The Battery management system in newer cars is a little misunderstood………
Most of the time you can get away with just swapping the battery and
have no issues at all, but what happens when a simple battery install
turns into a nightmare because the old Alternator isn’t working
properly with the new battery you had installed? So lets take a quick
look at the tech and see whats happening.
The battery management system watches the state of charge of the
battery and tells the alternator when to charge or not charge, to keep
up with the power demands of the vehicle, and to make sure the
Alternator isn’t just jamming along at 100% charge rate and making your
fuel economy go to crap and all sorts of electrical tomfoolery.
On some newer vehicles the current State of charge (SOC) requires
registration via a scan tool and some models have various tricks or
can automatically deduce this information if the vehicle sits for X
hours etc etc.
So all battery registration does is tell the Battery management
system about whats going on with the charging system. And this extra
piece of programming tells the car to be prepared for this different
and or new battery, that may have a smaller or larger power capacity.
If the car battery is dead or close to being dead and you replace the
battery, the BMS might think there is still a shit battery and may
have the old “state of charge” stored in the BMS. This old Information
being incorrect causes the Battery Management System to keep pushing
more unneeded power into the battery thinking its still helping out
the crippled old battery that’s no longer there!
In turn this excess power can create a little havoc by overcharging
or not charging a battery at all….who knows what a none calibrated
component will do? Or maybe the car came with a standard wet cell
battery and you swap an AGM battery into the Car, Mr. Sensitive
battery Management system thinks its charging a regular battery, The
AGM may have different charge cycle requirements and with its brain
out of wack could damage the alternator or battery.
My question is, would you hook a charger made for flooded batteries up to a AGM battery to charge it? I think there is more to the battery registration than we think.
So lets talk about some basic BmS……
A charging scheme, does NOT maximize the battery ON time per charge.
The charging scheme has the ability to reduce the battery’s lifetime if
it needs more charge and discharge cycles. A weaker battery discharges
faster. This also occurs on the discharge cycle, the weaker battery
trips the discharge limit faster on most BMS systems than a good
So weak battery more charge time, Good battery less charge time.
So if you replace a 700 Amp hour battery with a 400 amp hour battery
what happens? the charge/discharge cycle changes and the Alternator
turns ON and OFF more to keep up with the reduced power capability
of the smaller battery. And vice versa, If we change from a AGM battery
with a longer charge/discharge capability to a lead acid battery with
30 or 40 percent less power storing capacity and the Battery
Management system still thinks it is trying to keep a Larger Capacity AGM battery in top
shape! So whats wrong?
The BMS is programmed for a BIGGER battery!
The BMS goes through its normal discharge/charge ON/OFF cycle and now your new
battery is dead, your trying to price a new Alternator when that’s not the issue!
Now lets look at going from a weaker flooded battery to a bigger
battery, The charge discharge cycle is set at a faster rate, So the
BMS is still trying to keep a smaller battery charged, this faster
charge/discharge ON/OFF cycle keeps the battery charged but reduces
the life of the battery, batteries can only charge and discharge so
many times before they are crap! so there is no issue until you need
a new battery every year and you are wondering why new batteries are
The main issue I have seen with the BMS systems on newer vehicles, is not very many mechanics and car owners know that there is a battery control system at all! So when the car starts to act up and there actually IS an issue with the battery to save money and time they run to the nearest parts store and get a battery installed. Parts stores have been installing batteries for years for free (unless its buried in a wheel well or in an odd place) And after installing a battery in a BMW or a Newer Ford (most other makes also) the customer goes along their happy way and the next day the battery is dead or weak, they run back to the parts store and the battery is checked and charged and put back into the car. This process continues without any mention of Resetting the BMS in the Computer at all! A few makes and models are known for needing a proper reset of the battery system (Ford and BMW are big culprits) to make everything sync up and work right again. So next time your vehicle starts acting stupid and you change the Alternator and or Battery 4 times and the battery is still dead, Might be a different issue all together.
Does your BMW need a New Battery Every year?
The Car more than likely requires the Battery Management system to be programmed to see a new or different type of battery! Skipping the Battery registration your car might think its trying to keep an 8 year old 600 CCA Flooded battery charged, when you just had a shiny new AGM battery put in…….
On most of the Newer BMS systems the BMS itself can deduce the state of charge/depth of charge and program itself according to the current it measures and a few other variables, Sometimes “programming the battery” is as easy as leaving your vehicle off and locked for 8 hours.
Battery Management Systems have been around for many years and are not “new tech”. Uninterruptible Power Supplies, Android phones and Laptops have Battery Management Systems so Don’t take a Simple thing and turn it into a witch hunt……
Below is a picture of the voltage reading of a battery not charging because of an issue, Many times more modern charging regulators and BMS systems will be at the bottom of the charging cycle and the voltage output reading below 13 volts will trick mechanics into thinking there is some kind of issue when there is not.
In the picture below, After repairing the issue in the charging system, charge voltage is back to a normal level, but this higher charge voltage can be missing at the bottom of a charge cycle, once again a perfectly working Alternator and/or charging system tricking you into thinking there is an issue. On many newer vehicles the Alternator output will be negligible if the Battery is at 100% charge, and a reading as low as 12 volts may be seen. Once the charge cycle starts you will see the Alternator start to activate and voltage will be somewhere around 13-14 volts on most Average cars.
So……. What car brands have the Battery Initialization or coding function?
Now Lets look at a List of supported Vehicles on a popular Battery Reset tool!