Batteries
Lithium vs lead acid comparison chart: Peukert, depth of discharge and a worked example
Quick answer
At a 50 amp draw, published Peukert math shows a 100 Ah AGM battery delivers about 35 usable amp hours to its 50 percent depth of discharge guideline, while a 100 Ah LiFePO4 pack delivers about 76, more than double, because lithium's Peukert exponent sits close to 1.
A 100 amp hour rating means the same thing on the label of an AGM battery and a LiFePO4 battery: it is the capacity delivered discharging the battery over the published 20 hour rate. It does not mean the two batteries deliver anywhere near the same usable capacity at the higher, faster draw a trolling motor actually asks for, and the gap between them is published battery engineering rather than a lithium marketing claim.
Two separate figures explain the gap, and this page keeps them in separate tables on purpose. Peukert's equation, with a chemistry dependent exponent, is published engineering describing how delivered capacity falls as discharge current rises. Depth of discharge, the fraction of that delivered capacity actually used before recharging, is convention on lead acid and manufacturer guidance on lithium. Mixing the two into one figure hides which part is a fixed physical relationship and which part is a planning choice.
On this page
Peukert exponents by chemistry
LiFePO4's Peukert exponent of 1.02 sits close enough to 1 that its delivered capacity barely falls as the draw rises, while flooded lead acid at 1.25 falls hardest of the four.
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 |
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% |
50 percent on lead acid protects cycle life and is convention rather than a standard. 80 percent on LiFePO4 is the conservative planning figure lithium makers publish directly. Kept in a separate table from the Peukert exponents above because Peukert is published physics and depth of discharge is a guideline, and the two should never be read as the same kind of number.
What that means at a real trolling motor draw
Running both figures through the published run time formula, twenty hours times the ratio of rated to actual amp hours raised to the Peukert exponent, then applying the depth of discharge guideline, gives the following at a 50 amp draw from a 100 Ah rated battery in each chemistry.
At the same 50 amp draw, LiFePO4 returns about 76 usable amp hours against about 35 for AGM, both from a 100 Ah rated battery.
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 | Safe run time to DOD (hours) | Usable amp hours (of 100 Ah rated) |
|---|---|---|
| Flooded lead acid | 0.56 | 28.1 |
| AGM lead acid | 0.71 | 35.4 |
| Gel lead acid | 0.63 | 31.5 |
| LiFePO4 lithium | 1.53 | 76.4 |
The Peukert exponent behind this table is published. The depth of discharge fraction applied to it is convention on lead acid and manufacturer guidance on lithium, which is why this combined table is labelled convention rather than standard.
Weight and capacity per pound, from published specs
Using each product's own published rated capacity and shipping weight, a 100 Ah LiFePO4 battery works out to roughly three times the amp hours per pound of a 120 Ah AGM in the same rough size class.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Product | Chemistry | Rated Ah | Weight (published) | Amp hours per pound |
|---|---|---|---|---|
| VMAX MR137-120 12V 120Ah AGM Group 31 battery | AGM | 120 Ah | 75 lb | 1.6 |
| Renogy 12V 100Ah trolling motor LiFePO4 battery | LiFePO4 | 100 Ah | 21.8 lb | 4.6 |
| LiTime 12V 100Ah trolling motor LiFePO4 battery | LiFePO4 | 100 Ah | 22.16 lb | 4.5 |
Amp hours per pound is arithmetic on each product's own published rated capacity and shipping weight, not an estimate. The AGM row is a larger 120 Ah case rather than a size matched 100 Ah, and it is still the heaviest per amp hour by a wide margin.
Charging the two chemistries
A lithium battery needs a charger with a lithium profile. A lead acid charging profile either never reaches the voltage a LiFePO4 pack needs to finish charging, or holds it at a voltage it should not sit at once full, and both outcomes shorten its life. Chargers built for both chemistries, such as the Minn Kota PCL series, let each bank be set independently rather than assuming one profile fits everything on the boat.
Frequently asked questions
Why does a lithium battery run a trolling motor longer than the same rated amp hours in AGM?
The gap comes from the Peukert exponent, a published figure describing how delivered capacity falls as discharge current rises. LiFePO4 sits at 1.02, close enough to 1 that its delivered capacity barely changes with a faster draw. AGM sits at 1.15, which costs it noticeably more of its rated capacity at the same current. At a 50 amp draw that difference alone accounts for most of the usable capacity gap between the two chemistries.
Is 50 percent depth of discharge a hard limit on a lead acid battery?
No, it is convention rather than a hard stop. A lead acid battery keeps delivering current well past 50 percent depth of discharge, but repeatedly drawing it deeper shortens its cycle life significantly. The 50 percent figure is the planning guideline the deep cycle battery industry has settled on to balance usable capacity against how many seasons the battery survives, not a specification any standards body publishes.
Can I use a lead acid onboard charger on a LiFePO4 battery?
Not safely. A lithium battery needs a charger with a lithium profile. A lead acid profile either never reaches the absorption voltage a LiFePO4 pack needs to finish charging, leaving it perpetually undercharged, or holds it at a float voltage once full that a lithium cell should not sit at indefinitely. Both outcomes damage the battery over time even though it may appear to charge normally at first.
Does LiFePO4 always deliver its full rated capacity?
No, but it comes close. LiFePO4's Peukert exponent of 1.02 means delivered capacity still falls slightly as the discharge current rises, the same physical relationship that governs lead acid, just with far less effect. At a 50 amp draw from a 100 Ah rated pack, published Peukert math still predicts a small loss before the 80 percent depth of discharge guideline is even applied.
Why is the depth of discharge table separate from the Peukert table?
Because the two figures come from different places and should never be read as the same kind of number. The Peukert exponent is published battery engineering that does not change because someone disagrees with it. Depth of discharge is convention on lead acid, protecting cycle life rather than a hard limit, and manufacturer guidance on lithium. Keeping them apart stops a reader from treating a guideline as if it were a law of physics.
How much lighter is a lithium trolling motor battery than lead acid?
Published specs in this catalog show a 100 Ah LiFePO4 battery at roughly 22 pounds against a 120 Ah AGM at 75 pounds, working out to about three times the amp hours per pound for lithium. The exact ratio depends on the specific products compared, since case size and construction vary, but the weight advantage is consistently large across the catalog rather than a one-off figure.
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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.