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Sunday, October 1, 2017

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

Part 2 of 2

Power draw from the battery

If all 4 amps is being used on the Amazon inverter, how many amp hours are actually being drawn from your battery? 4 amps / 0.8 (efficiency rate), or about 5 amps per hour. Worst case scenario for an inverter that provides 4 amps: If your battery holds 20 amp hours, it will last for 20 amp hours / 5 amps per hour, or 4 hours if there is no power being added to the battery. If your battery holds 80 amp hours, it will last for 80 / 5 amps per hour, or 16 hours of continuous use with no power being added to the battery.

Examples of batteries

Below is a variety of batteries. I don't know about their quality, but let's compare the cost per amp hour of capacity. Do not use "cold cranking amps" for capacity, find the amp hours rating.

- Optima Marine battery, $200 for 55 amp hours. Cost per amp hour: $3.64usd
- 12v battery, $100 for 35 amp hours: $2.86usd

Charging batteries

Different battery chemistries need different chargers. A 12v lead acid battery needs a different charger than a lithium battery. Lithium batteries can be put in series. So if you put two 3.7vdc lithium batteries in series, the output voltage is 2 * 3.7, or 7.4 vdc.

Charging rate

The charging rate is how many amps is used to charge a battery. It is measured in "C", or percent of capacity. If the battery capacity is 1000mAh, then one charging rate is 500mA, or 0.5C, which is half the capacity of the battery.

Charging a battery with a "fast charger" decreases the time to charge that battery, but also decreases the number of charges the battery will take. This decreases the life of your batteries. I didn't know this and my NiMh batteries were only lasting for 15 charges, because I was using a fast charger!

Charging a battery with lower amps (current) takes longer to charge, but the battery will also be able to be recharged many more times. The recharge times for a battery is probably based on a very low C value.

Ideally, for maximum rechargability, a 1500mAh AA battery should be charged at about 0.25C, or 1500 * .25 = 375mA. On a similar note, a 12vdc battery with 35Ah, can be charge with 3.3 amps, which is 0.09C, or 9% of capacity. Trickle chargers generally have a very low C charge rate.

Sizing solar panel

Now what solar panel do we need to output enough power to charge the battery at least 1 amp per hour?

Solar panels are rated on ideal laboratory conditions. A 1 amp solar panel will only get 80% of that on the sunniest day of the year with the panel in the perfect angle to the sun. The perfect angle means the sun's rays hit the panel at 90 degrees. So your panel should form an angle with the ground equal to your latitude. If you live directly on the equator at 0 degrees latitude, you will lay your PV panel flat on the ground!

Now what about worst case scenario? On average, through all seasons, your PV panel will output 0.4 of the amp rating. So a 1 amp panel will output 0.4amps, and charge the battery up 0.4amps per hour. For a 35Ah battery fully depleted, and not used at all while charging, it will take 35 / 0.4, or 87.5 hours to charge. In my area, the winters are very cloudy for 3 straight months, so I would size the panel at 20% of it's rating in the winter, which is my worst case scenario. So for my worst case in the winter, I need a panel which generates 1 amp / 0.2, 5 amps per hour. Under this condition a 35aH battery takes 35 / 5, or 7 hours to charge in the winter. That's not bad since I only use my TV about 3 hours per day on work days.

Here is a 5 amp panel for $60usd. Even on a cloudy winter day this should produce 1 amp per hour to charge my 35mah battery in 7 hours.


So what do we need to power my TV?
  1. Battery: 35 amp hour capacity, because I might want to power my Roku from this battery also. TV requires (.412 / .80) = .515, round up to 0.6 amps per hour. This will run the TV for (35 / 06) or 58 hours, but that's only for the TV and does not include powering my Roku from the battery. $90
  2. Solar panel which generates 12vdc: it must be rated 5 amps to charge up at least 1 amp per hour in the winter. This one will work because it generates 5.8 amps. $180
  3. Charge controller: $60-80. This needs to handle 12vdc and 5amps. (I didn't talk about these.)
  4. Inverter: We want to add some expansion room to I will choose a 110vac inverter that provides 6amps of power. $60
So this gives you an idea of how to size your inverter, and your battery. Please post any questions in the comments.

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