Charging
Onboard charger sizing chart: amps per bank, not amps total
Quick answer
Onboard chargers are sized per bank, not by total output. A 100 Ah AGM bank wants roughly a 15 amp charger per bank at the typical 13 percent rate, while a 100 Ah LiFePO4 bank can take about 20 amps, and a lithium bank always needs a charger with a lithium profile.
An onboard charger's rating is a per bank figure. A three bank charger rated 30 amps total delivers roughly 10 amps to each of three batteries, not 30 amps to any one of them, and sizing a charger for a boat means sizing each bank's own output against that bank's own capacity and chemistry.
Lead acid charges at a conservative rate to protect plate life. LiFePO4 accepts a much higher rate, and more importantly needs a charger with a lithium profile: a lead acid 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.
On this page
Charge rate by chemistry, as a percentage of bank capacity
LiFePO4 accepts a typical charge rate of 20 percent of bank capacity, roughly half again as fast as the 13 percent typical rate for AGM, and up to 50 percent at its published maximum.
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 | Minimum rate | Typical rate | Maximum rate |
|---|---|---|---|
| Flooded lead acid | 10% | 13% | 25% |
| AGM lead acid | 10% | 13% | 30% |
| Gel lead acid | 10% | 12% | 20% |
| LiFePO4 lithium | 10% | 20% | 50% |
Convention on lead acid, manufacturer guidance on LiFePO4. Either way, not a published standard, and the rate a charger and battery maker publishes for a specific pair governs over this table.
Charge efficiency by chemistry
AGM needs about 1.15 amp hours in for every amp hour actually stored, while LiFePO4 needs only about 1.03, losing far less to heat during charging.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Chemistry | Charge efficiency (Ah in per Ah out) |
|---|---|
| Flooded lead acid | 1.20 |
| AGM lead acid | 1.15 |
| Gel lead acid | 1.18 |
| LiFePO4 lithium | 1.03 |
Published battery engineering: it takes more amp hours in than come back out, because some energy is lost to heat and to reactions other than storing charge, and lithium loses far less of it than any lead acid chemistry.
Charger amps needed per bank, worked examples
A 100 Ah bank wants roughly a 15 amp charger per bank on AGM or a 20 amp charger per bank on LiFePO4, snapped to the nearest stocked charger size.
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.
| Bank size | AGM charger amps (per bank) | LiFePO4 charger amps (per bank) |
|---|---|---|
| 50 Ah | 8 | 10 |
| 100 Ah | 15 | 20 |
| 150 Ah | 20 | 30 |
These are stocked charger sizes snapped up from the typical charge rate for each chemistry. A three bank motor draws three of these outputs, one per battery, from a charger whose total rating is the sum of its banks, not the single number stamped on the front.
Recharge time for the amp hours actually used
Continuing the 50 amp draw worked example from the battery run time chart, an AGM bank used about 35 usable amp hours and a LiFePO4 bank used about 76, over the same trip.
Even with a larger charger, the LiFePO4 bank in this example takes longer to recharge, about 3.9 hours against 2.7, simply because it delivered more than double the usable amp hours the day before.
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 | Amp hours used (from a 50 amp fishing day) | Charger output used | Recharge time |
|---|---|---|---|
| AGM lead acid | 35 Ah | 15A | 2.7 hours |
| LiFePO4 lithium | 76 Ah | 20A | 3.9 hours |
This combines the published charge efficiency figure with the charger amps from the table above and the usable amp hours worked out on the battery run time chart.
Real chargers and their per-bank rating
Most onboard chargers here publish amps per bank directly, but at least one major brand publishes a total rating shared across banks on demand instead, which is worth reading carefully before assuming a number.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Charger | Banks | How it publishes output | Lithium profile available |
|---|---|---|---|
| NOCO GEN5X1 12V 5A 1-bank onboard charger | 1 | 5 amps to its one bank | Yes |
| NOCO GEN5X3 12V 15A 3-bank onboard charger | 3 | 5 amps to each of three banks | Yes |
| NOCO GENPRO10X3 12V 30A 3-bank onboard charger | 3 | 10 amps to each of three banks | Yes |
| Minn Kota MK-345PCL onboard charger, 3 banks 15 amps | 3 | 15 amps to each of three banks | Yes |
| Minn Kota MK-440PCL onboard charger, 4 banks 10 amps | 4 | 10 amps to each of four banks | Yes |
| ProMariner ProSport HD 20A 3-bank waterproof onboard charger | 3 | 20 amps total, shared across three banks on demand | Yes |
Most onboard chargers state amps per bank. At least one major brand states a total instead and shares it across banks as needed. Read the listing carefully before assuming a number is per bank.
Frequently asked questions
Is an onboard charger's amp rating per bank or total?
It depends on the brand, and reading the listing carefully matters. Most onboard chargers in this catalog, including the NOCO GEN and Minn Kota PCL lines, state amps per bank directly, so a three bank 15 amp charger delivers 15 amps to each battery. At least one major brand instead states a single total figure that its electronics distribute across banks as needed, which is a genuinely different specification to compare against.
What amp charger does a 100 Ah AGM battery need?
Using the typical 13 percent charge rate convention for AGM, a 100 Ah bank wants roughly 13 amps, which snaps up to a commonly stocked 15 amp charger output per bank. A smaller charger still works, it simply takes longer to fully recharge the bank, and a larger one is not harmful as long as the charger's own multi-stage profile manages the taper correctly.
Can a lithium battery use a faster charge rate than lead acid?
Yes. LiFePO4's typical charge rate convention sits at 20 percent of bank capacity against 13 percent for AGM, and it can accept up to 50 percent at its published maximum against roughly 30 percent for AGM. The faster acceptable rate does not remove the more important requirement: a lithium battery still needs a charger with a lithium profile regardless of how fast it charges.
What happens if I charge a LiFePO4 battery on a lead acid profile?
One of two bad outcomes follows. A lead acid absorption and float profile either never reaches the voltage the LiFePO4 battery management system expects to see, leaving the pack chronically undercharged, or it holds the battery at a float voltage once full that a lithium cell should not sit at indefinitely, which shortens its life. Either way, the fix is a charger with a genuine lithium profile, not a faster lead acid setting.
Why does charging efficiency matter for recharge time?
Charge efficiency is the published ratio of amp hours put in to amp hours actually stored, because some energy is lost to heat rather than to the battery during charging. AGM needs about 1.15 amp hours in for every amp hour recovered, while LiFePO4 needs about 1.03. Ignoring that ratio and dividing amp hours used by charger amps alone understates how long a full recharge actually takes.
How many banks does a 36 volt trolling motor bank need on the charger?
Three 12 volt lead acid batteries wired in series for 36 volts need three separate charger banks, one per battery, because each battery still charges individually even though they are wired together for the motor. A single 36 volt LiFePO4 pack with its own internal battery management system typically needs only one charger bank, which is one of the wiring simplifications lithium brings to a 36 volt system.
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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.