Wanted to give you an accurate, comprehensive answer so I asked chatgpt. Not something I use often, but it does have it's place, and I feel this is one of those times where it excels.
I hope this helps you out:
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A 36-V trolling motor with
three 12-V batteries in series should draw
the same current through all three batteries (that's a rule of series circuits). So seeing
-59 A, -61 A, and -26 A at the same moment is a big clue:
either the wiring isn't a true 3-battery series string, something is "tapping" part of the string, or the per-battery current readings aren't trustworthy in your setup.
Here are the most common real-world causes of "one battery drains faster" in a 3×12V→36V trolling motor setup, and how to prove which one you have.
This is
the #1 cause of one battery being way lower than the others after a trip.
Examples:
- Fish finder / graph
- Livewell / bilge / lights
- USB/12V outlet
- Anchor winch
- Any accessory that someone "temporarily" clipped onto one battery
If a 12-V accessory is connected to (say) Battery #2, then
Battery #2 will discharge faster, and you can absolutely end up with something like
86% / 3% / 49%.
How to check
- Confirm the only things connected to the trolling battery bank are:
- the trolling motor leads (at the very ends of the series string)
- the two series jumpers between batteries
- the charger leads (one bank per battery)
- If you need 12 V on the boat, the "right" way off a 36-V trolling bank is a 36→12V DC-DC converter connected across the whole 36-V pack (with proper fuse), not tapping one battery.
Correct series for 36 V:
- Motor positive goes to the positive of Battery #1
- Jumpers connect:
- Battery #1 negative → Battery #2 positive
- Battery #2 negative → Battery #3 positive
- Motor negative goes to the negative of Battery #3
If the motor negative (or positive) is accidentally landed on the
middle battery or a jumper point, you can create a situation where the motor is effectively using only part of the pack (or stressing one battery more).
Quick proof
- With the motor OFF, measure with a multimeter:
- Across the two motor leads you should see about ~38–40+ V when fully charged (resting voltage depends on LFP state/charger).
- Then measure each battery individually (~13.2–13.6 V resting depending on SOC).
A 3-bank charger wired wrong (or one bank failing) can make one battery start the day at a lower SOC even though "everything looks fine."
How to check
- After charging to "full," let them rest 30–60 minutes and compare resting voltage per battery in the app and/or multimeter.
- Swap charger leads between batteries (bank #2 to battery #1, etc.) and see if the "low battery" changes. If it follows the charger bank, you found it.
Loose lug, corrosion, undersized jumper, or a weak breaker can cause one battery to see more voltage sag under load or trigger BMS protections sooner. Sometimes the "problem battery" is just the one with the worst connection.
Signs
- One battery voltage drops more under load than the others.
- Heat at a lug/breaker after running hard.
- Random shutoffs at high thrust.
What to do
- Physically torque and clean every connection.
- Make sure series jumpers are the same gauge and good quality.
- Inspect breaker/plug connections (these are frequent resistance points).
Less common with new-ish packs and "99% health," but possible:
- A battery's BMS may limit discharge current if it detects internal cell voltage imbalance, temp issue, or protection threshold.
- It may "recover" after resting, making SOC look odd.
But note: if the batteries truly are in a pure series string, the
current cannot actually be different through each battery. So if your wiring is definitely correct, those different amp readings strongly suggest the
readings are being affected, not the physics.
In a proper series-only 36-V setup, current should match across all 3. So one of these is true:
- Wiring/tapping is creating multiple current paths (most likely), or
- The app's per-battery current sensor is not accurate when the batteries are in series (some BMS/shunt designs can display odd current values depending on where the measurement is taken and internal referencing), or
- One battery is intermittently disconnecting/limiting and the system is doing something unintended (less likely unless you're seeing motor cutouts).
Do these two tests:
- Label the batteries A/B/C.
- Put them in the boat as A-B-C (your current order). Run a trip, note ending SOCs.
- Next trip, swap positions (e.g., C-A-B) but keep wiring correct for series.
- If the same physical battery always ends up low → battery/BMS issue.
- If the middle position (or a particular position) always ends up low → wiring/tap/connection issue.
With the trolling motor on high:
- Measure voltage of each battery at its terminals while running.
- They should all sag somewhat similarly.
- If one sags a lot more, you likely have a bad connection or a weak battery/BMS limiting.
That earlier result (
#2 at 3% while others much higher) screams
extra load on one battery or
one battery not getting charged fully.
Your "constant/full thrust" live data showing one battery at
~half the current suggests either:
- the system isn't actually a pure series circuit as connected, or
- the current readings aren't reliable in-series and you should trust pack voltage + individual voltage sag more than per-battery amps.
- Do you have any 12V accessories connected to the trolling batteries (even "just sometimes")?
- And when you measure across the trolling motor plug/leads with a meter at full charge, do you see ~39–40 V?
If you don't have a meter, you can still do the "swap positions" test and a careful look for any mid-string taps/accessory grounds.
