Batteries are such a solid (often made primarily of lead, after all), quiet and passive object, that their “wearing out” seems hard to believe. Underneath their cases of rubber and plastic, however, is a cauldron of electro-chemical processes that can turn motors, power lights, heat water and make life more comfortable. After doing that, providing the batteries with appropriate electricity power can reverse those electro-chemical processes so that the cycle can be repeated.
Our current lithium ferro phosphate (LFP) batteries are the third set of batteries we’ve used. The previous two battery banks were both made up of AGM lead acid batteries. For AGM batteries, typical lifespan is 500-1,000 cycles and four to eight years. Our two AGM battery banks lasted about six years. One of the claims of LFP batteries is greater longevity. Our Battleborn Batteries are rated at 3,000 to 5,000 discharge cycles so we are hoping they will last the rest of our ownership of Alpenglow. Ignoring the small discharge cycles accompanying the transition between shore power and engine alternator, we have had 261 discharge cycles of more than 50 Ah in years 2023 through 2025. Assuming our first full year, 2022, with the new batteries was similar, I estimate that we’ve had fewer than 400 significant discharge cycles so far.
While batteries can fail suddenly, normal “wear and tear” usually shows up as reduced capacity. Our LFP battery bank is rated at 500 amp-hours (Ah), but we have never intentionally consumed that much energy (more about an unintentional discharge later) before recharging. Searching the Internet for methods to determine a batteries capacity usually give solutions that are impractical for “working” batteries because they require they usually require the such the isolation of some or all the batteries from loads for a period of time followed by monitoring the battery while under a constant modest load.
The method I am using to assess the battery bank is monitoring the bank’s voltage over the course of many discharge cycles. While each discharge cycle will be of different duration, depth of discharge, and rate of discharge, I am hoping the aggregation of data will reveal patterns and trends.
In my dataset, for the cruising seasons 2023, 2024 and 2025, I identified 201 discharge cycles that started from a nearly full battery (<2 Ah of discharge) and resulted in a discharge of more than 50 Ah. I had 37,758 measurements of the battery during those discharge cycles. Each measurement included the battery bank’s voltage, state of charge, amp-hours discharged and rate of discharge. These measurements were the average of instantaneous values during the previous six minutes (five minutes in the case of the 2023 cruise year). The discharge cycle ended when the next charge cycle began. In most cases, the charge cycle was the main engine starting, but it could have been the generator starting or even the shore power breaker being switched on if I was purposely cycling the battery bank while at a dock.
Besides the aggregated data from three cruising seasons, I also have one case of the total discharge of the battery bank. This unfortunate and unintentional “test” occurred over about 4-1/2 days during the New Year’s holiday of 2023/2024. The shorepower to the boat was turned off and the battery bank depleted until the battery management system (BMS) incorporated in the individual batteries shut things down to prevent any further depletion. The boat sat for over 3 weeks with dead batteries until I returned to the boat near the end of January for my regular mid-winter visit. I restored shore power and the inverter-charger then recharged the battery. In a call to the battery manufacturer’s technical support, they told us that the accidental complete depletion ought not have any long-term impact on the battery.
Below is the graph of the house bank battery voltage versus the amp-hours discharged for that unfortunate incident. We had most things shut down so the typical discharge was only a little over 4 amps. The discharge curve is relatively flat, although it steepened at about 150 Ah discharged until about 180 Ah whereupon it flattened again, creating a slight “S” shape. A few bumps in the curve are when the diesel furnace came on to heat the boat (we keep the thermostats at 45°) and increased the discharge rate, The discharge curve steepened again at about 350 Ah discharged, then plummeted after about 470 Ah. The BMS turned off the batteries at 509 Ah after 4-1/2 days.
The next graph shows the aggregated discharge curves for the 201 discharge cycles in the cruising seasons of 2023, 2024, and 2025. The scales are different because the range of voltages and amp-hours is more constrained. I have also included the relevant portion of the 2023 total battery discharge as a reference curve since its low, steady discharge rate is closer to what a bench test might be.
Two things strike me about the curves. First, the steepening of the discharge curve (the “S” bend) at about 150 Ah is consistent across all discharge cycles and not an artifact of a particular cycle (e.g., the discharge rate increased because of a device being turned on). It must be inherent in the battery, either something to do with the BMS or the batteries construction (e.g., as the discharge continues additional/different internal portions of the battery are called into action). The second observation is that as the battery is aging, the discharge curve is lowering (i.e., the battery voltage is less for the same level of amp-hours discharged). In the flatter portions of the curve, we are only dealing with 10 millivolts (mV) or less. In the steeper portions of the “S” bend, it is much easier to see because it is closer to a 50 mV difference between the 2025 curve and the 2023 curve. The aggregate data at higher amp-hour discharge is noiser because I have fewer discharges going that deep and therefore the varying loads (e.g., furnace, inverter, Starlink) that come on or drop off can swing the voltage.
We are still very happy with the LFP battery bank we installed in 2021 and have no regrets. They make cruising more pleasant in not having to worry about managing loads or charging schedules. Compared to our previous AGM batteries they charge dramatically faster and reduce the time we must run the generator when we are spending multiple days at anchor. As to their longevity, from our previous experience with AGM batteries, I believe I would be noticing far more reduction in capacity in the AGM batteries at this point, four cruising seasons, than I’ve detected in the LFP batteries.
All of the measurements I’ve used in the analysis, battery voltage, state of charge, amp-hours discharged and rate discharge were obtained from our battery monitor, a Victron BMV-712 installed in 2021 as part of the LFP battery installation. The BMV-712 directly measures the battery voltage. The current going into or out of the battery bank is computed by the BMV-712 via measuring the tiny voltage drop across a 500A shunt (a known resistance) and applying Ohm’s Law. The BMV-712 computes the other values from tracking the current flows over time. At the time of installation, the BMV-712 was configured with the appropriate values for our battery bank.
