Continuation...
**Steps**
5. Choose system components that meet your
current needs and make the best use of each
other.
The cheaper parts, such as the batteries,
should have enough capacity to make full use of the more-expensive parts, such as the solar cells. If you're buying an all-in-one device, check that the compromises its components reflect are reasonable: any missing capabilities or reduced durability should be acceptable and compensated for with a significantly lower price and/or (if convenience is important) weight.
**Avoid vendor lock-in.
Make sure major system components follow design and compatibility standards not limited by any particular manufacturer. For instance, they should work at voltages and with connection types many charge controllers and appliances accept.
6. Choose solar panels.
There are several types of solar cells that vary in efficiency per unit area and tendency to decay over time. But, unless you're buying so many that having enough space for them is a concern, it's generally more important that the assembled panels be durable (since they will be out in the rain), have a good price per watt (rate of electricity production).
**Check that the solar panel on an all-in-one
device has sufficient capacity. Because solar
panels are expensive manufacturers may skimp on them. A single day's charging should suffice for all you might want to do with the device in a day, such as run a light for four hours and also, if the device has a phone charger attached, charge the phone once.
**Choose an appropriate battery. Their capacity is measured in watt-hours; for instance, a 100watt-hour battery can accept 5 watts for 20 hours to charge it (or a little more, due to losses) and then release 25 watts for four hours for use. Large systems often use multiple batteries in parallel. There are many different kinds, with markedly different
characteristics -- unlike with solar panels, this isn't just a matter of power per unit of size or money. The battery should be at least big enough to hold a full day's charge from the solar panels; more, if you want to be able to miss a sunny day and still use the system or device fully. Batteries should not be charged too fast; check that the batteries over which
the energy from your solar panels will be spread can handle their maximum charging rate.
**Lead-acid batteries are inexpensive for their
capacity and durable but not suitable for
portable devices because they are filled with
dangerous acid. Some need to be "topped off"
with water from time to time.
**Don't use ordinary car ("starting") batteries.
Deep-cycle batteries, some of which are used
for trolling motors and RVs, are the kind of lead-acid battery to use for solar power, since they are built for repeated substantial discharge rather than brief large engine-starting bursts only.
**Sealed lead-acid "gel cell" batteries are sealed, with the acid inside contained within material to reduce hazards even if they do break. They are inexpensive for their capacity, though not quite so inexpensive as regular lead-acid batteries. They are also relatively heavy compared to non-lead-acid batteries.
**Lead-acid batteries can be damaged by
excessive discharging, overcharging, or leaving in the discharged state. The charge controller should prevent overcharging and over-the-counter discharging; portable devices should be recharged promptly after use.
Most batteries should be recharged promptly after any use. Nickel-cadmium batteries, which are obsolete for most uses, are the kind that was important to discharge before recharging.
Nickel-metal-hydride (NiMH) batteries are
sealed, durable, and relatively safe generally.
They are more expensive and lighter than lead-
acid per unit of energy stored, so they are good
for portable devices. They are available in
standard "AAA", "AA", "C" and "D" sizes, so, if
you have a solar power system, you don't need
portable solar-powered gadgets too, just a much-cheaper charger and batteries.
Most NiMH batteries run down on their own within a month even if not used; "low-self-discharge" ones take a year or more to.
Check that standard-size rechargeable batteries bigger than AA, such as C and D size, have commensurately larger capacity ratings of several thousand mAH or are cheap and sufficient for your needs. Some are AA cells in larger cases.
Lithium-ion batteries are very light and
expensive for their capacity, and mainly suited
to expensive portable devices like laptops.
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