Sizing Flooded Lead Acid and Lithium (LIFEPO4) Battery Banks
Backwoods Solar knows batteries. For over 40 years, our office and our employees have used them on a daily basis. During these four plus decades, we have fine-tuned the art of sizing and maintaining flooded lead acid (FLA) batteries, and for the last four years, we have come to understand the ins and outs of Lithium batteries as we use them in our office’s emergency backup system. Additionally, within the last two years, we’ve had two employees install Lithium batteries in their home’s off-grid systems and we’ve added another Lithium setup in the office.
Given the incredible interest we see in Lithium batteries and the ongoing use of FLA batteries, we thought we’d share our approach to sizing a FLA battery bank versus a Lithium battery bank. Let’s start with a FLA battery bank.
To get the longest life out of a FLA battery bank, ideally, that bank gets fully recharged every day. Realistically that does not happen. In our experience, a FLA battery bank should not get discharged below 50% AND it should be fully recharged every three days. Therefore, when we size a FLA battery bank, we choose to power your home for three days; we assume zero charging of the battery bank over those three days; and we insist the battery bank does not get discharged more than 50%. A battery bank is usually described in terms of your system’s nominal voltage (12, 24, or 48) and the battery bank’s amp-hour capacity at 72 degrees F. Energy consumed by your home is usually described in watt-hours. To square those two descriptions, we multiply a battery banks nominal voltage by its rated amp-hour capacity and the product we get is watt-hours of capacity. For example, a 12v 375 amp-hour battery bank has 4500 watt-hours of rated energy capacity (12 x 375 = 4500). Re-worked, 4500 watt-hours divided by 12 volts = 375 amp-hours.
With that understanding, let’s size a battery bank to support a home that consumes 6000 watt-hours of energy per day and has a nominal system voltage of 48. (We choose to ignore battery room temperatures above or below 72 degrees F as such specificity just clouds the picture and doesn’t adversely affect most systems.) To avoid discharging the FLA battery bank by more than 50% after three days without any charging, we size the battery bank for six days of energy consumption. So, 6000 watt-hours consumed per day times six days equals 36,000 watt-hours of energy capacity needed. By dividing the 36,000 watt-hours of energy capacity needed by the system’s nominal 48 volts, we tentatively determine we need a battery bank with 750 amp-hours of capacity. (36,000 divided by 48 = 750)
However, Backwoods Solar also believes the “rated” capacity of most FLA batteries exceeds the actual capacity by 20%. Therefore, in the above example, we would multiply the 750 amp-hour battery bank by 1.2 and conclude that one needs a 900 amp-hour 48v FLA battery bank to support 6000 watt-hours per day of energy consumed. Trojan offers a 6v 370 amp-hour FLA battery. Eight of these 6v batteries wired in series gives us a 48v 370 amp-hour battery bank. Since we’ve determined we need a 48v 900 amp-hour battery bank to support our loads, Backwoods would suggest twenty four of these 6v 370 amp-hour batteries (three strings wired in parallel with eight batteries per string) which gives us a 48v 1110 amp-hour capacity battery bank.
Lithium batteries, like the Eclipse LIFEPO4 that Backwoods Solar offers, can be discharged by 90% but we do not recommend doing so regularly. For the purpose of sizing a Lithium battery bank, Backwoods chooses a maximum discharge of 80%. For an apples-to-apples comparison with FLA batteries which use nominal voltages of 12, 24, or 48 to calculate battery capacity, we will use the same voltages for our Eclipse Lithium battery calculations.
Eclipse offers a 48v 100 amp-hour battery which translates into a battery with 4800 watt-hours of rated capacity (48 x 100 = 4800). If we discharge this battery by 80% for battery bank sizing purposes, we have consumed 3840 watt-hours of energy. In our above example, we wanted a three day, 18,000 watt-hour, supply of energy during which time the batteries would receive zero charge. If one 48v 100 amp-hour Eclipse offers 3840 watt-hours of usable energy, then we would need five of these batteries (18,000 divided by 3840 = 4.6875 and we round up to five).
