Radio Draw on the Drive Battery

Discussion in 'Clarity' started by CIvan, Apr 30, 2021.

To remove this ad click here.

  1. CIvan

    CIvan New Member

    I sometimes like to just sit in my car and listen to music. However, I don’t like draining the 12V battery, so I wanted to know how long I could listen to music with the car “running” and the drive battery charging the 12V.

    When the GOM turned to 47.5 miles, I started a timer on my phone. At 47.4 miles, I stopped the timer with five minutes and 29 seconds showing. So basically I got 5.5 minutes for every tenth of a mile of range, or 55 minutes for every mile. Which means on a fully charged drive battery, you can run the radio for a LONG time.

    Noted test conditions:

    1. About 70 degrees in my garage
    2. Volume set at 8
    3. Parking brake was on before the car was started, thus the DRLs were off.
    4. Fan was turned off
    5. Driver display was turned to minimum brightness. For even longer listening pleasure, the driver display can be dimmed to black, but I needed to see the GOM in tenths of a mile for the test

    Technically, you can also black out the infotainment display by dimming it completely if you just want to keep listening to whatever source you have chosen. I imagine there are still quite a few electronics running (including the red “P”ark LEDs and passenger seat air bag light), but 55 minutes per mile is still pretty good.
  2. To remove this ad click here.

  3. petteyg359

    petteyg359 Well-Known Member

    Radios use very little power regardless. You'll never manage to kill your 12v battery by listening to the radio. People have been listening to the radio in old ICE cars with the engine off for hours at a time for decades.

    If the car is fully on instead of in accessory mode you're just wasting power if all you want is the radio.
  4. MrFixit

    MrFixit Well-Known Member

    Although well intentioned, your experiment seems off.

    Roughly speaking, the Clarity can go approximately 3.5 miles per kWh. During your experiment, you seemed to be draining the battery at a rate of ~ 1 mile in an hour, therefore the drain on the battery would have to have been 285 Watts

    The most a radio could possibly draw is a couple of watts. Somehow you are off by a factor of 100 !

    When in "Ready" mode (as you were, because the HV battery was engaged and charging the 12V battery) there are a lot more things running than just the radio, but 285 Watts does seem somewhat high...

    My suspicion is that the GOM fooled you (particularly because you only watched it for a one tick change). I think your conclusion that you can run the radio for a really long time is true... It's just that it is really MUCH longer than a LONG time.

    The best way to know what the 'idle' power is (radio or no radio) would be to look at the HV Battery Current using the OBD2 port and Car Scanner. Although there would likely not be enough resolution to notice whether the radio was on or off, it would be able to tell you whether the Clarity is 'idling' at 285 Watts (even with everything off that you have control over). I will look at this when I have a chance. 285 Watts seems high, but if true, it is not the radio that is wasting this power. I am inclined to agree with @petteyg359 that you would be better off in accessory mode, running just on the 12V battery.

    Edit: My original post was wrong (had an inversion)... Numbers above have been corrected !
    Last edited: Apr 30, 2021
  5. CIvan

    CIvan New Member

    Thanks petteyg359 and MrFixit! The reason I post stuff like this is so someone can tell me if I’ve fallen completely off my rocker. Or if there’s a better way to go.

    petteyg359: I know people have used their 12V battery forever to listen to the radio, but it seems a lot of Clarity’s have had their batteries replaced after 2 or 3 years, and I didn’t want to stress it any more than I had to. Not that I think listening to the radio was their problem. I think you’re right about wasting power though. See below.

    MrFixit: I took your excellent advice and went out to my car with my CarScanner to look at the power draw. Voltage was around 372.4 [HV Battery Voltage (LineA/B)] and current was between .58 and .78 amps [HV Total Battery Current – Trial 1]. (And the radio made no discernable difference). That provides a range of 216 to 290 watts. That includes the OBDII sensor power draw. If those are not the parameters I should be looking at, please let me know.

    So maybe my experiment was correct. And the power draw at “idle” is pretty high for just running a radio.
  6. petteyg359

    petteyg359 Well-Known Member

    Again, "on" vs "accessory mode" vs "ready" or whatever they differentiate that middle state and the drivable one with matters

    Compare those.
  7. To remove this ad click here.

