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 are compulsory to be installed for 24V or 48V AGM/Gel battery banks.
AGM/Gel VRLA batteries do not want to be discharged completely. Ensure that the cut off voltages is set correctly for off-grid systems or back-to-utility voltages for hybrid installations.
AGM/Gel batteries do not have a built-in BMS. Hence, the actual state of charge of the battery can not be accurately be determined with voltage only. To ensure decent use-ability from AGM/Gel batteries without a SoC controller, the load on the batteries needs to be less than the following:

100AH: ~ 300W @ 12V, ~600W @ 24V, ~1200W @ 48V
150AH: ~ 420W @ 12V, ~840W @ 24V, ~1680W @ 48V
200AH: ~ 540W @ 12V,~1080W @ 24V,~2160W @ 48V

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 Gel batteries 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:

4x 150AH Gel batteries 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:

4x 200AH Gel batteries 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:

1x100AH LiPO4 for 48V Inverter utilized 80% DoD
Time to 80% 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%).