Energy
Storage
The
only practical way to store electric energy is in chemical form, in batteries.
There is loss of energy while it is being deposited into batteries, converted
into chemical energy, and then also while the battery sits in storage
(self-discharge).
The
output will be DC current. Batteries are built from units of two volts each.
Six of these units make a 12-Volt battery, et cetera. Physically larger
batteries are more susceptible to damage from rough handling (drops, vibration)
because they use larger plates. Some batteries are of sealed type and require
no maintenance; some are of open type and will evaporate water while being
charged, requiring a periodic refill with distilled water (manual or
automatic). The open type also emit hydrogen gas, which may accumulate in the
battery compartment and explode with a spark; therefore, adequate ventilation
is needed, and because hydrogen is lighter than air, the battery compartment
should be ventilated at the top rather than at the bottom. These open batteries
can also spill acid more easily.
Due
to low voltages, when supplying power to pumps, microwave ovens, welding
equipment, and other large loads, the current flowing through the cables is
very high, easily in hundreds of Amperes. This requires cables with very little
resistance; these are beefy expensive ones. There is some good info on cables
here: http://www.solar-electric.com/wire-cable-information.html. Generally,
anything below 1.0AWG will be inadequate to power a 2kW or larger inverter. I
suggest using welding
cables,
because they are pure copper with less resistance, instead of alloys; they’re
built of hundreds of thin wires, which makes them very flexible, relatively
speaking of course, and easy to work with. Also, their outer shell is
multi-layered and much more durable. Thick cables require a capable crimping
tool.
If by any chance your connectors will be exposed to salt water, only use tinned
copper; copper “rusts” in salt water quickly, while the tin coating protects
the connectors and wire. Alternatively, you can cover the surface of your
connections with dielectric
silicone grease,
but using the tinned copper will make re-arranging your batteries a little
messy. You can buy premade
connecting cables,
but I suggest developing the skill and acquiring the equipment to do this
yourself.
Batteries
can generate, without damage, several hundred amperes of DC current for short
periods of time. In fact, you can arc weld using a battery. There are
welders
designed to run, away from utilities, using battery power alone or are able to
run either from internal batteries and/or supplementing utility power with
internal battery power, which is useful if you wish to achieve higher amps than
is possible via a single 120V household outlet. The higher the battery’s
amperage, the easier the battery can start a car engine, but this requires a
large surface area for chemical reaction to take place; therefore, these
batteries tend to have thinner, less durable plates, leading to faster
deterioration of battery over time. The batteries more suitable for power
backup are the deep
cycle variety,
which have more robust architecture and can withstand many hundreds of cycles
of deep discharge (below 50% of their full capacity). In the best case, good,
quality, deep-cycle batteries will last about 10 years in a typical, daily
charge-discharge scenario. Don’t forget to factor in the cost of replacement of
your entire battery bank every 10 years. You don’t want to regularly deplete
your batteries below 50% of their rated capacity, because that shortens their
life significantly– 2x-3x times– so the useful total capacity is half of
nominal amp/hours of your bank. Plan accordingly. The self-discharge rate, even
for the best lead-acid batteries, is 3-5% weekly. Other battery technologies
(lithium?) may have lower self-discharge, but they haven’t yet proved
themselves in power backup systems.
When
connecting multiple batteries for higher capacity or higher output voltage,
wire them such that there is an equal number of batteries and length of wire in
between the last battery terminal and the inverter input. There are multiple
configurations possible, each with their own advantages and disadvantages. Some
are better for running high loads. Some are better for more equal charging.
Always put a DC
breaker,
using one at minimum, before the inverter. Size it so it is just a bit larger,
in terms of amps, than your inverter. If you put a breaker on each battery,
make them small enough so that their sum is just about equal the breaker in
front of the inverter. You can use automatic breakers that you can reset after
they are tripped, or use an ANL wafer fuse. None of these types will trip when
you accidentally touch positive to negative and see sparks flying; they are not
that sensitive, but they will abort a short that is longer than a second or
two, preventing a meltdown in your cabling.
Batteries
are heavy and will eventually need to be moved around. After you have connected
them, it will be even more difficult to do so. Invest in a heavy
duty cart,
and prepare for the hefty shipping cost. Get a battery cart with six 12V
batteries at the bottom, a shelf with a 2000W inverter that can be fully
retracted to allow for easy access to your batteries, and two chargers mounted
internally on the back. The top compartment will be used to add six more
batteries in the future.
Here
is a good page to look up battery
manufacturers,
and here You can
simultaneously charge batteries and draw current from them. The appliance will
be drawing current directly from the charger; whatever is left, the difference
between the charging current and consumption of appliance, will be deposited in
the battery. If the appliance uses more current than the charger can supply,
then the battery may supplement the difference, depending on your system setup.
There
is only one way to test the battery properly– with a significant
load
and a voltmeter. All other methods only estimate the condition of the battery.
