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Deep Cycle Batteries

Deep Cycle Batteries

Solar batteries provide energy storage for solar, wind power, or other renewable energy systems. A solar battery is just a deep cycle battery - a battery designed for the prolonged, repeated, and deep charging/discharging cycles needed to store and distribute energy generated by intermittent renewable sources like solar panels. For this reason, car batteries cannot be used as solar batteries.

Grid tied systems do not need batteries unless you want to maintain power during utility grid outages. But for off grid systems, deep cycle solar batteries are essential and will likely be providing 100% of your electricity. This makes correctly sizing a solar battery bank among the most important steps of off grid system design - watch our video below for more.

Find more deep cycle battery information below, on our blog, in our DIY Solar Resources Library, or by talking with our energy storage experts at 877-878-4060.

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Sizing a Solar Battery Bank

A solar battery bank consists of one or more deep cycle batteries. Just like an individual battery, a battery bank’s energy storage capacity is measured in amp-hours (Ah) or kilowatt-hours (kWh). To determine how much capacity you need, use our kWh calculator.

In this video you will learn how to correctly size a solar battery bank.

Solar Battery Bank Sizing Tips

We strongly recommend watching the solar battery bank sizing video above, but some of the key takeaways are:

diagram of solar batteries connected in series and parallel
  • Connecting batteries in series (positive terminal of one battery to negative terminal of the next) increases voltage but keeps amp-hour capacity the same.
  • Connecting batteries in parallel (positive to positive, negative to negative) increases amp-hour capacity but keeps voltage the same.
  • Limiting the number of parallel battery strings minimizes the problems from uneven charging/discharging between strings.
  • Don't use batteries of different voltages or ages in the same battery bank. In fact, using multiples of the same exact battery to create your bank is recommended.
  • You can convert back and forth between a battery’s Ah and Wh (or kWh) by using the battery’s voltage since Watt-hours = Amp-hours x Volts.
  • Caveats about that the energy storage capacity number you got from our kWh calculator:
    • The number provided by the calculator is your daily energy use. A battery bank based on that number will only provide enough power for one “day of autonomy”. It’s a good idea to double or triple your battery bank’s capacity and consider incorporating a generator into the system to ensure you’ve got enough power for prolonged periods of no solar/wind power generation.
    • The recommended Depth of Discharge (DoD) on the deep cycle battery model your bank uses must be accounted for. For example, many lead acid batteries recommend discharging no deeper than 50% to get the most cycles out of them - meaning you should plan to only ever use half their rated capacity. Pay close attention to recommended DoD when comparing battery options to use for your bank.
    • Ambient temperature and the efficiency of the system’s inverter also affect how a solar battery bank should be sized.
    • If you expect your daily kWh use to increase soon (electric vehicle purchase, more people living in the house, etc.), consider oversizing your battery bank. Expanding a deep cycle battery bank later can be done in some cases, but is generally not recommended.

After determining the capacity and voltage of your battery bank (12V, 24V, or 48V DC), you can start thinking about the specific deep cycle batteries that will make up the bank. Need help making those determinations? Call us at 877-878-4060 or request a free off-grid solar power system quote.



Types of Solar Batteries

A deep cycle battery is the only kind of battery that makes sense for a solar or wind system, but what about the different types of deep cycle batteries - lithium, flooded lead acid, AGM, and gel? Which kind is best?

While it’s true that each different cell chemistry has its pros and cons, it’s also true that lithium batteries are easily the best choice for most solar panel systems. Compared to all the other chemistries, lithium batteries are deeper discharging, longer-lasting, lighter weight, safer, and maintenance-free. Yes, they are more expensive up front than the other types, but in the long run, the cost per kWh cycle is the best metric to look at - and with both longer cycle life and deeper Depth of Discharge than the alternatives, the cost per kWh cycle you’ll get from a lithium solar battery bank is unbeatable - and you won’t have to replace them as often.

Lithium

Flooded Lead Acid

Sealed AGM

Sealed Gel

Upfront Cost High Low Moderate High
Cost per kWh Cycle Lowest Low Low to moderate Moderate
Expected Lifespan 10+ years 3-5 years 4-5 years 5-6 years
Max Recommended DoD 80% 50% 50% 50%
Regular Maintenance None Watering, equalizing, cleaning None None
Best Applications All renewable energy systems Full-time residences with committed, hands-on owners willing to do regular maintenance and replacement Part-time residences with intermittent use Part-time residences without many high-surge loads
Worst Applications Projects on a tight budget Part-time residences with intermittent use Systems requiring deep discharges Systems requiring high-amperage charging and discharging


Adding Solar Batteries to a Grid Tied System

If your solar power system is connected to the grid, it will shut down during grid outages as a safety precaution for the workers who will be repairing the utility equipment. To keep a grid tied solar system online during a grid outage, you will need to add a battery bank and a second inverter to create what is known as a hybrid solar system.

This video explains the two main ways to add battery storage to an existing grid-tied solar system.


Adding batteries to a grid-tied solar system is becoming increasingly popular - especially in areas where the utility grid is unreliable due to excessive demand (rolling blackouts) or frequent extreme weather events. For a new hybrid solar system or to retrofit an existing grid-tied system with battery storage, use our battery backup power system quote.

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