Thread: D4's charging / battery problem

Originally Posted by Ferret

My experience also after monitoring its behaviour. Once it gets above 80% it just losses a bit of interest in charging it further. It will get to 100% but only after hours of driving.

Hi Ferret, this is actually quite normal charging.

The last 20% of the charging cycle is controlled by the battery, not the charging source. It is usually termed the ABSORPTION stage of the charge.

The condition of the battery will also have an effect the charging of any lead acid battery. If a battery is in a poor condition, it will take longer to charge.

I'll cover this more in my next reply.

A couple of points to cover here.


First off, the “Low Battery, Start Motor” message is usually supplied by your infotainment unit, and in many cases, when this message is displayed, you will usually have some form of limited operation of the entertainment system, like a dramatic reduction of the volume.


The warning is usually at battery voltage levels of 12.2v or lower.


Your motor can be stated at 11.6v ( 20% SoC ) so you will normally still have plenty in reserve.


As in my last reply, the final stage of charging is controlled by the battery and if the battery is in a poor condition, it will not only take longer to fully charge but you will most likely have a reduced total usable capacity.


This is commonly caused by lots of short drives and no maintenance charging before you go on your trip.


A good practice to get into is to carry out an over night charge, every night for up to a week before you go on a trip.


This type of maintenance charging will both increase the amount of battery capacity you will have when you need it and will get the batteries back to a better condition.


The additional advantage of this is that if you stop in one place for a while and use a fair bit of battery capacity when you move to your next stop-over, because your batteries are in good condition, you will replace far more used capacity in a shorter drive time.


There is a lot more to this, but some basic maintenance charging has some good benefits.

Originally Posted by drivesafe

Hi Ferret, this is actually quite normal charging.

The last 20% of the charging cycle is controlled by the battery, not the charging source. It is usually termed the ABSORPTION stage of the charge.

Yes I'm aware charging current is controlled by the battery state but what I'm saying is, at times the BMS seems to not want to maximise the SOC.

For instance, I see it at times drawing from the battery rather than charging the battery when SOC > 80%, ie it only tries to charge the battery when coasting or under brakes. So at times, it is not a simple absorption stage, it's doing something else during this phase - monitoring / testing, I don't know what. Point is, the BMS sometimes shows little interest in getting the battery to the max SOC possible until the duration of whatever is going on in that phase finishes.

I have posted this picture before showing current draw from the battery while driving. I thought at the time it was associated with fitting a new battery or maybe resetting the battery with the GAP tool but since I have become aware of it and look I have noticed it doing it periodically. And as stated above, that battery draw reverses to a battery charge when braking or coasting which means effectively there is no net change in SOC, even after driving for a while.

Might be just some weirdness with my vehicle but it happens.

Hi again Ferret, and the reading you see in your screen snap is of a battery that is near fully charged and the vehicle is drawing on the cranking battery to reducing the load on the alternator while you have your foot on the accelerator peddle.

This is called Kinetic Charging and if you have a look at the link below, I describe it in more detail.

Again, your screen snap is of a battery that is near fully charged. If your vehicle has one of my isolators fitted and if you have been camping for a day or two and have been drawing on your cranking battery, when you start driving again, your vehicle's BMS will monitor the cranking battery as you start your motor.

In this case, as the cranking battery is not at a fully charged state, the BMS will disregard the Kinetic operation and just runs the alternator at 14.v until the BMS ascertains that the cranking battery is at a near fully charged state again.

The BMS only controls the VOLTAGE level of the alternator and current output of the alternator is dependant on the current load being applied to it.

So in vehicles fitted with my isolators, where the cranking battery is used to assist the auxiliary battery, if the cranking battery is anything but near fully charged, while the alternator voltage is held high to charge the cranking battery, all batteries connected to the alternator, including batteries in a camper trailer or caravan, will all be receiving a charge.

Again, there is a lot more to it and I will be adding to the thread below, shortly.

On-Line auto electrical info

Originally Posted by drivesafe

...This is called Kinetic Charging

Yes accept all that.

The problem is the BMS does not know you have a fridge full of food in the back nor that you are going way and ideally want to arrive at your camp site several hours later in a higher SOC than what you initially set out with. On long trips along flat roads there is less opportunity to be coasting under brakes or going downhill so that type of charging behaviour just tends to draw down your battery till it drops to whatever SOC (or other conditions are reached) which kicks it out of that type of charging behaviour and the BMS resumes normal service.

And I might add, it does not behave that way all the time. Sometimes, even when at a high initial SOC, The BMS does not display that kinetic charging behaviour, it just displays a normal 'adsorption' type charging behaviour and the battery eventually reaches 100% SOC.

I guess I would rather it concentrated on just charging the battery rather than saving me some fuel.

You know it’s not allowed to though - all done to meet emissions.

Originally Posted by Tombie

You know it’s not allowed to though - all done to meet emissions.

There should be a law against hot beer.

Hi again Ferret, I must be missing something, why is your fridge not working?