Choosing a Lead Acid Battery for Solar ChargingBy Joe O'Connor
Lead acid batteries have large capacities and are often available in many places around the world. But which lead acid battery should you use with solar panels? I recommend using sealed AGM lead acid batteries wherever possible and will describe in this post the trade-offs of using different battery types with portable solar panels.
Starter vs. Deep-Cycle Batteries
Starter batteries are designed to deliver short, high-current bursts for starting the engine, and are designed to discharge only a very small part of their capacity. A daily cycle of using most the capacity would corrode a car battery very quickly, the plates and the chemistry are designed to stay nearly 100% fully charged most of the time.
For solar charging applications, you instead want a deep cycle battery, similar to those used for marine vessels or golf carts – a typical car battery will not work. Deep cycle batteries are designed with larger plates and different chemistry to avoid the corrosive effect of frequently using the full capacity. They are designed to be regularly deeply discharged using most of its capacity.
Flooded, vs. Gel vs AGM
There are a few types of lead acid deep cycle batteries: flooded, sealed gelled, or sealed AGM. For most situations a sealed AGM (Absorbed Glass Mat) is the safest and best option. AGM batteries require little maintenance. There is no need for ventilation and they will not spill. Flooded batteries have the advantage of being significantly less expensive, but they require adequate ventilation, maintenance, and also have the potential liability of tipping or spilling.
AGM batteries are typically lighter and less expensive per amp-hour compared to gel. As an example, on AltE Store a Trojan AGM 12V 100 amp-hour battery is 64 pounds and $241, or 0.64 lbs/AH and $2.41/AH. A Trojan Gel 12V 77 amp-hour battery is 52 pounds and $231, or 0.68 lbs/AH and $3.00/AH. In addition gel batteries need to be recharged in a specific way that is not optimal for solar. Basically, AGM batteries are more forgiving in the way that they are recharged.
If lead acid batteries are maintained properly, they will function at 80-90% efficiency. It is important to maintain a full charge when ever possible, because it will extend its life and maintain a higher efficiency. Always use a charge controller with solar panels, so they don’t over charge the battery or apply the wrong voltage. We’ll discuss more about how to select a charge controller in a future post.
Charging Station Application
I recently built a portable but powerful solar battery system to power some LED string lights, a stereo, and an evaporation cooler. I used a Xantrex PowerPack 1500 AGM battery, MorningStar SunSaver charge controller, and two 25-Watt solar modules. The Xantrex PowerPack comes with a built-in AC inverter, surge protector, low voltage disconnect (LVD), and can be wheeled around. The wheels and handles are helpful because it is quite heavy at 57 lbs.
A less expensive alternative is the Trojan sealed AGM deep cycle batteries. The Trojan 22-AGM batteries are similar in size to the Xantrex PowerPack, but it does not have all of the extra features listed above.
The final touch is to attach a cigarette lighter socket to the load terminal on the charge controller. This allows you to draw a load directly from the solar panels when there is enough sunlight or from the battery when there is no solar production. You can use any 12Vdc devices straight from the cigarette lighter or you can use a DC/AC inverter to power 120Vac devices.
I used a similar system to build a solar powered cell phone charging station at the 2012 Dumbo Arts Festival. In this project we used two of the Voltaic 16.8 Watt, 18 Volt solar panels, which were embedded into a dodecahedron shaped sculpture. The cell phones all charged from the DC side of the battery with cigarette lighter chargers. This is more efficient than using the AC inverter because we are simply stepping down the voltage from 12Vdc to 5Vdc. Also, by avoiding the use of the AC inverter, we don’t need to convert the DC to AC and back to DC again.
Joe O’Connor is a guest blogger for Voltaic. In addition to making cool things with solar, he does consulting on alternative energy and is pursuing his Master’s Degree in Manufacturing Engineering at NYU Poly.