Limitations to Battery Size

1. Battery Bank Size. With solar panels as the primary energy source, it is usually recommended to have a minimum of 5 days battery storage with the battery bank still retaining a minimum of 50% charge after the end of those 5 days. The largest single battery bank available will provide 550 amp-hours over a 100 hour period to be 50% discharged at the end of that period. It is not recommended to increase storage capacity by connecting two or more battery banks side by side (in parallel). By doubling the battery voltage, the current (amps) from the loads is effectively halved, so doubling the voltage has the same effect as doubling the amp-hour storage capacity of the battery bank without having the battery bank connected in parallel.

2. Size of Inverter required to meet expected 240VAC loads. For any particular battery voltage there is a limit as to how large an inverter is available. With higher battery voltages larger inverters are available.

3. Cable size and length to carry DC loads. Doubling the voltage effectively halves the DC loads and halves the voltage drop. Because the battery voltage is doubled the percentage of the voltage drop in relation to the battery voltage is only a quarter of the percentage drop with the lower battery voltage. Unless the cable runs are exceptionally long or the power draw (amps) of the loads is exceptionally high this consideration would not be an issue.

4. Number of solar panels required. Solar regulators are generally limited to 30 amps maximum. With a large 12 volt system you may require twice as much cabling and twice as many regulators as with an equivalent 24 volt system.

5. Maximum Charging Rate. The maximum charging rate for a battery bank is usually 10% of its amp-hour capacity measured at the 10 hour rate. A 600 Ah battery should therefore not be charged at more than 60 amps.

6. Voltage of charging source. If a large wind turbine or large DC generator is incorporated into the system then the system voltage will be dictated by the availability and voltage of these charging sources.

Recommendations
to Overcome Limitations

Some techniques for overcoming some of the aforementioned limitations:

1.1 Batteries may be placed in parallel with a battery isolator between the charging source and the batteries. You would then use one battery bank for some of the loads and the other battery bank for the rest of the loads. You may, for example connect all DC loads to one battery bank and inverter loads to the other.

1.2 Batteries may be placed in series with separate charging sources, regulators and loads. With this technique you can also have the advantage of being able to use both the individual and the combined voltages. You may, for instance, have 12VDC and 24VDC loads and/or use a 24V to 240VAC inverter. You may also have solar panels to charge either or both 12V banks and a 24V wind turbine to charge both banks.

1.3 Less battery storage and more reliance on generator back-up.

2. You may be able to overcome the inverter shortcoming by having several inverters or having inverters that can operate in tandem such as the larger model of InvertaPower.

3. Instead of opting for a higher voltage, an increase in cable size could also have solved the problem.

4. This limitation can be overcome by having several solar arrays separately wired through separate regulators. It must be remembered that maximum charging rate of most battery banks is 10% of their amp-hour capacity (see limitation 6.).

5. This limitation can be partially overcome by adopting the recommendation 1.1 above. If one battery bank is full and the other is not, you would still have to throttle down the charging rate to 10% of the capacity of the one battery bank.

6. See recommendation 1.2 above.