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The two main battery chemistries used for backup power are Lead acid (Pb) and Lithium (Li).
Both batteries come in two variations: Lead acid is either “wet” or “sealed”, and for this article, we will address the “sealed” versions, which aren’t really sealed, but vent when internal pressure builds up.
The common term for this type of battery is AGM, which stands for Absorbent Glass Mat. The battery electrolyte is suspended in an absorbent glass mat, as opposed to liquid electrolyte “wet” battery.
The biggest benefit of AGM batteries is they never require watering, eliminating the sometimes messy and dangerous task of adding water to the battery periodically
Lithium batteries also come in two versions: Lithium Ion, typically the small cylindrical cells, and Lithium Iron Phosphate (LiFePO4), which is gaining in popularity as it is safer and doesn’t present a fire or explosion hazard.
For the sake of comparison, we’re comparing AGM with LiFePO4, the two prevalent battery technologies for backup power in use today.
Which is best?
There is no hard and fast answer. Both have advantages and disadvantages.
Lead acid is less expensive, well understood and is the traditional choice for golf carts. It does have a shorter life than lithium, and is not the best choice for cyclic service, but for backup power, it works very well.
Lead acid can be occasionally discharged down to 20% capacity, per manufacturer recommendations, with no effect on battery life, as long as the batteries are recharged promptly.
Lithium batteries are typically discharged to 10% capacity, per manufacturer recommendations, for maximum battery life. Lithium is better suited to frequent cyclic service.
The important thing to remember is a Kwh is a Kwh, regardless of battery chemistry. Your backup power system inverter simply does not care where the quadrillions of electrons come from.
Therefore, CAPACITY is more important than chemistry.
Capacity is affected by the rate of discharge, so for the sake of this article, we are rating both chemistries at the 100 hour discharge rate.
Getting back to golf carts, lead acid has been the battery technology of choice for decades. Lithium is coming on strong, but usually at the expense of capacity, due to lithium’s higher cost.
Generally speaking, lithium costs 2-3X lead acid for comparable capacity.
Many of the new lithium batteries for golf carts have about 5Kwh of energy storage for the same price as lead acid. Lead acid capacity, on the other hand, is usually twice that amount.
For example, new Trojan T105 AGM batteries, 230Ah at the 100 hour rate, provide 11Kwh of energy, enough to power critical loads in a home for almost a week. (230Ah x 48Volts = 11.04Kwh)
So, what is the best chemistry for backup power?
Neither chemistry is a clear choice, as they both work equally well.
For the dollar, lead acid is the best choice
For longevity, lithium works better.
The question is, will lithium last 2-3X the life of lead acid? The jury is still out, as many external factors affect the lifetime of any battery, such as temperature, daily depth of discharge, charge and discharge rate, etc….
So, to decide, we suggest asking yourself several questions:
The short term, 5 year answer is probably lead acid.
For longer term storage, above 32 degrees F, lithium is probably the best choice.
Please note, this is article is not an engineering document, but attempts to discuss the benefits and limitations in layman’s terms. The opinions herein are those of the author, based on 30+ years of solar and backup power experience, and don’t necessarily reflect the noise and hype often found online.