Ben James
Well-Known Member
- Joined
- Oct 19, 2018
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- Location
- Queensland, Australia
- First Name
- Ben
- Vehicle(s)
- 2018 S550 Mustang GT
I’m actually heading out today to meet up with a large group of other Mustang enthusiasts that want to know the ins and outs of this system for performance testing as well.
long story short:
near the battery is a coil type current sensor on the negative lead. Any earth cables you want to connect directly to the negative terminal of the battery have to go through this coil for the BMS to accurately guage power usage.
near the main fuse box in the engine bay is another smaller coil sensor coming from the alternator positive lead, and this tells the BMS how much current is being created.
when replacing with a new battery you should reset the BMS via 5 high beam/3 brake method or Forscan. If replacing with a Lithium it is a must, as you have to clear the days in service and measured floating voltage history.
there is a way to remove the Bms control altogether and run the system as per a standard old School alternator that does 14.4v at all times (disconnect negative sensor and positive) but I haven’t done it to check the results long term (possible over temp/current of alternator)
instead, you go into Forscan, reset the Bms, then under the body control module settings (not as built, the tab underneath) change the battery type to an AGM variant (similar charge attributes, I chose the 70Ah one), and select the BMS to be NONE (three choices, the default is PWM, then there is linear and lastly NONE-OFF).
On first startup after the changes you will see your voltage charge rate going up to 14.4v, indicating the BMS is actually still doing things, however now it’s job is just simply to make sure that the battery is fully charged (at what ever voltage the two sensors indicate equaibrium). For a Lithiumthis ends up being around 13.5-13.8 volts.
After 8 hours of monitoring the battery for history to be accumulated, the BMS will now charge up the battery to full, and not stop charging until the negative sensor and positive sensor (plus other currents) equalize indicating a fully charged battery state.
it will then monitor and try to keep the current perfectly matched so you battery isn’t being used, and isn’t being overcharged.
every time you start your car, regardless of whether you drive off straight away or sit parked for a while, the BMS will ensure the battery is fully charged first, and it monitors the alternator to give a measured current that keeps both the battery and alternator in operating limits (good for hot climates).
Quick explanation of BMS modes;
PWM: a pulse width modulation signal is used to negotiate charging and the BMS will charge only if it thinks the system needs it. Because the system does not cater for lithiums, it can’t quite figure out the battery charge state, and assumes if it’s over 12.8v it’s ok… for lithiums this voltage is under 20% left (not ok). Typically the BMS will charge up the battery a little on startup, then stop charging when you start to move, only charging on decelerating and if the voltage falls below a predetermined rate deduced by a combination of the historic data and selected battery type.
Linear: Same as PWM but charges at a linear rate (not required on Mustangs)
NONE (off): charges until the current draw into the battery is nulled out and then holds that voltage regardless of acceleration/deceleration.
Today with our Mustang group we are going to see how much actual load the alternator presents on the engine on acceleration considering the lithium floating voltage is almost the same as the maximum voltage the alternator puts out (200amp alternator is used for this testing).
we suspect that ford puts crappy batteries in our system to save money, and turns off the charging rate on acceleration to meet the 339kw figure on paper…
I’ll get photos of the Forscan menus while I’m out to explain the settings better then report back.
hope this helps!
p.s. don’t take the voltage reading the vehicle dash reports as gospel either. The reading you see on your vehicle LCD guage is typically 0.5-1.0 volts under what’s actually being seen at the battery. This is because the display is designed to give an indication of the battery charge state, not the alternator charge state. It assumes we are all ignorant and only understand that the guage at the halfway point is good.
