The imformation of solar inverters and battery size

There are more and more problems in solar power system, it is serious because there is real danger of fire when relationship of inverter power, battery capacity, and cable size, and contact resistance are blown out of proportion, especially at the extremely low voltages battery pv inverter  use of 12, 24 and 48 volt batteries.

What is important when operating at low voltages it take high current to obtain high power. High current means large expensive conductors and electrical components. It also means a considerable increase in risk of fire at higher current levels. When you operate at high current levels requires special attention to termination resistance and voltage losses.

Ever wonder why electric companies use extremely high voltages to transport power? It is because of the relationship of power, voltage and current. Power or Watts = Voltage x Current. Example of the extreme is to transport 1 million watts : At 1 million volts takes 1 amp of current or a conductor the size of a toothpick to transport a hundred miles. At 100 volts takes 10,000 amps a conductor the size of your fat asses.

Same logic applies to battery inverters. There are limits on what you the DIY types are completely unaware of. Those limits are battery size, cable/wire sizes, voltage/current relationship, heating, tooling, connectors, and most importantly FIRE. The largest amperage charge controller is 80 Amps! A 80 amp MPPT Charge Controller has the generic Power input limitations vs.

A 12 volt 1000 watt solor pv inverter  requires roughly 90 amps of current at full power. If you push the limit of copper cable requires a minimum of a #6 AWG conductor, in reality a #4 AWG or larger conductor. Well unless you have a $1000 special tool and training to terminate conductors for #4 AWG and larger is out of reach for most DIY. So 6 AWG and 90 amps is a limit you do not want to mess with. Once you get above 100 amps, you had better know what you are doing and have specialized training with a several years of supervised experience. Otherwise you are playing with Fire !

The same relationships extend into batteries of how much current you push into them and take out of them in a given amount of time. Which brings up the question of TIME. Energy and Batteries measurements are expressed in units of Time of Watt Hours and Amp Hours respectively. Watt Hours = Watts x Hours and Amp Hours = Amps x Hours.

Batteries are rated in Amp Hour, usually specified at a 20 hour charge or discharge rate. So if you have a 100 AH battery means it can be discharged for 20 hours at 5 amps until exhausted. Mathematically expressed as C/20 where C = the battery AH rating and 20 is the number of hours. It is important to understand that going forward. 100 AH / 20 hours = 5 amps.

As stated batteries have maximum charge and discharge rates they can withstand. It is related to the batteries Internal Resistance. The maximum rate depends on the battery chemistry type like Lead, Nickel, Iron, or Lithium, and how they are constructed. For Renewable Energy applications we use secondary lead acid chemistry. Lead Acid Chemistry comes in two flavors of Flooded Leas Acid  and Sealed Lead Acid . In those two groups are more sub categories which I will not go into other than to say for RE we want true Deep Cycle Batteries of either FLA preferable because they have longer life cycles, and AGM. Both have advantages and disadvantages over each other. 

As a general rule the maximum charge/discharge current for FLA batteries is C/8, and SLA/AGM is C/4. Some AGM’s can Charge/Discharge at higher rates of C/2 to 1C, but those are exceptions. Now lets tie it all together as it relates to Battery Inverters, cable size, and current.

A FLA battery you can go as high as C/8. Got it? So how much current can a 12 volt 720 AH battery provide? C= 720 AH right? So C/8 = 720 AH / 8 H = 90 amps. Where have we heard 90 amps before? Using Ohm’s Law we know 12 volts x 90 amps = 1080 watts. Just about exactly what a 1000 watt Battery Inverter uses at full power. How many of you have a 1000 watt inverter with a 12 volt 720 AH battery? To clue you in that is about 7 car batteries.