Backwoods has not determined that the “rated” battery capacity is exaggerated for the Eclipse Lithium battery. However, if you wanted to add the same 20% to the calculated battery bank size as we did with our FLA battery bank example, then you would want six of the Eclipse 48v 100 amp-hour batteries (4.6875 x 1.2 = 5.625 and we round up to six).
Because Lithium based batteries have little to no battery memory, they can sit at less than a fully charged state for long periods of time and still fully recharge normally. This feature is a huge advantage for an off-grid household. Lithium batteries can be run at a 40%-80% state of charge for days on end as you wait for a sunny day to provide a good charge. This advantage significantly reduces generator run time and consequently fuel consumption. This “Lithium strategy” should also incorporate a larger solar array capable of recharging a discharged battery bank in as little as one to two days of full sun. With today’s solar panels historically much less expensive relative to Lithium batteries, it is common to have larger solar arrays and smaller battery banks. The Lithium strategy works well as long as the operator understands the system. If the sun simply does not shine, more frequent, but shorter charges from a generator will be needed to prevent the battery bank from getting too low. It is very important that Lithium batteries don’t get too low, as they may need a “jumpstart” to turn the batteries back on prior to their being able to take a charge. (As of December 2020, some of our Eclipse Lithium batteries now have an onboard “jumpstart” button!)
It is also important to pay attention to the charge/discharge rate of Lithium batteries. Many Lithium batteries on the market can only accept or deliver a relatively small amount of current. Too much current going into or out of a Lithium battery can cause the battery to shut off or possibly damage the battery. In these cases, programming your equipment to deliver appropriate charge and discharge rates is the only way to avoid shut down and/or damage. Backwoods knows the Eclipse battery current parameters and we will help you ensure your equipment is programmed correctly so you get the longest possible life out of your Eclipse battery bank.
Our employee, Brian Betz, has tested two of the Eclipse 48v 100 amp-hour batteries since July of 2019 in his off-grid home’s system. The battery bank is undersized for his roughly 3000-5000 watt-hours of energy consumed per day. He purposefully designed a smaller battery bank in order to more aggressively test these Eclipse batteries. In the summer, when his micro-hydro system is not producing power, he frequently consumes 20% of the battery bank’s capacity from evening to morning. Fortunately, the following day usually offers sunshine and his solar array recharges the battery bank. He could certainly use and would highly recommend a larger battery bank for his amount of electricity usage. However, given the battery bank he does have, he faithfully keeps a close eye on his Midnite Whizbang Jr. battery meter to monitor accurate battery bank state of charge information. With careful observation and a willingness to run a generator, his small battery bank works well and with the long cycle life of the Eclipse batteries, his Lithium battery bank should last for many years to come.
That's a lot of information to absorb but the friendly staff at Backwoods Solar offers decades of experience to help you design a new battery bank, or replace an older off-grid battery system that will provide years of reliable energy storage. Call us today at 208-263-4290 for a quote.
Backwoods Solar will be closed Friday, December 25th and Friday, January 1st to observe Christmas Day and New Years Day. We wish everyone a safe and healthy holiday season.
Only 21 Days Left 26% Federal Solar Tax Credit Ends Soon
While we're on the topic of sizing battery banks, it's a good time to remind you that purchasing a new battery bank for your current solar electric system can qualify for the 2020 26% Federal Solar Tax Credit. But time is short, only three weeks remain to take advantage of this tax credit and save money.
The Residential Renewable Energy Tax Credit is a dollar-for-dollar reduction in income taxes when homeowners purchase and install solar and/or wind electric systems. Click here to get more information. Many states and local jurisdictions also offer incentives in the form of rebates, tax credits, grant programs, and access policies. You can search herefor policies and incentives in your state. By combining these incentives with federal tax credits a homeowner can achieve tremendous savings on their solar electric system installation.
As always, consult your accounting or tax professional to determine your actual tax liability and how this tax policy can benefit you. Don't wait, call 208-263-4290 today to take advantage of these savings.
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