Batteries
Battery run time chart: the published 20 hour rate against real draws
Quick answer
The published 20 hour rate means a 100 Ah battery's label capacity assumes a slow discharge. At a realistic 42 amp trolling motor draw, Peukert math shows an AGM delivers about 36 usable amp hours while LiFePO4 delivers about 77 from the same 100 Ah rating, before either recharges.
A battery's printed amp hour rating is measured at the published 20 hour rate, discharging it slowly enough that the full number is technically achievable. A trolling motor never draws anywhere near that slowly, and Peukert's equation, published battery engineering rather than a manufacturer trick, describes exactly how much of that rated capacity actually shows up at a faster draw.
This page runs the Peukert equation and the depth of discharge guideline through real trolling motor draws for both major chemistries, then works the same math backward to show how many rated amp hours are actually needed to hold a target run time. Whichever chemistry the run time numbers below point toward, recharging a lithium bank afterward needs a charger with a lithium profile rather than a lead acid setting.
On this page
The 20 hour rate
The 20 hour rate means a 100 Ah battery is tested by drawing 5 amps continuously for 20 hours. Draw the same battery faster and Peukert's equation predicts it delivers less than the rated number, because internal resistance and plate chemistry both perform worse under a heavier load. This is published battery engineering, not a manufacturer disclaimer, and it applies to every lead acid chemistry to some degree and to LiFePO4 only slightly.
Peukert exponents by chemistry
LiFePO4's Peukert exponent of 1.02 is close enough to 1 that fast draws barely dent its delivered capacity, while flooded lead acid at 1.25 loses the most.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Chemistry | Peukert exponent (published) |
|---|---|
| Flooded lead acid | 1.25 |
| AGM lead acid | 1.15 |
| Gel lead acid | 1.20 |
| LiFePO4 lithium | 1.02 |
Run time and usable capacity at real trolling motor draws
The draws below match the published maximum current of common stocked trolling motors: 30 amps for a 30 lb thrust motor, 42 amps for the 40 to 55 lb thrust range, and 50 amps for a 55 lb thrust motor at its published maximum.
At every draw in this table, a 100 Ah LiFePO4 battery returns more than double the usable amp hours of a 100 Ah AGM battery, because its Peukert exponent sits close to 1 and its depth of discharge guideline is higher.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Draw (amps) | AGM safe run time (hours) | AGM usable Ah | LiFePO4 safe run time (hours) | LiFePO4 usable Ah |
|---|---|---|---|---|
| 30 | 1.27 | 38.2 | 2.57 | 77.2 |
| 42 | 0.87 | 36.3 | 1.83 | 76.7 |
| 50 | 0.71 | 35.4 | 1.53 | 76.4 |
The Peukert exponent behind this table is published. The depth of discharge fraction applied to reach a safe run time is convention on lead acid and manufacturer guidance on lithium, which is why the combined figures here are labelled convention rather than standard.
Depth of discharge by chemistry
Lead acid convention holds at 50 percent depth of discharge to protect cycle life, while LiFePO4 makers publish 80 percent as the conservative planning figure.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Chemistry | Recommended depth of discharge |
|---|---|
| Flooded lead acid | 50% |
| AGM lead acid | 50% |
| Gel lead acid | 50% |
| LiFePO4 lithium | 80% |
Kept separate from the Peukert table above on purpose. Peukert is published physics describing how delivered capacity falls with current. Depth of discharge is a guideline about how much of that delivered capacity to actually use before recharging, and the two should never be read as the same kind of number.
How many rated amp hours you need for a target run time
Inverting the same formula answers a different question: given a target run time at a fixed draw, how many rated amp hours does the battery need to start with.
Holding 30 amps for 4 hours needs about 270 rated amp hours in AGM but only about 154 in LiFePO4, which is why a single lithium pack often replaces two or three lead acid batteries.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Target run time at 30 amps | AGM rated Ah needed | LiFePO4 rated Ah needed |
|---|---|---|
| 2 hours | 148 | 78 |
| 4 hours | 270 | 154 |
This inverts the same Peukert and depth of discharge relationship used above, so the same convention label applies to the depth of discharge component.
Frequently asked questions
What is the 20 hour rate?
The 20 hour rate is the published discharge standard used to rate deep cycle batteries: drawing a fixed current continuously for 20 hours until the battery is empty. A 100 Ah battery rated this way is being drawn at 5 amps for those 20 hours. Discharge the same battery faster and Peukert's equation predicts it delivers less than the full 100 Ah, which is exactly why a trolling motor never gets the printed number.
Does a trolling motor really draw its maximum published current the whole trip?
No. The published maximum draw figure is measured at maximum thrust, and most fishing happens at a lower speed setting where the actual current is significantly less. No manufacturer publishes a draw table by speed setting, so the honest way to know an actual draw is to measure it with a clamp meter at the speed typically fished, rather than assuming maximum draw for an entire trip.
Why does LiFePO4 barely lose capacity at a fast draw?
Its Peukert exponent, the published figure describing how delivered capacity falls as current rises, sits at 1.02, which is close enough to a value of exactly 1 that speed has almost no practical effect. Lead acid chemistries sit further from 1, between 1.15 and 1.25 depending on the type, which is why they lose noticeably more of their rated capacity the harder they are drawn.
How many amp hours do I need for a 4 hour trip at 30 amps?
Working the Peukert and depth of discharge relationship backward, holding 30 amps for 4 hours needs roughly 270 rated amp hours in AGM, which usually means multiple batteries, or roughly 154 rated amp hours in LiFePO4, which a single large pack can cover. The exact number shifts with the chemistry's specific Peukert exponent and depth of discharge guideline.
Is depth of discharge the same for every lead acid chemistry?
Yes, by convention. Flooded, AGM and gel lead acid all use the same 50 percent depth of discharge guideline to protect cycle life, even though their Peukert exponents differ slightly. LiFePO4 uses a higher 80 percent figure because lithium chemistry tolerates a deeper regular discharge without the same cycle life penalty that lead acid plates suffer.
Can I just multiply amp hours by hours to get run time?
No, because that ignores Peukert's equation, the published relationship showing that delivered capacity falls as discharge current rises. A 100 Ah battery does not simply run a 50 amp load for 2 hours. Depending on chemistry it may deliver noticeably less than that before even applying the depth of discharge guideline, which is why this chart works through both steps rather than a single division.
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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.