Choosing a battery: how to pick the best battery for you

scanning: time:2021-09-28 classify:Technology info

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When comparing quotes for different solar battery systems, it can be difficult to determine which characteristics and technical specifications matter most and with good reason: the home energy storage industry is so new that you probably don’t know anybody with a battery who you can ask about their experience. While every battery has to meet certain reliability and safety requirements in order to be sold and installed in the US, outside of those standards there is very little standardization of specs and characteristics across the batteries available on the market today. We’ve provided some tips on what to look for when comparing different battery quotes.

What to look for in a home solar battery: six measurements to consider

A solar battery stores electricity for later use, so you can keep appliances running during a power outage, use more of the solar energy you produce at your home, and even save money on electricity in some cases. They are often referred to as "deep cycle batteries", due to their ability to charge and discharge a significant amount of electricity compared to something like a car battery.

Energy storage systems provide a number of different benefits, from emergency backup power to even financial savings. But they also bring technical complexity and a new set of unfamiliar terminology. Here’s what to focus on and look for in a deep cycle solar battery: 

How to decide which battery specs matter for your needs

There are a number of different potential decision criteria and comparison points to make when evaluating your energy storage options. Here are a few of the most common decision criteria, as well as which battery specs matter most if these criteria match your situation:  

  • If you want to power more of your home at once, look for a solar battery with a high power rating

  • If you want to be able to power a more energy-intensive appliance (like a sump pump), look for a battery with a high instantaneous power rating

  • If you want to run your home with your solar battery for a longer amount of time, look for a battery with a higher usable capacity

  • If you want to get the most out of every kilowatt-hour of electricity you put into your battery, look for batteries with a higher roundtrip efficiency 

  • If you are space constrained and want to get the most amount of storage out of the least amount of space, look for lithium-ion nickel manganese cobalt (NMC) solar batteries

  • If you want a battery with the longest lifetime that you can cycle the most amount of times, look for lithium iron phosphate (LFP) batteries

  • If you want a battery with the absolute highest safety rating possible (don’t worry, they’re all safe!), look to LFP solar batteries

The power rating of a battery refers to the kilowatts (kW) of power that the battery can provide at once. In other words, 
a battery’s power rating tells you both how many appliances your battery can power at once and which appliances those are

Power rating

Power is expressed either in kilowatts (thousands of Watts) or in Amps, and different appliances use different amounts of power. For instance, a typical compact fluorescent lightbulb will use 12 Watts (or 0.012 kW) of power, while a 3-ton AC unit will draw 20 Amps, which is equivalent to 4.8 kW. Most of the batteries available on the market today have a continuous power output of around 5 kW. 

Importantly, solar batteries often have two different power ratings–a continuous power rating and a 5-minute or instantaneous power rating–meaning they can provide more power in short bursts. This is important if you have an appliance like a sump pump that requires a large amount of power to turn on, but then runs at lower power.

A battery’s capacity (or size) is the amount of electricity that a battery is able to store and supply to your home. While power is expressed in kW, battery size is expressed in kilowatt-hours (kWh), which is power multiplied by time. As a result, 
a battery’s storage capacity tells you how long your battery can power parts of your home. Be sure to look for the usable capacity of a battery, as that number represents the amount of stored electricity that you can actually access in a battery.

Battery size/usable storage capacity

Since electricity usage is power multiplied by time, if you are using more power, then you’ll run out of stored electricity faster. Conversely, if you’re only using your battery to backup a few appliances with relatively small power consumption, you can keep them running for a longer amount of time. This makes the size of a battery slightly misleading, because the length of time a battery's charge will last is directly influenced by how much power it's outputting.

Think about the example above of the difference between a light bulb and an AC unit. If you have a 5 kW, 10 kWh battery, you can only run your AC unit for two hours (4.8 kW * 2 hours = 9.6 kWh). However, that same battery would be able to keep 20 lightbulbs on for 2 full days (0.012 kW * 20 lightbulbs * 42 hours = 10 kWh).

How many solar batteries do you need to power your home?

Depending on what you want to use your energy storage system for and the characteristics of your household appliances, the number of batteries you'll need can vary widely. There are several questions to answer: how long you'll want to stay battery powered, what kind of production your solar panels get, what appliances you absolutely need to keep on, and the list goes on and on.

