • battery weight
  • capacity
Just like in your car, a battery may or may not need maintenance, depending on the type. When I started driving, batteries needed distilled water every so often to maintain their vitality. [We did not turn a crank then - just stop it.] Now, chances are your car battery is maintenance-free, which means that it is sealed and the water does not evaporate. Hydrogen and oxygen are still produced in the process, but they recombine inside the battery to form water, so it's all good.

We want this feature. The not messing around with battery acid part appeals.

If your battery isn't pooting hydrogen, ventilation is less of an issue. Presumably, it's also emitting oxygen, but that's not talked about much. Maybe because we don't notice a little more? I'm not clear on that part. But, forget the emissions anyway, because a sealed battery isn't gassing off.

We want this, too.

Sealed (maintenance-free) can be either gel or absorbent glass mat (AGM). Most people seem to prefer AGM, although some people say gel is great. AGMs are more expensive.

AGMs can be starting batteries (for your car or truck), deep-cycle, or marine.
  1. Starting batteries have the power to crank a big engine, but they don't have staying power. They are the sprinters of the battery world.
  2. Deep-cycle batteries are built to be deeply (up to 80%) discharged before they must be recharged. Less discharge is better, though, as in delicate medical issues. These are the long-distance runners.
  3. Deep-cycle marine batteries are a hybrid of the first two. Some extra cranking muscle, some sustained capability, but they must not be discharged below 50%. These are those guys who sort of run fast but can run longer if they have to. [This entry is a FAILED SPORTS METAPHOR. Help Blogipedia by cleaning it up.]
We choose Door #2, for the sustained power, Monty.

Size matters, but bigger is not better. These high AH batteries pack some serious weight. The takeaway is that one of us (Annie) needs to be able to lift a battery, should the need arise. But, if you go too light, you sacrifice too much capacity.

Here's what that looks like:

battery specs

battery ratingss

We could wire a pair of 6V in series, and then wire that pair in parallel with another pair wired in series. Remember, if you wire two batteries in series, you double their voltage, but amperage stays the same. If you wire two batteries in parallel, the amperage doubles, but the voltage stays the same.

(220AH)(6volts) X 2 in series = 220 AH at 12 volts

(220AH)(12volts) X 2 in parallel = 440AH at 12 volts

If you just tuned out, it just means we need 4-6volt batteries with 220 AH capacity. Since (Amp-Hours)(Volts) = Watt-hours,

(440AH)(12V) = 5280 watt-hours = 5.28 Kwh

This doesn't account for efficiency issues between storage and available power.

But, the battery bank can't fall below 20% capacity, so that means that usage could never exceed 4.2 Kwh between charges. Will we really use that much power? What kind of solar harvesting capacity would we need to supply that much energy? What do the generator ratings mean in this context? Which cereal do I want this morning? Because that sound was my brain restarting in safety mode.

If you actually passed Physics 102, we need a wet clean-up in Aisle 4 by the cerebral cortex.

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