Batteries for RE Systems: 100 vs 20 Hour Rate
When choosing batteries for renewable energy, you may think it's best to buy the biggest Amp Hour battery available. However, in this case, bigger is not always best. You don’t need the 100 Amp Hour (AH) rated battery; here we explain why 20 hours is the appropriate rating for a renewable energy battery.
We all want a good deal, right? This is no exception when purchasing batteries for renewable energy. Some may think it's best to buy the biggest Amp Hour (AH) battery available; the easiest way for manufacturers to provide a big AH number is to rate the battery at the 100 hour rate. However, with batteries for a renewable energy system, bigger is not always best. In fact, 20 AH is the most appropriate rating for a renewable energy battery (based on 24 hours in a day).
This is not an apples-to-apples comparison. The big box store battery at the 100 hour rate is not 220 AH. They can say it is a 6v 220 AH. In reality at the 20 hour rate, it is only 170 AH.
Generally, batteries are designed and made for a particular use. For solar applications, consider that the plates are the most expensive part of the battery. Quality battery manufacturers such as USA-based companies Trojan and Crown cast their plates with little micro-hooks designed to hold the paste on the plate for the life of the battery. This helps their 20 hour rated batteries have at least 4 times the life of other batteries (such as those sold by big box stores). We recommend choosing a battery that will last the longest, for less cost in the long run.
Battery Plates and Ratings
Batteries designed for low price points are manufactured using plates made out of thin stamped lead sheets. These sheets are called the “grid” and are painted with active material. Unfortunately, this doesn’t give the paste anything to on to. Double pasting it to get a higher test specification ensures a short life as the paste starts to slide off the plate in 8-14 months. These are typically 6v 220 AH batteries sold by big box stores. Also, while these batteries may be rated at the 100 hour rate, they may not be 220 AH. Although packaging may be confusing and say it’s a 6v 220 AH, in reality at the 20 hour rate, it is only 170 AH. grip
To avoid being outright fraudulent, a battery must reach the desired specification at some point during testing. Some manufacturers may double paste the plates, run them through 30 cycles and test at that moment, resulting in the peak rating the battery will ever achieve. This is a marginally legitimate way they provide low-cost batteries and claim the rating, because at one point it did achieve the rating. Unfortunately, with a low-cost battery, that rating tends to go downhill rather quickly after testing, as is reflected in the long-term performance of the battery.
If you have any questions, we’re happy to provide guidance. Call us at 888-826-0939.