|Table of Contents:
volts = watts / amps
Convert all the loads you want to run into amps. If it's a 12-voltdc device, it's already rated in amps. The charging source should have the amps ratings on the back of it as well . AC devices are usually expressed in watts,but the conversion doesnt take into account that motorized loads will draw 2-3 times more during startup which means inverters will need to be sized for the surge . (A 500-watt blender ,full,will surge to 1000-1500 watts momentarily,a inverter capable of 1500 watt surges and appropriate fuse would be needed) Divide the watts by 12 volts, since that's the voltage the inverter runs on (500 / 12 = about 42 amps), and then multiply that by 1.15 to factor in inverter inefficiency (42 * 1.15 = 48.3). Now multiply the loads you want to run by the number of hours you want to run them (let's say a half hour every day: 48.3 * .5 = 24.15), which gives you the amp hours you need. Your battery should be rated at least twice the size of your needs (so we need a 50 amp hour battery, minimum). Finally, figure on a means of charging the battery that will supply that many amp hours back into the system every day.
Another example: we want to run a radio that draws 4 amps at 12 volts, and we plan on using it 3 hours every day, and we want to run 2 lights that draw 2 amps each for 4 hours each night. We have
4 amps * 3 hours = 12 amp hours, and (2 amps * 4 hours) * 2 = 16 amp hours. -- Total 28 ==
We need a battery with a capacity of at least 58 amp hours. We must replace 28 amps every day, and a solar day will be around 4-6 hours of charging time, so we need a solar panel that will put out 28 amps in 6 hours, which is about 4 and a half amps per hour. The wattage required is the output voltage of the solar panel (usually 17.1 volts) times our 4.5 amps, which is roughly 80 watts (round up to be safe). Solar panels are price- compared by looking at dollars per watt, and small panels sell for anywhere from $7 to $16 per watt, and larger 75 to 100 watt panels average $5.50 per watt. Estimate then that an 80 watt panel will cost $5.50 per watt or $440. If you are saying ouch, then don't despair, because you have two options:
1. You can leave out some of the loads and live with a smaller system, or
2. If mobile or boat,you can use your own alternator to supplement some of the charging.
I would not recommend cheap AC generators as an option because they are time bombs, noisy, and for charging DC will only supply a few amps of DC output unless coupled to a large AC charger.
There are a few different types of panels to choose from, so choose carefully
as some are a better value. There are mono- and poly-crystallized panels
composed of between 30 and 36 cells (round or square black discs mounted
behind the glass). There are thin film(Triple junction)amorphous panels where
the light- sensitive material is deposited on non- tempered glass or sometimes
stainless steel in the case of flexible panels. Thin film is 30% less efficient
than crystalline panels, and they degrade rapidly. They have the advantage
of being producible in smaller sizes than crystalline panels and are common
in cheap solar recharging products. For true portability, nothing can beat
tedlar coated crystalline panels that are on aluminum plate with the cells
mounted to the aluminum and covered with shatter proof plastic. In terms
of power output, an important concern is voltage. The panel's peak output
voltage is determined by the number of cells connected in series. To charge
a 12-volt battery to 14.7 charge volts, one needs to have a panel that puts
out at least 16.9 volts peak output. That number multiplied by the maximum
current gives the wattage of the panel. Fewer cells = lower output voltage
and works only in cooler climates. As cell temperature increases, the panel
voltage drops; any fewer than 36 cells wont' get the voltage high enough
to do a full charge. Stay away from self- regulating panels, as their fewer
cells limit their voltage; when they heat up, they're too low to charge to
the max. Be sure to always use a separate regulator to automatically prevent
the opposite problem; overcharging. The addition of an ammeter coming off
the panel will show amps going to the battery, and a voltmeter will show
the battery charging voltage, hopefully as high as 14.5 to 14.8 volts. An
hour or two at these voltages is considered a full charge.
Your car or boats alternator supplies 35-100 amps and typically tops off
the starting battery in the first ten minutes of running. After that it could
easily supply power to a second deep cycle auxiliary battery. The easiest
way to do that is to come off the positive of the starting battery and go
through a solenoid or electrically controlled gate that passes power on to
a second battery only when the engine's running, thereby preventing drain
down of the starting battery when the engine is off. They are simple systems
to install, work well, and a solar panel is easy to add for those times when
you don't anticipate running the engine. I suggest such a system unless you
dont plan on using the engine at all, in which case you are looking
at what's known as a "stand alone system" or a solar- only charging
system.Warning: Some alternators are too wimpy for heavy charging . Check
their temperature by touching the case,too hot to keep a hand on is a sign
The best small regulators are pulse width modulated or PWM controlled. Look
for that feature no matter what size regulator youre using. Sizes are
based on the maximum amps the regulator can handle; allow for future expansion
when deciding on a size. If you had a 60-watt panel that put out 3.5 amps,
you'd be looking at least for a 6-amp regulator to be on the safe side. Be
sure to add a fuse between the panel and the regulator, if the unit doesn't
have one. The better regulators come with temperature compensation and ideally
with some means to equalize the batteries.
