What’s the most expensive component in a small off-grid solar setup? You might assume it would be the solar panels, but the cost of panels can be surprisingly low. For many people, the batteries are the single most expensive element in the system. So what batteries offer the best value?

Lead acid batteries and lithium batteries are both popular options for solar energy storage. When discussing solar, it’s important to note that “lithium” usually means lithium iron phosphate – LiFePO4. This particular lithium battery chemistry is different from the “lithium ion” batteries commonly used in laptops and mobile phones, which is typically something like lithium cobalt or lithium manganese. It’s also different from the LiPo (lithium polymer) batteries used in applications like RC cars and airplanes. While these can both be used with solar, LiFePO4 works at a different voltage level that’s a better match for common 12V equipment. It’s also safer and less explosion-prone.

From a technical standpoint, LiFePO4 batteries are superior to lead acid batteries in nearly every respect.

Usable Capacity – A deep cycle lead acid battery can be regularly discharged to a depth of about 50%. Frequent discharges below that level will harm the battery and shorten its lifetime. If I’ve got a 100 Ah lead acid battery, only about 50 Ah is usable in practice. LiFePO4 batteries can be regularly discharged to a depth of 80-100%.

Lifetime Charge Cycles – When cared for properly, a sealed lead acid battery should last about 600 charge/discharge cycles. If charged and discharged every day with a solar setup, that’s a lifetime of less than two years. A LiFePO4 battery should last about 4000 charge/discharge cycles, for a lifetime of roughly 11 years with daily use.

Charging Efficiency – A charge/discharge cycle with a lead acid battery is about 85-90% efficient. I’ll get out about 85-90% of the power I put in. A LiFePO4 battery is 99+% efficient.

High Current Capacity Loss – A typical “35 Ah” lead acid battery is usually measured at a C/20 discharge rate: depleting the battery in 20 hours at a rate of 1.75 amps. Discharging the battery at a faster rate than C/20 will reduce the battery’s effective capacity. At a C/5 discharge rate of 7 amps, the effective capacity is reduced to only 28 Ah. A LiFePO4 battery is much less sensitive to the discharge rate, and will provide close to the rated capacity even at C/2 or faster.

Weight – A lead acid battery weighs more than twice as much as a LiFePO4 battery with the same Ah rating.

 
Hunting for Battery Bargains

If somebody’s giving me a free battery, then LiFePO4 is the obvious choice. But is it worth the extra cost? Hunting Amazon for inexpensive choices, I found this Weize 35 Ah 12 volt lead acid battery that’s currently $63. I couldn’t find a 35 Ah LiFePO4 battery, but I could buy three of these TalentCell 12 Ah 12 volt LiFePO4 batteries for a total of $249. That’s about 4x more expensive. Ouch.

To compare the options, instead of measuring the battery capacities in amp hours, it’s helpful to measure capacity in watt hours (volts times amp hours) instead. This is because lead acid and LiFePO4 batteries operate at slightly different voltages. Then I can compare the cost per watt hour of nameplate battery capacity.

lead acid 35 Ah x 12 volts is 420 Wh, $0.15/Wh
LifePO4 12 Ah x 12.8 volts is 153.6 Wh, $0.54/Wh

The simple difference in upfront cost per Wh is 3.6x more for LiFePO4, then. But what about the differences in usable capacity, lifetime charge cycles, and charging efficiency? Let’s calculate the cost per actual usable Wh over the lifetime of the battery.

lead acid 420 Wh x 600 lifetime cycles x 50% per cycle x 90% efficiency = 113.4 kWh = $0.55/kWh lifetime
LiFePO4 153.6 Wh x 4000 lifetime cycles x 80% per cycle x 99% efficiency = 486.6 kWh = $0.17/kWh lifetime

Over the battery’s lifetime, the LiFePO4 battery represents a much better value. My cost to charge and discharge a kWh is less than one third the cost with a lead acid battery.

Still, there’s that painful upfront cost. Can I do even better?

 
Used LiFePO4 Batteries

For the home DIY enthusiast and backyard tinkerer, used LiFePO4 batteries are an interesting prospect. If you’re like I was, the concept of buying used batteries may seem about as desirable as buying used toothbrushes. However, with the very long lifetimes of LiFePO4 batteries, pre-owned batteries can make good economic sense.

Many LiFePO4 batteries are used in medical and industrial applications where they’re replaced on a fixed schedule every 2-3 years. These batteries are probably not used every day, nor routinely discharged fully. A gently used three year old LiFePO4 battery might only have 500 cycles on it, and still retain 80% or more of its originally rated capacity.

After scouring eBay for a few days, I purchased this used 45 Ah / 576 Wh LiFePO4 battery for $167.50 shipped. It advertised “scheduled removal from well maintained equipment”. I’m taking a chance of course, but if this battery retains 80% of its original capacity, then $167.50 for what’s effectively a high-quality 36 Ah / 461 Wh LiFePO4 is a bargain. At $0.36/Wh it’s about two-thirds the cost of el-cheapo brand LiFePO4 batteries available new.

The only puzzling thing about this battery is the maximum charge and discharge rate of 20 amps. While it’s more than I ever anticipate needing, it seems curiously low for a 45 Ah LiFePO4 battery, representing a charge/discharge rate of 0.44C. From the reading I’ve done, a LiFePO4 battery should easily support 1C discharge and probably even higher.

I’ll report more about this used battery once I get my hands on it.