Better 12V battery option?

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Well-known member
Aug 16, 2011
Toledo, Ohio
I can't find any specifications for the 12V battery. It's model# is BTX 67R 390. It says 390 cold cranking amps on it. The "vehicle modifier" manual or whatever it's called says it's a standard automotive type battery. This would seem less than ideal since a standard automotive battery is engineered to provide a high current surge to start a gas engine. The engineering trade off for this is the battery has less overall capacity than it could and does not handle significant discharges well. It would seem a deep cycle battery would be more ideal for our car.
Does anyone have the specifications for the OEM 12V battery? I would specifically be interested in the capacity.
If you go with just a deep discharge, you're likely ok with any that will fit. I think a deep discharge is more geared for capacity rather than current. The biggest rate of discharge will likely be when the headlights are on, the climate control blower motor, the heated seats, and the radio blasting. An easy way could be to add up all the fuses for those devices and you will get a higher than needed current draw; but still likely within any deep cycle battery you are looking at. As long as you stick with lead-acid, you likely don't need to worry about the DC to DC charger since charging is regulated by voltage level alone...unlike other technologies.

My guess is that the Ford battery is a simple 36mo battery...cost reduced to meet the minimum requirements for an automotive battery. If you are thinking to get something different because the battery doesn't last very long, you'll likely be ok by simply buying a better high quality battery.
Good article here:

Anyone try one of these lithium alternatives?
In some consumer Lithium rechargeable batteries, there are electronics that can shut the battery down when the level gets to a certain threshold that would otherwise cause harm to the battery. I would have to believe that an automotive lithium battery would have similar protection to protect itself from the alternator. The alternator was designed for use with lead-acid, not lithium. In the case of an FFE, the DC to DC converter was also designed for use with lead-acid. I do believe that a 12V lithium replacement automotive battery would likely work ok, but I would also have to believe there to be some added risk. With electrical components that separate the battery output to the car from the internal lithium storage, there are more possible points of failure. Also, as I mentioned, there may be circumstances that would case the battery to shut down due to the behavior of the DC to DC converter...or other condition. A lead-acid battery is just a dumb chemistry battery with no intelligence to screw things up.

I'd be curious if someone tries it, but for me, I think sticking with the lead-acid chemistry is the safest bet.
I had used a LiFePO4 battery pack in my motorcycle. The cost of the Lithium battery is slightly more but actually had less capacity than the lead battery. also below freezing temps the lithium battery all but shuts down. I would not use another lithium battery as a "starter" battery since it gets cold where I live. If I lived down south it might be a viable option.
According to the MotorCraft service manual, the battery is rated at 43 amp hours. Deep cycle batteries are usually rated in Amp hrs. I recently had to remove my 12V battery and tray to check a brake booster issue. Pretty light for a lead acid battery. Deep cycle batteries are much heavier due to their thicker plates. Might be tricky to find a deep cycle in the size and Ahr rating. Using a lithium battery would be risky, in my opinion, because of the charge is limited to 80-90% of full capacity. That dc-dc converter might not charge lithium properly.
Some Fusion Hybrid/Energi owners have gone with an AGM deep cycle battery. The issue has been getting the new battery to fit. For those owners the deep cycle battery has fixed their low 12V battery issues.
redcelt007 said:
According to the MotorCraft service manual, the battery is rated at 43 amp hours.... Using a lithium battery would be risky, in my opinion, because of the charge is limited to 80-90% of full capacity. That dc-dc converter might not charge lithium properly.
Thanks for the info. Is there an actual manual other than the web based stuff? I'd like to get my hands on one.

I will probably buy a deep cycle AGM as hybridbear pointed out hybrid/energi owners have done.

At least the A123 based "12V" automotive batteries would be fine with the DC-DC charging. These packs are made by putting four cells in series and then adding addition cells in parallel to get usable capacity. You will often see them denoted as 4s2p, the number before the "s" indicates how many cells are in series and the number before "p" indicates how many cells are in parallel. So in the example I gave (4s2p) it would have 4 groups of 2 parallel cells in series for a total of 8 cells. The following specs are from memory while researching the feasibility f using a lithium pack in my motorcycle so I may be slightly off. A 4cell A123 based pack is nominally 13.2V. Each cell can safely handle up to 4v charging so as long as the DC-DC converter doesn't put out 16V+ (max converter output ~14.5V) it would be ok. Since the cell voltage never reaches 4v the battery is never charged to the theoretical 100% This video of me starting my motorcycle in cold weather demonstrates why lithium starter batteries (at least for ICE vehicles) have not caught on. Additional reasons not to use a pack like this is the potential for cells to not be balanced (all cells have same state of charge). It would take a complicated battery management system to keep things in balance. Also without battery protection circuitry, if you ever were to accidentally run the battery down, it would be destroyed and never hold a charge again. To build a 41.4Ah (each cell is 2.3Ah) pack for our car would be a 4s18p battery. That's 72cells! An 8 cell pack is about $100 so I imagine a 72 cell pack would be in the neighborhood of $900!!! Not a viable option!

