Batteries and run time
What battery capacity runs 30 amps for 2 hours?
Quick answer
Holding 30 amps for 2 hours needs roughly 148 Ah of AGM or about 78 Ah of LiFePO4. In stocked sizes that is a 150 Ah lead acid battery against a 100 Ah lithium one.
The naive answer is 30 times 2, or 60 Ah, and it is short by a wide margin on lead acid and by a little on lithium. Two effects push the requirement up.
Peukert says a battery delivers less than its rating when discharged faster than its twenty hour basis, and the depth of discharge guideline says only half a lead acid battery was ever on the table. Run both backwards and the capacity you have to buy is considerably larger than the energy you plan to use.
Capacity needed for 30 amps over 2 hours
Holding 30 amps for 2 hours needs about 148 Ah of AGM or 78 Ah of LiFePO4.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Chemistry | Energy used | Rated capacity needed | Nearest stocked size |
|---|---|---|---|
| Flooded lead acid | 60 Ah | 166 Ah | 200 Ah |
| AGM lead acid | 60 Ah | 148 Ah | 150 Ah |
| Gel lead acid | 60 Ah | 157 Ah | 200 Ah |
| LiFePO4 lithium | 60 Ah | 78 Ah | 100 Ah |
Energy used is the same in every row. Everything else is the difference between what you use and what you have to buy to be able to use it.
The same draw over other durations
A longer day does not scale the requirement linearly on lead acid, because a larger battery is also being discharged at a gentler rate relative to its capacity.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Target hours | AGM capacity needed | LiFePO4 capacity needed | AGM stocked size |
|---|---|---|---|
| 2 h | 148 Ah | 78 Ah | 150 Ah |
| 4 h | 270 Ah | 154 Ah | 300 Ah |
| 6 h | 385 Ah | 229 Ah | more than one battery |
| 8 h | 494 Ah | 304 Ah | more than one battery |
Is a continuous draw the right thing to plan for?
Usually not, and planning from a published maximum draw buys a bigger battery than the day needs. A published maximum happens at maximum thrust, and a fishing day is mostly low settings.
The right input here is your own average draw across a normal day, measured with a clamp meter on the motor positive cable. That figure is frequently a third to a half of the rating, which changes this answer completely. No manufacturer publishes current draw by speed setting, so the only honest source for an average is your own measurement.
The requirement, worked backwards
Published figurefull discharge time t = target hours / depth of discharge rated capacity C = I x H x ( t / H ) ^ ( 1 / k ) I = 30 A, target = 2 h, H = 20 h
This is the Peukert relation inverted with the depth of discharge guideline folded in, in that order.
Batteries that cover 100 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
Check price on Amazon
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.
Check price on Amazon
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.
Check price on Amazon
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 many amp hours do I need for 30 amps over 2 hours?
About 148 Ah in AGM or 78 Ah in LiFePO4. The energy actually used is 60 Ah in both cases. The gap is the Peukert effect at that discharge rate plus the depth of discharge guideline, which on lead acid means half the nameplate was never available to begin with.
Why is the lithium figure so much smaller?
Two reasons that compound. LiFePO4 has a Peukert exponent near 1.02, so it delivers essentially its rated capacity even at a heavy draw, while AGM at 1.15 gives up a real fraction of it. And lithium is planned at eighty percent depth of discharge rather than fifty. Together those turn a 148 Ah lead acid requirement into roughly 78 Ah of lithium.
Should I size from the published maximum draw?
Only if you genuinely run the motor at full power for the whole trip, which almost nobody does. A published maximum draw is measured at maximum thrust. Sizing from it is conservative and expensive, and measuring your own average draw across a normal day with a clamp meter usually shows a figure well under half the rating.
Does adding a second battery double the run time?
In parallel at the same voltage, close to it, and slightly better than double on lead acid because the pair is discharged at a gentler rate relative to its combined capacity. In series for a higher voltage motor it does not: two 12 volt batteries wired in series make 24 volts at the same amp hours, which buys thrust rather than time.
How much does the extra capacity weigh?
That is usually the real constraint rather than the money. A 100 Ah lead acid battery runs around 60 lb and a 100 Ah LiFePO4 pack around a third of that, and on a small boat or a kayak the weight decides the answer before the price does. The exact figure is a manufacturer specification for each battery rather than a general rule.
What charger do I need for a bank this size?
Roughly ten to thirteen percent of the bank capacity per bank is the lead acid convention, so a 150 Ah bank wants somewhere around 20 amps. Lithium accepts far more and twenty percent is a comfortable onboard figure. What matters more than the amps is that a lithium pack gets a charger with a lithium profile.
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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.