Battery Capacity Basics for Solar and Storage

When choosing the right energy storage device, we must consider what we are doing with them, where we will put them, how much we will work on them, and how long we want them to last.

Of course, all of these factors will be affected by budget. Batteries are like (oh no, here comes the fine wine analogy!) anything you pay for: more money spent often means higher quality. If we’re going to use batteries we should understand how they are built and what they can (and cannot) do for us. We will want to know an amp-hour or watt-hour from the specific gravity and how they relate. 

Buying a battery is a long-term investment.

Similar to designing any renewable energy (RE) system, with energy storage I begin by assessing the owners’ goals. What are we trying to accomplish? If we are building a battery to live “off-the-grid” we will need a large unit and we will want to use large batteries that deeply cycle for long cycle lives. If we simply want to back up a few lights and some communications for a few hours during a storm-induced outage, we can use fairly small energy storage devices.

Background on Energy Storage Devices (ESD’s) in Solar PV Systems

Typical RE system ESD’s are either Lead Acid (LA) batteries (which are all made up of 2-volt cells) or Lithium Ion batteries (made up of 3.2 volt cells for Lithium Iron Phosphate (LFP) and 3.7 volt cells for Lithium Manganese Cobalt Aluminum Oxide(NMC)). 

LA Cells are combined in series to build individual batteries (i.e. Golf Cart Battery (GC) = 3-2VDC cells in one 6VDC unit typically storing 220 Ampere hours (Ahrs.)  Individual GC batteries are combined in series and parallel to create the correct size for the battery “bank” (i.e.: 4 GC’s in series to create a 24 VDC series string, and 2 strings of 4 GC’s in parallel to create a 440 Ahr 24VDC battery “bank). 

With LiOn batteries are similarly made up of cell combinations such as 3 or 4 – 3.7 vdc cells to make up a 12 vdc battery.

Battery Banks should be sized to provide the energy storage for the work you want to do (average daily watt/hour consumption) for the time needed (‘hours of autonomy’ for grid-backup, ‘days of autonomy’ for off-grid).

Making up the right battery bank for your needs will require this basic design pattern and a few additional tricks. Batteries live and die by how well we “cycle” them. With LA battery packs discharging a battery 80% and then charging it back to full with a large industrial battery charger is a typical “cycle” for lead acid deep cycle batteries. However in RE systems we cycle lightly, often living in the top 25% of the batteries capacity. The depth to which we discharge and the number of times we cycle them will determine how long our battery bank lives. 

With LiOn batteries manufacturers suggest that you may discharge the battery 80% to 100% of its capacity depending on how it is rated (LiOn batteries cannot be fully discharged without becoming thermally unstable and therefor have Battery Management Systems (BMS’s) to prevent over discharge or over charge of the unit).

We want our system to recharge our bank to full on a daily basis (within reason) and we generally don’t want to cycle LA RE system batteries below 50% depth of discharge (DOD) before they get filled back up. However, LiOn ESD’s generally will not age greatly when they are discharged to 80% DOD. 

With any battery, discharging a battery deeply and leaving it discharged for extended periods is what reduces battery life faster.

We can talk about battery types (not considering exotics such as liquid pocket plate NiCad’s, Nickel Iron, and Nickel Metal Hydride) in order of increasing cost. 

With LA batteries it so happens that this is also the inverse of life span. As we pay more per amp hour, we get a shorter-lived lead-acid battery.

• Flooded lead Acid (FLA) batteries have been the most commonly used in RE systems, they require the most maintenance, are the least expensive per amp hour, die at a predictable rate, and need to be recycled and replaced regularly.
• AGM (absorbed glass mat) lead-acid batteries are similar in chemistry to FLA cells, and they have “glass mats” placed between the plates (anodes and cathodes) to reduce gassing during charge and discharge cycles. This reduces maintenance – you cannot add water to these cells – and increases cost while potentially shortening the life span, by the end of which they need to be recycled and replaced.
• “Sealed cells” may use a ‘gel’ type electrolyte and be completely sealed. This means no added water and less gassing, less maintenance, a short life, and higher cost after which they need to be recycled and replaced.

LiOn batteries most often are more costly than the Lead acid batteries.  They also most often have a significantly longer cycle life than Lead acid batteries.  Currently Lithium Nickel Manganese Cobalt Oxide (NMC) batteries cost about 20% more than Lithium Iron Phosphate (LFP) batteries.

See a theme? 

Let me point out – I highlight recycling and replacement for a reason – how this is done is important. For the LA batteries I use go back to my U.S.-based factory for recycling at a facility that is regulated by our EPA. This costs more than offshoring recycling to China or Africa, but it means that the batteries plastic case, electrolyte, and lead are all recycled in the cleanest way possible, where we produce so much pollution in the first place. 

