Backup Electric Power Design Considerations- Part 3, by Duliskov


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.

You can simultaneously charge batteries and draw current from them. The appliance will be drawing current directly from the charger, and 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.

I had my NOCO chargers plugged in while individually charging six disconnected Trojan AGM 27 batteries from 50% state of charge to full. I noted that it took eight hours to fully charge the batteries, during which time a total of 4 kWhatt/hour energy was deposited into the batteries. The peak power need was around 560 watts. Considering that I was running a 2kW inverter generator for eight hours, not just two hours, some fuel was wasted, despite the fact that this generator has the ability to adapt to various loads.

There are good online resources where you can look up battery manufacturers.

There are two ways to test a battery’s state of health properly. Let’s call these two methods the old-fashioned DC way and the new AC way. The old way is to load your battery with a resistive load; think of a heater, for example. It is recommended to load the battery to half of its maximum “cold cranking power” for 10-15 seconds; most personal vehicles use something in the range of 400-800 Amps “cold cranking power” or CCA, so you need to load the battery with 200-400 Amps of load to simulate a realistic drain on the battery for 10-15 seconds and measure the voltage drop in the process. 400 Amps by 12 Volts is almost 5,000 Watts that will need to be dissipated by the instrument between measurements. In battery shops, you could typically come by something like a 100 Amp load tester, which is not enough. To properly test a larger battery, you will need a 500 Amp load tester. If the voltage drop is more than what is presumed healthy, corrected for temperature of the battery, then it is time to get a new one. The proper test procedure involves fully charging the battery, waiting at least six hours, applying load, measuring 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. Alternatively you can also apply any known resistive load (per standard 25 Amps) and simply measure the time it takes to discharge the battery from full charge (give it six hours of rest period after charging it up) to 10.5 Volts (assuming lead acid battery), that will give you the actual reserve capacity at a specific load. A repeat timed discharge measurement down the road will reveal weakening performance; just make sure to use the same load/Amps again.

The newer and more accurate method of assessing your battery state is using of so-called conductance analyzer, which also works on partially discharged batteries (above 60% state of charge), and generates negligible amount of heat, because it uses less than 1 Amp of current. Technically it has only been tested on sealed valve regulated lead acid batteries. Practically, it appears, that low frequency (80-100Hz) conductance correlates directly with battery capacity as measured in a timed discharge test. This correlation is nearly linear allowing for state of health, a.k.a. timed-discharge capacity or reserve capacity, estimation. Keep in mind that increasing the temperature of an electrolyte always increases the conductivity between 1.5 and 5.0% per degree Celsius. To compensate for temperature changes, conductivity analyzers have integral temperature sensors that allow the analyzer to correct the raw conductivity measurement. Let the analyzer sit next to the battery a bit to reach the ambient temperature before measuring or you could get an incorrect assessment result. Measuring of conductance at multiple current frequencies (20Hz to 2kHz) allows for even more accurate assessment. Technically you need a reference value to compare the actual battery to for each temperature, which is either available in a database somewhere or not. Without a set reference value, conductance testing can still be utilized to trend state of health, but it is important to test each time the same way and at the same battery temperature.

Test your batteries periodically, at least once a year to ensure you don’t have deteriorating ones in your bank. If your bank is approaching end of life – knowing each battery’s performance allows you to regroup them into smaller banks to extend the life a bit. If one of the batteries in your bank is dead and they are connected in a string and you charge them together, then the dead battery will draw all the charge current due to its high internal resistance and your bank will charge very slowly. The solution to the above problem is to disconnect batteries before charging (doable, if you have a manual system and circuit breakers on each of them) and charge with a multiple-port charger or use a battery isolator. This can get expansive with large banks quickly. A good charger can efficiently charge multiple batteries simultaneously and simplifies installation. Charging many batteries with an cheap charger (low output current and only one or two ports) will require using the generator for longer periods of time.

Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.05–2.10 Volt/cell. (This equals to about 80-90% state of charge at room temperatures.) A lower charge state would cause sulfation. With AGM, these requirements can be somewhat relaxed.

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. Fully charged battery will not freeze in the harshest of winter; however, it will seemingly “lose” part of its capacity, and the colder it is 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 degrees Celsius, for example, a fully charged battery may measure 12 Volt instead of 12.7 Volt. Don’t overcharge them. If your charger supports an external temperature sensor, it makes sense to install those near the batteries to prevent overcharging, which is very damaging to batteries.

There is some good online information for deep cycle batteries on various websites. (Scroll down to the white papers.)

DC to AC

So how can the energy stored in batteries and available in DC form power tools that require AC? It’s done via inverters. The cheaper version of inverter is generating an 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 electronic 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 radiofrequencies that will interfere with wireless phones, cell phones, Ham radios, satellite communication, WiFi routers, and terrestrial TV signal.

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

Internal inverters may have totally isolated inputs and outputs or they may have one of the leads connected “through” and/or to common ground. The latter can present a problem with some inductive loads, for example isolation transformers, because the DC voltage offset may saturate the windings of the transformer, resulting in full power load on the transformer without anything plugged in 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 oxygen concentrator, it is best to charge the battery and power it from battery or 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 an entire house.

Inverters usually generate one phase AC. There are expensive models that can generate split phase having 240 Volts outputs, just like a typical gasoline or propane generator. Also, there are inverters in the few thousand range that can generate two-three phase AC current, too. However running a dryer or a powerful motor requiring 240V or multi-phase also requires a compatible battery bank, which would not be in the 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, but most cannot. The voltage at which inverters shut down are between 10.5-11Volts, which essentially corresponds to a totally depleted battery bank, which is no good; see 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, and there is also a battery protector, which is another option.

Check 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.

Many inverters are equipped with ground fault protected outlets (GFCI – circuit interrupters). Those are handy if you happen to touch a hot wire, as they will shut the circuit open in less than 30 milliseconds and may 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 clams to fixate your electrical cables or plastic clams for lighter wires, and use cushioned clams to protect your wires.

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