The NTM Tech Center has been approached on several occasions to recommend batteries that can be purchased in-country. This is nearly an impossible task. Depending on the country, there may be a hundred different possibilities. Many batteries come from countries that do not follow the same testing, design, or quality control procedures that US, Canadian or European battery manufacturers use. The specification datasheets can be misleading because many do not rate batteries on a 20 hour discharge rate, or they just lie. Other factors include Depth of Discharge (DOD), usage cycles, plate thickness, electrolyte composition, quality (purity) of the lead used, and specific gravity of the acid. When you consider all of the variables, it is very difficult to make a reasonable recommendation. There are a few things to keep in mind when looking at batteries. Find a copy of the manufacturer’s data sheet for any battery you are considering buying, this is often found on the manufacturer’s website. The battery distributer may have a copy as well.
* NOTE: The only Batteries that the NTM Tech Center recommends are the batteries that we have positive experience with. At this time this includes AGM batteries from SunXtender and dry charged liquid electrolyte batteries from Rolls/Surrette. All other choices you make are at your own risk. Please be aware that in most cases you are likely going to pay more for a comparable battery if you purchase it in-country even after you add in shipping and customs costs.
- Cost vs. Longevity – In our experience, the batteries that are inexpensive do NOT last very long. This can be attributed to many factors including thin lead plates, poor quality control during the manufacturing process, poor quality electrolyte, improper storage and etc. You may want to consider spending a bit more on a quality battery to avoid changing them out ever year or so and the poorer performance they provide during their shorter life span. Don’t just buy the cheapest battery you can find, look for quality as well.
- Battery Design – Batteries are designed for many different purposes. Automotive/Truck Batteries are designed to give up a lot of energy quickly and then be recharged very quickly; they have a low depth of discharge. Automotive / Truck batteries are NOT recommended for a PV system. Standby Batteries are designed for continuous charging and may only need to cycle once every month, they have a low cycle life. Frequent discharges or cycles on a standby battery will destroy it quickly and are NOT recommended for a PV system. PV system batteries are designed to give up energy slowly over a long period of time, and have a long cycle life.
- Depth of Discharge, this is the amount of power that can be discharged from a battery without damaging the battery or shorting its life. For example if you have a 100 Ah battery and use 25 Ah’s worth of power you have discharged the battery by 25%.
- Cycle life, this is the amount of times a battery can be discharged to a specific discharge depth and still have 80% of its amp capacity left. For example if you have a 100 Ah battery and discharge it to a depth of discharge of 25% then recharge it 1000 times. After this if it has a capacity of 80 Ah then it has a cycle life of 1000 at 25% depth of discharge.
A battery designed for the purpose you are intending to use it for will give you longer life and fewer frustrations. Look for a battery that is designed for a deep discharge (25% – 50%) and has a cycle life of at least 1,200 when discharged 25% every day.
- Battery Type – There are three main types available: Liquid Electrolyte, Gel (Gelled Electrolyte) and AGM (Absorbed Glass Mat).
- Liquid electrolyte batteries are the oldest type and the most common batteries found worldwide. They MUST be stored/used outside of a house due to the harmful and explosive gases that are emitted during the charging process. Most Liquid electrolyte batteries do not have a very deep depth of discharge and a very low cycle life. They also require quite a bit of maintenance each month. One advantage of Liquid electrolyte batteries is the ability to equalize them. This helps to equalize the voltage in each battery cell and improve battery performance. Another advantage is that they can be purchased dry (without acid), and if kept sealed will store for several months. A very important consideration with liquid electrolyte batteries is the acid that will be added to the battery. Poor quality acid will be a detriment and can cause permanent damage. Be sure to test all acid with a temperature compensated hydrometer (BAT024) before adding it to any battery. The Specific Gravity of new battery acid should be at least 1.250 but not more than 1.275 @ 77 deg F. (25 deg C) Understand the requirements of liquid electrolyte batteries before purchasing.
- Gel and AGM batteries are similar to each other, because they are both sealed batteries and can be stored/used inside of a house. When using Gel or AGM batteries it is VERY IMPORTANT to understand their unique characteristics. Most Gel and AGM batteries are not designed for PV systems. You must choose batteries that are designed for PV systems when using Gel or AGM. In most countries, the Gel and AGM batteries that are available have been designed for Telecom or Computer standby power. These batteries will not have a long cycle life. Gel batteries will give long life, but are sensitive to over/undercharging. True PV Gel batteries are very uncommon. AGM batteries are a bit more durable, but can have a limited life in extremely hot climates. AGM and Gel batteries must be fully charged every 30 – 90 days while in storage. If purchasing Gel or AGM batteries, be sure that they are designed for PV systems with a long Cycle Life.
- Battery Capacity – Battery capacity is measured in Amp Hour (Ah), this is the number of amps that can be drawn for a number of hours. For example a battery can be rated 80 Ah @ 10 hours and 100 Ah @ 20 hour. This means that power is used slower (20 hours instead of 10 hours) more power is available. Battery Amp Hour (Ah) Capacity is important because it allows you to compare different batteries. Be sure you are looking at the 20 hour Amp-hour rating capacity when considering purchasing batteries.
- Batteries in Parallel – No more than four batteries should be connected in parallel. This is not just an NTM Tech Center recommendation, it is the battery industry standard. Breaking this rule can lead to many problems including catastrophic battery failure, or damaging your entire battery bank. If you must connect more than four batteries in parallel, you should fuse each battery individually; this way if one battery fails it should burn the fuse and disconnect the failed battery. (FUS120) Do not connect more than four batteries in parallel.
- Battery Shelf Life – Batteries need to be charged on a consistent basis. If a battery is sitting on a supplier’s shelf for more than 3 months it is likely that it has suffered damage. when the sides of a battery are sunken in it has likely been sitting on the shelf for several months. It would be wise to measure the voltage of each battery you are considering buying before you purchase them. Some batteries have their manufactured date stamped on them, any battery with a manufactured date older than six months should be rejected. The exception to this is if the battery is dry (without acid). If you can special order batteries from a supplier this may be best. Do not purchase batteries that have been sitting on the shelf for a long time.
The NTM Tech Center can evaluate batteries you are consider buying, if you include a link to the manufacture datasheet, where we can see the specific data for the battery you are considering buying. Without this data it is not possible for us to give any battery evaluation. Please understand this evaluation is not a recommendation since we have not had direct experience with the specific battery.