The battery voltage on my Focus MK3 is 11.9 volts after the car has been parked and off over night. Stop/start is not working. I figure it's time for a battery swap.

Currently the car has the original battery, a 60 Ah EFB T6 format battery with 650 CCA.  

I can get an Exide EL752 EFB battery for a good price. This is a 75 Ah T7 battery with 730 CCA. It has the same height and width as the original battery, but is a few cm longer. 31.5 cm vs 27.8 cm. It willfit in the battery box and nothing will obstruct the cover from closing as with taller batteries.

FoCCCus reports #210 Battery as "T6 IF 60 Ah 650 Cold Cranking Ampere". 

My plan now is to leave the battery configuration as it is after the battery swap. I will reset the BMS as discussed in other threads here. 

My guess is that the BMS operates something like this: 

• When the car is in use, the BMS counts the coulombs/electrons that are leaving the battery as well as those sent to the battery while charging. This is what the sensor connected to the negative battery terminal is there for. Based on this the BMS has a good estimate of the battery state of charge at all times. 
• After the car has been parked for several hours and the battery voltage has stabilised to a value that is a function of the battery state of charge (SOH), the BMS measures the battery voltage and determines the actual battery charge level. 
• The BMS tries to keep the battery at 80% state of charge in order to not accidentally overcharge and damage the battery.

With a larger battery than the OE one, the Depth of Discharge (DOD) experienced by the battery during normal operation should be less than with a smaller original size battery. This will lessen the wear on the battery and prolong the battery life. 

If what I write on how the BMS manages the battery is correct, I can see no problem with installing a larger battery than the OE one and keeping the #210 Battery setting as it is. 

One negative consequence of installing a larger battery is of course that the car will have to carry a few extra kg at all times. 

Anyone have any thoughts on this? Are my thoughts on this correct?

Your thoughts on how the BMS works are broadly correct.

Then you need to think about how the smart charging works. It will be trying to put enough amps into a 60Ah battery to bring it up to 80% capacity. Will that bring a 75Ah battery up to 80% capacity or only 64%? If it only brings the 75Ah battery up to 64% will the BMS reset overnight when it reads the voltage and result in your stop-start not working because the battery is at less than 80% charge?

I don't know the answers because I've never fitted the wrong size battery. 

 

 

I have the Exide EL752 EFB in mine, no probs