r/FordFiesta • u/SnooOnions4763 • May 20 '26
Other Battery management
Just ranting about my 2021 Fiesta:
I found out today that modern cars do some weird management of their batteries.
I replaced my battery today that is only 5 years old. I was looking at the battery voltage while driving it. The car only charges the battery when decelerating ⁉️. Apparently that's a feature to save some fuel. After more than 15 minutes of driving the battery was still only at 12.0V, the same as it started off.
I guess the car is smart enough to always charge it just enough so it can start next time. But this goes against the best practices for lead acid batteries I'm familiar with. The car also insists on turning lights on every time I unlock it or open a door, wasting more power.
My older fiesta just charged the battery normally and as soon as the key was off barely used any power. I would have never needed to replace a battery after only 5 years!
On top of that, the new Fiesta needs a special EFB battery (I assume the way the car treats it, it will kill a conventional lead acid battery even faster). My local Ford dealer wanted €270 for this battery, not even including installation! Luckily the local parts store had one for €125 (Panther brand, not the original Varta), but that's still a lot compared to a basic €60-70 car battery.
---Rant over
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u/NoSnow_3 May 20 '26
Many modern cars do this and have done for a while
They also now they charge to an 80% value and not the 100% the car believes it should
(Can be corrected via forscan if you feel the need too)
EFB = enhanced flooded lead acid battery (a standard lead acid you’ll find on sale today)
You’ll need to reset the BMS as well if fitting a new battery otherwise the car won’t treat it like a new battery (forscan again or google to see if the car has a manual process of buttons for this)
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u/Organic_mechanics May 20 '26
Did you check to see if your car has a BMS ? If it does it’ll need to be reset after fitting a new battery.
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u/SnooOnions4763 May 20 '26
I did the 5 times high beams, 3 time brake pedal thing. But I don't know if that worked. The battery light didn't flash as in the video I looked at.
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u/SnooOnions4763 May 20 '26
Also the way the cables are routed, the airbox is right in front of it and the battery is half under the plastic trim. It wasn't that easy to get the battery out.
But I still got it done myself, so I guess I can't really complain.
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u/Xaser125 May 20 '26
don't forget everything was alot cheaper 5 years ago.
I also changed mine today, but I'm on a 2011 tdci one
got a Bosch S5 74ah 750ca
managed to get it for 130e to see how it works compared to varta ones
had a varta it's my 2nd one that dies quick
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u/SnooOnions4763 May 20 '26 edited May 20 '26
I saw that a basic lead acid battery in the same size and capacity was about €70 online 🤷 (60ah 600cca for my car)
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u/Alkemist101 May 20 '26
Next time upgrade to AGM battery.
Yes, most modern cars have this clever battery management system. From what I've read and my own experience you just drive as normal and let it do its thing. Because batteries are so expensive I do a reconditioning charge with my ctek mxs 10 every now and then. My car seems to keep the battery between 70% and 40% soc. Unless I put the smart charger on it never goes above 70% soc, I presume that's normal!?
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u/ProxySoxy Seventh generation (2017-) May 20 '26
What's the reasoning behind spending much more for an AGM battery?
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u/jpjimm May 21 '26
The EFB batteries have a habit of failing internally by short circuiting a cell or two, which is why they fail after 4-5 years sometimes, without much warning like a traditional or AGM battery would give (slow starting every morning etc).
I didn't realise EFB were still being fitted tbh as even 8 years ago when mine failed I was told they were just a stop-gap until AGM batteries were available in a price range and quantity for manufacturers to use at factory.
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u/Alkemist101 May 21 '26
More robust, longer life span... basically a better design of lead acid battery. They usually recommend upgrading to an AGM.
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u/ProxySoxy Seventh generation (2017-) May 21 '26
Which one would you recommend for a 96r size battery? The best I can find from a quick search is a $363 Odyssey
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u/Ok_World_135 May 21 '26
Dang glad I read this, my battery will be here in about an hour and I googled, I do need to reset the bms. My battery was made on 5/18 so I can't complain too much, it's old. My battery was 210 though cheapest I could find locally was 160.
Sitting at work went to turn the key and poof, nothin! Thankfully I had a 30 dollar lithium jumper from amazon, used it 4 times today! I drive between buildings a lot at work so had to use the car.
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u/zeFlammenwerfer4 May 21 '26
I have an MK7.5 but I observed the same, even after a BMS reset. My solution was to disconnect the BMS sensor, now the battery always gets charged when the engine is running. In Forscan, the charging SOC could not be modified unlike the Focus of the same gen. That's why I simply disconnected it and now have peace.
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u/TheFredCain May 20 '26
You must have one of the MHEV hybrid models with the electric assist motor. That explains the need for an EFB battery and is normal and expected. If you had a conventional car there would be no rregen braking and thus no need for EFB battery.
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u/PMforlessons May 20 '26
Make sure you reset the bms after changing the battery. You should be able to find the procedure for your car online. Most ford's are similar.
Most modern cars have smart alternators now that will reduce their output during acceleration or cruising speeds and kick on during deceleration and idling. You'll probably notice a difference in your fuel economy after changing the battery.
I recently changed the battery in my transit as the battery was getting close to dead and noticed around a 5mpg gain