LiOn recycling is becoming more widely available and some services offer pick-up service for them.

That said, why do we use shorter-lived, more expensive LA Batteries? The reasons vary and often labor is a key factor. More expensive AGM or Gel cells are advertised as being “maintenance free.” While this is a bit of an exaggeration, it does sell these types of batteries. Every lead acid battery will require some maintenance – cleaning terminals (at a minimum) will always need to be done. In addition, with flooded lead acid cells we’ll be adding distilled water to the cells on a regular basis dependent of how often we cycle the bank and how deeply we do it. AGMs don’t get watered and they have to be charged more lightly to avoid using up the finite amount of electrolyte on board. Sealed and Gel cells also don’t get watered and are charged even more lightly to avoid drying out the cell and killing it. AGM’s require lower charge rates on equalization charges (if they are allowed by the manufacturer), and that makes this battery maintenance regime even more challenging.

LiOn batteries have come down in price significantly and with very low maintenance and long cycle life they now are taking up more and more market share in the Solar+ Storage market.

Battery Voltage

Lead acid batteries, as mentioned previously, are built up from cells with a “nominal” voltage of 2 volts. Battery banks for RE systems are made up of combinations of cells to achieve nominal battery bank voltages of 12, 24, or 48.

• When designing small systems (loads under 1000 watt hours/day) we still use 12 VDC as a nominal battery bank voltage, if we think that system will not grow over its lifespan. So, a cabin system for a hunting shack that’s used only a few times a year (and will never become a vacation home or permanent residence) will keep costs down by having this low voltage design.
• When the system has a larger load profile, we move into the larger (and more electrically efficient) battery voltages of 24 and 48. With medium size deep cycle batteries (GC’s and L16’s) the unit is often a 6VDC device made up of 3-2VDC cells.
• In the medium-to–large systems we will combine the 6vdc units in series (4 for a 24vdc “string”, and 8 for a 48vdc “string”) and then parallel the series “strings” to get greater amp hour capacity at that voltage. To fully charge a bank, we will limit the number of strings in parallel…

…but first we will need to understand how LA battery capacity is rated.

Battery Capacity – 

The Ampere Hour – the metric for LA capacity

So what is an amp hour in relation to battery capacity? One ampere-hour (Ahr) is a unit of electrical charge and represents the amount of electric charge transferred by a current of one ampere for one hour. The Ahr capacity of a given battery depends on the rate at which it is being discharged and the amount of time it takes to discharge it (usually to 80% state of discharge for industrial deep-cycle use). Large industrial batteries like the fork-truck type are often rated at the “6-hour” rate, indicating a large ampere discharge rate bringing the battery to a terminal voltage (often 20% state of charge) in 6 hours. For RE systems, we usually employ the “20 hour” rate indicating a modest discharge rate that brings the battery to a terminal voltage (again often 20% state of charge) over 20 hours.

Kilowatt Hour – the metric for LiOn capacity

LiOn batteries are manufactured at many voltages – 12,24,48vdc and also at much higher voltages – such as 350 to 450 Vdc.  The battery packs are rated in Kilowatt hours of capacity.

Battery Temperature

Batteries are specified with the assumption of operation at 70° Fahrenheit. When we move out of the 60°F to 80°F range, the battery performance changes. At somewhat higher temperatures, batteries may perform better but at extremely high temperatures, (125°F +) batteries can be severely damaged.

Batteries at temperatures below the 60°F range begin to show reduced capacity. Below 32°F batteries will show significantly lower capacity and while the battery has not actually lost the energy it is storing, the cold temperatures restrain the batteries chemical activity resulting in lower capacity. Remember: a discharged batteries electrolyte is no longer highly acidic and, being more like water, it can easily freeze. Freezing a battery can destroy the cells.

LiOn batteries have similar temperature characteristics.  While cold will reduce accessible capacity high temperatures may improve the battery performance.  However, operation at significantly high temperatures can reduce cycle life and potentially damage the battery.

To have your battery to last a long time – read the warrantee – and identify the ideal temperature, depth of discharge and current (C) charge and discharge rates before you purchase the battery for your system.

Battery Specifications

When we charge batteries, we want to put as much current as we have into the battery efficiently. The internal temperature increase during high charge rates can mean that LA batteries gas wildly. To keep from harming the battery in this process, the charge controllers we use in RE systems limit the charge rate based on the batteries voltage. As the voltage increases the charge rate (number of amperes allowed in) is ramped back to prevent overcharging. The initial phase when all available current is allowed into the battery is referred to as the “bulk” charge phase. Once the battery has reached its initial “bulk charge” voltage, (in FLA 12VDC language approximately 14.6VDC) the intelligent charge controller will hold the voltage there for a programmed period of time referred to as the “absorption” charge phase — often 2 hours. This is done to assure full charging throughout the many cells of the battery. After the “absorption” period, the charge controller ramps the allowed current down to achieve the “float” phase, which is a lower voltage (approximately 13.2VDC in FLA 12VDC terms) that greatly reduces the batteries’ gassing while keeping the unit full. Specific “bulk”, “float” and “equalizing” voltages should be supplied with the deep-cycle batteries from their distributors to avoid over or under-charging the particular battery and shortening its life. As mentioned earlier, both AGM and Sealed units have lower charge voltage tolerance and the systems charge controller must be programmed appropriately, to avoid damaging the units.

