12V battery replacement

FloridaBreen · Friday at 12:14 PM

How do you Remove and Replace the 12V battery?

longer story:
So I was going to be away for 26 days and in the owners manual (Storing your vehicle) it said to disconnect the 12V battery if going to be stored for over 30 days - and I didnt disconnect the 12V battery - of course when i returned home the truck was 'dead' - no lights etc. After manually opening the FRUNK (thanks Ford for a poor design, 2 people are needed to open Frunk manually - one to hold the spring loaded manual release in the cab and the other to lift the frunk). I then checked the voltage directly to the 12V battery terminals, it measured 3.38v, I connected my portable jump start and immediately the Frunk lights came on and since driver door was open so did the dash. Truck "started" normally and I disconnected the jump battery with truck staying in "ready" mode. I then let the truck stay on for a little over an hour to charge the 12V battery. I also found that an update was in progress once the truck was on. In my experience 12V lead acid batteries do NOT like to be run down to very low voltages (under 10V) and it greatly impacts battery life. Looking more deeply in the owners manual I found "12V Battery - Troubleshooting" where it says

12V BATTERY
CHANGING THE 12V BATTERY
If the 12 volt battery needs servicing, see
your authorized dealer.
CHARGING THE 12V BATTERY
Your vehicle has a high voltage to low
voltage energy transfer feature that keeps
the 12-volt battery charged by the high
voltage battery. If the 12-volt battery level
is low, the high voltage battery transfers
energy to the 12-volt battery when the
vehicle is off.
12-volt battery depletion can still occur if
the high voltage battery state of charge is
low. See Preserving Your High Voltage
Battery (page 203).
Always take care not to deep discharge
the 12-volt battery as it may cause
premature wear.

In my case the HV battery was around 61% IIRC when I parked it and near the same when I returned - so the HVB should have had enough juice to keep the LVB charged even when OFF (see above)

searching previous posts regarding the 12V battery I didnt find any that actually removed and replaced the 12V battery, and as the manual states if it needs servicing see your authorized dealer. I am wondering why, is there some esoteric procedure to disconnect a 12V battery that is required by the Lightning?

Maquis · Friday at 12:24 PM

Your 12V battery failed. It should be replaced by the dealer under warranty. You have a 24, are you still under 36K miles?
Once you are out of warranty, it’s not a big job to replace it yourself. The most tedious part is the BMS reset procedure. It’s been posted on here previously.

On the Road with Ralph · 2025-06-14T09:06:38-0700

Two things: Didn’t I recently see that Ford did an OTA update that addressed keeping the 12 V battery charged from the main battery pack? Also, why is the LVB such a weak point in the Lightning? It is a stupid place to try to save money.

HOTAS · 2025-06-14T09:26:07-0700

FloridaBreen said:
How do you Remove and Replace the 12V battery?

longer story:
So I was going to be away for 26 days and in the owners manual (Storing your vehicle) it said to disconnect the 12V battery if going to be stored for over 30 days - and I didnt disconnect the 12V battery - of course when i returned home the truck was 'dead' - no lights etc.

In my case the HV battery was around 61% IIRC when I parked it and near the same when I returned - I am wondering why, is there some esoteric procedure to disconnect a 12V battery that is required by the Lightning?

My first question is, did you leave the truck plugged in?
I get the impression you did not.
So if not, why not just leave the truck plugged in?
With the charge level set to say 62%, since you were leaving it at 61%. That would have avoided any problems.

Also, as you near the end of your absence you could initiate a charge to say 80-85% from the app, so the Truck is all ready when you get home.

The owners manual guidance on “storage” pertains to leaving the truck somewhere it can’t be plugged in.
was that your situation?

If so, then couldn’t you put a little battery tender on the 12v battery while you were gone?