While we can't say for certain how many batteries you'll need, we've outlined the steps you can take to start doing that math in our article about how much of your house you can power on batteries. You can also check out our article on going off-grid with solar and batteries, where we do some example math on what it would take to keep your home completely solar and battery powered.

Roundtrip efficiency is a system-level metric that measures how well your energy storage system (battery + inverter) converts and stores electricity. There are losses associated with any electrical process, meaning you’ll lose some kWh of electricity when you invert it from direct current (DC) electricity to alternating current (AC) electricity, or when you put electricity into a battery and take it out again. 
A solar battery’s roundtrip efficiency tells you how many units of electricity you’ll get out of a battery for every unit of electricity you put into it.  Roundtrip efficiency

Battery lifetime: throughput and cycles

Battery lifetimes are measured with three different metrics: expected years of operation, expected throughput and expected cycles. A battery’s expected throughput and cycles are like a car’s mileage warranty. Throughput lets you compare how much electricity you’ll be able to move through your battery over its lifetime. Cycles measure how many times you can charge and discharge a battery. 

To convert a battery’s expected or warranted throughput into an expected lifespan, divide the throughput (expressed in kWh) by the usable capacity of the battery to estimate how many full cycles you’ll get from your battery, and divide that number of full cycles by the number of days in the year: a 20,000 kWh throughput warranty on a 10 kWh battery means 2,000 expected cycles, or a cycle per day for 5.5 years. 

To convert a battery’s expected or warranted number of cycles into an expected lifespan, divide the number of cycles by the number of days per year: a 4,000 cycle warranty equates to a cycle per day for 11 years.

All solar batteries have to meet certain safety requirements in order to be certified for installation in homes and businesses: every battery that you receive a quote for on EnergySage is safe and meets these safety requirements! There are, however, some battery chemistries that have been 
tested for safety to different levels, going even beyond the government-mandated safety requirements for batteries, meaning some battery chemistries are slightly safer than others. But the most important thing to remember is that all batteries installed in the US are very safe!Safety

A battery’s chemistry refers to the primary compound that’s used to store electricity inside the battery.
 Chemistry may be the most important characteristic to compare since it ultimately determines a lot of the characteristics of batteries listed above. For instance, different  lithium ion chemistries may be more power dense–meaning they store more electricity in a smaller amount of space–or may do a better job of cycling–meaning they are able to perform at a higher level for more years. And those are just the differences within lithium-ion chemistries, to say nothing of the differences between lithium ion batteries and lead acid batteries, or vanadium flow batteries, or other experimental battery chemistries. As is the case with most things, different solar battery chemistries come at (often significantly) different price points.Chemistry

What is the best solar battery?

It's hard to say. The best battery for you will depend on a number of factors, from the size of your home to the characteristics of your solar installation, and all the way to what you even want to get from an energy storage system. If you have a large home with lots of appliances, you'll want to look for a high-capacity battery that can keep pumping out electricity for hours on end. If you're price-conscious and care more about optimizing your solar energy system, a smaller battery with great battery integration may be the best choice.

Curious how some of the top brands stack up against one another? Check out our article about the best solar batteries from popular brands.

In conclusion: there's a lot to consider

These are just a few of the most common decision criteria for installing a battery. If you’re making a decision based on other criteria, let us know! Feel free to reach out to our Energy Advisor team, who are happy to answer any questions you may have about the differences between the batteries available on the market today.

Find the right solar-plus-storage system on EnergySage

EnergySage is the nation's online marketplace for solar and storage: when you sign up for a free account, we connect you with companies in your area, who compete for your business with custom solar-plus-storage quotes tailored to fit your needs. Over 10 million people come to EnergySage each year to learn about, shop for and invest in solar and home batteries. Sign up today to see how much you can save.

I use solar battery charging every day for lead-acid car batteries, deep-cycle batteries and lithium iron phosphate batteries .

Running costs are zero once the solar panel kit is paid for and every little bit of renewable energy energy used in the world helps our environment.

One battery takes very little power to charge but multiply that by 2 billion! That’s how many cars are estimated to exist on the planet.

It’s a good idea to understand some of the basics about solar charging before connecting a solar panel directly to a battery.

Let’s assume that most people are interested in charging the average 12 volt car battery – in this post I’ll answer some basic questions relating to charging batteries with solar power.

