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How to Size a Lithium Battery Bank

Batteries are a great addition to backup power when you don’t have access to power from the grid. And if you ever lose power due to a storm or any other emergency situation, it is crucial that you have enough storage capacity in your battery bank. A battery bank should run all of your essential appliances for at least three days without interruption. This article will discuss four key factors for sizing a battery bank: calculating total amp-hours (Ah), estimating appliance power usage in watts, estimating maximum watt-hours per day

A battery bank is simply a group of batteries that are wired together with the same voltage and capacity rating. In most cases, you wire two or more 12-volt lead-acid batteries together in series, positive terminal to negative terminal to increase the voltage.

Lithium-ion batteries and specifically Lithium Iron Phosphate batteries have become common in Battery Banks as opposed to Lead Acid batteries. This is because they are lighter, don’t sulfate when they sit, have about half the self-discharge rate of lead-acid batteries, and can be discharged deeper before causing damage to them. They also have minimal internal resistance which means they can be recharged quicker than Lead-acid batteries. 

So how can you size a lithium battery bank?

Sizing a lithium battery bank is similar to how you would a typical lead-acid bank. The most important part is knowing your requirements in terms of how much power your appliances require, the maximum draw you’ll need, and the amount of space you have to store the batteries. 

But first, let’s take you through of battery specifications you need to be aware of: 

Voltage Requirements

Voltage in a battery refers to the difference in electric potential between the positive and negative terminals.

Lithium Phosphate batteries are made of cells with a nominal cell voltage of 3.65.  A typical 12V battery for a solar system sits at 14.6V that’s 4 cells while a 6V Golf battery type will be at 7.3V. 

Most solar panels or power inverters use 12 Volts and cannot use less than that (9 volts) and if you wire more batteries in series to double up the voltage at 24 volts, 48 volts, 96 volts. 

Lithium Cells should not be stored at high or too low voltage and are usually stored at around 3.4V.

Amp-Hour Rating

Batteries are rated in Ah (amp-hours). The amp hour rating is a measurement of the current output over time. For example, a 1 amp-hour battery should be able to continuously supply 1 amp for an hour.

Capacity is relative to the size of the Battery, this is usually printed on its side. The Amp-hour rating dictates how long you can use the battery and what types of loads you can place a battery on. Remember the goal is to run the battery over a long period of time.

For example, if you have a 100 amp-hour battery you can draw out 100 amps in an hour but that is not advisable, you can also draw 10 amps over 10 hours or 20 amps over 5 hours. We discuss next on also why it may not be possible to draw a high amount of power from some batteries.

Battery “C” Rating

“C” rating is how many amps your battery can deliver at one time continuously, without the battery overheating or being damaged. For example, if your battery is 100 ah capacity it would be safe to use an inverter that can draw up to 150 amps continuously without the battery overheating or being damaged over time. This is written on the side of most batteries. 

As an example, an 11Ah battery with a 10C discharge rating can deliver a maximum current of

11A (10 x C=10x 1.1 =11A).

Storage Capacity (Power)

Battery Capacity or power is measure in Kilowatt-hours.  It is different from the Amp-hour rating as this gives you the total storage capacity derived from Voltage and amp-hours.

Capacity (Watt-hours) = Voltage x Ah

Cycle Life

This refers to how many times the battery can be charged and discharged before it starts to degrade or loses its capacity to hold a charge. Most of the Lithium-ion batteries are rated at more than 2,000 cycles depending on the brand, whereas most lead-acid are only rated at 400-500 cycles under normal conditions.

The cycle life is also based on how deep you discharge the battery each time. If you use 80% of your battery’s capacity, this is considered a deep discharge and will reduce cycle life. The best way to look at it is if you want your bank to last 10 years, divide the rated lifespan by 10 and that’s how many times you can fully charge and or completely discharge the battery before its life expires.

For example, a 100ah Lithium Ion battery is rated for 2,000 cycles. That means if you want your bank to last 10 years it would need to be charged or discharged 1,000 times.

Wiring 

This can vary depending on the voltage you are trying to achieve. If you’re looking to wire batteries in parallel to get more AH capacity, multiply your Voltage x Amps = Total Capacity of the bank. So if you have a 24 Volt system and need 200 ah of capacity, two 100 ah 12-volt batteries wired in parallel will give you the desired 200 ah capacity.

Wiring batteries in series will give you more voltage, so if you’re looking to increase your system voltage you would wire batteries up this way. If you’re looking for more 12 volt capacity than 24-volt capacity, which is what most appliances run on, just multiply your Voltage x Amps = Total Capacity of the bank and that will be your 12-volt capacity. You would just buy the batteries that are already wired in series to achieve this.

So how Many Batteries will I need?

This is how many amps you need 24 hours a day for the appliances you plan on running during an emergency, storm, or outage. If your refrigerator uses 5 amps and your freezer uses 4 amps and they both run 8 hours a day, add all these up and that’s what you should be looking at to

To calculate how many batteries you will need, simply divide your wattage (watts) by the voltage (volts) and multiply it times how many hours you want them to last. For example, if you have a solar panel that produces 100 watts at 12 volts, that equals 1.2 amps per hour. So if you want to have the batteries last 10 hours it would be 120 amp-Hours or 5 of the 20 amp-Hour batteries needed for this example.

Note: Capacity (ah) is how many amps you can pull from the battery at one time and Voltage (v) is how much energy the battery produces to supply those amps.

You can also find out the total watt-hours for your appliances if you know their power consumption in amps and watts. All it takes is multiplying one by the other, then dividing that amount into how many hours each device will run per day. For example:

A refrigerator uses 300 watts of energy per hour, so to determine its daily average, take 300 watts and multiply by the number of hours it will be running each day.

300w x 24h = a daily usage of  just over 8000 watt-hours (8000 wH) per day

To find out how many amp hours your battery bank needs, divide this amount by 12 volts to get its AH capacity:

How much do Lithium-ion Batteries Weigh?

A typical 100 AH lithium battery bank weighs about 110 pounds, but when you add in the weight of its case (which is usually made out of durable ABS plastic), it can double in size and weigh close to 200 pounds. If this seems like too much for your back to handle, look into smaller 60-80 AH models that will save you a lot of weight.

Lithium-ion batteries are getting lighter every year, but you can expect to see an average figure of about 50 pounds for each 100 AH battery bank. As technology advances and production becomes more efficient, this number is likely to go down even further in future years.

How many batteries do I need for a 3000-watt inverter?

A 3000 watts inverter draws about 33 amps per hour

So in one day, you’ll be drawing 24*33=792 amp-hours (AH) of power.

You will need at least a 1200 AH battery bank to run this inverter for an entire day without recharging or running the generator.

How many batteries are needed for a 5kw solar system?

A typical 120-watt panel produces about 12.57 AH in one hour of direct sunlight, so you need at least 16 panels to produce 100 AH per day (minus what is lost due to shading and angle).

In order to have a full charge after 24 hours of solar production, you will need approximately 2000 AH worth of batteries .

Replacing Lead Acid batteries with Lithium

Generally to get the same runtime as a Lead-Acid battery on Lithium battery you just need 60% capacity of the Lead-Acid battery you are replacing. This is because with lithium-ion batteries a battery can be discharged up to 90% of its capacity but you can only manage 50% with lead-acid batteries. So technically you can replace a 200ah lead-acid battery with a 110 ah Lithium-ion battery.

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