The Phaserl


Backup Electric Power Design Considerations

by Duliskov, Survival Blog:

Batteries can generate, without damage, several hundred amperes of DC current for short periods of time. In fact, you can arc weld using a battery. There are welders designed to run from battery power alone or able to run either from internal batteries and/or supplementing utility power with internal battery power. Though the Hobart Trek 180 welder, which I recommend, may have been discontinued or currently unavailable, it is useful if you wish to achieve higher amps than is possible via a single 120V household outlet. The higher the battery’s amperage, the easier the battery can start a car engine, but this requires large surface area for chemical reaction to take place. Therefore, these batteries tend to have thinner, less durable plates, leading to faster deterioration of battery over time. Batteries more suitable for power backup are the deep cycle variety, which have more robust architecture and can withstand many hundreds of cycles of deep discharge (below 50% of their full capacity). In the best case, good quality deep-cycle batteries will last about 10 years in a typical daily charge-discharge scenario.

Don’t forget to factor in the cost of replacement of your entire battery bank every ten years. You don’t want to regularly deplete your batteries below 50% of their rated capacity, because that shortens their life significantly (2x-3x times), so the useful total capacity is half of nominal amp/hours of your bank. Plan accordingly. The self-discharge rate even for the best lead-acid batteries is 3-5% weekly. Other battery technologies may have lower self-discharge.

When connecting multiple batteries for higher capacity or higher output voltage, wire them such that there is least or at least an equal number of batteries and length of wire in between the last battery terminal and the inverter input. There are multiple configurations possible, each with their own advantages and disadvantages; some are better for running high loads and some better for more equal charging. Always put a DC breaker, at a minimum of one, before the inverter. Size it so it is just a bit larger in terms of amps than your inverter. If you put a breaker on each battery, make them small enough so that their sum just about equals the breaker in front of the inverter. You can use automatic breakers that you can reset after they are tripped or an ANL wafer fuse. None of these types will trip, when you accidentally touch positive to negative, and cause sparks to fly; they are not that sensitive, but they will abort a short that is longer than a second or two, preventing a meltdown in your cabling.

Batteries are heavy and will eventually need to be moved around. After you have connected them, it will be even more difficult to do so. Invest in a heavy duty cart, and prepare for the hefty shipping cost.

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