Batteries (Overview)

Batteries General

Renogy specializes in off-grid systems, which are solar system components that utilize batteries. Batteries are needed as a method of storing the energy generated by the solar panels, for when the power is needed. They come in multiple forms and operate on multiple principles. They generally have a voltage level rating (typically 6v or 12V) as well as an Amp-hour rating, denoted as “Ah”. Renogy offers 4 main battery types: Sealed Lead Acid (SLA), Flooded Lead Acid (FLA), Absorbed Glass Mat (AGM), and Gel, each with their own set of unique properties. When choosing batteries attention should be made to lifespan, technology type, and the cost. Finding the balance between these three factors will help users find a battery specifically for their needs.

Battery Types

The most common and basic battery type is Sealed Lead Acid (SLA). They are the oldest battery technology. They are the first maintenance-free battery and due to their composition can be typically mounted in other physical orientations without leaking. In all, SLA is designed to reduce maintenance, reduce explosive risk, and foul odor that can be created by other battery types.

Flooded Lead Acid (FLA) batteries are referred to as “wet” batteries because of the liquid solution they have inside. These type of batteries require more maintenance as one needs to be conscious of their water levels. These batteries are sensitive to vibrations and shocks due to their water levels and have a high discharge rate. However, FLA batteries usually have the lowest cost per AH. These batteries also have one of the longest track records with alternative energy storage. For safety concerns, this means FLA batteries should not be placed in the same enclosed space as charge controllers or other electrical devices prone to sparking. Otherwise, heavier ventilation is required to minimize this risk 

Absorbed Glass Mat (AGM) batteries is another maintenance free battery that has a glass fiber mat material in its chemistry for flow. This material is special and can render the battery completely sealed and can do well against gassing due to the plates. In many cases, they typically charge faster than FLA batteries and are vibration resistant. These batteries tend to perform better in colder temperatures. However, these batteries are usually higher in cost than FLA and are more sensitive to overcharging. Over-time they have a gradual decline in capacity, and this is intensified if the battery is not properly cared for.

Gel Batteries (Gel) are another maintenance free battery thanks to the gel-like material inside the battery making it completely sealed. Gel batteries are excellent for extreme conditions because they have higher boiling points. Characteristics of gel batteries include high performance until the battery’s end, larger battery sizes availability, and performs better in warmer temperatures. However, gel batteries are typically the most expensive battery types and equally sensitive to overcharging. 

Deep Cycle vs Starter Battery

For effective compatibility in off-grid systems, batteries utilizing solar panels need a “deep cycle” battery. Deep cycle batteries are designed for continuous charging and discharging, which is the exact solar application. Starter batteries are designed to crank an engine with momentary high loads and for a few seconds. This means that starter batteries cannot be deep cycle because the emphasis is on immediate power and not capacity.

Deep cycle batteries will have an Ah rating, whereas starter batteries might also have an Ah rating or a reserve capacity (RS) rating as well as a Cold Cranking Amp (CCA) rating. CCA refers to the starter battery being able to deliver power at cold temperatures. Starter batteries cannot be swapped out for deep cycle battery and vice versa. The battery chemistry inside a starter battery would fail or dissolve in a deep cycle application.

Battery Usage in System
The overall characteristics of a battery will determine its usage in system designs. There are three main characteristics of concern, the capacity, the voltage, and the charging/discharging specifications. Battery capacity is measured in a unit called ampere-hours (Ah). This represents the amount of amps that is transferred to a battery in one hour. If one were to think of a battery as if it were a glass capable of storing liquid, the capacity of the glass is essentially the amount of liquid capable of being stored (ounces). If the glass were then to be poured out, the rate at which this water left the glass would be the flow rate, analogous to current (ounces/minute). If one wanted to know how long it would take the glass to empty, you could divide its capacity by the flow rate.

A final understanding of battery capacity behavior is to note that the voltage of a battery that is listed is what is referred to as the nominal voltage. The actual voltage of the battery varies with time as described above and is typically never at the nominal voltage. For example, a 12 V battery when full usually has an actual voltage of around 12.7 V when full. This arises from the cell voltages not being quite even. It should be also noted that when charging the battery, since the voltage needs to be higher to move charge, the actual charging voltage is typically 13.2-14.6 V.

Battery Banks
Individual batteries are typically too small in terms of either storage capacity or voltage. Storage capacities often need to be increased to deal with battery maintenance issues or to extend operating times for attached loads. Voltages may need to be increased to reduce system amperages through various components or to meet charge controller requirements. Reaching the necessary electrical system requirements can be easily accomplished by connecting the batteries in the appropriate manner of series or parallel connections.
Wiring the batteries up to achieve the necessary capacity is akin to the internal battery wiring used to create the battery itself from the individual cells. Special consideration must be paid to this external interconnection, however. A key design goal for battery banks is to maintain all components to be as identical as possible to reduce wear on the batteries. This includes:

o Interconnection cable length
o Battery capacities
o Interconnection fuse ratings

Addressing the above concerns, variation in cable length will cause different resistances between batteries. This will lead to disproportionate charging between bank members. Likewise, differing capacities will cause the batteries to constantly attempt to equalize with one another leading to early battery death.

Battery Sizing
The battery bank should be based on intended usage. Sometimes this could be for daily appliances or for extended periods of energy storage, perhaps 2 or 3 days of backup power. Smaller systems typically utilize 12V systems and bigger systems such as 1200W or more utilize 24V battery banks. The main reason is efficiency. Unless the charge controller allows it, it is better to have just one controller on a battery bank and not multiple, for they can interfere with charging efficiently.

Let’s say you have a 200W tv that you’d like to use for 10 hours per day. That’s 2000 watt-hours or 2KWh of power need per day.

You take your needed power and multiply it by 2, since you want to preserve at least 50% of a deep cycle battery for long battery life. That means you actually need 4000Wh but should use 2000Wh without damaging your battery.

Since Volts x Amps = Watts, Volts x Amp-hours = Watt-hours, and Watt-hours/Volts = Amp-hours So divide your 4,000 Watt-hour need by a 12V battery, and you have 333.33 Amp-hours. You could get this in one big battery or multiple smaller 12V that adds up to your need.

Renogy is the author of this solution article.

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