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-   -   Battery change and BMS reset? (https://www.focusfanatics.com/forum/mk3-focus/752153-battery-change-bms-reset.html)

saintsfins 06-09-2017 11:43 AM

Battery change and BMS reset?
 
My OEM battery died a couple of days ago, so I got a new one at Advance Auto. I've read that the BMS resets itself if the car is left alone for 8+ hours, which mine has been for the last two nights. Can I assume that the BMS was automatically reset? Car starts/runs fine. No warning lights on the dash.

Is there any indication that it wasn't reset, like the batt light coming on?

stayboogy 06-09-2017 10:18 PM

no.

if yours has the BMS, not positive all of them do, then it will need to be reset.

same thing happened here 3 months ago. OE battery bit the dust, put new battery in and sometimes it would say battery there was low battery if I had the key on engine off, even after sitting over a weekend and checking the battery. i realize this may not be happening to you, but it doesn't reset itself.

reset it with forscan, and immediately the radio seemed louder at a lower volume and I noticed my key fob works fast again like it hasn't in forever, and security light and lock light actually stay activated for longer again after the car is parked and locked.

definitely not placebo. my radio come on loud as hell automatically as soon as it reset and the locks locked and I think it honked the horn.

Jburks 06-09-2017 10:38 PM

that's odd. i've had my battery out for various reasons multiple times and i've never once had any issues when i put the battery back in the car.

dibar74 06-09-2017 10:52 PM

Needs to be reset. If not ecu still thinks there`s poor battery in and charges incorrectly. Kills the new battery.

stayboogy 06-09-2017 10:52 PM

me either. never got a warning for low battery before the OE battery died and wouldn't recharge--6 years old BTW.

after driving on a brand new battery for a week, letting it sit for 24+ hours, I noticed if I just left the key on a few moments before cranking it would warn for low battery. tested new battery, fully charged before testing, and put it back in, same stuff.

I know it doesn't reset itself though. after resetting BMS it hasn't happened and the one thing that was pissing me off for like the previous 6 months or so was the key fob not being as responsive even after it got a new battery. slightest touch it works now immediately

Jburks 06-09-2017 11:03 PM

Do these issues only happen if you change the battery? I've had mine completely disconnected and out of the car multiple times and put back in and never had a light or had anything act weird.

stayboogy 06-09-2017 11:28 PM

have no idea.

I attributed mine to the battery going completely dead like so dead it wouldn't jump.

I think it sets a flag in the bcm.

from what I've read the BMS monitors current and uses a formula in the bcm to calculate battery life and health and uses this formula to cut power as it sees appropriate to accessory powered devices to preserve a stabilized current.

I say if you replace the battery or it goes completely dead, it needs to be reset cause it can't reset itself.

I know of at least one instance where a bad battery killed a TCM in a focus and we at work had it all fixed for free from ford. battery went dead and the car kept acting like it was shifting into neutral after being unused off and in park, and without any user interaction. it was like it was possessed. ford said battery dying, being jumped off, and the car being driven with a weak battery fried the TCM. all fixed under warranty and they weren't surprised and seemed to act like it was common.

Jburks 06-09-2017 11:30 PM

well damn. how do you reset the BMS?

stayboogy 06-09-2017 11:35 PM

Quote:

Originally Posted by Jburks (Post 10786001)
well damn. how do you reset the BMS?

forscan, under services tab. you might have to have the extended license, but that's fast and free, just have to register and be approved at the forum and then ask for a code. takes up to a day.

if you do it, verify if the horn honks or not. I really can't remember.

Jburks 06-09-2017 11:41 PM

sorry, i guess i'm a newb, but what is Forscan?


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