HOTAS · 2025-06-14T09:51:21-0700

On the Road with Ralph said:
Two things: Didn’t I recently see that Ford did an OTA update that addressed keeping the 12 V battery charged from the main battery pack? Also, why is the LVB such a weak point in the Lightning? It is a stupid place to try to save money.

It‘s a ‘weak’ point in all Hybrid, PHEV, & EV’s.
Mostly because the small 12v battery, and it‘s purpose, are misunderstood. It’s purpose is to boot a larger 12v system.
There is no legit reason to have a large 12v battery with such cars.

A small battery will always have a shorter life than a larger one.
Especailly when sitting idle. 1000’s of motorcycle, ATV, jet ski, snow mobile & riding mower batteries are replaced every year due to poor maintenance but nobody raises a brow. That’s essentially what’s in this truck and every P,H & EV.
Education and a little battery tender can avoid any problems

Maquis · 2025-06-14T10:57:53-0700

HOTAS said:
My first question is, did you leave the truck plugged in?
I get the impression you did not.
So if not, why not just leave the truck plugged in?
With the charge level set to say 62%, since you were leaving it at 61%. That would have avoided any problems.

Also, as you near the end of your absence you could initiate a charge to say 80-85% from the app, so the Truck is all ready when you get home.

The owners manual guidance on “storage” pertains to leaving the truck somewhere it can’t be plugged in.
was that your situation?

If so, then couldn’t you put a little battery tender on the 12v battery while you were gone?

Leaving it plugged in has no effect once the target charge level is reached.

After a few (several?) days, the vehicle will go into deep sleep and you won’t be able to access it from FordPass.

Ford’s long-term storage instructions make no mention of whether or not it is stored in a location with plug-in capability.

johnnyonetime · 2025-06-14T11:13:04-0700

It’s always been a weak link, not fooling with a trickle charger. I replaced mine very early at the first sign of failure. Didn’t wait to get stranded somewhere. Bought the Ohmmu and 2 years on haven’t looked back.

chl · 2025-06-14T19:53:46-0700

I'd guess your 12v battery was low before you left to begin with.

It's an AGM battery so yes sensitive to low voltages, anything below 50% will shorten the life.
But also when an OTA is being processed it has to be maintained at a voltage above 13 volts which may be higher than some other types of vehicles for OTA's.

So sometime in the summer of 2024, I could not receive and install an OTA until I plugged the truck in over night to boost the LV battery level to over 80% SOC.

Anyway, I had a better experieince than you for some reason, I got my 2023 Lightning new in Jan 2024, but had a trip planned to be away for nearly 2 months from mid-March to mid-May.

I did not leave my truck on a charger - had not installed the FCSP yet.

I did not put it on a trickle charger, I hadn't figured out how I'd do that inside the Frunk.

I did not know about the battery removal recommendation - I would have ignored it even if I had because what a hassle for me to remove and replace a battery - how would I close the Frunk after removing it? Go figure.

So all I did was let the HV battery get down to about 50% because the truck was going to be sitting for nearly 2 months to protect it somewhat.

When I returned I half expected the 12v battery to be low, that happened to me with my 2012 Nissan Leaf in 2012 when I went on a trip, so I had my jump cables handy, but it started without any trouble.

One thing about the HV to LV charging that may have contributed to your issue is the OTA update you mentioned because:

"When HV to LV Energy Transfer Feature is in the process of an energy transfer, the OTA update shall stop the energy transfer and proceed with the OTA software update."

