Knowing what battery you should integrate with your solar design can make or break a solar system. Too many times a new off-grid client will inherit an older system that lacked forethought or was designed based on a YouTube video. That is not to say there aren’t great people out there willing to help you along, just remember that every solar system is a little different. There are questions you need to ask before you spend the $$$ for a battery style that might advertise great results, but not work in your system due to something as simple as the temperature your battery bank will be exposed to. Batteries can easily become the largest expense for a new system. This isn’t always necessary.
On the market today there are a much wider variety of battery options than there were even ten years ago. Be wary of batteries advertised with extremely long lifespans, as there is nearly always more to the story if you read the maintenance requirements. Let’s dig into a few simple examples.
Off-grid systems in an environment that will be cold (under 50-55 degrees) on a constant basis can be a challenge. In most cases to design a system under those conditions it actually makes more sense NOT to go with Lithium batteries. Lithium batteries have higher energy capacities than many other battery chemistries, and if you can maintain a constant temperature that allows them to operate efficiently they can be the best choice for an off-grid design. However, if you are building your first solar system on your own, I recommend that you instead start off with absorbed glass mat (AGM) batteries. Most lithium batteries will not accept a charge below freezing. Imagine a scenario where you have an emergency and have to leave your cabin, to return a week later to your homestead with a lithium bank below the threshold.
Battery bank maintenance, regardless of battery chemistry, is vital. You must go into off-grid living with that in mind. With older flooded lead acid (FLA) batteries you will need to regularly and carefully maintain water levels in the cells, and ensure that you equalize the batteries to ensure they are all operating at equal capacity or your bank will suffer. With AGM batteries this requirement is lessened in that they are sealed. Again, know what you are capable of doing and if you will have the awareness to monitor and care for your batteries.
With every battery chemistry you should also consider the lifespan. Example: the manufacturer’s listed lifespan for Nickel Iron (Ni/Fe) batteries is 20 years. In some cases, with care the expectation is that they could last longer or shorter based upon how carefully you care for them. With Ni/Fe batteries you also need to restore the battery chemistry at the ten year mark. Ask yourself if you have the patience to go through the process to empty, clean, then restore the alkaline compound and water to 20 sizeable batteries. If the answer is YES then perhaps they will be a great battery for you (considering the much greater expense to purchase them).
Saltwater batteries are a newcomer to home energy storage. Once manufactured by the now bankrupt Aquion (prior to 2017), they appear to be making a comeback and are available for installers under a new manufacturer. As a battery chemistry they offer the advantage of not relying on heavy metals and instead relying on saltwater electrolytes. The main disadvantage with this and some of the more exotic battery chemistries is that they are less proven and sometimes less efficient (albeit safer). More on this in a future article.
Another considering that often gets overlooked is the charging requirement for each battery chemistry. A perfect example again is Ni/Fe. In my example I will be using the calculations for a 24V battery bank (20×1.2V Iron Edison cells). In order to charge them at the manufacturer’s recommended 1.65V per cell you need a charger capable of adjustment to allow 33V. It is possible but not as easy as you might think. Most charge controllers are designed for chemistries that max out at 32V. The old Trace, Xantrex C40’s have an adjustment to allow for this. If you go this route though you sacrifice the ability to integrate a more robust MPPT (more on this in an article on charge controllers soon).
The calculations used to build a battery bank are important as well. If you learn nothing else from this article, remember that batteries in a series equals higher voltage, while batteries in parallel equals higher ampere hours (AH) and thus capacity to sustain loads longer. See the chart below for a basic understanding of this. Price is variable based upon selected options.
To summarize, never let yourself be afraid to ask questions from either an installer, electrician, or more experienced off-grid homesteader. Most of us have learned first hand (sometimes the hard way) what works in real practice and can save you years of frustration. Remember the goal should be to build your system and enjoy the freedom of having power when you need it.
~ Rick