Fully charge the battery, wait at least six hours, apply load, and then measure
the voltage as you apply the load. I suggest you record the video of the
voltmeter as you may miss the reading in the 10-15 seconds that the test runs.
Test your batteries periodically– at least once a year– to ensure you don’t
have deteriorating ones in your bank.
If
one of the batteries in your bank is dead and they are connected and you charge
them together, the dead battery will draw all the charge current and cause your
bank to charge very slowly. The solution to the above problem is to disconnect
batteries before charging them. (This is doable if you have a manual system and
circuit breakers on each of them.) Alternatively, you can use a battery
isolator.
This can get expensive with large banks quickly. A good
charger
can analyze and optimally charge multiple batteries simultaneously and
simplifies installation. Charging many batteries with a poor quality charger
(low output current and only one or two ports) will require using a generator
for longer periods of time.
Partially
discharged batteries can freeze in winter cold. I don’t know if this will
actually damage them or not, but I am assuming it is not beneficial. A fully
charged battery will not freeze in the harshest winter weather; however, it
will seemingly “lose” part of its capacity, and the colder, the weaker it will
be. Do not keep your battery bank in an outside, unheated box, if you live in
the north. In cold weather, the voltage will also drop; at 0 Celsius, for
example, a fully charged battery may measure 12 Volt instead of 12.7 Volt, so
don’t overcharge them. If your charger supports external temperature sensors,
it makes sense to install those near the batteries, to prevent overcharging,
which is very damaging to batteries.
For
a good source of information on deep cycle batteries, scroll down to the white
papers.
Another source of information for charging
cycles.
DC
to AC
So
how can the energy stored in batteries and available in DC form power tools
requiring AC? The answer is “via inverters”. The cheaper version of an inverter
is generating alternating current that has significantly different waveform
from utility power. This may be sufficient to run resistive type appliances and
lights, but motors will run less efficiently and heat up quicker and
electronics and computers may or may not run at all. If uninterruptable power
systems (UPS) is used to protect sensitive electronics from brownouts or
voltage fluctuations, they may not like this type of “dirty” input and will
switch to internal batteries, depleting them despite availability of AC power.
These cheaper inverters may also generate radio frequencies that will interfere
with wireless phones, cell phones, Ham radios, satellite communication, WiFi
routers, and terrestrial TV signal.
The
more expensive type, typically three to five times more expensive, of pure sine
wave inverters generate AC that is as good as utility power and will not cause
any of the problems discussed above.
Internally,
inverters may have
totally isolated inputs and outputs, or they may have one of the leads
connected “through” to common ground. The later can
present a problem with some inductive loads, for example, with isolation
transformers, because the DC voltage offset may saturate the windings of the
transformer, resulting in full power load on the transformer, if there is not
anything plugged into it. The transformer may burn out rather quickly, not to
mention it will consume maximum power constantly. So, if you need to use an
isolation transformer for a medical appliance, like an oxygen concentrator, it
is best to charge the battery and then power it from battery, or you should be
sure to use a fully-isolated inverter.
An
inverter that has common ground and “through” connection between input and
output is not suitable for feeding into a transfer switch to distribute the
power to the entire house.
Inverters
usually generate one phase AC. There are expensive models that can generate
split phase by having 240
Volts outputs,
just like a typical gasoline or propane generator. Also, there are inverters in
the few thousand dollar range that can generate a two-three phase AC current,
too. However, to operate a dryer or a powerful motor that runs on 240V or
multi-phase also requires a compatible battery bank, which would not be in
price range of an average person.
To
measure DC current flowing through a wire, you will need a clamp
meter,
and to measure an AC current without splitting the power cord you will also
need a line
splitter.
Inverters
typically monitor the charge condition of the battery and shut themselves down
when the voltage drops significantly. Some inverters can be configured by the
end user to shut at a specific voltage threshold; most can not. The voltage at
which inverters shut down are between 10.5-11Volts, which essentially
corresponds to a totally depleted battery bank; this is no good for reasons
explained above. A simple voltmeter will allow
constant monitoring of the battery status. There are automated
tools
that can do that for you at a more useful 11.7V threshold. Here is another
option.
Check
to see if the inverter fans are triggered by the load or internal temperature.
If they are triggered at a certain load, they will kick in, make noise, and
consume your precious energy even when the inverter is ice cold, which of
course is not ideal.
Many
inverters are equipped with ground fault protected outlets (GFCI– circuit
interrupters). These are handy if you happen to touch a hot wire; they will
shut the circuit open in less than 30 milliseconds, which might save your life.
However, they can also keep randomly tripping, if there are other GFCI devices on
the same circuit or you have a very small leakage into the ground somewhere. A tester comes in handy
if you want to be sure that your ground fault protection works. Use cushioned
clamps
to fixate your electrical cables or plastic clams for lighter wires. Use
cushioned clams to protect your wires.
From
the Survival Blog
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