One of the main reasons people set up a solar backup system in their homes is for use in case of an emergency. In many emergencies, the electricity from the power grid is no longer accessible. A backup will ensure you remain relatively comfortable while being able to run basic loads in your home.
Electricity needs is what we call the load in the set up. An automatic transfer switch will switch your power from the grid to the storage backup in case of a black out. It is important to note that a solar battery backup may not power your entire home.
It should however keep your critical loads running like lights or refrigerator running in case of a blackout. During the day the power bank can power your critical loads in case of emergency and during night time it can power important house loads. In this article we go through various types of batteries and how each will suit you according to your needs and experience with Solar Systems.
The best type by far are the Lithium Deep Cycle Iron Phosphate Batteries. They have a higher density in terms of capacity, require little maintenance and last longer than Lead Acid Batteries. However in terms of upfront costs they need you to dig deeper in your pockets.
- Terms Used in Batteries
- Why you need a Deep Cycle Battery
- Lithium Deep Cycle Battery
- Lead Acid Battery
- Calculating Your Battery Size Needs
- Frequently Asked Questions
Terms Used in Batteries
Large systems use small voltages typically 6v while small systems use higher voltages. Most small solar power systems use 12v.
Batteries are rated in Ah (amp-hours). This is a measurement of the current output over time. Voltages are fixed but according to type of battery. However capacity is relative to the type of Battery, this is usually printed on its side.
Battery Capacity or power is measure in in Kilowatt hours. This gives the total storage capacity derived from Voltage and amp hours. Capacity (Watt hours) = Voltage x Ah
Every time a battery is discharged and recharged this is called a cycle. Cycle life is the number of times a battery can be charged and discharged. This is used to calculate the life of battery.
Depth of discharge (DOD)
DOD is measured as a percentage of energy that can be pulled out of Battery during each cycle.
Batteries are not 100% efficient; Lithium Batteries have 95-98% efficiency while lead Acid batteries have a 80-85% efficiency rate.
Why you need a Deep Cycle Battery
The battery type most used, by far, for solar applications is the 12 volt Deep Cycle battery. Sometimes it’s referred to as a Deep Cell battery or a Deep Discharge battery. Deep cycle batteries are designed to produce a steady amount of current over a long period of time.
While regular batteries (for example a car battery) are made to provide a large amount of current over a short period of time. These batteries are designed to be repeatedly discharged up to 80% time after time, for years.
For example when you’re starting your car you only need that short burst of power to get the motor running and once it’s running the alternator takes over from there and gives the motor all the power it needs.
‘Starting’ batteries have thin “plates” to provide lots of surface area for that initial cranking oomph. Deep Cycle batteries have thicker plates. Less initial oomph, but better long term cycling capabilities (a cycle means a discharge with subsequent charge).
There are many kinds of Deep cycle Batteries and, many are not ‘True Deep Cycle batteries’. And more than that, it’s hard to tell. Some of the lower end ones are called Deep Cycle Marine batteries. A more accurate term for them might be ‘hybrids’. They’re sort of a ‘Deep Cycle – Starting’ combination.
But they’re less expensive and they work, especially on smaller systems. You can pretty much assume that if a battery is on the lower end expense wise it’s probably a hybrid of some sort whether it’s mentioned or not. But that’s not a horrible thing.
Lithium Deep Cycle Battery
Lithium batteries are the best option for domestic grid connected homes. These batteries are commonly used in appliances we use everyday from phones to laptops. It has a higher density of energy compared to Lead Acid batteries. This means that you get more power with smaller lithium ion battery than you would with a lead acid battery of the same size or even bigger.
Lithium batteries can tolerate huge temperature changes and can be use between 320 – 1000 F. Lithium batteries have a long lifetime can last up to 10 years and do not require maintenance. However they have higher upfront costs and must use a controller as it poses a slight risk of fire in case of overcharging.
