Why the Price per Ampere-Hour is Misleading
A datasheet compares nominal capacities. However, you are not buying nominal capacity; you are buying usable energy over a period of use. Between these two figures are two factors that differ significantly between technologies: the permissible depth of discharge and the number of cycles.
A lead-acid battery — whether AGM, gel, or flooded — can be discharged by about half in continuous operation. So, out of 200 Ah nominal capacity, about 100 Ah are usable. Discharging it deeper won't be immediately noticeable, but will result in a significantly shorter lifespan. A LiFePO4 cell can tolerate 80 to 90 percent depth of discharge without significant losses; from 200 Ah, this becomes about 170 Ah.
The second factor is the number of cycles until 80 percent remaining capacity. And this is where the error lies that makes almost every comparison useless: A cycle count without the corresponding depth of discharge is not information. The same AGM battery achieves around 1,500 cycles at 30 percent depth of discharge, about 600 at 50 percent, and only about 400 at 80 percent. Comparing the 1,500 from the brochure with a lithium value at full discharge means comparing two different operating modes.
| Depth of Discharge | AGM | Gel | LiFePO4 |
|---|---|---|---|
| 30 % | approx. 1,500 | approx. 1,800 | well over 5,000 |
| 50 % | approx. 600 | approx. 750 | over 5,000 |
| 80 % | approx. 400 | approx. 500 | 2,500–3,500 |
Cycles until 80 percent remaining capacity, typical ranges for onboard applications at 25 °C. At the same depth of discharge, LiFePO4 is ahead by a factor of six to eight — that's the honest comparison.
Third, charge acceptance. Lead-acid only accepts limited charging current, typically 0.2 to 0.3 C, and spends the last 20 percent in a long absorption phase. If you have two engine hours daily, you will practically never fully charge a lead-acid battery bank — and a chronically partially charged lead-acid battery will sulfate and not even come close to its datasheet cycle count. This is not a marginal case: in practice, it is the most common reason why lead-acid battery banks die earlier than expected. LiFePO4 accepts full current almost until the end.
First Calculation: Cost per Ampere-Hour Withdrawn
FormulaCost per Ah = Purchase Price ÷ (Nominal Capacity × Depth of Discharge × Cycles)
All four values are in the datasheet or offer — the important thing is that the cycle count and depth of discharge match. A calculation example for a battery bank with 300 Ah usable capacity. The prices are assumptions and only serve to illustrate the method; use your own quotes.
| Size | AGM | LiFePO4 |
|---|---|---|
| Required Nominal Capacity | 600 Ah | 375 Ah |
| Assumed Depth of Discharge | 50 % | 80 % |
| Cycles at this Depth of Discharge | approx. 600 | approx. 2,500 |
| Purchase Price (Assumption) | €1,200 | €2,900 |
| Usable Energy over Lifespan | 180,000 Ah | 750,000 Ah |
| Cost per Ah Withdrawn | 0.67 Cents | 0.39 Cents |
| Weight of the Battery Bank | approx. 180 kg | approx. 45 kg |
Factor 1.7 in favor of LiFePO4 — significantly less than usually stated in sales arguments. The reason: the AGM is calculated here at 50 percent, its most favorable sensible operating point.
Second Calculation: Cost over a Period of Use
The first calculation assumes that the cycles are actually used. This is precisely where the argument fails in practice. Batteries are not paid for in cycles, but in replacements over a period of time. It is therefore crucial how often you have to buy a new battery bank during this period.
| Full Cycles per Year | In 15 Years | AGM | LiFePO4 | Cheaper |
|---|---|---|---|---|
| 30 (Long-term berth) | 450 | 1 Battery Bank · €1,200 | 1 Battery Bank · €2,900 | AGM |
| 50 (Weekend trips) | 750 | 2 Battery Banks · €2,400 | 1 Battery Bank · €2,900 | AGM |
| 80 (Longer trips) | 1,200 | 2 Battery Banks · €2,400 | 1 Battery Bank · €2,900 | AGM |
| 100 (Anchoring) | 1,500 | 3 Battery Banks · €3,600 | 1 Battery Bank · €2,900 | LiFePO4 |
| 150 (Long voyage) | 2,250 | 4 Battery Banks · €4,800 | 1 Battery Bank · €2,900 | LiFePO4 |
With the price assumptions from above and a 15-year consideration period. Installation and removal, disposal, and additional charging technology for the switch are not included — they further shift the picture in favor of the longer-lasting battery bank.
The break-even point is therefore around 100 full cycles per year, i.e. about one hundred nights at anchor or standing autonomously. Below this, the pure cost calculation is not an argument for lithium — then weight, space, charging time, and the question of whether you even have enough charging power to fully charge a large lead-acid battery bank become decisive.
Two corrections to this calculation, both from practice: First, lead-acid battery banks rarely achieve their datasheet cycles because they are chronically operated in a partially charged state — this shifts the break-even point downwards. Second, LiFePO4 also ages calendarically, regardless of use. If you only do 30 cycles a year, you would only reach 2,500 cycles mathematically after over eighty years — the cell will be at the end of its life long before that. The calculated savings will then never materialize.
Exceptions: When AGM Remains the Better Choice
Long-term berth with shore power. Few cycles, uncritical consumption profile, existing charging infrastructure. Here, the surcharge does not pay off.
Operation in frost. LiFePO4 must not be charged at low temperatures — manufacturers' specifications for the lower charging limit range from −5 to +5 °C. If the boat remains in operation during winter and a heated battery is not an option, lead-acid is the more robust solution.
Starter battery. Lead-acid is designed and proven for short, very high currents and continuous charging from the alternator. A change is not technically worthwhile here.
Existing system without conversion budget. The battery price is not the only item when changing technology. Charger or charge controller with lithium characteristic, external controller or charging converter for the alternator, solar charge controller with suitable charging end voltage, battery monitor with shunt, and fuse protection designed for higher short-circuit currents are added. If you only have the battery in your budget, you are buying a system that will not work.
WarningA battery bank of LiFePO4 cells will suddenly disappear from the onboard power supply in case of a fault if the BMS disconnects under load. The alternator will then no longer be working against a battery, and the resulting voltage spike regularly destroys the rectifier. An external controller with a buffer battery or a charging converter are part of the planning, not an optional extra.
Typical Mistakes in Comparison
- Comparing nominal capacities. 200 Ah AGM and 200 Ah LiFePO4 are not the same product. Compare usable capacity.
- Citing cycles without corresponding depth of discharge. The most common trick in sales literature — on both sides.
- Only calculating the purchase. The decisive factor is the number of replacements over your period of use.
- Ignoring the follow-up costs of the switch. Charge controllers, fusing, and monitoring must be included in the calculation.
- Forgetting calendar aging. With low usage, it limits the lifespan, not the cycle count.
- Not evaluating charging time. If you only have short engine phases, you will never fully charge a large lead-acid battery bank — the calculated capacity will then never be available.
- Overlooking weight and trim. 180 kg versus 45 kg noticeably changes the situation for smaller vessels.
How to Proceed
First, determine your daily consumption and from that, the usable capacity. Only then does the technology comparison have a basis — without this figure, you are comparing products that do not fit the system at all. Then estimate the full cycles per year based on your actual usage profile, not the planned one: One cycle is a complete discharge within the permissible depth of discharge; two half nights at anchor are one cycle. Finally, obtain quotes for both variants, including the necessary charging technology, and plug the numbers into both calculations above.