The program flow (which may have been modified recently by an update) controlling the HV to LV charging was as follows:

ENTRY CONDITIONS FOR HIGH VOLAGE TO LOW VOLTAGE ENERGY TRANSFER
Normal Entry Conditions:
• The following are entry conditions that must be met to initiate an Energy Transfer Request:
• Ignition = Off
• Parked Gear
• No Error or Faults Detected in HV to LV Energy Transfer System (e.g. DTCs etc.)
• No OTA software update in process
• HV Battery SOC and Power Limits > 15% HV SOC
• LV Battery Temp > -15°C
• *** LV Battery SOC <= 40%
• *** Minimum Lock Out Period – 48 hours
• Since ignition off, and last energy transfer
*** Indicate special conditions where can be over written

ENTRY CONDITIONS FOR HIGH VOLAGE TO LOW VOLTAGE ENERGY TRANSFER (continued)
Special Conditions:

• 1.) If Customer On-Demand Request is detected (e.g. OTA On-Demand)
• Then request will be honored with any LV Battery SOC.
• [Overrides LV Battery SOC <= 40% condition.]
• But other conditions must apply before sending energy transfer request
• OTA (Over The Air) On-Demand Request provides option for LV battery energy top off before OTA software update

• 2.) If LV Battery SOC =< 40% and detection of any of the following special error conditions:
• Battery End of Life (EOL)
• High Key Off Load (KOL)
• Park Lamps Active / Hazard Lamps Active
• Then will initiate energy transfer request.
• [Overrides 48 hour delay limit]
• But other conditions must apply before sending energy transfer request.
• Special error conditions and customer on-demand noted above are excluded from 48 hour lock
out period.

• 3.) If LV Battery SOC =< 30%; extreme threshold - will initiate transfer request
• [Overrides 48 hour delay limit]
----
Further information from the attached document section:

12V Battery Charging System
The 12V batter is charged by the DC-DC converter control module. The Battery Management System (BMS) continuously monitors the battery state of charge condition and provides the BCM with this information. The BCM communicates this information to the PCM over the HS-CAN1. The PCM communicates the battery desire setpoint to the DC-DC converter control module which supplies the necessary charge voltage to the 12V battery.

12V Battery Management System
NOTICE: When any vehicle module is being programmed, connect an external battery charger to make sure the module programming is complete without interruption due to the load shedding feature becoming active. The external battery charger must maintain a system voltage above 13 volts. This may require a charger setting higher than the lowest charge setting. The
external battery charger negative connection must be made to an engine or vehicle chassis ground and not the negative battery terminal.. If the connection is to the negative battery terminal, load shedding may begin and module programming may be corrupted. After charging has begun, start the engine to clear any load shed states and then turn the engine off the proceed with programming.

Charging and Jumping
Do not charge or jump start the vehicle by connecting to the battery negative terminal. Refer to the Owners Guide for more information.
If the vehicle has been jump started, test the battery condition.
If the 12V vehicle battery has been charged by connecting to the battery negative terminal, do not reset the Battery Monitoring System.

Battery Monitoring Sensor
The Battery Monitoring Sensor continuously monitors the conditionand state of charge of the 12V battery and provides the BCM with this informatino. The Battery Monitoring Sensor also estimates losses in the battery capacity over time. The Battery Monitoring Sensor should only be reset when the batter is replaced. It is urgently recommended that the replacement battery has
the exact same specification as the origina battery. If it does not, the accuracy of the Battery Monitoring Sensor outputs will be compromised.

The Battery Monitoring is clamped directly to the negative terminal of the battery and grounds to the vehicle at the chassis ground connection point through the negative battery cable and eyelet. It is part of the negative battery cable and cannot be serviced separately.

External customer loads must only be connectred to the vehicle at the customer battery connection point. If an eternal customer load is connected at the negative battery post, the Battery Monitoring Sensor accuracy cannot be guaranteed. It is recommended that the Battery Monitoring Sensor pole clamp is not removed unless a battery replacment is required. Shoud the battery need to be isolated, this should be done by disconnecting the ground eyelet at the chassis ground.

chl · 2025-06-14T19:56:35-0700

Maquis said:
Your 12V battery failed. It should be replaced by the dealer under warranty. You have a 24, are you still under 36K miles?
Once you are out of warranty, it’s not a big job to replace it yourself. The most tedious part is the BMS reset procedure. It’s been posted on here previously.

Agreed. A 2024 Flash should still be covered unless you put too many miles on it already.