I'm not nearly as knowledgeable about the larger higher capacity prismatic lithium cells, but I suppose it would be possible to make a 4 cell battery with them where you wouldn't really have to worry too much about the cells being balanced assuming they are balanced when the pack is assembled. From some quick googling for a 40Ah battery it would be 4x$120 cells = $480 just for the cells to build a "12v" battery! I don't know anyone who would spend that much instead of buying a cheap lead battery. It would cost even more to add protection from an accidental complete discharge. Additionally who knows if these cells can tolerate the 14.5V charging voltage from the DC-DC converter? It may be necessary to add a protection circuit for overcharging adding even more cost!

I just saw another site that listed the 40Ah prismatic cells at about $50 which makes a 12v lithium pack more reasonable.
Another thing to keep in mind regarding the dc-dc converter is that it can deliver as much as 100 amps of charge current to that 12 volt battery, according to the service manual. That's almost a 3C charge rate. I know of no other service manuals other than the online version. I paid for a year's subscription. It has really come in handy.
I the tech manual says that it needs to be replaced with a battery of equal capacity and size. You can buy a CD version of the manual for about $160 from
So I got a bit over 3 years out of my warranty replacement Motorcraft battery. I'm not sure what the FFE does to abuse these so much besides running them flat every so often.

Anyway I had planned on shoehorning in an Optima Yellow Top deep cycle AGM but I know how bad it is for the battery every time our FFE's run the battery flat dead. However I think I'm gonna go LiFePO4 for my next 12V battery. I've had a bad experience with LiFePO4 batteries in my motorcycle but even riding that almost daily in winter temps down to about 10F for 3 or 4 winters. That battery is still going strong today after 8ish years. I actually don't remember when it was that I bought it. The cells on it have never been balanced and repeatedly abused by charging at subfreezing temps yet they keep on keeping on.

I made this decision in light of the following:
- FFE occasionally completely draining the battery which is very bad for lead acid (even worse for lithium without a BMS)
- The claim that modern LiFePO4 batteries can provide cranking amps for an ICE in sub freezing temps.
- I found a deep cycle LiFePO4 battery with a BMS that cuts off at 10 volts to provide over discharge protection.

I decided to go with this battery:

My only concern is the max charge rate of 50A since the DC-DC converter will supply up to 100A. If you are not aware, Lithium batteries have very little internal resistance and will take about as much current as you can feed them. This can be a bad thing if you're giving it more current than what it is rated for.

Well I'll find out what happens. Supposedly the BMS won't allow over current charging, but I'm not sure how that will all work out. I might end up frying the BMS.
It's my understanding that the BMS is very picky about voltage. As soon as it hits mid-summer, my AGM droops just enough that it will refuse to remote start because of the low 12V warning that triggers. Be prepared for a different chemistry to register incorrect status. Even different types of lead acid seem to do it.

I'm actually considering feeding the voltage sensor with a boost circuit that will prevent it from reading low enough to trigger that 12V error. I'll still let it drop just above that, so it doesn't think the battery is fully charged when it isn't, just never let it see that last .1V that makes it think the world is ending.
So why not a deep cycle battery? Doesn't have to be Lithium based. Aren't those supposed to be able to go flat repeatedly?
And I thought pulling fuse 3 stops the TCU from draining the battery? Also, didn't Ford release a firmware update to fix the TCU draining the battery problem?

Considering the new Tesla 12V batteries are not much more than a laptop battery pack, I wonder if a laptop battery pack, or one of those jump starters could be used as the 12V battery for our cars.
Heima said:
So why not a deep cycle battery?

I was going to go with a cylindrical cell AGM (Optima Yellow Top) These are about as good as it gets as far as lead acid goes. They are deep cycle batteries. I opted for the LiFePO4 because they are about the same cost and the LiFePO4 suppsoedly has about 10x the cycles, so neglecting calendar life the LFP battery should last much longer and they are similarly priced. Also no they are not able to go flat repeatedly. IIRC a normal starter battery shouldn't be drained more than 20% A deep cycle just extends that to 80% so running them flat is still very bad for their longevity. The LFP has a BMS that disconnects at 10V so you literally cannot run them flat. That is why I went LFP.

