DIY Solar Energy System, Part 2: Batteries

Author: oakwhiz (33 Articles)

I run things around here at Minousoft Software. I enjoy developing games and building DIY electronics.

Yesterday, I wrote an article about basic PV (photovoltaic) system design and I talked about charge controllers. Today, in Part 2 of the series, I’m going to be explaining the many different types of batteries and how to maintain them.

By far, the most-used battery chemistry for PV applications is lead-acid. Thusly, most PV parts, such as charge controllers or inverters, will be designed for lead-acid batteries. In this article, I’m going to be explaining lead-acid batteries the most, but before we start, I’m going to provide a very brief note on electrical calculations.

I’m going to explain some battery chemistries that people have used in their PV systems:

• Car batteries intended for starting the engine – “shallow cycle flooded cell” types.
  • Don’t use these! They are not designed for energy storage and won’t work in a solar energy system. They are meant for providing a lot of current for a short amount of time – thus the name “shallow cycle.” If you use them for deep-cycle purposes, you will kill them quickly.
  • They tend to be expensive and heavier than other battery designs.
• Deep-cycle lead-acid, aka flooded cell.
  • Deep-cycle lead-acid is basically the storage version of a car battery.
  • The flooded cell design is dangerous because the battery caps aren’t sealed. Explosive hydrogen gas vents from the top of the battery – if there is a spark near the battery, it will explode the cloud of gas. Additionally, if a battery is sloshed, tipped, or charged improperly, dangerous sufuric acid will spill out of the battery.
  • Having access to the electrolyte can be advantageous if you want to use a hygrometer to check electrolyte density, or if you want to add your own chemicals to the battery acid.
  • Flooded cell batteries tend to be designed with high current ratings, but this is not always the case. They do have higher specific energy than the next type.
• Deep-cycle sealed lead acid (SLA), aka Absorbed Glass Mat (AGM), Gel Cell, Valve Regulated Lead Acid (VRLA), or “Maintenance-Free.”
  • SLA batteries are safer to handle and are spill-proof. You can mount an SLA in any position, even upside-down.
  • SLA batteries come in two types: Gel and AGM. Gel batteries and AGM batteries are similar but have different properties. In a gel battery, the sulfuric acid electrolyte is mixed with a block of gel, which immobilizes the acid and prevents spillage and gassing even if the case is cracked. In an AGM battery, the sulfuric acid is absorbed into a mesh of glass fibers, much like water in a tissue paper.
  • SLA batteries need different chargers from flooded-cell batteries.
  • Gel batteries have different charge characteristics from AGM batteries.
  • SLA batteries recombine any gassed electrolyte back into water, so you don’t need to open them to maintain the acid.
  • Any excess gas is vented.
  • SLA batteries can store energy for a very long time without losing it (lower self-discharge rate.) Flooded-cell batteries can lose 1% of their energy per day, whereas SLA batteries lose around 1% per month.
• Rechargeable Lithium-Ion (Li-Ion)
  • Lithium-ion batteries are different from one-time use lithium batteries.
  • Lithium-ion batteries are more expensive than other chemistries.
  • They provide a high power-to-weight ratio.
  • They must be charged specially.
  • They have a very low self-discharge rate of less than 0.1% per month.
  • They lose capacity every charge cycle.
  • Lithium is flammable, explosive and highly reactive with water. Lithium can explode when exposed to the water vapor in normal air. Additionally, lithium-ion batteries.
• Rechargeable Lithium-Polymer (LiPo)
  • Lithium-Polymer is basically the same as lithium-ion but more advanced and more expensive.
  • LiPo has different charging requirements than Li-Ion.
  • LiPo cannot be float-charged.
• Nickel-Cadmium (NiCad/NiCd) and Nickel Metal Hydride (NiMH)
  • NiCad and NiMH are older chemistries. NiCad is not often used anymore due to its generally poor performance. NiMH is used in rechargeable AA batteries and the like. It is possible to use these and other chemistries in a PV system but this will probably require custom parts.

I recommend using AGM batteries for PV systems, especially small ones. My PV system is going to start out with a single PowerSonic PS-12120 12v 12Ah AGM battery, which is enough to run a normal household oscillating fan on an inverter for about 5 hours. The price tends to be around $30-$45 each.

Batteries need proper chargers. This goes for both AC wall power chargers and solar charge controllers. All of the battery types listed above have different charge curve characteristics and thus require different voltages and currents. For example, a flooded-cell charger can’t be used with an SLA because they have slightly different voltages.

However, some special chargers exist which automatically sense the characteristics of the battery and adjust themselves accordingly.

You might remember from my last article that a 3-stage charger should be used with lead-acid batteries to maximize their lifespan. I also mentioned something called ’sulfation’ which you must watch out for. Below is the excerpt:

If using lead-acid battery types, it may be useful to consider a charge controller that does simultaneous desulfation. If a lead-acid battery is pushed hard, mistreated, or not charged for a long time, a layer of deposit (sulfation) begins to form on the surface of the internal battery plates, which prevents electricity from moving through the battery. A desulfation system sends a specially-tuned high frequency pulse through the battery which will actually cause sulfation to flake off and dissolve back into the battery acid, essentially breathing renewed life into a poorly-performing battery. If the battery is desulfated while it is float charged, any sulfation that forms during normal use will be dissolved soon after.

For AC charging, I bought the BatteryMINDer Plus #12117. It has 3-stage charging capability, automatically senses battery type and voltage, and desulfates the battery when on float-charge mode. It works great and I haven’t noticed any drop in performance since I first bought the battery. It charges the battery in around 1-6 hours with light use, and with heavy use, it would theoretically take a maximum of 13 hours to fully charge.

Next time, I will be talking about sun tracking for your PV array! If you enjoy these articles, show your support by subscribing to our blog, bookmarking us, sharing this article with your friends, or donating. Any comments, questions, or suggestions you have would be greatly appreciated.

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