Advantages of Lithium Batteries
Lithium battery is becoming the battery of choice for many reasons and it is also they are used in portable battery power stations: If you are a Beginner in Solar Setups or looking for a Portable system consider a Power Station that will house a battery, controllers, inverters and more and save you the hassle.
This is because of they add more functionalities to a battery bank to allow it for more than just for home use.
When it comes to weight, the Lithium battery wins hands down. It is typically half to one third lighter than the original lead acid battery weight (for an equivalent run time/capacity lead acid battery bank).
Higher electrical efficiency.
The Lithium battery has a much higher electrical efficiency compared with lead acid batteries. LiFeP04 lithium batteries are typically 98% charge efficient compared to between 70% and 85% with the lead acid batteries. They also have significantly faster charging times than lead acid batteries.
What this means is the Lithium battery will charge not only more efficiently, but more quickly than a lead acid battery. This can be particularly important in situations with limited charging methods available, such as from solar panels.
For example, camper van owners have reported that their LiFePO4 batteries are fully charged from solar panels by midday compared to 4 to 5pm using their old lead acid batteries.
Longer working life.
While good quality Lithium batteries such as C&C Power Supply batteries are far more expensive to buy and set up initially, the actual cost of ownership is considerably cheaper than lead acid batteries (calculated in terms of cents per charge cycle).
In a lead acid battery, voltage will continue to drop over time as the internal circuit within the battery will self discharge. This forces the connected electrical device to eventually shut down at its minimum operating voltage.
In the Lithium battery, however, voltage tends not to drop under load until the battery is almost depleted. For example, an ignition coil in a motor vehicle will have a consistent spark due to the battery’s ability to delivery a consistent voltage.
This means electrical equipment tends to run more efficiently as they are receiving constant voltage. Ultimately, using Lithium batteries gives electronic equipment a longer working life before failure than the lead acid battery option.
Expect to spend much less time recharging Lithium batteries than lead acid batteries.
Lithium batteries are the safest batteries for consumers and the environment. Internally, they do not contain corrosive sulfuric acid or any toxic heavy metals such as lead or cadmium which can be environmentally harmful if leaked.
Lead Acid Battery
Lead Acid batteries are the common types of batteries we find in Cars, tractors and sub marines. There are two types; Flooded Lead Acid batteries and Sealed Lead Acid batteries.
Flooded Lead Acid Batteries
Flooded Lead Acid batteries are an old school deep cycle battery. They are high maintenance and need to be stored in a place with good ventilation as they emit gas. They are referred to as flooded as because it needs to be submerged in distilled water regularly. It is an economical option for off grid systems but need hands on maintenance.
Sealed Lead Acid batteries are the better option if you don’t intend to do regular maintenance. While the liquid inside the standard flooded battery looks like water, the liquid in a gelled deep cycle battery looks like jello and they’re sealed.
The whole concept of spilling, a hazard, is removed so that’s really nice in addition to the no maintenance. But, a disadvantage of these is that a regular automotive type charge could easily damage them. The charging current has to go by the manufacturers recommendations.
Too much current too fast will damage the battery. If you see yourself putting a nice pump up charge on your battery before (or even during) your battery use “solar season” or some intermittent charging over the off season, this could be an added hassle factor. These batteries also typically cost more.
AGM batteries, or Absorbed Glass Mat, are the non-spill type that you find (normally) as small 10 amp hour alarm system back-up batteries. They are also sometimes called “dry” or “starved electrolyte”.
There’s a fine fiber between the plates holding the acid. They’re sealed, maintenance free, more efficient, can’t spill, don’t freeze easily – lots of great qualities. Some applications need these features.
This type of battery is also available in larger capacities for solar applications, and have advantages, such as a bit more available energy per pound of battery, and are safer because there is no electrolyte (battery acid) to deal with, there are also disadvantages such as cost, and, for some, the fact that they are ‘maintenance free’ means that once they are done, there is not much you can do.