Can I charge a battery directly from a solar panel?

A solar panel can be connected directly to a 12 volt car battery, but must be monitored if it’s more than 5 watts. Solar panels rated higher than 5 watts must not be connected directly to a battery, but only through a solar charge controller to protect against over-charging.

In my experience theory never quite stands up to actual real-live tests so I’ll hook up a solar panel directly to a partially discharged deep-cycle lead-acid, comparing the voltage and current with a solar charge controller. Jump to test results.

Before that, I’ll go through some theory – it’s good to learn, it clears things up!

Solar Battery Charging Questions:

  • Why is battery voltage important?

  • What is the battery Depth of Discharge (DoD)?

  • What is the open circuit voltage of a solar panel (Voc)?

  • How do you find the maximum current of a solar panel?

  • What does solar panel Maximum Power Point (MPP) really mean?

  • Can you ever connect a solar panel directly to a battery?

  • Do I need a solar charge controller?

  • What does Maximum Power Point Tracking (MPPT) and Pulse Width Modulation (PWM) mean? Which is better and does it matter?

  • How to choose a solar charge controller

  • How do you connect a solar charge controller to a battery?

I think of the term ‘nominal‘ as almost the same as ‘designed for‘. When you buy an appliance rated for 12 volts, this is an indication that it’s designed to operate at 12 volts and will draw a certain amount of power.

The same appliance could operate on 11 volts or 13 volts, which is quite in order if the volts doesn’t deviate more than 10 to 12%.

If the voltage is too low, then the speed and power will suffer. If it’s too high, the appliance pulls too much power and could overheat.

Car batteries also have a nominal voltage, which is 12v. Like an appliance, it’s terminal voltage can be below or above 12 volts.

In fact the terminal voltage of a 12 volt lead-acid car battery is a rough indication of it’s state of discharge, also known as Depth of Discharge. The chart below shows this relationship:

Battery depth of discharge chart (lead acid)

So you can see that the battery is fully charged at 12.73 volts and 90% charged when the voltage reads 12.62 volts.

Once the voltage drops down to 12 and under, a lead acid battery is pretty much discharged and should be charged up as soon as possible to avoid damage.

Note that the voltage reading should only be measured after the battery has not been used for at least 3 hours, so there is no chemical activity within the cells.

Difference between deep cycle and regular battery

Regular, or auto, lead-acid batteries are primarily designed for one thing; to provide a lot of power to crank a cold car engine until it fires. Current draw is hundreds of amps for a short time.

After the engine has started the alternator quickly recharges the battery and maintains it fully charged.

Deep-cycle batteries are designed to deliver light to medium currents over long periods. Most batteries are rated over 20 hours.

The performance of both types goes down significantly if the current draw is high. Neither type will supply it’s rated capacity. Auto batteries should never be discharged more than 15% and deep-cycle batteries between 50% and 80%.

A deep-cycle lead-acid batteries may last 5 times longer if regularly discharged to only 50%. This means that when you buy a 100Ah deep-cycle battery, you’re only getting 50Ah to use!

LiFepo4 vs lead acid – are lithium batteries better than lead-acid?

Lithium iron phosphate batteries are inherently deep-cycle. They can be discharged up to 95% without damage, but the number of charge-discharge cycles reduces, shortening its life.

This is important because you only need to buy a lithium iron phosphate battery with an Ah capacity of 60Ah to benefit from 57Ah of usable capacity.

Also, if you discharged a LiFeP04 to only 80%, the battery would last half a life-time! For many applications, a lithium phosphate battery is a once-a-life-time purchase.

Solar panel voltage and current output

Solar panels are unusual devices with special operating features. The 4 most important operating parameters are:

  • maximum power (Pmax)

  • open circuit voltage (Voc)

  • short circuit current (Isc)

  • maximum power current (Imp)

  • maximum power voltage (Vpm)

What is solar panel Voc?

Open circuit voltage is the value of DC volts measured across solar panel leads. For a typical 36 cell ’12 volt’ panel it’s between 21-22 volts.

What is solar panel Isc?

Short circuit current is measured by connecting the panel leads together through an ammeter. This isn’t dangerous as the circuit voltage falls to zero, so there is no power to cause overheating or damage.

How much current can a solar panel produce?