LiOn batteries have a much narrower voltage charge window.  While, in 12Vdc terms, LA batteries can have a charge window ranging from 10.5 to 15.9 LiOn batteries may only have a window between 11.9 and 13.9.  The window varies between manufacturers and is critical for proper operation.  For inverter charging LiOn batteries generally use a 2-stage charging regime.  The Bulk phase followed with a very brief absorption phase (usually only a few minutes) and no float phase. For charging with a solar array the charge controller will often be set to a three stage regime with the same short absorption and a low float voltage set point (below the resting voltage of the battery bank).  LiOn batteries do not require Equalizing charges.

So with LA batteries what is “Equalizing” in battery parlance? An “Equalizing” charge cycle is a controlled over-charging of the battery bank. This removes secondary sulfate ion bonds on the battery’s plates, to regain that capacity before tertiary bonds develop and create lost capacity in the unit. First the battery is charged to full by completing a “bulk” and “absorption” charge cycle and then the battery is charged for an extended period (6-12 hours) at a controlled rate of the batteries capacity divided by 20 (C/20 rate). By controlling the charge rate, the battery is kept from harm while the secondary level sulfate ions are put back into solution and the batteries full capacity is restored. BEWARE: uncontrolled over-charging can warp the batteries plates, causing it to short out, or (in extreme cases) explode!

Equalizing a flooded LA battery is essential to maintain battery life and can be hard to achieve with the limited current available from a PV array. Often, a back-up generator (in off-grid applications) or the utility grid and a battery charger is employed to get the desired amp charge rate for the extended periods necessary to fully equalize the battery. During this cycle, no loads can be allowed to discharge the battery or the whole cycle will need to be re-done. Obviously, equalizing a battery is quite challenging.

Battery Size – Buying or Building Containment

The size of the batteries you use determines the size of the containment that you’ll need to buy or build. In addition to the dimensions of the individual batteries, you need to plan extra room for air space around the individual units, so that they can perform as they were designed to and so that they shed heat correctly during heavy charging. I like to keep 1/2” to 1” space between each unit, so I add that to the unit dimensions when I am considering the size of the containment.

Battery Weight

Moving batteries and containing them needs to be done carefully. Proper trucks, skids, palette jacks and fork trucks all become essential as our battery grows in size. Containing the battery requires both size and weight considerations. Building or buying a battery box means getting the battery units into the containment. Large batteries will be easier to handle if the containment can open at floor level to allow for moving units in and out. Containment must be strong enough to hold up to the moving of units and the long-term weight of the bank.

Lead prices affect LA battery costs dramatically. Batteries represent over 70% of the lead market. Some analysts site China’s growing demand for non-ferris metals as the reason for the recent spike in lead costs. Between 2014 and 2023, lead prices have varied modestly. Lead is produced from roughly 50% new ore and 50% recycled stock. 

As with any investment, we want to abide in frugality and meet our true needs.

Grid-tied battery back-up systems can have an incredibly small battery and still meet our needs. If we’re off grid and survive off of our battery we can buy a long-lived battery and maintain it well. Batteries have their place and with caution and care we can use them responsibly.

RE has been a reality at SunFarm where I live and work for 30+ years because of batteries. I still wish I had the grid. There is nothing like having someone else maintain your storage for you. If I could give my extra electrical production to my neighbors – I would! That is the beauty of the grid. When I am gone teaching, and more than 50% of my load profile is on the road, my charge controllers turn my generators OFF. Grid-tied systems put that unused capacity into my neighbors hands… even if they don’t care about RE. Now that is my kind of battery – the Grid!

Click here to sign up for “ Residential and Commercial Solar + Storage System Design”  with Christopher LaForge. This course provides in-depth analysis of the issues surrounding the use of batteries for residential PV applications. It covers battery design, specification, use, and maintenance. The course capstone project is creating the design for a battery-based solar PV system.

For a free look at battery free and battery back-up PV systems, check out the “Kroska Case Study: Solar PV Systems with Storage.” This case study outlines a blend of two PV system designs (battery free and battery back-up) that accommodates two roof planes and types. The design provides for great cost-effectiveness and high aesthetic quality.