How to Choose a Battery Bank for a Solar PV System

Battery bank for a solar PV system

Battery bank for a solar PV system

[box]Part 4 of the Buyer’s Guide to Solar Photovoltaic Systems. Read part 1>>[/box]

With all that juice you’re sucking up from the sun you’ll need somewhere to store it, if you’re going off-grid that is. A battery bank lets you store your energy for nighttime and cloudy days so you don’t have to rely on the sun to show off every day.

Lead-acid batteries, the dominant battery type for solar PV systems, have been around for years. Though they look similar to car batteries, solar PV batteries operate on a deep-cycle principle—they’re intended to deliver small charges for a long time rather than short bursts of intense energy as with car batteries. Aside from the most common flooded lead-acid type of battery, a couple of other options exist: gel-cell and sealed absorbent glass mat (AGM). Batteries are often considered the weakest link in a solar PV system since they’ll wear out quicker than the other parts, reducing the system’s performance. Keep the chain as strong as possible by purchasing the right product and purchasing high quality.

Deciding on a battery bank

1. Determine how many watt-hours your system will consume by following Part 2 of this guide, “How to Size your Solar PV System.”

2. Decide how many days of stored energy you would like for your system. Ask yourself the following questions:

» How important autonomy is to you?
» Are you fine with being grid-tied? If so, you don’t even need batteries.
» Are you OK with using a backup fossil fuel energy source or do you want your system to be powered by renewable energy at all times?
» Are you going off the grid completely or not?
» Do you want to be prepared for long blackouts?

Once answering those questions decide how many days of storage you need. Most systems are sized for one to five days of storage capacity.

3. Determine what type of battery is best for your application:

Flooded lead-acid




charges and discharges at 35% amperagehighly efficient—charges and discharges at 100% amperagecharges and discharges at 35% amperage


requires regular maintenance—adding distilled water, venting and cleaning corrosive acid from terminalsmaintenance-freemaintenance-free


can leak and will corrode without maintenancespill-proof, vibration and impact-resistantdurable—even if the battery cracks it will still function


cheapest, most widely usedmore expensive than flooded lead-acidmore expensive than flooded lead-acid

4. Since lead-acid batteries typically drop 20 to 25 percent of their capacity while operating in 30ºF (-1ºC) temperatures, plan to house them somewhere warm (ideally between 50 and 85ºF [10 to 29ºC], with 77ºF [25ºC] as the optimal temperature). If you can’t keep them somewhere warm, purchase additional batteries to compensate.

6. Simple is best when deciding on a battery bank. Choose a single series string of cells because there’s less that can go wrong than with multiple strings and they’re easier to replace when broken.

7. If you foresee yourself expanding your system, buy a larger battery bank than you think you’ll need since battery banks cannot be easily expanded. Overtaxing your battery bank also leads to a chronic undercharged condition that can cause the batteries to fail prematurely. As a general rule of thumb, aim for 30 to 40 percent more energy than the load demands.

Additional components

1. Buy a battery monitor and install it where it’s easily visible. Using these devices to manage your batteries can increase their lifespan and make maintenance easier.

2. Purchase a charge controller that includes temperature compensation as one of its features. This is important since it can prevent warm batteries from overcharging and assist cold batteries in getting the extra charge they need. In order for temperature compensation to work, you have to purchase a temperature sensor and place it next to the batteries.

3. Assess how likely you are to regularly water your batteries. Just like your plants, if you neglect your batteries, they will die. In that case, buy a battery watering system.

[box]by UB Hawthorn[/box]
 image: Eddie Codel (Creative Commons BY-NC-SA)
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