A battery’s capacity, the amount of energy stored in it, plays a large role in the overall design of PV systems and the use of batteries within them. In order to select the right battery for your PV system, you need to have a solid understanding of how batteries’ capacity values are determined and reported and how to apply those ratings to the system you’re designing.
Batteries are rated according to the number of amp-hours (Ah) stored in them. Although this seems like an easy concept to grasp, multiple factors change this value.
Considering the C Rate for Capacity
Batteries used in PV systems are typically rated at a temperature of 25 degrees Celsius and at a discharge rate over a time period of 20 hours. This rate is referred to as the C rate and is notated as C/20 (C over 20). The C represents the capacity value, and the 20 represents the number of hours of discharge time.
Here’s a numerical example to help you make sense of the C rate: If a battery manufacturer lists its battery as a 100 Ah battery at the C/20 rate, you can do the math to figure out what kind of load that represents:
100Ah ÷ 20hrs = 5A
This means that if a 5A load was placed on that battery, in 20 hours the battery would deliver all of its stored energy and would no longer have any capacity. In reality, getting all 100Ah out of that battery isn’t realistic because the battery can’t really deliver all of its capacity.
The C rate can also describe the charging half of the cycle, but in this case we’re talking about replacing the capacity rather than drawing it out. For example: If we say that the battery charger has a C/10 rate, we’re saying that we’ll replace the entire capacity in a matter of ten hours. The best rate of recharge varies with battery technologies and even manufacturers. Finding the correct value is very important.
Recognizing Factors that Affect Capacity
A battery is somewhat at the mercy of its surroundings when it comes to energy delivery. Environmental conditions dictate a battery’s ability to deliver energy, as do the age and discharge rate of the battery. By understanding these factors, you can avoid costly mistakes that will require a battery replacement sooner than necessary.
The temperature has a very significant effect on a battery’s ability to deliver and accept current.
- A cold battery has a reduced capacity due to the fact that the chemical reaction inside the battery is slowed down and isn’t as efficient.
- A hot battery has the ability to deliver a greater number of amp-hours because the heat actually aids the chemical process. However, increased temperature ultimately reduces a battery’s life, so intentionally keeping a battery hot to increase its capacity in the short term will only have the effect of reducing its life in the long term.
Ideally, the battery bank should be located in a temperature-controlled climate that’s around 25 degrees Celsius. In reality, this task is difficult. When helping decide where to place the battery bank, aim for a location that doesn’t experience extreme temperatures.
As a battery ages, the chemical reactions don’t work as well as they did when the battery was young and fresh from the factory. The active material on the batteries becomes layered with parts of the electrolyte that didn’t return into acid solution during the recharging process.
This aging process is evident in both the charging and discharging parts of a battery’s cycle. Aged batteries discharge quickly due to their reduced capacity. They also seem to finish recharging quickly due to the reduced amount of active material that’s actually available to the battery, allowing the battery to appear fully charged when it really isn’t.
A common question asked about battery banks and aging batteries is this: Can we install a new battery into a bank of older batteries? Adding a new battery to an existing battery bank isn’t a great idea because the old batteries drag the new battery down to their level in a short period, but depending on your reality, it may be workable. So the question becomes this: What’s the state of the battery bank? If the bank is relatively young and well-maintained, the idea of adding a new battery to replace a failed unit may seem reasonable. If, on the other hand, the entire bank is on the verge of replacement in a short period, adding a new battery may not make any sense. A bad battery makes itself known relatively quickly by dragging down the whole bank almost instantly. An aging battery bank, on the other hand, has a slow decline. When an aged battery bank can’t hold a charge for a required amount of time, the whole bank needs to be replaced.
Battery Discharge Rate
A battery’s capacity is directly affected by the rate at which the battery is discharged. This rate is the theoretical number of amp-hours a battery could deliver to a load. So, for example, a 100 Ah battery could, theoretically, deliver 1 A for 100 hours (1 A × 100 hours = 100 Ah) or 100 A for 1 hour before it was completely discharged. Because a battery delivers power via a chemical reaction, the slower the reaction, the greater the battery’s capacity. The opposite is also true: The faster the reaction, the smaller the capacity.
As we mentioned that battery manufacturers typically rate their batteries at the C/20 rate (or something very close to that). They then generally report the capacity values for their batteries at other C rates for your information so you can establish the discharge rate that most closely reflects your load profiles and use the C rate in your designs accordingly.