A lithium battery's runtime comes down to one simple relationship: usable capacity divided by the power your equipment draws. For most 12V systems, divide the battery's amp hour (Ah) rating by your device's amp draw to get hours of runtime, then adjust for the depth of discharge your battery chemistry allows. A properly sized 100Ah lithium iron phosphate (LiFePO4) battery running a 5 amp load will typically run for around 20 hours before recharging is needed, a useful baseline to keep in mind when comparing LiFePO4 batteries for your own setup.
What Determines How Long a Lithium Battery Will Run
Every runtime question comes down to three variables: how much energy the battery stores, how much power your equipment draws, and how much of that stored energy you can safely use. Get those three numbers right and the math becomes simple. It is also worth keeping runtime separate from charge time: runtime is how long a battery powers your gear, while charge time is how long it takes to refill that capacity, and the two are independent of each other.
Amp hours, watt hours, and why the units matter
Battery capacity is usually listed in amp hours (Ah), and appliance or motor draw is usually listed in amps or watts. If your equipment spec sheet only lists watts, convert it to amps by dividing watts by voltage (amps = watts ÷ volts). The same relationship applies to capacity: amp hours equal watt hours divided by voltage, as outlined in this FAA guidance for calculating battery watt-hours. Mixing up amp hours and watt hours is one of the most common reasons a quick calculation ends up wrong.
Voltage and how it changes your math
A 100Ah battery at 12V and a 100Ah battery at 24V do not store the same energy. The 24V battery holds twice the watt hours, because watt hours equal amp hours multiplied by voltage. This matters most for trolling motors, where 12V, 24V, and 36V systems are common, and for RV setups that mix a 12V house battery with higher voltage appliances.
Depth of discharge and why LiFePO4 changes the equation
Depth of discharge (DoD) is the percentage of a battery's capacity you actually use before recharging. Many lead-acid batteries lose cycle life quickly past 50 percent DoD, while LiFePO4 batteries are commonly rated to handle a much deeper discharge without the same long-term wear. That difference is why a 100Ah LiFePO4 battery often delivers close to its full rated runtime, while a 100Ah lead-acid (deep cycle) battery may only give you about half of that before recharging is recommended. That gap is also why the answer to how long a deep cycle battery lasts depends heavily on chemistry: well-maintained lead-acid deep cycle batteries typically hold up for a few years of regular use, while Bioenno's LiFePO4 batteries are commonly rated for many more charge cycles over their lifespan.
The Simple Runtime Calculator You Can Do By Hand
You do not need special software to estimate runtime. The calculation only requires three numbers, and you likely already have two of them on a spec sheet or product label.
To estimate runtime, gather:
- Your battery's amp hour (Ah) rating, from the label or spec sheet
- Your equipment's amp draw, from the device manual or a clamp meter
- Your usable depth of discharge, based on battery chemistry (LiFePO4 vs lead-acid)
The basic runtime formula
Runtime in hours equals usable amp hours divided by the amp draw of your equipment. Usable amp hours equal the battery's rated Ah multiplied by its usable depth of discharge. For a 100Ah LiFePO4 battery, that is close to 100Ah of usable capacity. For a 100Ah lead-acid battery kept to 50 percent DoD, that is only 50Ah of usable capacity.
A worked example for a trolling motor
A 24V trolling motor drawing 30 amps on medium speed, paired with a 100Ah 24V LiFePO4 battery from Bioenno's marine batteries lineup, gives roughly 100Ah divided by 30A, or about 3.3 hours of continuous runtime at that speed. Most anglers do not run a trolling motor at full draw all day, so actual time on the water is often longer, but this baseline number is what to plan around for a full day of fishing.
A worked example for RV boondocking
A camper running a 12V refrigerator (about 4 amps), LED lighting (about 2 amps), a phone charger (about 1 amp), and a water pump intermittently (averaging about 1 amp) totals roughly 8 amps of continuous draw. A single 100Ah LiFePO4 battery from Bioenno's RV and overlanding batteries lineup would provide close to 12 hours of runtime at that draw, and two batteries in parallel would roughly double that, which is why many off-grid travelers build up capacity in stages rather than buying one oversized battery up front.
