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Battery guide

A variety of battery types can be used with off-grid solar systems. The two most common types are lead acid batteries and lithium ion batteries. 

Which battery should you choose Lithium ion or AGM/Gel?

Lithium ion, LiFePO4 batteries:

Lasts longer (10 year design life, dependent on usage)
Higher energy density (Smaller and lighter than the same capacity Lead acid batteries)
Higher upfront cost than AGM/Gel
Higher chance of fire/explosion (Dependent on the chemistry type)
Notes:
  • Battery management system (BMS) is usually included. DO NOT buy a Lithium ion battery without a battery management system.
  • LiFePO4 batteries should not be discharged completely. It is recommended to keep at least 20% capacity left after a discharge cycle, hence up to 80% of the battery capacity can be used to ensure optimal life cycles.
AGM/Gel VRLA
Lower upfront cost
Does not last as long as the Li-ion batteries (3 years, dependent on usage)
- Lead acid batteries last much longer if they are fully charged. 
Notes:
Battery balancers that balances the load between the different cells are recommended to be installed for large battery banks.
AGM/Gel VRLA batteries do not want to be discharged completely.

 

Tips for buying batteries

  • The weight of a lead acid battery is a good indicator of the battery quality. A heavier battery usually indicates a better quality battery.
  • Beware for manufacturers who overestimate their battery cycle life. Batteries with thousands of cycles have likely not been tested. The cycle rating is usually an estimate. Tests to determine the battery life can take years to complete.

Determining battery storage capacity:

To determine the ideal storage capacity of a battery, the power that the battery can supply must be multiplied by the time that the battery can deliver the power:

Energy (Wh) = Power (W) x Time (h)

We don't have a power rating for the battery, but we can approximate it by multiplying the nominal voltage of the cell with the Ah rating.

Energy (Wh) = Nominal voltage (V) x Battery capacity rating (Ah)

For example: A 12 V, 200 Ah battery will be able to supply 12 V x 200 Ah = 2 400 Wh or 2.4 kWh. That means that one ideal 200Ah battery contains enough energy to power a 2.4 kW element for an hour. If it is a Gel/AGM VRLA deep cycle battery it is recommended that you do not discharge the battery less than 50% of its capacity to ensure long capacity life, so it is recommended that only 1.2 kWh is used.

 

Another import factor to consider is the discharge rate of a battery. The rate of discharge affects the total capacity of the battery. This effect is known as Peukert's Law. To determine a realistic battery capacity, one should consider the discharge rate, the inverter power consumption and DC-AC conversion efficiency.

 

The tables below can give an indication how long a battery bank (4 in series) of AGM/Gel batteries can last. The LiFePO4 battery capacity information is also given below. 
4x 100AH in series for 48V Inverter utilized 50% DoD
Time to 50% DoD 30min 1h 3h 4h 5h 8h 10h 20h
Constant Load on inverter display 1960W 1140W 480W 370W 310W 200W 160W 60W
Values above includes the AGM/Gel battery Peukert effect, inverter power consumption (60W) and DC to AC efficiency (90%).

For values not listed in the table above: 

Watt =hours on 4x 100AH AGM/Gel batteries in series

 

4x 150AH in series for 48V Inverter utilized 50% DoD
Time to 50% DoD 30min 1h 2h 3h 4h 5h 8h 10h 20h
Constant Load on inverter display 2960W 1870W 1110W 800W 630W 520W 340W 270W 120W
Values above includes the AGM/Gel battery Peukert effect, inverter power consumption (60W) and DC to AC efficiency (90%).

For values not listed in the table above: 

Watt =hours on 4x 150AH AGM/Gel batteries in series

 

4x 200AH in series for 48V Inverter utilized 50% DoD
Time to 50% DoD 30min 1h 3h 4h 5h 8h 10h 20h
Constant Load on inverter display 3690W 2020W 910W 720W 570W 370W 340W 160W
Values above includes the AGM/Gel battery Peukert effect, inverter power consumption (60W) and DC to AC efficiency (90%).

For values not listed in the table above: 

Watt =hours on 4x 200AH AGM/Gel batteries in series

 

1x100AH LiPO4 for 48V Inverter utilized 80% DoD
Time to 50% DoD 1h 2h 4h 6h 10h 14h 16h 20h
Constant Load on inverter display 2510W 1270W 630W 410W 230W 150W 130W 90W
Values above includes the LiFePO4 battery Peukert effect, inverter power consumption (60W) and DC to AC efficiency (90%).

For values not listed in the table above: 

Watt =hours on 1x 100AH LiFePO4 battery

 

For use with offgrid inverters:

Offgrid/hybrid type inverters can be set to:

  • limit battery charge rate, 
  • limit battery discharge amount and
  • switch to use the grid as a backup (depending on the inverter).

We do sell inverters that do not require batteries to function.