Backup Electric Power Design Considerations, by Duliskov – Part 2


Energy Storage

The only practical way to store electric energy is in chemical form, in batteries. There is loss of energy while it is being deposited into batteries, converted into chemical energy, and then also while the battery sits in storage (self-discharge).

The output will be DC current. Batteries are built from units of two volts each. Six of these units make a 12-Volt battery, et cetera. Physically larger batteries are more susceptible to damage from rough handling (drops, vibration) because they use larger plates. Some batteries are of sealed type and require no maintenance; some are of open type and will evaporate water while being charged, requiring a periodic refill with distilled water (manual or automatic). The open type also emit hydrogen gas, which may accumulate in the battery compartment and explode with a spark; therefore, adequate ventilation is needed, and because hydrogen is lighter than air, the battery compartment should be ventilated at the top rather than at the bottom. These open batteries can also spill acid more easily.

Due to low voltages, when supplying power to pumps, microwave ovens, welding equipment, and other large loads, the current flowing through the cables is very high, easily in hundreds of Amperes. This requires cables with very little resistance; these are beefy expensive ones. There is some good info on cables here: Generally, anything below 1.0AWG will be inadequate to power a 2kW or larger inverter. I suggest using welding cables, because they are pure copper with less resistance, instead of alloys; they’re built of hundreds of thin wires, which makes them very flexible, relatively speaking of course, and easy to work with. Also, their outer shell is multi-layered and much more durable. Thick cables require a capable crimping tool. If by any chance your connectors will be exposed to salt water, only use tinned copper; copper “rusts” in salt water quickly, while the tin coating protects the connectors and wire. Alternatively, you can cover the surface of your connections with dielectric silicone grease, but using the tinned copper will make re-arranging your batteries a little messy. You can buy premade connecting cables, but I suggest developing the skill and acquiring the equipment to do this yourself.

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, away from utilities, using battery power alone or are able to run either from internal batteries and/or supplementing utility power with internal battery power, which 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 a 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. The 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 10 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 (lithium?) may have lower self-discharge, but they haven’t yet proved themselves in power backup systems.

When connecting multiple batteries for higher capacity or higher output voltage, wire them such that there is 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. Some are better for more equal charging. Always put a DC breaker, using one at minimum, 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 is just about equal the breaker in front of the inverter. You can use automatic breakers that you can reset after they are tripped, or use an ANL wafer fuse. None of these types will trip when you accidentally touch positive to negative and see sparks flying; 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. Get a battery cart with six 12V batteries at the bottom, a shelf with a 2000W inverter that can be fully retracted to allow for easy access to your batteries, and two chargers mounted internally on the back. The top compartment will be used to add six more batteries in the future.

Here is a good page to look up battery manufacturers, and here You can simultaneously charge batteries and draw current from them. The appliance will be drawing current directly from the charger; whatever is left, the difference between the charging current and consumption of appliance, will be deposited in the battery. If the appliance uses more current than the charger can supply, then the battery may supplement the difference, depending on your system setup.

There is only one way to test the battery properly– with a significant load and a voltmeter. All other methods only estimate the condition of the battery. Fully charge the battery, wait at least six hours, apply load, and then measure the voltage as you apply the load. I suggest you record the video of the voltmeter as you may miss the reading in the 10-15 seconds that the test runs. Test your batteries periodically– at least once a year– to ensure you don’t have deteriorating ones in your bank.

If one of the batteries in your bank is dead and they are connected and you charge them together, the dead battery will draw all the charge current and cause your bank to charge very slowly. The solution to the above problem is to disconnect batteries before charging them. (This is doable if you have a manual system and circuit breakers on each of them.) Alternatively, you can use a battery isolator. This can get expensive with large banks quickly. A good charger can analyze and optimally charge multiple batteries simultaneously and simplifies installation. Charging many batteries with a poor quality charger (low output current and only one or two ports) will require using a generator for longer periods of time.

