Solar Inverter and Controller Battery Voltage

I would instead choose both options, on paper. Go thru each and every required selection, without purchasing, in order to make each a working system for. Along the way identify the financial and other procurement (lead time) costs for all of the components. Next identify the expected longevity, ease of expansion and other possible drawbacks for each approach. Now you can make an informed selection instead of hoping for insightful wisdom from uninformed strangers.

When specifying a battery based solar inverter/charging system, after performing a detailed facility load and system reliability and regulatory compliance and safety analysis, we create a preliminary set of system level power flow block diagrams with a top level BOM.
Some of the major factors are;

- battery chemistry to fulfill the roundtrip charge/discharge efficiency, cycle life, MX, reliability, safety, and total lifecycle cost requirements vs expected environmental conditions

- charge controller type and efficiency

- solar array string configuration and expected daily panel solar insolence and expected daily cell min to max temperature range.

- distance and location from the solar array to battery bank to determine the conductor type and amp capacity and expected I2R/voltage drop losses.

- inverter efficiency @ average base, 3+ hours continuous and peak loads.

An often problem with 12 or 24 Vdc DC/AC inverters is the normal DC ripple current created by the inverter @ its input significantly increases with higher normal AC loads. The low voltage shutdown circuit of the inverter can easily cause the inverter to halt its output and either will attempt automatic restart or will require manual restart once the overload occurs. This can readily occur well within the power delivery specification of the inverter due to equipment design and installation practices. The lower input voltage increases the ripple current at a given AC load. 12v input inverters are more prone to ripple current shutdowns than 24 to 48v+ inverters at similar loads.

The key causal factors are;

- insufficient instantaneous battery power/amps caused by the battery internal impedance, the Peukert constant, cell state of charge, temperature and aging. LiPO4 greatly out performs lead acid batteries at a similar appropriate Ah capacity in this aspect.

- insufficient wire gauge and/or excessive length of DC current conductors, not running battery to inverter conductors together, not twisting them together

A typical rule of thumb that many use with usual success is the Ah (amp hour) capacity vs the inverter peak and continuous KVA output ratings of the inverter. Our rule is to provide 100ah Battery capacity per 1KVA inverter output @ 25deg C cell temp when the expected battery bank reaches 60% for lead acid chemistry and 80% for Lithium Iron Phosphate of remaining capacity towards the respective bank chemistries end of life. This method of specifying battery bank capacity provides excess margin when new. This typically provides enough reserve instantaneous power from new install until the battery is weaker but still provides adequate performance before one might soon consider replacement.

This initial daily Ah load capacity is then multiplied by the lowest bank temp vs capacity constant and then multiplied by the inverter efficiency at the average AC load and then multiplied by the days of desired/required autonomy for days without sufficient solar insolence.

12vdc powered inverters are generally not as efficient as 24vdc or higher powered. The difference can easily be 10% or more less efficient depending on make and model and average load point. Also the higher the step up ratio between the DC input and the AC Rms output, the inverter efficiency is often significantly lower due to higher amperage heat loss of the power transistors and transformer(s) primary winding(s) and core magnetic saturation losses for a given steady AC LOAD wattage. Reactive loads such as motors are a further significant degradation that needs to be addressed in the choice of the battery bank voltage and capacity for the chosen inverter power requirements.

Another major factor is excessive DC ripple at the inverter input terminals caused by the wire gauge or cross-sectional circular area of the battery conductors, their length and routing and the integrity of each terminal and cable connection at circuit breakers/fuses and buss bars. Lack of adequate design and build in this area is the primary root cause of system failures we have found and corrected in many thousands of new and modified systems. 12vdc systems @ 1 KVA output and above are typically 4-8 times or more likely to trip offline vs the 24v or 48v battery inverters.

Our well established guidance as many experienced competent techs and engineers routinely employ is to keep the battery conductors wire circular area as big as possible within the battery and input terminal limitations. If feasible, consider doubling the + and - conductors from the battery to the inverter vice the equivalent larger circular area single conductors. Don’t use building wire for the battery to inverter connections. Use Fine Stranded flexible battery/MTW cable, not welding cable for long term safety of the wire insulation) to meet any required installation’s code and/or environmental requirements with the properly crimped and sealed battery cable terminal lugs.

Try to mildly twist the + and - power conductors together and then routed with the protective earth conductor to significantly lower the undesired inductance of the power input conductors and therefore minimize the DC ripple (AC current superimposed on the DC input also helps lower the undesired but generally unavoidable AC Ripple current in the inverter’s DC BUSS FILTER capacitors. At a given AC load, the Lower DC INPUT voltage inverters typically require much larger input capacitance. These large caps are most often aluminum polarized electrolytic types that do not have long lifespans compared to other components inside the inverter. The higher ripple current and ambient heat cause the aqueous electrolyte to overheat and escape thru pressure vents which then increases the ripple current induced voltages at progressively lower AC output power that typically creates more frequent nuisance trips of the inverter which then typically results in permanent major failure and possible internal equipment fire and/or acrid smoke release before the DC overcurrent circuit protection clears the internal fault. Many if not most such failures often also destroy the primary switching transistors and associated drive circuitry.

- Abrasively clean with 3m Red aluminum oxide pads or equivalent and wipe off all abrasive and metal oxide and grease and moisture residues So that each mating surface is clean/dry bare metal. For plated terminals, go easy on the corrosion resistant typically tin plating.

- to minimize conductor oxidation, soon tighten each connection with a calibrated torque wrench and then coat the outside hardware and terminals with a thin layer of dielectric grease. WARNING: Do Not apply Non conductive grease between the electrical connection mating surfaces as we find in some failed systems. This bad practice can easily cause higher resistance thermal overloads of the wire and equipment terminals. Visually inspect monthly and repeat the corrosion prevention and connection tightening checks and voltage drop measurements at 80% to full load annually as a minimum and more often depending on corrosion, thermally cycling and vibration environments.

For a given power input/output, the lower the voltage the more critical the electrical and thermal conductive connections become. This is a key reason why many good or poorly designed and/or inadequately installed and very often poorly maintained inverters and charge controllers perform poorly upon commissioning and after being in service and fail prematurely internally or externally especially when loaded frequently for medium to long periods at or above 50-60% of rated load.

JT

Whichever voltage you choose to go with, make sure you select deep cycle batteries and not standard batteries. I have installed mobile radio repeaters, mobile and one base station, with solar panels charging deep cycle lead acid batteries either 600AH or 1000AH.

As for the 230VAC output, you need to determine the maximum load and constant load of your system and then work back from there to determine the KW size of inverter and how long you need your backup system to supply power.

Once that is done, you can select the battery AH and voltage to provide you with the required power. Remember that solar is not always available unless you are in a desert, for there will be cloudy days so an alternate means of charging will have to be considered. I found that after three cloudy, rainy days the batteries on the mobile repeaters needed charging by a generator charger combination which determined that the best option was 12V for the situation described.

Choose well and research all available options before investing.