Batteries

In this knowledge article we will focus on the battery basics required to help you make an informed decision on which battery is right for you.

Battery types can be broken into two major subsets:

  1. Starting Batteries 
  2. Solar Batteries

Start batteries can endure much larger instantaneous current draw which is required during the starting process. These will not be covered in depth here.

Solar batteries are deep cycle batteries which are designed to last longer and deliver current for a longer period.

In this knowledge article we will mainly focus on lead acid deep cycle solar batteries. Basic topics covered will be:

  1. Deep cycle battery types
  2. Selecting your battery
  3. Sizing your battery
  4. Preserving battery life

1. Deep Cycle Battery Types

Deep cycle lead acid batteries can be broken into two major formats

  • Flooded lead acid
  • Sealed lead acid

Flooded lead acid batteries  date back to the 19th century and are the most common type in the market. Flooded batteries use an electrolyte as the conduction medium which requires monitoring and top up when necessary.

Sealed lead acid batteries are maintenance free and include gel type and absorbent glass mat (AGM) .

2. Selecting your Battery

First of all you need to determine your system voltage(Vsystem). In most marine cases this will be 12 or 24 volts. If more storage is required or if you are installing a larger system with an inverter then you may want to consider a higher voltage system. Selecting your system voltage is covered more indepth in the advanced battery section.

 

3. Sizing Your Battery

Sizing your battery is dependent on multiple factors such as the solar array size and the charge controller size. The main factor is how many appliances you want to run. First you need to calculate your total power consumption. This can be done using Ohms law. First multiply the appliance voltage (Vapp ) and the current consumption (Iapp) to obtain the power rating, or just read the label on the appliance for power consumption in Watts. Work out approximately how many hours you would like to run this for each day and multiple the Watts and hours. This will give you an answer in Watt hours(Wh). Do this for every appliance and add all the numbers to total the power consumption per day. For example if you run a 20 watt fridge for 5 hours you will require 100 Wh of storage.

If you were to discharge your battery 100 percent of its capacity the calculation for the battery size in Amp hours (Ah) would be Ah =Total power consumption per day  divided by Vsystem. However discharging the battery to 100 percent will have very detrimental effects on its life span. The relationship between depth of discharge(DoD) and life cycles of the battery will be covered more in depth in the advanced battery section. For simplicity we will specify the DoD to be 30 percent. That means we will aim to never allow the battery to get below 70 percent of its full capacity. Our calculation now becomes Ah = Total power consumption per day /(DoD) times Vsystem. If we were to use the simple example above with our 100 watt hours per day consumed by our fridge, the calculations would be as follows. 

100 percent discharge( Included for understanding purposes. Never calculate battery size using this method)

Ah = Total power consumption/Vsystem

= 100 W / 12 V = 5 Ah

30 percent discharge

Ah = (Total power consumption / (DoD x Vsystem) 

= 100 W / (0.3 x 12 V) = 28 Ah

4. Preserving battery life

Battery life span can be increased by limiting things that are detrimental to the battery. The main factors are ambient temperature, depth of discharge, the solar array size and charging.

The higher the temperature, the lower the number of cycles you will achieve with your battery. It is best to keep your battery in a well-ventilated cool environment if possible. Increasing the temperature will slightly increase the  short term capacity of your battery. This is discussed in the advanced section.

As previously discussed, depth of discharge will have a very negative effect on the number of cycles achievable. This information can be found on most battery datasheets. One example battery lists 1600 cycles at 30 percent discharge and just 200 cycles at 100 percent discharge. Leaving a battery in a discharged state can even further reduce its life span. If a battery is left discharged for a long period it may not ever be possible to recharge it again.

The solar panel or solar array should be sized so that it easily replenishes the battery everyday. This will reduce the depth of discharge experienced by the battery ensuring maximum lifetime. See our solar panel knowledge article for selecting the correct panel.

Battery charging is very important. Choosing the right sized charger for your battery will increase its lifespan. If a battery is charged too slowly in comparison to your consumption needs it may suffer depth of discharge issues. If it is charged too quickly it will increase in temperature and reduce the life span. It is also important to choose a charger designed for your battery type (flooded, gel or AGM ) as they all have different charging characteristics.

If you have any questions or feedback on this knowledge article please feel free to contact us at info@luxsolar.co.nz .