Batteries; Deep Cycle AGM (Sealed) & Flooded Lead Acid (Vented)

Batteries for off-grid or grid tied solar powered systems are containers of electrical energy or a collection of electro chemical cells connected together in series. Electro chemical reactions produce a flow of electrons from the negative terminals to the positive terminals inside the battery. Factors affecting the capacity of a battery and how much energy you can expect to reap from the system includes the quality of active material and electrolyte concentration, the number of batteries, type, design and size of the battery plates. If you have ever asked "what is a deep cycle battery", Blue Pacific Solar® energy consultants have put this page together for you to help you understand and make an informed buying decision.

Solar panels produce electricity but without the proper battery storage, the electricity produced is wasted or given back to the utility company at much lower rates than you are buying it for. If your solar panel system is off-grid, you will need to select the correct battery bank wired together with the proper cable size to store electricity to use later. Grid-tied systems also can utilize battery banks to store power for days when the grid goes down during blackouts.

Rechargeable deep cycle batteries are a group of cells that store power using a chemical reaction as the catalyst. Each battery cell has a specific voltage output depending on the battery technology which is separate from the cell size. Batteries used for solar panels are similar to car batteries but constructed differently so they release their energy slowly rather than in a burst which is what car batteries do. Every cell in a deep cycle battery has either a single positive plate with a negative side or multiple positive and negative plates. The plates sit in a electrolyte solution, either gel (AGM) or liquid. This solution allows the negative and positive particles to flow between the plates. As the battery bank is discharged, the lead oxide becomes lead sulfate the sulfuric acid becomes water. In a fully discharged battery there is a very watery acid solution where the positive and negative electrons don't go away, they change into a different form and are stored in the lead sulfate solution.

In solar rechargeable batteries during the charging cycle, the sulfate breaks up forming lead oxide and the water solution becomes a strong acid again. Hydrogen gas is produced which can vary with the different battery technologies used by the various battery manufactures. Electrons are forced back into the plates where they started and the whole process is repeated when a load is provided to draw out the electrons again.

The most common battery used in off-grid solar systems are lead acid batteries because they are usually the least expensive. The downside to flooded lead acid batteries is the monthly maintenance over say sealed AGM batteries. Absorbed Glass Mat (AGM) is the latest in off-grid battery technology. Sealed AGM batteries completely eliminate the hassles of flooded lead acid battery maintenance. There is no liquid acid in the AGM battery. All of the acid is absorbed into the compressed glass matting in the cells. They are completely Non-Spillable, so they can be shipped by UPS or LTL freight with no hazardous warnings or extra fees. Blue Pacific Solar's AGM batteries re-combine the oxygen and hydrogen as the batteries charge and discharge to make them truly maintenance free. No liquid ever needs to be added, No leaks, spills or vent tubes. (Cheaper) Flooded lead acid batteries have a liquid electrolyte inside the container that floods the battery cells. The flooded lead acid battery typically has a cell cap that is removed once a month and check. There are automatic battery watering systems that are available now to help alleviate that monthly chore, but they are not cheap by themselves.

Before we get more into battery types, we need to say a few words about battery DOD or depth of discharge. This is an important topic when planning for any off-grid solar system because it affects not only if your home or cabin will have enough power to see it through the night, but also days without any sunshine. The average depth of discharge (DOD) is the amount of energy that is drawn from a battery bank each day. The electrical load that is drawn during the winter is usually higher than during the summer simply because of the shorter days. That is an important consideration, but no less important is the fact that in most locations, winter has the shortest days and the most days without sunlight or minimal sunlight. Days of autonomy or days without sunshine are expressed in the amount of time a fully charged battery system can supply power without further charging. We have customers ask repeatedly about how much much power will I need, but the truth is that answer always lies with each individual customer to control. The depth of discharge (DOD) is not only controlled by the owner, but is also conversely related to the lifespan of any flooded lead acid or AGM battery system. Batteries do not last forever and at some stage of your solar panel system, grid tied or off-grid, you will need to replace them. Every time you discharge and recharge a battery you cycle that battery the same as if you were driving your car and putting miles on the engine. After a number of cycles, the chemical reaction inside the battery cell will start to break down and eventually the battery will need replacing. A typical battery life cycle may look like this: 20% DOD = 1600 life cycles; 40% DOD = 1200 life cycles; 50% DOD = 1000 life cycles; 80% DOD = 350 life cycles. The life cycle of your battery bank is always controlled by the owner so hence the life of the battery is a function of the owners ability to control their home or cabin energy demand. You should always install a large enough battery bank that you will never discharge your batteries beyond 50%.

This page is not about charge controllers, but you cannot have a discussion about batteries without taking a quick look at the charge control process. A charge controller is a the piece of equipment that does exactly what the name implies, controlls the rate of battery charge. This process is mission critical to ensure the life of your investment and to maximize your life cycles. Simply put, the charge controller role in a solar system is to control the current (amps) and voltage from the solar array to your battery bank. Depending on the charge controller you select, it is sometimes necessary to set the the charge set points on the controller to match your batteries. The three main levels of recharging a battery are bulk charge, absorption charge and float charge. The first stage of charging that occurs when the sun comes up and wakes up your solar panels (system) is the bulk charge stage. This happens first thing in the morning after the batteries DOD has been drained down from the previous day or usage. The bulk charge stage of recharging your batteries is to push as many amps as possible back into the battery bank from your solar system and quickly get the voltage back up in the process. This is similar to filling a glass with water in that when the glass is empty you turn the faucet on full then slowly close the valve when he glass gets full. Same fundamental principle in battery charging only when the battery starts to accept the higher amps being pushed at it, it tends to heat up. Many solar charge controllers also have a temperature probe that is connected to the negative terminal of one of the batteries in the bank. When the probe senses the battery is heating up, it will act as one of the several controls switches to slow down the recharging of your batteries.

The next stage in the process of solar rechargeable batteries is the absorption stage. When your battery bank reaches it's manufactures voltage set point of nearly charged, most of the material in the batteries has been changed to its original form and the amps must be slowed down to limit the amount of overcharge applied. This occurs around the 80% level of DOD. During the absorption stage the battery charge amps slowly tapers off working towards the final stage of the battery charging process. The float stage is the final stage for solar powered batteries and is the process where the controller keeps the batteries in a full state of charge.

Now that we have covered the basics of batteries we can get to the real heart of the off-grid and backup systems, the battery bank. A battery bank is a group of batteries wired together with series or parallel connections (or a combination of both) to provide a specific overall voltage and power capacity. Though the size of the battery bank voltage is often determined by the inverter, for larger inverters and larger loads it is recommended that battery banks be sized for 48 volt rather than the 12 or 24 volt option available with most solar systems. By engineering larger volt battery banks you can usually lower the amps required for the solar system inverter. The lower a system amps are the lower the size and cost of conductors, fuses, disconnects and other balance of system components of a bimodal or off grid battery system. When choosing the type of battery you should consider: sealed or unsealed, allowable depth of discharge (DOD), charging characteristics of the inverter and charge controller, maintenance requirements cost and transportation costs. Additionally, it is recommended that when engineering a battery bank you connect your batteries in as few parallel strings as practical. The primary reason is slight voltage differences and lead to inconsistencies in the charge received by each string eventually causing, in some extreme cases, inconsistent charging.