12V (AUX 18L) new battery voltage shows 11.9V

[i3Houston]

All,
The OEM batter was over 56 months old. Cig. lighter adapter car volt meter [Amazon link ] showed 11.8volt. Clearly it was time to replace. So after fully charging Remy AUX 18L and verifying with volt meter that battery is producing 12.9V, battery was then installed in the car.

With the new battery in, i3 turned on and drove normally. Only prompt was to set the time (date was retained somehow). But, the voltage meter in the cigarette lighter adapter (front, center, under console) is still showing 11.8V. What could be causing this?

The 56 months old battery on testing with volt meter shows 12.9V as if its new but I think voltage under no load can be higher.

 

[EvanstonI3]

I would try some other way to measure the voltage than with a cheap cig lighter adapter volt meter then. Maybe just get a different one. The one you have could be bad. I have one plugged in just for reference and when the car is ON and ready to drive it shows 14.4-14.9 volts. Also try plugging into the port under the armrest instead to see if there is any difference. That's where mine is.

 

[mojo]

It would be a bit unusual, but perhaps BMW regulate the 12V outlets to be no more than 12V? TBH I've never checked the exact voltage at mine.

Was the 11.8V at rest and powered down, or with the car in "drive ready" state? My car is just over 2 years old, and the battery voltage on that drops to 11.8V if I unlock it and the "leaving home" headlights turn on. It seems that this voltage is just too high to force the car's built-in DC-DC converter to turn on.

But, as EvanstonI3 says, as soon as you put the car into "drive ready" state, the voltage should go up to around 14.4V. I should add that I measure the voltage directly from the battery, with a display in the car that is permanently on. That's probably a more accurate way to do it than via a cig lighter socket voltmeter.

[Edit: I should add that a lead/acid battery should not show more than 12.9V under "steady state" conditions. If yours does, it's probably because it had been on charge a few minutes earlier. If left for 30 minutes without any input or output, it should settle down to its natural resting voltage.]

 

[i3Houston]

It would be a bit unusual, but perhaps BMW regulate the 12V outlets to be no more than 12V? TBH I've never checked the exact voltage at mine.

Was the 11.8V at rest and powered down, or with the car in "drive ready" state? My car is just over 2 years old, and the battery voltage on that drops to 11.8V if I unlock it and the "leaving home" headlights turn on. It seems that this voltage is just too high to force the car's built-in DC-DC converter to turn on.

But, as EvanstonI3 says, as soon as you put the car into "drive ready" state, the voltage should go up to around 14.4V. I should add that I measure the voltage directly from the battery, with a display in the car that is permanently on. That's probably a more accurate way to do it than via a cig lighter socket voltmeter.

[Edit: I should add that a lead/acid battery should not show more than 12.9V under "steady state" conditions. If yours does, it's probably because it had been on charge a few minutes earlier. If left for 30 minutes without any input or output, it should settle down to its natural resting voltage.]

In Drive state voltage jumps to 14.0V with new battery (old battery showed 14.4V).

In rest mode (after unlocking), voltage is showing 11.8V just like yours. So this seems normal.
The cheap cig lighter seems to working though, in my other car it did report the voltage correctly. But I will try another outlet to test the voltage.

 

[EvanstonI3]

Under resting voltage, mine will show 12.4 because there is an accessory draw on the 12v when you unlock and open the doors to activate the power ports. If you measured the voltage at the battery terminals without the car in accessory mode, you would find the voltage to be higher, closer to 12.9. 11.8 still seems really low to me from the power port.

 

[i3Houston]

The resting voltage has climbed up to 12V now. Sometimes it shows 12.2V but then drops to 12V after Dashcam turns on. I was hoping to see 12.5V since Remy battery is new (11/2023) and fully charged.

I wil measure the voltage at the battery terminals when car is in ready state and post an update.


Voltage jumps to 14.0V and (not 14.4V ) when car is put in Ready state. I am wondering if there is need to register the battery via Bimmer link in this situation, thoughts?

 

[EvanstonI3]

Registering the battery does nothing. Nothing. The last 12v I installed where I registered the battery showed a lower voltage when "ON" (maybe 14.2) for awhile and then gradually went up to 14.8 where it is now.

If you are going to measure the voltage of the 12v at the battery terminals, don't do it when the car is ON and in READY state. You should pop the frunk, close the doors, keep the fob away from you and wait about 30 minutes. The accessories will shut off after about 15 minutes, but you should let it sit a bit longer to rest. Then remove the Frunk tray if necessary and measure the voltage at the terminals to see what the voltage is at a resting state.

 

[i3Houston]

ON state voltage at Cig lighter voltmeter - 12.0V. And at terminals of the battery - 12.44V.

In Gas car,
Accessories OFF - 12.55V at battery terminals. Cig lighter is OFF
Accessories ON - 12.42V at battery terminals. Cig lighter shows 12V

Cig lighter appears to be under reporting by 0.5V vs. battery terminal OR other assumption is that both i3 and Gas car are adjusting voltage to 12V at cigarette lighter socket.

 

[EvanstonI3]

When it is "On" there is a draw on the power so the voltage reading will drop. But it SHOULD read the same at the terminals and the cig lighter port. Really looks like the cig lighter volt meter is not functioning properly.

 

[alohart]

When it is "On" there is a draw on the power so the voltage reading will drop. But it SHOULD read the same at the terminals and the cig lighter port. Really looks like the cig lighter volt meter is not functioning properly.