The label above shows that a current of 5.75 amps flows when the panel voltage is 17.4 volts. If you multiply these together you get the rated power of the panel, which is 100 watts.

These two values are very important and is known as the Maximum Power Point or MPP.

What is the MPP of a solar panel?

The Maximum Power Point is the point at which the voltage and current combine to deliver the maximum power (Pmax) that a solar panel can deliver.

This occurs when the load resistance matches the internal resistance of the solar panel, called the panel’s Characteristic Resistance. A typical value for a 100 watt solar panel may be about 3 ohms.

Can you charge a 12V battery directly from a solar panel?

The internal resistance of a lead-acid battery fluctuates according to its state of discharge, but let’s assume it’s about 1 ohm.

We know that maximum power flows when the load resistance equals the panel’s Characteristic Resistance. In this example the battery may charge, but very inefficiently.

Also, we have no idea what the voltage is across the terminals, which determines how much current is flowing into the battery. It had better not be left alone for long!

How do you connect a solar panel to a battery?

Unless the solar panel is very small,like a trickle charger, a solar charge controller should always be used for battery charging. Modern controllers have settings for all types of batteries, lead-acid and lithium.

What kind of solar charge controller do I need?

Remember I talked about the Maximum Power Point of a solar panel? An MPPT charge controller adjusts its internal resistance to match that of the panel, thereby drawing maximum power from the panel safely.

The chart below shows results of a MPPT vs PWM solar charger tests I carried out on a lead-acid deep-cycle battery – MPPT shoed an increase in power of 22%:

MPPT solar charge controllers are proven to be about 30% more efficient than PWM, so will charge your battery 30% quicker.

PWM controllers are more primitive and cheaper. It depends if you’re looking for maximum efficiency or if you have plenty of time to charge your batteries.

Check out this page on DIYSolarShack to read a detailed comparison – MPPT vs PWM Test.

Battery Charging With Solar Panel Direct Connection Vs Solar Charge Controller – Test Results Compared

Battery Charging - Solar Panel Direct Connection Voltage and Current.

Solar panel direct connection to battery – what happens to the voltage and current?

The proof of the pudding is in the doing; I always like to run tests to verify solar theory and sometimes the theory comes down in flames – let’s see:

  • I measured the terminal voltage of a partially discharged 90Ah deep-cycle lead-acid marine battery to determine its State of Charge.

  • First, I hooked up a 100 watt solar panel directly, and measured the voltage & current.

  • Next, I connected the panel to the battery through an Brava Battery solar charge controller, and measured volts & amps.

The table below compares the battery charging readings in both cases:

Surprisingly, there isn’t a great deal of difference. The battery was 35% (31Ah) discharged and pulled a healthy current as would be expected.

It’s interesting that the MPPT controller is outperforming the raw solar panel output. This is because the controller is adjusting its internal resistance searching for the Maximum Power Point.

The question is: If the charge currents are similar, why bother with a solar charge controller at all?

The answer is to be found at the end of the charge cycle and the battery is fully charged. I’ll leave it on the Brava Battery controller and then connect up the solar panel direct again to check voltage and current.

See you in 11 hours time!

When the battery was fully charged by the MPPT controller the volts reading was 13.5V and current was under 0.5A. The voltage would eventually creep up to 12.73V with hardly any current going into the battery.

I disconnected the MPPT and connected the 100W solar panel directly to the battery. Here’s what I found:

  • Terminal battery voltage = 15.5V

  • Charge current = 2.5A

So even though the battery was fully charged, a directly connected solar panel will still push current into it. If left connected, the battery will overheat and become damaged.

DIY home made battery maintainer – buck-boost converter

Hom made buck-boost battery maintainer

Connect solar panel to battery with a buck-boost converter set to 13.5 volts.

If you just want to maintain a battery in full charge condition, there is a way to connect any high-power solar panel to your batteries without danger of over-charging and damaging it internally.

A buck-boost converter accepts DC voltage input and the user can limit the DC output voltage. This buck-booster model (I’m not an affiliate) takes up to 30 volts, so perfect for a single solar panel.

If you set the output volts to a typical float voltage 13.5V the battery will stay in tip-top condition in complete safety.

Wind the output up to 14 volts and the device will increase the charge current if you need it – but remember to monitor it and don’t leave it connected indefinitely.

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