Power outages are no stranger to Atlanta residents. Spring and summer bring lightning storms, strong winds, and sometimes tornadoes. Winter brings ice storms and in the autumn, we’re close enough to both the Atlantic Ocean and the Gulf of Mexico to feel the impacts from the tail of hurricane season. Even on cloudless, sunny days an errant driver can knock out a transformer and then there are the mysterious outages that last just a few minutes for no discernible reason. So, a battery backup seemed prudent to power my medical equipment and keep all my addictive gadgets charged. If you are an RV traveler or a camper with a vehicle at hand, then you might also find the following setup useful.
Inverters, which turn DC current into AC current, are both dangerous and horribly inefficient devices. The conversion process can easily waste 40% of a battery’s energy. It’s much more efficient to use a device’s DC adapter when one is available. Also, inverters can damage fine electronics because the “affordable” units output power that has a stepped or squared waveform. Simple devices like power tools, water boilers, fans, etc. can handle that rough output. But sophisticated electronics like TVs and laptops require a smooth waveform. The inverters that can produce the required sine wave output cost a lot more. If you need an inverter between your battery and device, make sure that it’s providing the correct type of power and is adequately fused.
No Car Batteries!
Car batteries are designed to provide a surge of power, all at once, and up-front to get the vehicle started. Once the vehicle is running on gasoline, the engine produces electricity through its alternator and recharges the car battery. These lead-acid batteries are not designed to handle a continuous or long-term power drain. So, you’ll want a Deep Cycle battery instead. Deep Cycle batteries are designed for an extended electrical pull and are what are used in golf carts, scooters, RVs, and boats and are often called “marine” batteries. Buy an AGM (absorbent glass mat) battery because they are sealed and can be used indoors. This prevents corrosive acid and awful fumes that could come from an unsealed battery.
Although lithium ion batteries can deliver power in a smooth sine wave and over an extended period, they cannot match the capacity or voltages of an AGM battery at the same cost point. Lithium battery capacities are typically rated in mAh (milliampere hour), which is one-thousandth smaller than the Ah (ampere hour) used to rate AGM battery capacities. A lithium battery with a 100 Ah capacity battery is insanely expensive compared to an AGM version of the same capacity.
What is Ah?
The unit of Ah (ampere hour) is the number of amps that can be delivered in an hour. So a 100 Ah battery, in theory, delivers 1 amp of electricity for 100 hours, or 2 amps for 50 hours, or 4 amps for 25 hours, etc. These are theoretical maximums because a battery’s age will reduce its capacity. Also, batteries do not discharge at the same rate for the entire duration. And environmental factors such as cold can reduce capacity. Nor would you use a battery's full capacity (i.e., discharge it completely) since that severely damages its ability to recharge. An AGM battery should not be discharged below 50% or its longevity will be curtailed. For my scenario, my medical device draws less than 10Ah per day, so a 100Ah AGM battery provides several days of service without going below the 50% threshold.
Wow! That's Heavy!
Also, the size and weight of a 100Ah is at the upper limits of portability. The model I purchased is a hefty 64 pounds and fits in a standard “group 27” battery case with barely space to spare. The case is important because of the danger of shorting out the battery terminals. There's enough energy in these batteries that a short can melt metal or start a fire. Also, the case I purchased has two convenient 12V plugs for connecting my devices, two insulated posts for permanently connecting the charger, a battery meter, and circuit breakers. And speaking of chargers, I selected one that is specifically safe for AGM batteries. I can leave the battery connected to the charger so it is always ready for use. The charger I purchased automatically cuts power to prevent overcharging while also preventing the battery charge from dropping too low. The external terminals also allow me to connect multiple batteries in parallel (to increase capacity) and/or in series (for higher voltages) should I ever need those. Or they can be used to attach solar panels to recharge the battery using sunlight.
Connecting multiple batteries
These diagrams are for clarity. In an actual implementation, I would ensure the cable lengths were equal length so that resistance was consistent. And the power connections would be arranged so that each battery had an even distribution for charging and for usage. That way no one battery is impacted significantly more than the others and thus prevent premature death of a battery. See this article on balancing connections.