The least expensive decent meter is the handheld multimeter. After that,
a panel mounted digital will be the least costly. Do not buy or use the dial
type or analog meters, as they have terrible accuracy. Look at the battery
capacity chart voltage figures below and imagine trying to get 0.1 volt accuracy
with an analog meter -- impossible!
An ammeter is useful to measure current output from the solar panel. Put
it on the battery side of the regulator. The simple analog types are fine.
They come in 0-5, 0-10, 0-20, 0-30, and 0-60 amps. Pick one that's just above
your maximum output. Fuses,Wiring & Lights:Fuses can be purchased at
electronics, auto parts, and marine stores, at RV shops, and of course mail
order. Class T fuses are generally used between inverters and batteries,
if the inverter is 500 watts or greater. While fuses are easy to find, good
holders are harder to come by.. Match fuses to loads by fusing to 25% to
50% more than the maximum load. Use flexible wire that is sized for the current
and the length of run. This is very important. If there is any doubt, use
oversize wire. Keep wire runs short to limit voltage drops. Most alternative
energy catalogs have wire sizing charts in the back. The most efficient and
expensive lights are white LEDs. Next in efficiency are compact fluorescent
lights. They come in 12-volt DC, but the AC ones are the most common and
least expensive. Next are halogens which put out a very white light and make
great spotlights. Standard incandescents are the least efficient and have
a short lifespan. Reflectors can be used to add to any light's
Fuses can be purchased at electronics, auto parts, and marine stores, at RV shops, and of course mail order. Class F fuses are generally used between inverters and batteries, if the inverter is 500 watts or greater. A type R hardware store fuse could be used. If you drill 5/16- inch holes in the blades coming off its ends, you can bolt the fuse directly to the battery. Otherwise a holder is necessary. While fuses are easy to find, good holders are harder to come by. Mail order may be the best bet. Match fuses to loads by fusing to 25% to 50% more than the maximum load.
Use flexible wire that is sized for the current and the length of run. This is very important. If there is any doubt, use oversize wire. Keep wire runs short to limit voltage drops. Most alternative energy catalogs have wire sizing charts in the back.
The most efficient and expensive lights are white LEDs. A good compromise
is compact fluorescent lights. They come in 12-volt DC, but the AC ones are
the most common and least expensive. Next are halogens which put out a very
white light and make great spotlights. Standard incandescents are the least
efficient and have a short lifespan. Reflectors can add to the light's
Inverters are the devices that take the battery power (DC) and convert it
to 120 volts AC or household power, which is what most tools and appliances
run on. Inversion is done one of two ways: 1) transformers, 2) high speed
switching circuits. The quality of the output is defined as: Modified square
wave and Sine wave, which is what grid power runs in. The modified square
wave inverters are less expensive and more efficient, but the sine wave inverters
have a cleaner output which allows one to run certain delicate electronics
and play audio equipment without the hum associated with modified square
wave inverters. For running compact fluorescent lights, tools, laptop computers,
and most appliances, the modifieds are fine. The cheap small inverters are
all modified square wave, and use switches to make the power. More expensive
but far superior for running motorized loads are the transformer type inverters.
These start at 500 watts and go up to 5,000-watt sizes and come with optional
battery chargers built in. The Trace UX series are real work horses in the
500W and 1100W sizes. Stat Power builds good small cigarette lighter plug-in
switcher type inverters. "You get what you pay for," and it applies to all
the equipment in an alternative energy system, especially inverters. Make
sure to put the appropriate fuse between the battery and inverter.
Batteries are the oldest equipment in the system, and even with many newer
battery technologies the plain old flooded lead-acid deep cycle battery is
hard to beat for weight, cost, availability, and endurance. Buy quality if
you want longevity. The purer the lead used, the better the battery. The
best are Rolls & Surrette, with Trojan and Deka building good mid-range
batteries, followed by U.S. Battery, with Exide being cheapest and least
durable. What makes a deep cycle battery vs. a car starting battery? The
number and thickness of lead plates and the amount of space below the plates.
