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Saturday, September 30, 2017

How to power things using batteries and alt power, part 1 of 2

I've been looking into using solar power and batteries to power things. My first kit is used to charge a battery using a small panel, about 12" x 8", and the battery powers an LED light. The battery box also has a 5vdc USB port for charging USB things. Or simply for powering a USB device like a USB fan.


Here are some terms to get you started.

  • Amps: how much current a device draws, per hour. A 1 amp device uses 1 amp per hour. 
  • Amp hours: the capacity of a battery. A 2 amp hour (Ah) battery provides 2 amps for 1 hour, or 1 amp for 2 hours, or 0.5 amps for 4 hours.
  • Volts: volts is another measurement of a battery. Devices have a minimum voltage they need to run. A radio powered by 2 AA batteries needs about 3.0 volts to run (2 * 1.5 for each AA). An LED can work on a range of voltages. If the ideal voltage for maximum brightness of an LED is 3.3vdc, then as the battery is used up, the voltage drops, and the LED gets dimmer. The LED might go out at about 2.9vdc.
  • Watts: A general unit of measure which is volts * amps used. A 12vdc device that uses 0.5 amps per hour uses 12 * 0.5, or 6 watts per hour. Electricity in the US is charged by 1000 watts per hour, or 1 kw per hour.

My next step was looking into using a solar and battery system to power my flat screen TV.

Alternative power usually uses an energy producing device to charge up a battery. If you use solar power (photovoltaics) you are using a solar panel to charge up a battery, and the battery stores energy for later use. You won't need lights during the daytime when the sun is out, so the battery provides power for nighttime use.

In the case of wind power or hydro power, in both cases they turn a generator to produce power, which then charges a battery for energy storage.

But how do you size the system to meet your needs? A battery provides DC power but your TV needs AC power. So you will need an inverter to change from DC to AC power. Buying an inverter based on watts it supports is not helpful. You need to look at relevant energy units, volts and amps. While watts = volts * amps, watts is just a very general way of looking at things. The inverter must provide both the volts and amps our TV needs.

My TV is rated at 12vac and 50 watts (labeled "50w" on the back of the TV). This does not tells us the amps that it draws/uses so we must calculate that. 50/120vac = 0.417 amps, or 417 milliamps (called "mA" with a big A). Amps is how much power it uses per hour. "Amp hours" is for battery capacity, and is how many hours a battery will provide 1 amp. A 1 amp hour (Ah) battery provides 1 amp for 1 hour before the voltage gets too low to be useful.

Volts and amps

Sidenote: A "nominal voltage" of a battery is an average voltage that battery can produce. As the amp hours (capacity) of a battery is used up, the voltage the battery can deliver decreases. A 3.7 lithium battery has 3.7 vdc as the nominal/average voltage. But a fully charged 3.7v battery is about 4.2 volts! Most lithium batteries are rated for 3.7vdc nominal voltage. A lithium battery can be damaged as it's discharged, so battery controllers will usually shut off the battery when it's voltage reaches 3.2 volts. So the useful voltage range of the battery is 4.2 to 3.2 vdc.

During this useful voltage range is where the amp hours is determined. So from 4.2vdc to 3.2vdc, a lithium battery might contain 1000mAh (which is 1 amp hour). Below 3.2vdc you risk permanently damaging the lithium battery, which is why it's good to have a Battery Management System, which prevents over discharge. This same concept applies to 12v lead acid batteries but I don't have the exact safe range for them. Some lead acid batteries can be discharged even more, so they are called "deep discharge" batteries.

The above inverter has a digital display, two 12vac outlets with a GFCI, and no visible USB port.


So now we know that my TV uses 120vac at .412amps, or 50 watts. So do we need a 50 watt inverter? No! There are inefficiencies when converting from DC to AC, and the efficiency rate is only about 80% (sometimes higher). We need to over-engineer this just for the TV, and also if we want to power more items at the same time, through this inverter. So .412 amps / .80 = 0.515 amps minimum. Now for "just in case" we should get an inverter that provides 0.8 amps minimum just for the TV.  (Remember, amps is current flowing around, amp hours is battery capacity.)

(An inverter that provides 110vac is also acceptable. Labeling in the US can range from 110-120vac, and both are acceptable.)

The above is a USB car charger, not really an inverter. It converts the car's 12vdc to 5vdc. It does not have to change anything from AC to DC as the power source, the car battery, is already DC.

Here's an inverter on which provides 4amps. It has 2 110vac ports and 4 USB ports (which provide 5vdc and probably 1 amp each, at least).

Part 2 of 2 coming tomorrow.

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