Runtime for Boats and Marine Electronics
Trolling motors on 12V, 24V, and 36V systems
Trolling motor runtime depends heavily on speed setting, not just battery size. Running at full throttle can draw two to three times the amps of a medium setting, which shortens runtime significantly. Anglers who fish long days on bass boats or kayaks often benefit from sizing their marine batteries around their highest realistic draw, not just an average speed, so the battery does not run low before the trip is over.
Fish finders, livewells, and other marine electronics
Fish finders, livewells, and aerators typically draw far less than a trolling motor, often under 2 amps combined, but they run continuously for the entire outing. On a shared battery bank, these smaller continuous loads add up over a full day and should be included in any runtime estimate, not just the trolling motor's draw.
Runtime for RVs, Van Life, and Off-Grid Setups
Everyday RV and van appliance loads
Runtime planning for RVs and vans usually starts with a simple load list. Typical draws look like this:
- LED lighting: 1 to 3 amps total
- 12V refrigerator: 3 to 6 amps
- Water pump: 3 to 5 amps when running
- Laptop or device charging: 1 to 3 amps
- Vent fan on low: 1 to 2 amps
Adding up expected daily hours of use for each item, then converting to total amp hours needed, is a more accurate way to size a battery bank than guessing based on trip length alone.
How solar charging reduces how often you recharge
Solar panels do not change how long a battery runs on a single charge, but they change how often you need to recharge it, which is often the more important number for extended boondocking or off-grid living. Readers planning a solar setup can walk through sizing decisions in more detail in this battery sizing guide for first-time solar users.
Common Sizing Mistakes That Lead to Under Buying or Overpaying
Underestimating startup and surge loads
Some equipment, particularly pumps, compressors, and certain motors, draws a short burst of higher current when it first turns on. If a runtime calculation only accounts for steady-state draw, the battery may struggle during that startup moment even though the average math looked fine.
Ignoring temperature and charge cycle habits
Cold temperatures can temporarily reduce usable capacity in most battery chemistries, and frequent deep discharges can shorten a lead-acid battery's usable life faster than expected. These two factors are why the same 100Ah rating can feel different in practice depending on season and usage habits.
A few other patterns are worth watching for when sizing a battery:
- Buying based on physical battery size or price alone, without checking the Ah or Wh rating
- Sizing only for average draw and skipping the highest-draw equipment on the boat, rig, or setup
- Assuming a lead-acid and a LiFePO4 battery with the same Ah rating will deliver the same usable runtime
- Forgetting to add up multiple small continuous loads that run for the entire trip
Choosing the Right Battery Size for Your Setup
Matching capacity to your daily power draw
The most reliable way to choose a battery size is to total the amp hours your equipment needs for a full day, then match that to a battery's usable capacity rather than its advertised rating alone. Browsing the full lineup of LiFePO4 batteries side by side makes it easier to compare Ah ratings and voltage options against that daily total. The same math works for outdoor recreation gear: a 12V 40Ah battery running a 3 amp cooler or camp light setup will last a little over 13 hours between charges, which is a useful baseline for weekend trips.
When it makes sense to size up
Travelers who boondock for multiple days without recharging, or anglers running electronics all day on the water, generally benefit from sizing up rather than cutting it close. For RVs and overlanding rigs specifically, reviewing options built for that use case, such as RV and overlanding batteries, can help match capacity to trip length instead of just daily use. For smaller portable needs like charging devices, running lights, or powering a Field Day radio setup, a dedicated portable power option may be a better fit than a larger fixed battery bank.
Once you know your daily amp hour needs, choosing the right battery becomes a matter of matching capacity and voltage rather than guessing. Bioenno's LiFePO4 batteries are built and supported in the USA and backed by a comprehensive warranty, so you can size your setup with confidence. Shop now and choose the Bioenno LiFePO4 battery sized for your boat, RV, or off-grid setup. If you aren't sure what battery works for your application reach out to us, we're happy to help!
