Battery Backup Basics for Solar PV
When choosing the right battery, 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 can not) do for us. We will want to know an amp-hour from a specific gravity and how the two relate. Buying a battery is a long term investment.
Similar to designing any renewable energy (RE) system, 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. 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 batteries.
Background on Batteries in Solar PV Systems
Typical RE system batteries are Flooded Lead Acid (FLA) batteries which are all made up of 2-volt cells. 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). The Bank 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 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. Discharging a battery 80% and then charging it back to full with a large industrial battery charger is a typical “cycle” for deep cycle batteries. 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. We want our system to recharge our bank to full on a daily basis (within reason) and we generally don’t want to cycle RE system batteries below 50% depth of discharge before they get filled back up. Discharging a battery deeply and leaving it discharged for extended periods is what kills battery life quickly.
We can talk about battery types (not considering exotics such as Lithium Ion, liquid pocket plate NiCad’s, Nickel Iron, and Nickel Metal Hydride) in order of increasing cost. 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 are 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 can not 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.
See a theme? I highlight recycling and replacement for a reason – how this is done is important. The 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 off-shoring 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.
That said, why do we use shorter-lived, more expensive cells? 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 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. AGM’s 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. Both AGM’s and Gel’s require lower charge rates on equalization charges, and that makes this battery maintenance regime even more challenging.
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 battery capacity is rated.