Hello!

I first like to apologise for my terrible English so please bear with me 

I'm experiencing a BMS issue which is causing my battery to charge incorrectly. I have two aftermarket amplifiers installed and noticed that the battery voltage was not keeping up while the battery is as good as new. At first i thought it was a parasitic drain so I did a drain test using a multimeter. After the car went asleep the draw was 80mah which is on the high side but nothing to panic about. The alternator is also working correctly.

When using forscan i can monitor the current that goes in and out of the battery. I notice that the current draw gets higher when I do things like opening the electric windows or turning on the ventilation fans but when i turn up my sound system full blast the current drawdoes not change at all. So i think the BMS calculates the wrong SOC because it doesn’t read the extra load of the amplifiers. The battery is below 12v and according to forscan the SOC is 78%? lol? So BMS thinks the battery is 80% but in reality its drained.

I read somewhere that it’s important to ground auxiliary systems not directly to the battery terminal but on the other side of the BMS Sensor like the chassis ground so the current goes through the sensor. Both my amps are grounded to the chassis. One of them is even using a factory ground. How is it possible that the BMS cannot see the extra load of my amps?

Any advice is much appreciated! 

14 minutes ago, Mitchh91 said:

The battery is below 12v and according to forscan the SOC is 78%? lol? So BMS thinks the battery is 80% but in reality its drained.

This can happen if a new battery is fitted, BUT, it was not fully charged before it was installed and then the BMS was reset. This will give a false SOC.

To correct the problem you will need to fully charge the battery for at least 12 hours, probably longer using a smart battery charger. Then let the battery rest for a few hours and measure the battery voltage. If it is at least 12.60V then reset the BMS.  I normally would say that the BMS should only ever be reset when the battery is installed new, but I suspect it has already been reset a few times now.

If the car still has the original battery, then it's five years old, so could be coming to the end of its life.

2 hours ago, unofix said:

This can happen if a new battery is fitted, BUT, it was not fully charged before it was installed and then the BMS was reset. This will give a false SOC.

To correct the problem you will need to fully charge the battery for at least 12 hours, probably longer using a smart battery charger. Then let the battery rest for a few hours and measure the battery voltage. If it is at least 12.60V then reset the BMS.  I normally would say that the BMS should only ever be reset when the battery is installed new, but I suspect it has already been reset a few times now.

I bought an original ford battery a few weeks ago and measures voltage before installing. Voltage was 12.7v BMS was reset using forscan.

Do you think it is normal that the additional current draw from my amplifiers is not picked up by the BMS according to forscan sensor monitoring service?

1 hour ago, Alan G H said:

If the car still has the original battery, then it's five years old, so could be coming to the end of its life.

The battery is only a few weeks old and tested fully charged before installing