A car battery needs lots of up front power to turn the motor over and only
gets run down 10%.It will not last through repeated deep discharge. A deep
cycle battery is rated in amp hours at 80% of its capacity and has fewer
and thicker plates. For longevity, only cycle them down 50%. If the
battery is rated 80 amp hours, to play it safe only withdraw 50% of that
or 40 amp hours. So with this battery for example, a 4 amp load should be
shut off after 10 hours of use (4 amps x 10 hours = 40 amp hours, the maximum
safe discharge). There is an easy way to determine remaining capacity; with
an amp hour meter that counts the credits and debits to your battery, and
spits out a percent at the push of a button. The second cheapest alternative
is to use a voltmeter to read the voltage when the battery is resting (see
chart below); no charge/discharge occurring for at least an hour. For example,
100% = 12.7V, 75% = 12.4V, 50% = 12.2V, 25% = 12.0V, and discharged = 11.90V.
To use these figures, one must have an accurate digital voltmeter. Throw
away any dial type (analog) voltmeters, as they are worthless as battery
monitors. A cheap digital meter costs $30 to $60. Are your batteries worth
it? If not, then at least spend $10 on a cheap hydrometer and put up with
the mess of dipping it into all those holes and measuring the specific gravity
of the cells. Specific gravity readings will read more or less like this:
100% = 1.265, 75% = 1.225, 50% = 1.190, 25% = 1.155, discharged = 1.120.
Till now, I've only talked of flooded batteries. Of course, there are ni-cads,
nickle- metal- hydrides, gelled electrolyte lead acids, and A.G.M. or absorbed
glass mat lead acid batteries. If you are only looking for 10 to 50 amp hour
batteries to run a tape player or some small load, then these might be good
possibilities, but be forewarned they are all more expensive technologies
and more complicated to maintain correctly. I've seen enough gel batteries
hit the trash to know they are problematic. They absolutely will not tolerate
being charged beyond 14.2V. The subject of batteries is far too wide for
this primer, so I'm only focusing on flooded batteries. Monitoring can be
a slippery deal, almost like checking the air pressure in your tires without
coming to a stop. A few hints: we've already discussed reading a battery
at rest, but what if it's being charged while you're reading it? The voltage
will be driven higher as the battery is being charged and around 14.5 volts
depending on the temperature, the acid will start to bubble. A little bubbling
is good, but a strong bubbling is burning off the water, and most regulators
cut back the charging to prevent this. Typically most charge controllers
(or regulators as they are also called) will sense battery or ambient temperature
and compensate -- colder / more voltage and warmer / less voltage. So here's
your key: when you see the battery voltage peak out in the upper 14s then
you have more or less charged the battery fully and can consider the battery
at 100%. Below that is real "seat of the pants" gauging. On the other side
of the coin, if the battery is being discharged as you monitor it then the
numbers will read lower. There's no sliding scale to apply "seat of the pants,"
so try to stick with reading resting voltage and hopefully see that your
batteries are getting up to 14.5 - 14.8 every day or two. Battery size: The
most common sizes are group 24/80 amp hours and group 27/105 amp hours. I
have an 80 amp hour battery as an auxiliary in my truck that supplies my
800 watt inverter. If I'm running large loads then I start the engine and
let the alternator pour some current in to keep the volts high to give the
inverter more grunt. My alternator charges both the auxiliary and starting
batteries simultaneously while I drive. When I camp for days at a time, I
usually bring along a solar panel to supplement the charging. I use to have
the bigger 105 amp hour battery, but felt that I couldn't justify the extra
weight for the occasional camping trip so went smaller. The battery must
match the inverter to some extent; the larger the inverter and load on it,
the larger the battery needed. For larger systems, I'd recommend 6-volt golf
cart batteries or L16s hooked up in series. Even a 600 watt inverter running
at maximum output will draw the voltage down quickly on a group 27 battery.
When the inverter sees lower voltages, its output drops in proportion. Even
if you had a 1000 watt inverter and it was coupled to a small battery, you
would only see 1000 watts for a few minutes. So don't scrimp on capacity!
|Percentage of Charge||12V Battery Voltage||24V Battery Voltage||Specific Gravity|
Specific gravity values can vary + or - 0.015 points of the specified values.
This table is for the Trojan L-15 battery in a static condition, no charging
or discharging occurring, at 77 degrees F. Discharging or charging will vary
these voltages substantially.
Source: Trojan Battery Company
The following chart gives the maximum distance one-way in feet of various gauge two- conductor copper wire from power source to load for 2% voltage drop in a 12V system. Do not exceed the 2% drop for wire between PV modules and batteries. A 4 to 5 percent loss is acceptable between batteries and lighting circuits in most cases. To allow for a 4% loss, double the lengths given in the chart.
Our company, Land & Sea Solar, can provide
most of the components you'll need for a complete system at the fairest prices.
Feel free to call for prices: Land & Sea Solar, 831-335-4518, email@example.com
Copyright © 2000 Carl Reuter. All rights reserved.