If you see the voltage display on the dash waver, it’s an indication that the battery isn’t fully charged. The display can’t accurately show the charge state when the alternator is feeding the battery current so it will go up when charging and down when displaying the charge state. Lithiums will do this for a little while after installation as the BMS collects data, then after a few startups it will go to a rock solid figure in between 13-13.5 (which is actually about 13.8-14.2v)
with a lithium battery that can report to your app what actually going on, it gives a much better understanding of what’s happening.
long story short:
near the battery is a coil type current sensor on the negative lead. Any earth cables you want to connect directly to the negative terminal of the battery have to go through this coil for the BMS to accurately guage power usage.
near the main fuse box in the engine bay is another smaller coil sensor coming from the alternator positive lead, and this tells the BMS how much current is being created.
when replacing with a new battery you should reset the BMS via 5 high beam/3 brake method or Forscan. If replacing with a Lithium it is a must, as you have to clear the days in service and measured floating voltage history.
there is a way to remove the Bms control altogether and run the system as per a standard old School alternator that does 14.4v at all times (disconnect negative sensor and positive) but I haven’t done it to check the results long term (possible over temp/current of alternator)
instead, you go into Forscan, reset the Bms, then under the body control module settings (not as built, the tab underneath) change the battery type to an AGM variant (similar charge attributes, I chose the 70Ah one), and select the BMS to be NONE (three choices, the default is PWM, then there is linear and lastly NONE-OFF).
On first startup after the changes you will see your voltage charge rate going up to 14.4v, indicating the BMS is actually still doing things, however now it’s job is just simply to make sure that the battery is fully charged (at what ever voltage the two sensors indicate equaibrium). For a Lithiumthis ends up being around 13.5-13.8 volts.
After 8 hours of monitoring the battery for history to be accumulated, the BMS will now charge up the battery to full, and not stop charging until the negative sensor and positive sensor (plus other currents) equalize indicating a fully charged battery state.
it will then monitor and try to keep the current perfectly matched so you battery isn’t being used, and isn’t being overcharged.
every time you start your car, regardless of whether you drive off straight away or sit parked for a while, the BMS will ensure the battery is fully charged first, and it monitors the alternator to give a measured current that keeps both the battery and alternator in operating limits (good for hot climates).
Quick explanation of BMS modes;
PWM: a pulse width modulation signal is used to negotiate charging and the BMS will charge only if it thinks the system needs it. Because the system does not cater for lithiums, it can’t quite figure out the battery charge state, and assumes if it’s over 12.8v it’s ok… for lithiums this voltage is under 20% left (not ok). Typically the BMS will charge up the battery a little on startup, then stop charging when you start to move, only charging on decelerating and if the voltage falls below a predetermined rate deduced by a combination of the historic data and selected battery type.
Linear: Same as PWM but charges at a linear rate (not required on Mustangs)
NONE (off): charges until the current draw into the battery is nulled out and then holds that voltage regardless of acceleration/deceleration.
Today with our Mustang group we are going to see how much actual load the alternator presents on the engine on acceleration considering the lithium floating voltage is almost the same as the maximum voltage the alternator puts out (200amp alternator is used for this testing).
we suspect that ford puts crappy batteries in our system to save money, and turns off the charging rate on acceleration to meet the 339kw figure on paper…
I’ll get photos of the Forscan menus while I’m out to explain the settings better then report back.
hope this helps!
p.s. don’t take the voltage reading the vehicle dash reports as gospel either. The reading you see on your vehicle LCD guage is typically 0.5-1.0 volts under what’s actually being seen at the battery. This is because the display is designed to give an indication of the battery charge state, not the alternator charge state. It assumes we are all ignorant and only understand that the guage at the halfway point is good.
If you see the voltage display on the dash waver, it’s an indication that the battery isn’t fully charged. The display can’t accurately show the charge state when the alternator is feeding the battery current so it will go up when charging and down when displaying the charge state. Lithiums will do this for a little while after installation as the BMS collects data, then after a few startups it will go to a rock solid figure in between 13-13.5 (which is actually about 13.8-14.2v)
with a lithium battery that can report to your app what actually going on, it gives a much better understanding of what’s happening.
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