Batteries and run time
How long will a 120 Ah battery run a 56 amp draw?
Quick answer
A 120 Ah AGM battery holds a 56 amp draw for about 0.8 hours to the usual fifty percent depth of discharge. The same nameplate in LiFePO4 gives about 1.6 hours, roughly 2.1 times as long, because lithium is barely affected by discharge rate and is not held to half its capacity.
Dividing 120 by 56 gives 2.1 hours, and that answer is wrong for every chemistry on this page. Two separate effects cut it down.
The first is Peukert. A battery printed capacity is measured discharging it over 20 hours. Pull it faster and it delivers less, and how much less depends on chemistry. The second is depth of discharge. Lead acid is conventionally taken to half its capacity to protect cycle life, so half the rating was never available in the first place. Lithium is close to immune to the first and is planned at eighty percent on the second, which is most of why it costs three times as much.
What 120 Ah actually delivers at 56 amps
At 56 amps, a 120 Ah AGM battery gives about 0.8 hours and the same nameplate in LiFePO4 gives about 1.6 hours.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Chemistry | Delivered at 56 A | Usable to recommended depth | Run time |
|---|---|---|---|
| Flooded lead acid | 69 Ah | 34 Ah | 0.6 h |
| AGM lead acid | 86 Ah | 43 Ah | 0.8 h |
| Gel lead acid | 77 Ah | 38 Ah | 0.7 h |
| LiFePO4 lithium | 115 Ah | 92 Ah | 1.6 h |
Peukert exponents and the twenty hour rating basis are published battery engineering. The depth of discharge figures used alongside them are convention for lead acid and manufacturer guidance for lithium, and the two are combined here in that order.
The same draw at other capacities
Doubling capacity does not quite double run time on lead acid, because the discharge rate relative to capacity falls at the same time.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Rated capacity | AGM run time | LiFePO4 run time | Difference |
|---|---|---|---|
| 35 Ah | 0.2 h | 0.5 h | 0.3 h more |
| 50 Ah | 0.3 h | 0.7 h | 0.4 h more |
| 75 Ah | 0.4 h | 1.0 h | 0.6 h more |
| 100 Ah | 0.6 h | 1.4 h | 0.7 h more |
| 120 Ah | 0.8 h | 1.6 h | 0.9 h more |
| 150 Ah | 1.0 h | 2.1 h | 1.1 h more |
| 200 Ah | 1.4 h | 2.8 h | 1.4 h more |
What a fishing day actually draws
The figure above is a continuous 56 amp draw, which almost nobody produces. A published maximum draw happens at maximum thrust, and a day of fishing is mostly low settings with occasional bursts. That is why a motor that would flatten this battery in 0.8 hours at full power will happily fish all day.
The honest way to plan is to measure your own average draw with a clamp meter across a normal day, then run that figure rather than the rating. No manufacturer publishes a current draw table by speed setting, so any per-speed number you find, including the modelled one used in our calculator, is an estimate rather than a specification.
Peukert, then depth of discharge
Published figuret = H x ( C / ( I x H ) ) ^ k H = 20 hours, C = 120 Ah, I = 56 A usable run time = t x depth of discharge k = 1.25 flooded, 1.15 AGM, 1.20 gel, 1.02 LiFePO4 depth of discharge = 0.50 lead acid, 0.80 LiFePO4
Peukert equation and the twenty hour rating basis are published battery engineering. The depth of discharge multiplier is convention on lead acid and manufacturer guidance on lithium.
Frequently asked questions
How long will a 120 Ah battery run a 56 amp motor?
About 0.8 hours in AGM and about 1.6 hours in LiFePO4, taking each to its recommended depth of discharge. The simple division of 120 by 56 gives 2.1 hours and is wrong for both, because it ignores the Peukert effect at high discharge rates and ignores the fact that half a lead acid battery was never available.
Why does a lead acid battery deliver less than its rating?
Because the rating is measured over twenty hours. Peukert equation describes how delivered capacity falls as discharge current rises, and the effect is substantial on lead acid: a 120 Ah AGM battery delivers about 86 Ah at a 56 amp draw rather than 120. LiFePO4 sits at an exponent of roughly 1.02, which is close enough to no effect at all that the difference disappears into rounding.
Can I take a lead acid battery below fifty percent?
You can, and it will start the boat home. The fifty percent guideline exists to protect cycle life rather than to describe where the battery stops, and repeatedly taking a deep cycle lead acid battery to twenty percent shortens its life dramatically. It is convention rather than a published limit, and the manufacturer own cycle life curve for your specific battery is a better guide.
Is lithium worth three times the money?
On these numbers it buys about 2.1 times the run time from the same nameplate, plus roughly two thirds less weight and a charge that finishes in a fraction of the time. Whether that is worth it depends entirely on whether you run out of battery. If you finish a normal day with plenty left in an AGM, lithium buys you nothing you use.
Does temperature change the answer?
Considerably, and in both chemistries. Lead acid capacity falls noticeably in cold water, which is exactly when early season anglers are fishing. LiFePO4 holds capacity better in the cold but must not be charged below freezing without a battery management board that handles it, which is why low temperature protection appears in the specifications of the better packs.
Should I measure my own draw instead?
Yes, and it is the single most useful measurement you can take. A clamp meter on the motor positive cable across a normal fishing day gives an average draw that is usually far below the published maximum, and running the numbers on that figure rather than the rating is the difference between planning for the day you actually have and planning for one you never fish.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.