In this discussion, we are going to address the larger, more stable lithium-iron phosphate (LiFePO4) battery with usable outputs as the most ideal solution for temporary off grid power, and save the conversation on deep cycle gel and lead acid batteries for another day.
Why LiFePO4? That’s easy. It’s clean. It’s green. And it’s safe. A LiFePO4 battery is comprised of nontoxic materials, has superior chemical and thermal stability over other lithium ion batteries, enabling it to store larger quantities of energy, and remains cool at room temperature.
Before selecting your off grid portable battery power bank, there are a few things you need to decide upon:
Point 1 – Battery Wattage and Capacity
Round up all the electrical appliances that you intend to use with your off grid system: lights, toaster, coffee makers, microwaves, portable electronics, etc. Check the power supply for each item and write down the number of watts each one will draw. If only volts and amps are listed on your appliance, watts = volts x amps.
Next, write down the number of hours in the day you intend to use the appliance. Note that we are measuring watt hours because electronic devices and household appliances consume power in various voltages.
You will be surprised to discover how power hungry your electric appliances really are.
Let’s do a quick example.
Baseline Battery Power Need: 1137.3 Watt Hours
Ideally, you should aim to choose a battery that will provide at least 25% more watt hours than your baseline, to avoid draining your battery down to zero. In this example, watt hour requirements are:
1137.3Wh x 1.25 = 1421.6Wh
Note that you can opt to stretch available wattage further by using LED lights at significantly lower wattage requirements, and expand the use of your power supply for other appliances.
Point 2 – Inverter Options
Next, before you run out and buy a large portable battery power station, consider the inverter capability. The inverter’s job is to regulate the incoming energy load (varied amounts of solar power) down to stored energy in the battery, and distribute the energy at levels usable for home electronics via built-in AC, USB or other outputs. Commercial choices are pure sine wave or modified sine wave. Pure sine wave delivers a cleaner, smoother electrical current, typically required for larger appliances. Modified sine wave will suffice for smaller electronics.
You want to have a built-in inverter that provides a variety of electrical input/output options to maximize flexibility in how you can both charge the battery and use the power supply.
Typical power charge inputs are:
Typical power outputs are:
Be sure to read the product description of the battery/inverter combination before you buy, and remember that W = A x V. So, if a power outlet’s max amperage is 10A, and can use up to 120V, you would be using a maximum of 1,200 watts with this particular outlet, depending on the appliance being plugged in. If the battery capacity is higher than 1,200 watts, then you have extra wattage available to run other devices, such as low wattage lights, or cell phone charging, simultaneously.
Finally, please keep in mind that amps are how much electricity an appliance is using, so if your small appliance tries to use more amps than the inverter output is providing, the appliance won’t run.
Point 3 – Charge Controller Technology
Solar charge controllers are required for any solar panels that provide output options at or above 12V, and are typically built into portable solar power banks.
There are two types of charge controllers: Pulse width modulation (PWM) and Maximum Power Point Tracking (MPPT).
Portable battery power banks (aka “solar generators”) will have one or the other technology built-in. PWM capability is fairly simply in that it reduces the incoming solar power voltage down to that of the battery to which it is connected, and lowers the amount of power applied as the battery gets closer to a full charge.
Portable solar power stations with built-in MPPT technology are more sophisticated (and expensive). Solar panels with high wattage ratings often deliver far more voltage than needed. MPPT technology takes advantage of extra voltage to effectively reduce the amount of time needed to recharge batteries. Where you have multiple solar panels with wattage over 100 intended to charge one or more large batteries, MPPT technology is your best bet for efficient and timely solar charge.
When shopping for portable solar power stations, if the product description does not indicate MPPT technology, it is highly unlikely to be a feature as it is considerably more expensive to manufacture.
Point 4 – Opt Off Grid: Time to Charge Using Solar Panels
Charging your batteries using solar panels requires substantial solar panel wattage ratings for successful charging in a relatively reasonable amount of time. A 6W panel will do nothing more than trickle charge a 12V battery, if you are clever enough to properly angle that panel toward the sun during peek sun hours. A 100W panel is a good starting point, and additional solar wattage combined with an MPPT charge controller will greatly enhance your charge speed.
Though not a perfect calculation by any means, to roughly estimate time to charge with a set of given solar panels, look at 3 factors:
To determine approximately how many total watts of energy per day that you can get from your panels, multiple your solar irradiance figure (kWh/m2/day) by 75% to account for inefficiencies during the charge process, and then multiply that figure by the total wattage of your solar panel.
Let’s look at one example.
RESULT: 5.3kWh/m2/day x .75 x 180W = 715.5W per day.
APPROXIMATE TIME TO CHARGE: 1500W = 1500W/715.5W = 2.1 days of ideal Miami sunshine.
Of course, if your portable solar power bank allows simultaneous charge and discharge, you can use a bit of power during the day while keeping it charged for power reserve after dark.