I agree. The 12 V voltmeter I've inserted into the auxiliary power port beneath the center of the dashboard displays almost exactly the same voltage as the 12 V system monitor installed on the battery terminals. Both display 14.2±1 V when the DC-DC converter is on. I've never seen 14.8 V.

 

[i3Houston]

The proper voltmeter measured 12.50V at cigarette outlet when car was in ON state so that shows the new Remy battery is working normally. And the cig. lighter voltmeter adapter that was purchased from Amazon is faulty.

Thank y’all so much for your guidance!

FWIW, Dealership quoted around $350 for battery, $250 for labor, $100 for other work ( total $700, another dealership quoted $750).
DIY replacement - Remy Battery+shipping ($168) [Coupon for 15% is RDBC], 10mm wrenche and 10mm socket wrench extender (~$20). Total $190 and 45 minutes of labor excluding learning time on YT, preparing checklist, buying tools etc.

 

[weajd]

If you are not measuring voltage directly at the terminals of the battery, you should expect lower numbers due to voltage drops under cables and terminals before your voltmeter can measure. You are not likely getting low resistive contact to your battery by using the cigarette lighter port.

Your battery is likely to have a nominal 12 volt value due to the cells in the battery when the car is turned off.
When you turn the car on, a generator, alternator or in the i3 case maybe a voltage regulator, has to provide a few volts more than the nominal in order to charge the battery. It may be 14.4 volts directly at the battery terminals when the car is turned on so that power (current) is being sent back into the battery.

That i3 battery is more like a motorcycle battery because it is just for the accessories when the car is off. If I remember right, the i3 uses the Lithium Ion battery to start up the REX engine, if available, so that "Sealed Lead Acid" battery is used for accessories (e.g., alarm, comms., key lock) when the car is off.

Someone should look into using the frunk space to putting a larger lead acid battery in the i3 to avoid so frequent changes. It would weigh more and reduce range some, but it would eliminate some of the headaches from that small battery. I remember the engine compartment space in my BMW 2002Tii being so small, I moved the battery to the trunk and that improved its reliability.

 

[weajd]

I think the i3's engine microprocessor keeps track of voltage.
An OBD2 reader may be able to read out the voltage being sensed by a sensor closer to the battery.
That is probably a more accurate voltage measurement than what is being read by a voltmeter at the cigarette lighter.

 

[alohart]

If you are not measuring voltage directly at the terminals of the battery, you should expect lower numbers due to voltage drops under cables and terminals before your voltmeter can measure. You are not likely getting low resistive contact to your battery by using the cigarette lighter port.

The magnitude of a voltage drop across a resistance is proportional to the current. There is likely very little current flowing from the battery through the cigarette lighter port fuse to the cigarette lighter port. A voltmeter requires very little power, so I'd be surprised if there's a noticeable voltage drop in this case. The voltmeter in my auxiliary power port displays the same voltage as my 12 V system monitor installed across the battery terminals within 0.1 V.

Your battery is likely to have a nominal 12 volt value due to the cells in the battery when the car is turned off.
When you turn the car on, a generator, alternator or in the i3 case maybe a voltage regulator, has to provide a few volts more than the nominal in order to charge the battery. It may be 14.4 volts directly at the battery terminals when the car is turned on so that power (current) is being sent back into the battery.

The i3 doesn't have a generator or alternator. The 12 V battery is charged by a DC-DC converter that converts high voltage from the HV battery pack to ~14.3 V.

Someone should look into using the frunk space to putting a larger lead acid battery in the i3 to avoid so frequent changes. It would weigh more and reduce range some, but it would eliminate some of the headaches from that small battery.

A better solution might be to replace the original absorbent glass mat (AGM) battery with a lithium iron phosphate (LFP) battery as I have done. Tesla has been installing lithium chemistry 12 V batteries for a few years. This LFP battery weighs about 1/3 that of the AGM battery while having about twice the usable capacity. Its expected lifetime should be considerably greater which should justify its higher cost. This battery is self-heating so it works in cold weather. The battery includes a Bluetooth radio that communicates with a smartphone with the Ohmmu app installed. This app reports total voltage, current in and out, power, charge-discharge cycle count, age, temperature, and the voltage of each of its 4 cells. It supports turning off charging or discharging and updating the battery's firmware. I suspect that more vehicle manufacturers will replace lead chemistry batteries with lithium chemistry 12 V or 48 V batteries.

 

[mojo]

Someone should look into using the frunk space to putting a larger lead acid battery in the i3 to avoid so frequent changes.

As with everything weight-related, it's going to be a compromise. You only have to look at some of the weight-saving efforts that BMW has gone to on this car - such as the covers that sit on either side of the frunk box. They are feather-light. BMW could have put in solid pieces of plastic and that would have only added a few tens of grams extra, but it all adds up.

So yes - you can fit a 20 kilo battery instead of the factory 5-kilo battery, in the same way that you can fit bigger, heavier wheels, and a bigger fuel tank on the REx. It will all make a significant difference on what is essentially a 1200 kilo car.

Others have suggested replacing the lead/acid battery with a small 12V lithium ion battery, and that (to me) makes more sense. In the original design, BMW didn't do that and the reason was almost certainly cost - but LiFePO batteries have become significantly cheaper in recent years.