Jump to content
Do Not Sell My Personal Information

Could this be a Battery Management System issue?

Recommended Posts


I have a 2015 registered Mondeo 2.0 diesel. Over the last 18 months, I have had a few occasions where the battery appears to have died on me. On these occasions, there has usually been a message advising of an issue with the door locks or the handbrake.

When I have checked the battery using my rudimentary tester, it appears to be showing low voltage, but when the engine is running, it indicates that the battery is charging. I did have the battery replaced about 2 years ago, but with working from home quite a bit, I haven't been doing as much driving as I had previously.

On past occasions I have connected the battery to one of our other cars and started it without a problem. I also tried immediately stopping and restarting after this initial "jump start", and it then started without any problem.

Yesterday, same problem, but as I had no family car to connect to, I put the battery on charge then tried it after 15 minutes, and it started without any problem. Again, immediately stopping and restarting was done without a problem.

So given that it immediately restarts once I have "jump started", could this be a BMS issue? Is it worth trying the fixes for this issue (e.g. disconnecting the positive lead, flashing the light/pressing the brake etc)?

Many thanks.

Link to comment
Share on other sites

Did you reset the BMS when you fitted the new battery 2 years ago ?

Modern cars don't fully charge the battery (by design) and require regular topping up with a battery charger depending on how often they are driven.

The State Of Charge (SOC) must be low on the battery for you to have the issues that you describe.

On 4/30/2022 at 6:08 PM, unofix said:

I'll try and give you the quick version. 🤣

So due to emissions and save the dolphins, cars have been fitted with Stop/Start. At more or less at the same time also to reduce emissions and save more dolphins cars had there smart charging systems 'tweaked'  so that the battery is only ever charged to a maximum of 80% of capacity. Big problem is the two things work against each other.

When your car battery State Of Charge (SOC) falls below 70% the brains of the car starts to disable things to save battery power. One of the first things to become disabled is Stop/Start, followed closely by the heated front screen.

To fix the problem you can use FORScan software and adjust the SOC to 90 or even 95% and for most people this is enough to keep the Stop/Start function working. You do need to charge your battery fully with a smart battery charger @StephenFord has a recommendation I think.

So, change the SOC to 95%. Charge the car battery for 12 hours, make sure the negative battery charger lead is connected to the chassis and not the battery terminal and you can once again begin to enjoy the pointless annoyance of the Stop/Start system. 👍

See the attached table for battery SOC

Ford - Battery SOC.JPG

Link to comment
Share on other sites


The battery was replaced at ATS - they didn't mention anything about resetting the BMS, so in hindsight, I suspect they may not have done!!

Just checking that it the Stop/Start function is not working (mine has only worked very infrequently recently), would this also impact on starting the car?

Many thanks.

Link to comment
Share on other sites

Stop/Start is a pointless gimmick designed purely to get the vehicle through the type approval process, it's best forgotten about.

In your situation I would recommend using a Smart battery charger and charge the battery for at least 12 hours, longer if possible. Make sure that the positive charger lead is direct to the battery terminal and that the negative lead of the charger is connected to the cars chassis main earth point - NOT the battery negative.

After a good long charge then reset the BMS. Please note to all others reading this the BMS should only ever be reset when a new battery is installed, this is a special case.

Link to comment
Share on other sites

Many thanks, and apologies for not replying sooner - I didn't get a notification!!

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...