These can be great for an off grid system that you don’t visit regularly and battery bank for grid tied systems.
Using a Car Battery for Solar
The WORST type of battery you can use for alternative energy applications is the standard car battery. They will ‘work’, and some will argue that given the cost and availability, they are the best for someone just starting out, and in some limited way, I agree.
You may have an extra/old car battery you’re thinking of using. Just know if you use a ‘starting’ battery like you would a Deep Cycle battery (repeated discharges) it could fail after not too long, as few as 30 discharges, depending on its condition. Starting batteries are not designed for repeated deep discharge – doesn’t mean you can’t use one as long as you understand it’s limitations.
This however does not mean you can’t use a Car Battery with Solar. A trickle charger can be used to maintain a car battery charge so that it does not go flat. This set up does not include a load.
For someone in a garage that just wants to tinker with a 100 watt inverter to run a couple lights and that’s all they will ever want out of their system, I would say yes, grab any battery (A deep cycle if you can get you’re hands on one), a cheap inverter and panel hooked into a controller (usually available as a set) will do just fine, and for a couple hundred bucks, you have a working system.
Golf Cart Battery for Solar
With all that said about the Car Battery you might think using a Golf Cart Battery is a bad idea too. Not the Case. 6V Deep cycle batteries are one of the best types of batteries out there for long extended use in a solar system or without any type of charger nearby.
A typical golf cart battery costs about the same as a 12 volt car or deep cycle battery. You will need two of them to run on a 12V inverter. A set of golf cart batteries can run for much longer and they are designed for this type of use, so, at double the cost, I got 4 times the capacity.
A pair of 6V batteries running together to create a 12V battery bank can last up to three times longer than a single 12V deep cycle battery. On top of that they have a longer life span than a 12V battery and are made to be used for long periods of time.
Calculating Your Battery Size Needs
It’s helpful to estimate how much electricity you might consume when you’re trying to make decisions about battery size. Since batteries typically measure storage capacity in amps and consuming devices typically in watts, you might want to change the amp hour measurement associated with the battery into watt hours as it becomes more like an oranges to oranges comparison.
The equation for finding how many watt hours are available in a battery when the amp hours are known is voltage x amp hours. For instance, a 12 volt 105 amp hour (AH) battery can supply 1260 watt hours (12 volts x 105 amp hours =1260 watt hours).
That’s at 100% discharge without pumping in new energy (which means, if you actually do that, you’ve likely ruined your battery – you shouldn’t go lower than 80% with 50% being better). So, at say a 50% discharge its capacity might be closer to 630 watt hours (1260 divided by 2). Again, the general equation is watts equal volts x amps.
Going along with the example above, let’s say your “very safe” 50% capacity allows that 630 watt hours of total usage mentioned above. Let’s further say that maybe the biggest or longest in-use consuming device you want to run is your portable computer and it consumes about 95 watts an hour (look on the label).
630 total watt hours safely available divided by 95 watts used per hour = 6.63 hours usage with no incoming battery charge. (Although it’s good to know even cloudy or rainy days provide some electrical charge.)
If that’s a long enough use time for you, then perhaps in our example, a 105 amp hour battery is big enough for your needs. In reverse, watts divided by volts = amps. Or, 1260 watt hours divided by 12 volts = 105 amp hours, or using our 50% example, 630 watt hours divided by 12 volts = 52.5 amp hours available.
How many amps does a laptop computer draw? I think mine is around 7 or 8 amps per hour. 52.6 amp hours divided by 8 equals 6.56 amp hours. So, whether you compare by amp hours or watt hours the actual usage time comes out about the same. Comparing using watts is just a bit easier because that’s what consuming devices tend to come in.
The amp hour size of your battery matters. On the very low end, a 34 amp hour battery might be fine if you have strong solar panel input, short term use which allows for battery recovery time – good sun with no battery drain, or you don’t get many rainy days where you are.