Batteries and run time
How long will a 75 Ah battery run a 25 amp draw?
Quick answer
A 75 Ah AGM battery holds a 25 amp draw for about 1.1 hours to the usual fifty percent depth of discharge. The same nameplate in LiFePO4 gives about 2.3 hours, roughly 2.0 times as long, because lithium is barely affected by discharge rate and is not held to half its capacity.
Dividing 75 by 25 gives 3.0 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 75 Ah actually delivers at 25 amps
At 25 amps, a 75 Ah AGM battery gives about 1.1 hours and the same nameplate in LiFePO4 gives about 2.3 hours.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Chemistry | Delivered at 25 A | Usable to recommended depth | Run time |
|---|---|---|---|
| Flooded lead acid | 47 Ah | 23 Ah | 0.9 h |
| AGM lead acid | 56 Ah | 28 Ah | 1.1 h |
| Gel lead acid | 51 Ah | 26 Ah | 1.0 h |
| LiFePO4 lithium | 72 Ah | 58 Ah | 2.3 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 | 0.5 h | 1.1 h | 0.6 h more |
| 50 Ah | 0.7 h | 1.5 h | 0.8 h more |
| 75 Ah | 1.1 h | 2.3 h | 1.2 h more |
| 100 Ah | 1.6 h | 3.1 h | 1.5 h more |
| 120 Ah | 1.9 h | 3.7 h | 1.8 h more |
| 150 Ah | 2.5 h | 4.7 h | 2.2 h more |
| 200 Ah | 3.5 h | 6.3 h | 2.8 h more |
What a fishing day actually draws
The figure above is a continuous 25 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 1.1 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 = 75 Ah, I = 25 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.
Batteries rated at 75 Ah or more
Chosen against the numbers on this page rather than from a general list, so the recommendation cannot contradict the arithmetic above. Every figure quoted is the manufacturer own published specification.
ECO-WORTHY 12V 100Ah LiFePO4 battery with state of charge display
$229.99 Published specsA state of charge display on the case, which removes the single most annoying thing about lithium: it holds voltage until it does not.
Check price on Amazon
Mighty Max MM-G31M 12V 110Ah deep cycle marine AGM
$249.99 Published specsA dual purpose Group 31M that states deep cycle marine duty in its own title rather than automotive starting duty.
Check before buying: Dual purpose means it compromises both jobs. A dedicated deep cycle lasts longer on a trolling motor.
Check price on Amazon
VMAX MR127 12V 100Ah AGM deep cycle battery
$259.96 Published specsThe AGM most commonly recommended for 40 to 100 lb trolling motors, and its own listing says exactly that.
Best for: A conventional trolling motor bank where the budget rules out lithium
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VMAX MR137-120 12V 120Ah AGM Group 31 battery
$279.96 Published specsTwenty extra amp hours in the same Group 31 case, which is free capacity if the tray already fits a 31.
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Renogy 12V 100Ah trolling motor LiFePO4 battery
$279.99 Published specsIP67 rated with Bluetooth monitoring and a 100 A BMS, from a brand with a real support path when something goes wrong in year three.
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Interstate Batteries Marine and RV Group 31 AGM, 12V 100Ah
$284.95 Published specsA brand with a nationwide warranty counter, which is worth real money on a part that fails away from home.
Check price on AmazonHow we chose
We did not test this equipment in person and we never claim to. Picks are researched from manufacturer specification sheets, published engineering and regulatory figures, and the recurring themes in verified owner reviews. Every pick is matched to a real thrust rating, a real published amp draw, a real capacity or a real conductor size rather than to a price bracket, and it is placed in the tier where its published capability actually belongs. We rejected the unbranded flood that dominates several of these search results, because a lithium pack whose management board is the only thing between a cell fault and a fire, a breaker protecting a battery cable, a life jacket and a bracket holding a two thousand dollar motor to the bow at highway speed are all parts whose failure has a consequence nobody can walk away from. Where a figure is convention rather than a published standard we label it that way, and where a manufacturer does not publish a figure we leave it out instead of estimating it.
Frequently asked questions
How long will a 75 Ah battery run a 25 amp motor?
About 1.1 hours in AGM and about 2.3 hours in LiFePO4, taking each to its recommended depth of discharge. The simple division of 75 by 25 gives 3.0 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 75 Ah AGM battery delivers about 56 Ah at a 25 amp draw rather than 75. 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 2.0 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.