A Solar Cell Backup Power System - 3

If you're just coming across this series, we've had some truly great comments to the other parts, so make sure to read it all as you catch up. 

Battery Basics 

Unless you're using the power output from your solar panel in real time, a really inefficient thing to do, you'll need to store the output from the panel in a storage system of some kind.  The most common way is with a bank of batteries.  A bank of batteries can be a big portion of the total cost of your solar power system. Without proper care, it will have to be replaced too soon resulting in even more expense.  It's worthwhile to research this topic in depth. 

Keeping your world going after a grid-down event is an energy storage problem.  Energy can be stored in a few mechanical forms, such as running a flywheel, or pumping water up to higher levels so that it can fall and turn a generator.  Energy can be stored in gasoline, propane, alcohol or diesel fuel, but batteries are at the heart of every solar power system I've seen. 

At the moment, the most cost effective form of storage is a lead acid battery, of the type called a Deep Cycle battery.  Briefly, all lead acid batteries use the same chemistry, but a hundred years of engineering has optimized them for different applications and it's best not to substitute types.  A good overview of how they work is here.  All lead acid batteries have lead plates immersed in  sulfuric acid (more shortly) and the chemical combination gives a cell voltage of around 2.25 V when fully charged.  The battery that starts a car requires a large surge of current, but doesn't do much else until the next time you need to start it.  Since the current that can be drawn from a battery is related to the surface area of the lead plates, starting batteries have their plates made in a process that creates a spongy metal - lots of surface area.  They get a vigorous discharge, but it's not usually a large portion of the battery's charge, so that they are dis- and re- charged only 5% or so. That article on batteries at Cambridge provides this diagram:

In an RV or boat with a trolling motor or a solar power system, the load is more of a sustained heavy current drain, so these plates are made more solid.  This is called a deep cycle battery and is what you're looking for.  These can be discharged to very deep discharges, perhaps 80% of their capacity.  The exact amount to discharge in your system is a little controversial, but most authors seem to say while they can be discharged to that sort of level occasionally, shallower discharges extend their lives. 

Because of the common use in boats, deep cycle batteries are sometimes just labeled "Marine", but be careful.  The battery industry has come out with a hybrid of the two in which the plates are not as spongy as a starting battery, but not as solid as a deep cycle battery.  These will run the trolling motor or electronics on your boat better than a pure starting battery, while still giving a good surge for starting, but they shouldn't be discharged as heavily as a true deep cycle battery.  Make sure it says it's a deep cycle battery, not just a marine battery.   Batteries are rated in a variety of ways and starting batteries are rated differently than deep cycle batteries.  The key is to look for amp-hour ratings.  If the battery is rated in CCA (cold cranking amps) or MCA (marine cranking amps), it's being rated for starting service. 

Northern Arizona Wind & Sun has an excellent deep cycle battery FAQ here with the emphasis on solar backup.  A more generalized deep cycle battery FAQ is here.

A fairly recent innovation is the AGM or absorbed glass mat lead acid battery.  These batteries use less acid and don't require attention to the water level - in fact, they're permanently sealed and it's said they don't leak, even if the case breaks.  Rather than a flooded structure, like most batteries, the fiberglass mat is really just wet. 

The other very popular structure is a gell-cell, a permanently-sealed lead acid battery with a gelled acid electrolyte.  The trick here is that most of the batteries you'll find are really designed for float service, such as a UPS, attached to a charger for the vast majority of their life.  They can last up to 20 years in this service, although I've never had an APC UPS last more than a few years.

With the preliminaries out of the way, time to look at some practical aspects.  There are online calculators to help you figure out many of the details.

How big?

The best way to answer this is to really determine the power needs you have.  An anonymous commenter to part 2 recommended the "Kill A Watt" meter to help you determine true loads, and it's great advice.  I've been modelling a system based on providing 1800 W 24/7, so let's run some numbers based on that.  YMMV.  1800 W at 12V is 150A, and I need a battery that will deliver 150 A every hour for at least 12 hours (night).  Because I don't want to discharge this battery past 50%, I want to get a battery with twice this capacity. Beware that when a battery says it's 250 A-Hr, for example, that's at the 20 hour rate, so divide that by 20 to determine how many amps it can deliver per hour for that 20 hours (in this case 12.5 A).  A battery bank with 12 of these 250 AH batteries in parallel would deliver 150A for 20 hours or slightly more than 150A for 10 hours.  Congratulations: that's $5484 - not including shipping for 1800 pounds.  Ouch.  But as a reminder: this is more power than I bought from my power company last month, including running a central air conditioner and cooking on electric appliances without efforts to conserve. 

Voltage: 12 or 24?

For historical reasons, the vast majority of lead acid batteries are sold as 12V, (six cell) batteries.  Older cars use 6V batteries and some are still around, but systems that run on 24V usually just put two 12V batteries in series.  If you put two batteries in series, you double the voltage at the same current; if you put two batteries in parallel, you keep the same voltage and double the current.  In both cases you have twice the power of a single battery (voltage times current), so the higher voltage battery can possibly save you some wire cost, since the current is lower and losses in the wires are proportional to the current squared (twice the current gives four times the power loss in the same piece of wire).  Copper is expensive these days.

A simple example of a series connection is when you drop batteries into a flashlight positive on the second connecting with negative on the first.  You get 3 volts (instead of 1.5) with a pair of alkaline batteries this way.  Parallel is how you use one car's battery to jump a car with a weak battery.  Positive to positive, negative to the chassis ground of the car being jumped. 

Charging

Deep cycle batteries also differ from starting batteries in how they should be charged.  Solar panel output varies with temperature, the amount of sunlight, clouds, and some minor factors.  The optimum chargers are now said to be MPPT (maximum power point tracking).  The MPP varies, so the electronics community is grappling with different algorithms to find the optimum without wasting precious watts.  This plot shows how the maximum power point might move around from minute to minute.

 (source is a long semi-technical article at one of the EE trade magazines)
Some MPPT chargers actually have a microprocessor in them that's more powerful than early PCs just to do these relatively simple calculations.

Inverters 

I'm not exactly sure why switching power supplies that convert DC into AC output are called inverters while switching power supplies that convert DC to DC or AC to DC are just called power supplies, but I suppose as long as we all use the same vocabulary, any word will do.  The efficiency and cost of these supplies have been coming down as the industry refines the approaches.  In the most recent versions, the large transformers are gone, replaced by solid state switches, and electrical noise has been addressed better in the design.  The so-called "true sine wave" inverters are no longer a lot more expensive than the "modified sine wave" and worth the extra cost, should you find out something you depend on doesn't play well with the ugly waveform the modified sine wave inverters supply.

More important is the whole 12 VDC vs 120 VAC question.  It is obviously more efficient to not go through the switcher, but as RegT pointed out, 12V versions of a lot of things are quite a bit more expensive, and you kind of go with what you've got.  The whole comment is excellent - go read. In my case, I intend to get more 12V things, especially those things that might take in house AC and then use an internal supply to drop it down to 12V.   Most of the ham gear I'd want to run is 12V now, anyway. 

This is just a light brush over everything, but gives an idea of the various factors.  A little more practical stuff will follow in the days ahead.