Batteries and run time
What battery capacity do you actually need?
Quick answer
Holding 40 amps for 6 hours uses 240 Ah of energy but needs roughly 580 Ah of AGM or about 310 Ah of LiFePO4 to deliver it. The gap is the Peukert effect at that discharge rate plus the fact that half a lead acid battery was never available.
The naive answer is amps multiplied by hours, and it is short by a wide margin on lead acid and by a little on lithium. Two effects push the requirement up, and running them backwards is what this calculator does.
Peukert says a battery delivers less than its rating when discharged faster than its 20 hour basis. The depth of discharge guideline says only half a lead acid battery was ever on the table. The capacity you have to buy is considerably larger than the energy you plan to use, and how much larger depends entirely on the chemistry.
Run your own numbers
40 amps for 6 hours uses 240 Ah of energy and needs about 513 Ah of AGM to deliver it.
| Figure | Value |
|---|---|
| Energy actually used | 240 Ah |
| AGM capacity needed | 513 Ah |
| AGM nearest stocked size | more than one battery |
| LiFePO4 capacity needed | 306 Ah |
| LiFePO4 nearest stocked size | more than one battery |
Shown with the example figures above. Change any field to run your own numbers.
The working
Published figurefull discharge time t = target hours / depth of discharge rated capacity C = I x H x ( t / H ) ^ ( 1 / k ) H = 20 hours k = 1.25 flooded, 1.15 AGM, 1.20 gel, 1.02 LiFePO4
This is the Peukert relation inverted with the depth of discharge guideline folded in, in that order. A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
Batteries that cover this requirement
Ranked by published capacity against the requirement above. Where the answer exceeds a single stocked battery, a parallel pair of the same size, age and chemistry is the normal solution at 12 volts.
LiTime 24V 100Ah LiFePO4 battery
$519.99 Published specsReplaces a series pair of 12 volt batteries with one pack, which removes the interconnect and the risk of two batteries ageing unequally.
Best for: A 70 or 80 lb thrust 24 volt motor
Check price on Amazon
LiTime 24V 100Ah Bluetooth LiFePO4 battery
$499.99 Published specsBluetooth state of charge on a 24 volt pack, which is the only practical way to read a mid-bank battery you cannot get a meter onto.
Check price on Amazon
LiTime 12V 100Ah Group 24 LiFePO4 battery
$365.99 Published specsGroup 24 case, which matters because it drops into an existing battery tray without rebuilding the compartment.
Check price on Amazon
LiTime 12V 100Ah trolling motor LiFePO4 battery for marine and yacht
$355.99 Published specsThe same capacity in the marine housing, and the usual alternative listing when the other is out of stock.
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LiTime 12V 100Ah Xtra Mini LiFePO4 battery
$338.39 Published specsA physically smaller 100 Ah pack, which is the whole argument for lithium on a kayak where the compartment is the constraint.
Best for: Kayaks and small boats where the battery box space is fixed
Check price on Amazon
LiTime 12V 100Ah trolling motor LiFePO4 battery
$335.99 Published specsBuilt for the trolling motor case specifically, which mainly means a BMS sized to sustain the high continuous discharge a motor asks for.
Best for: A 55 lb thrust 12 volt motor on a boat that fishes full days
Check price on AmazonEvery figure shown is the manufacturer own published specification. The rating on the unit you receive is what governs, and the capacity plate on your own boat governs everything above it.
Energy used against capacity required
Every chemistry uses the same energy. What differs is how much you have to buy to deliver it, and lead acid needs roughly twice as much nameplate as lithium.
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 | 240 Ah | 532 Ah | beyond a single stocked battery |
| AGM lead acid | 240 Ah | 513 Ah | beyond a single stocked battery |
| Gel lead acid | 240 Ah | 523 Ah | beyond a single stocked battery |
| LiFePO4 lithium | 240 Ah | 306 Ah | beyond a single stocked battery |
Worked at 40 amps for 6 hours. Stocked capacities used here are 35, 50, 55, 75, 100, 105, 120, 150, 200, 300 Ah.
Do not size from a published maximum draw
This is where people buy a bank twice the size they need. A published maximum current draw happens at maximum thrust, and a fishing day is mostly low settings with occasional bursts.
Measure your own average with a clamp meter on the motor positive cable across a normal day. It is frequently a third to a half of the rating, and it changes this answer completely. No manufacturer publishes current draw by speed setting, so your own measurement is the only honest source for an average figure.
When the answer is more than one battery
Where the requirement exceeds a single stocked battery, a parallel pair of the same size, same age and same chemistry is the normal solution at 12 volts. It also gains slightly on lead acid, because the pair is discharged at a gentler rate relative to its combined capacity so the Peukert penalty falls.
Never parallel a new battery with an old one, or two different chemistries. The pair equalises through the interconnect and the weaker one drags the stronger one down, which ends with two damaged batteries rather than one.
Wiring batteries in series is a different thing entirely: it raises voltage at the same amp hours, which buys thrust rather than time.
Frequently asked questions
How many amp hours do I need for a full day of fishing?
It depends entirely on your average draw rather than your motor rating. At a measured 40 amp average over six hours, you need roughly 580 Ah of AGM or 310 Ah of LiFePO4. At a more typical 20 amp average the requirement roughly halves. Measure your own average with a clamp meter before buying, because the difference between the two is a lot of money.
Why is the capacity needed so much larger than the energy used?
Two effects that multiply. Peukert means a battery discharged faster than its twenty hour basis delivers less than its rating, and the effect is substantial on lead acid. Depth of discharge means only half a lead acid battery is conventionally used at all. Together they roughly double the nameplate you have to buy over the energy you plan to consume.
Why does lithium need so much less nameplate?
Because both effects are smaller. LiFePO4 has a Peukert exponent near 1.02, so it delivers essentially its rated capacity even at a heavy draw, where AGM at 1.15 gives up a real fraction. And it is planned at eighty percent depth of discharge rather than fifty. Those multiply, which is why the lithium requirement is close to half the lead acid one.
Is it better to buy one big battery or two smaller ones?
One larger battery is simpler: fewer connections, fewer failure points and nothing to equalise. Two in parallel gain slightly on lead acid because each is discharged more gently, and they fit compartments a single large case will not. The rule either way is same size, same age, same chemistry, because a mismatched pair damages both.
Does a 24 volt bank double my capacity?
No. Two 12 volt batteries wired in series make 24 volts at the same amp hours, which buys thrust rather than run time. The energy available roughly doubles because the voltage doubled, but the motor also draws that energy at a higher voltage. What actually changes is that you can run a 70 or 80 lb motor at all.
How much will the extra capacity weigh?
That is usually the binding constraint rather than the money, particularly on a kayak or a small boat. Lead acid and lithium differ by roughly a factor of three for the same nameplate, and packs vary within each chemistry. There is no reliable general figure, so check the published weight for the specific battery rather than any rule of thumb.
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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.