Battery discharge

[secretariat]

My 2016 X5M gave me a battery discharge drive for longer periods warning. Granted on Sunday morning it was 1 degree F outside. I took the car for a 45 mile run and haven't had any problems since. I'm going to assume the warning was because of the cold. Am I wrong, should I worry, etc????/

[onfireX5]

How cold was the M when you started it ? Outside ? Warning come on right at start up ?

[Skyracer]

These vehicles are very hard on the battery. In fact, I can say my X5M taxes its battery more than any other vehicle I have ever owned.

Mine was delivered in October 2017 and came with a trunk mounted charger installed by the dealer at no charge. To me, that was a big red flag from the get-go, and true to form, the vehicle is not happy if I don't use the charger regularly.

Example: Did a 15 km (9.5 mile) round trip on New Year's eve. The car had been driven 2 days before that, for about 60 km (40 miles), but I hadn't used the charger for a couple of weeks. I expected the short trip to be hard on the car as the temperature was about -28 C (-20 F). My wife and I used both front seat heaters and the steering wheel heater. There were no issues on the drive out, or on starting the car for the drive back, but after I turned the car off in the garage on arrival home, 'low battery warning' came up on the dash. Granted, we asked for a lot of electricity, but no more than I have done on previous vehicles.

Bottom line: trickle charging your battery (even if brand new!) if you are in a cold locale is mandatory.

[secretariat]

OnfireX5, it did not give me the warning until after I drove to the store and back to my driveway. When I turned the car off the warning came on, so I immediately turned the car back on and drove about 15 miles to get gas (7.5 miles each way) then I left the car on in my driveway took a shower and drove for another 30 or so miles. I have not had a warning since, and the temp. is now up to 28 Degrees Faren. To answer your question the temp on the gauge was 1 degree F. and the warning came on only after I drove about two miles to the store and back.

[onfireX5]

That makes sense. Was cold saturated. Then short cold start. Then short drive. You depleted more amperage than the alternator replenished and you received the warning. The long idle after the warning replenished the batteries. No worries.

[hinton]

hey guys, i think you might be missing how these things charge, the idle in the driveway would of done squat for recharging it. your vehilce has two batteries, the starter and the aux battery. a 92ah, and a 50ah one. look in the back trunk.

anyways i gave very accurate metrics about what happened to me and have a slew of data i have recorded for the last 10 days, i can expand if you have more questions about the inner workings, i am an engineer blah blah, anyways, here is my post

http://f15.bimmerpost.com/forums/sho....php?t=1452200

take care.

i used a series of conductance tests and wrote a program to predict the actual cca of the batteries based on F and C temp values, my numbers are usually accurate to about 10CCA when i plug them all into the formula.

[onfireX5]

What have you measured that shows the alternator does not replenish the batteries at idle conditions? I’m not saying that it is impossible, but it would be very unusual. I’ve built a couple of automotive charging systems. One, I did used an ECM control but later bypassed it since a properly designed alternator will only transfer what a battery requires. The battery is truly a filter that supplies the system with “clean” voltage....that’s why an alternator should never be used to supply electricity to the modern control circuits. It supplies the battery. Old systems could remove the battery and continue operating. The ECM does regulate the alternator for economy reasons, and you might have been testing during that “off cycle”, but I would be surprised if the alternator never replenished the batteries during idle conditions. That would cause a total discharge during a long idle in cold weather.

[hinton]

based on what secratariat said:

he went to the store, turned it off, got the error
that error probably triggered at like 43% state of charge, he then went home, and idled it. The idle would of only maintained the value of the over charge so he probably got a higher state of charge, but the idle would not of topped him up, if he had turned off the car and just left it alone witout the half hour idle he would of had the same value as if turning it off immediately. his temperature of -2 C is a negligible temp impact on an agm, so with better weather the issue went away, get it down to -28C and it would come right back the next day as soon as he drove away and turned the car off he would have the issue again.


from what i have read the way these cars are coded are as follows:

the batteries are only 80% charged when they are registered, the extra 20% is for the eff dynamics regen under coasting or compression.

the alternator at idle will only top up the aux battery to the level that it is currenlty at while idling, so whatever your cca value is or soc when you are at idle it will keep it at that value and engage the alternator. if you have a low battery, turning on the car and then just sitting at idle will not fully recharge your car nor driving for two hours would recharge the car either.

i read the comment as your battery was 40% or less of soc and that he sat their idling for 30 minutes and that topped him up back to the 80% threshold of the battery, which would not happen.

i do believe that the alternator will just keep your battery at the voltage level that it is at while idling but never going above and beyond and charging or topping it up.

if you sat there idling you would not drain your battery as the alternator would be engaging and keeping the same charge to maintain it but it would not be bringing the battery Ah to where it is considered normal or charged.

i have had many bmw people corroborate that statement hence why they push the trickle chargers. unless you code it out and the alternator engages all the time then that is how i understand this system to charge.

i have also gone out, let idle, come back, removed surface charge, and measured the batteries terminals and their conductance and no it was no where near even 60% charged back so that kind of proves the whole thing of idle and long drives under compression or in gear without automatic mode does not charge the batteries.

[onfireX5]

I’ll throw an analyzer on it for fun one day with the engine off, then idle and recheck. A 92AH battery at full charge at rest should be at roughly 12.7V. At total discharge, at 0AH, roughly 11.2V. A 40% charge and maintained at 40% is a poor design if true if it measures that level, then maintains that point. That would be a resting rate of roughly 11.8V and 37AH....ballparky as charge % isn’t perfectly linear. My analyzer should measure both values.

[hinton]

it would be interesting to see what you get back, i think you may be shocked at the results, as well report back the temperature outside when you do this as we can estimate the actual CCA based on battery temperature as that plays a role in the CCA seen. i would suggest doing this though like in the middle of winter cause i get good results in the spring and summer.

[secretariat]

Bimmerposters, I never said I let the car idle for 30 minutes, what I said was that the car was idling while I took a shower and got dressed which took me 15 minutes. Also, when I drove to the store I left the car on idling while I went into the store because it was so cold out. I got the battery discharge warning when I got back to my driveway after going to the store and turned the car off.

[hinton]

ok i agree with that statement and you should of gotten that error message. so did you trickle charge it after that? or was the weather too good the day after and the issue went away?

[secretariat]

The weather has been much better, and I have had no further problems. Thanks to everyone who chimed in.