Also if they did indeed fix all the 12V issues my car wouldn't have had a flat dead 12V the two times that is has unrelated to the TCU. It's the F1 fuse for the TCU. I had that out for over a year and had no 12V issues. I only enable the TCU in the winter for the GO times and I have it on a low voltage disconnect that cuts it off if the 12V drops below 12.1V which is about 40% SOC for lead acid.

Anti_Climax said:
It's my understanding that the BMS is very picky about voltage. As soon as it hits mid-summer, my AGM droops just enough that it will refuse to remote start because of the low 12V warning that triggers. Be prepared for a different chemistry to register incorrect status. Even different types of lead acid seem to do it.
Not sure what you're talking about there is no BMS for the 12v system. Also oddly enough after they "fixed" my 12V issues, I no longer get MFM warnings about low 12V when the battery dies. Also If your AGM voltage is low that means your battery is going bad or the DC-DC isn't charging it. My FFE will give no indication of a problem until you get down o 11.5-11.8V where it doesn't have enough voltage to turn on the car.

My LFP will be here Wednesday so I'll report back how it goes.
There is a 12V BMS that has separate settings for flooded lead acid, AGM, etc as they each have slightly different charge characteristics, giving slightly different voltages when fully charged. They've had to reset it each time they've replaced my 12V battery.

Folks that put optimas in can run into this awesome behavior where the car will see the normal fully charged voltage of the battery and treat it as if it's dangerously low.

You'll see this when you switch off the car but don't open the door. The fans and console will normally stay on for 30 minutes but it will almost immediately give a message about preserving the battery and cut off. I get this frequently even with the Ford standard AGM battery.

On the gas version, you can pull the 12V sensor plug off but on our EVs it will just refuse to work like that.

Feel free to mess around, just letting you know folks have tried exactly what you've been considering and it's not as simple as swapping them out.

In a perfect world we could find BMS commands that would set the proper float voltage values and low voltage cut off for any given battery but I'm not holding my breath.

If you have a copy of the service manual that's floating around, there's a little more detail in "414-00 Charging System - General Information". While it does mention a 12V battery management system it doesn't go into further detail. I believe the "BMS" they always check when my AGM droops is the Battery Monitoring Sensor that reads into the BCM.
Interesting. They must have disabled this on mine. Whatever "software update" they did my my TCU went bad must have eliminated it. My OEM battery has a resting voltage of about 11.8v. It still starts the car fine but doesn't have enough to jump my motorcycle.

Battery went in fine. There was a bit of a clearance issue with the negative terminal. Had to bend up the plastic above it to get the negative terminal on. The positive terminal just touches the Underwood insulation. This battery is almost too tall. But seems to work just fine the few minutes I've used it.

Other than i need to fab up a new hold down since the new battery is taller, there were surprisingly no issues installing it.

We'll see how it goes.
I'd have to double check, but I think my car pitches a fit when the AGM hits 12.5V (nominal for flooded lead acid being 13.8V). Maybe not that exact voltage but not nearly as low as I'd expect.

If yours goes that low and the car doesn't complain, something had to have been changed. If you ever figure out what, please - for the love of god - tell me.

It pisses me off to no end that my car will refuse to remote start during the hottest part of the Arizona summer WHILE PLUGGED IN and the best the dealer can do is make me waste time to bring it back for a warranty replacement battery that doesn't actually fix the root cause...
That sucks. I have never gotten any sort of low voltage indication other than reading DTCs or a notification thru MFM that the 12V is low AFTER the 12V has already expired. After they "fixed" it and replaced my TCU I never got a MFM notification that the battery had died (12V low). Even after the 3rd replacement TCU has failed. Other than mine being a '14 model, I wonder what is the difference.

You might try a LTO I was going to make a 6S LTO battery for the 12V I was going to go this route but LTO is still stupidly expensive. I was going to build a LTO with no BMS as they can tolerate being run flat without being ruined and can tolerate charging in freezing winter temperatures. Not sure about the other end that you would need to be concerned with. Not sure if that would make any sense for you but might be worth looking in to.

Oh and I'm pretty sure the nominal voltage for any lead acid battery is 12.6V or 2.1V/cell. so I don't see why it would make any difference for you, flooded, gel, or AGM. I could be wrong but I'm also pretty sure the gizmo on the negative terminal is a current sensor not a voltage sensor.
Apologies, it's float voltage, not nominal. Float varies between gel (13.05V), flooded (13.4V) and AGM (13.6V) but it also varies by temperature.

It's not much between flooded and AGM but it's enough for my car to complain. Strangely, the car doesn't post a DTC other than "I didn't start because something else errored". Doesn't post it when my HV battery overheats either.

I'm really starting to love-hate this car.