Partially discharged batteries can freeze in winter cold. I don’t know if this will actually damage them or not, but I am assuming it is not beneficial. A fully charged battery will not freeze in the harshest winter weather; however, it will seemingly “lose” part of its capacity, and the colder, the weaker it will be. Do not keep your battery bank in an outside, unheated box, if you live in the north. In cold weather, the voltage will also drop; at 0 Celsius, for example, a fully charged battery may measure 12 Volt instead of 12.7 Volt, so don’t overcharge them. If your charger supports external temperature sensors, it makes sense to install those near the batteries, to prevent overcharging, which is very damaging to batteries.

For a good source of information on deep cycle batteries, scroll down to the white papers. Another source of information for charging cycles.

DC to AC

So how can the energy stored in batteries and available in DC form power tools requiring AC? The answer is “via inverters”. The cheaper version of an inverter is generating alternating current that has significantly different waveform from utility power. This may be sufficient to run resistive type appliances and lights, but motors will run less efficiently and heat up quicker and electronics and computers may or may not run at all. If uninterruptable power systems (UPS) is used to protect sensitive electronics from brownouts or voltage fluctuations, they may not like this type of “dirty” input and will switch to internal batteries, depleting them despite availability of AC power. These cheaper inverters may also generate radio frequencies that will interfere with wireless phones, cell phones, Ham radios, satellite communication, WiFi routers, and terrestrial TV signal.

The more expensive type, typically three to five times more expensive, of pure sine wave inverters generate AC that is as good as utility power and will not cause any of the problems discussed above.

Internally, inverters may have totally isolated inputs and outputs, or they may have one of the leads connected “through” to common ground. The later can present a problem with some inductive loads, for example, with isolation transformers, because the DC voltage offset may saturate the windings of the transformer, resulting in full power load on the transformer, if there is not anything plugged into it. The transformer may burn out rather quickly, not to mention it will consume maximum power constantly. So, if you need to use an isolation transformer for a medical appliance, like an oxygen concentrator, it is best to charge the battery and then power it from battery, or you should be sure to use a fully-isolated inverter.

An inverter that has common ground and “through” connection between input and output is not suitable for feeding into a transfer switch to distribute the power to the entire house.

Inverters usually generate one phase AC. There are expensive models that can generate split phase by having 240 Volts outputs, just like a typical gasoline or propane generator. Also, there are inverters in the few thousand dollar range that can generate a two-three phase AC current, too. However, to operate a dryer or a powerful motor that runs on 240V or multi-phase also requires a compatible battery bank, which would not be in price range of an average person.

To measure DC current flowing through a wire, you will need a clamp meter, and to measure an AC current without splitting the power cord you will also need a line splitter.

Inverters typically monitor the charge condition of the battery and shut themselves down when the voltage drops significantly. Some inverters can be configured by the end user to shut at a specific voltage threshold; most can not. The voltage at which inverters shut down are between 10.5-11Volts, which essentially corresponds to a totally depleted battery bank; this is no good for reasons explained above. A simple voltmeter will allow constant monitoring of the battery status. There are automated tools that can do that for you at a more useful 11.7V threshold. Here is another option.

Check to see if the inverter fans are triggered by the load or internal temperature. If they are triggered at a certain load, they will kick in, make noise, and consume your precious energy even when the inverter is ice cold, which of course is not ideal.

Many inverters are equipped with ground fault protected outlets (GFCI– circuit interrupters). These are handy if you happen to touch a hot wire; they will shut the circuit open in less than 30 milliseconds, which might save your life. However, they can also keep randomly tripping, if there are other GFCI devices on the same circuit or you have a very small leakage into the ground somewhere. A tester comes in handy if you want to be sure that your ground fault protection works. Use cushioned clamps to fixate your electrical cables or plastic clams for lighter wires. Use cushioned clams to protect your wires.

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