  8. The Audio system is protected by a 15A, 12V fuse. The system most likely does not draw 15A. If it did, that would equal .18kW. The ~14,000 watts of available energy from the lithium battery would power that load, via the DC/DC converter, for ~78 hours.
  9. MrFixit

    MrFixit Well-Known Member

    I am not sure about this, but it is possible that the OBD2 might only work in "ready" mode.
    In order to know what is happening in "accessory" and "on" modes, it may be necessary to measure the 12V battery current directly.
    I have a clamp-on DC meter that would make this easy.

    I think the HV battery is not used at all for "accessory" or "on" mode, and therefore the HV current should be zero (even if the OBD2 is alive and functional).. Thus, the need for direct measurement in the 12V system.
  10. CIvan

    CIvan New Member

    Landshark, as noted above, the lithium battery is draining closer to .25kW when the radio is on in the Ready Mode (because of all the other systems in the car that are drawing power too).
  11. We did a double feature drive in (on the whole time) with occasional heat to clear the windscreen. Even with the two movies and heaters, we lost maybe 10 miles of range
  12. To remove this ad click here.

  13. CIvan

    CIvan New Member


    Here is my understanding of how the Power Button works:

    With foot NOT on the brake:

    1. First Push – Accessory Mode – Audio system on (and other accessories); 12V battery only; OBDII is not activated
    2. Second Push – On Mode – All electrical components can be used (except heavy ones I think, like AC and resistive heating); OBDII is activated; drive pack drawing 0 amps (-.16 - .08 on CarScanner)

    With foot on the brake:

    1. First Push – Ready Mode – All systems functional; drive battery charging 12V battery; OBDII is activated; drive battery discharging at slightly over 1 amp (so that means to me based on my earlier measurements that the OBDII sensor is using around .75 amps)

    I ran my tests earlier in this thread in Ready Mode. As MrFixit said, if you want to find out how much power the radio is using in Accessory Mode, you would need to measure the current coming out of the 12V battery with the radio on, and off.
  14. CIvan

    CIvan New Member

    That sounds perfectly feasible. 4 hours of sound from the stereo is roughly 4 miles, and resistive heating for defrost could have used at least that much. Plus if you were running the fan, any displays, lights, etc. it could easily add up to ten miles.
  15. OK. Now you’re down to 56 hours. If that isn’t enough, put it in HV and burn 7 gallons of gas.
  16. CIvan

    CIvan New Member

    MrFixit, at the very top of my CarScanner All sensors page, there is a parameter called “OBD Module Voltage” that I have seen read anywhere between 13.3 and 14.1V. Is that the current 12V battery voltage or the charge voltage (if the car is in Ready Mode)? Thanks.
  17. I’ve seen up to 14.8V in Ready mode, a minimum of 14.1V. The more stuff you turn on, the higher the charge voltage. 13.3V is probably fresh off a charge. It’s too low to be a charging voltage. Resting, open circuit voltage, will be 12.7-12.8v.
  18. CIvan

    CIvan New Member

    Thanks Landshark. I probably saw 14.1 when I was in Ready Mode and 13.3 in On Mode.
  19. MrFixit

    MrFixit Well-Known Member

    Throughout the OBD arena, there are quite a lot of renditions of the 12V battery voltage. Virtually all of the control electronics that are distributed throughout the vehicle use 12V, and I think the philosophy is that it is so fundamental to the operation of any electronics 'module' that each module includes its own sensor to measure it. There are probably dozens of "12v" monitor points.

    To me, these are just many copies of the same thing (the battery voltage). Although they all measure the same thing, they verify good connectivity (wiring harness). They may differ slightly because of any nominal (but small) voltage drops in the wiring depending on the current used by that module. They will also differ slightly because of the tolerance of the measurement.

    So, I think the "OBD Module Voltage" is simply the 12V battery as measured when it reaches the OBD Module (kind of where the Vgate plugs in).

    We have accepted that 'accessory' and 'on' modes both operate exclusively with the 12V battery (no charging from the HV side). Without any charging, the 12V battery will be 12.x volts (maybe as high as 12.7 initially and with a very light load). As the battery discharges, it may drop below 12V.

    My rule of thumb is that if you see 12.x volts, the battery is not being charged. When it is being charged, you will see 14.x volts. The only time you will see 13.x is if you are charging (14.x at the battery) and there is a 1V drop in the wiring harness due to a long wire with a heavy load to a given 'module', or similar).

    I think you can essentially run the radio 'forever' in accessory mode if your battery is good. The risk of course, is that over time your battery will become 'bad' at some point, and usually there is little or no warning. You could be stranded by running the radio for 10 minutes with a bad or failed battery !
    Last edited: May 1, 2021
  20. MrFixit

    MrFixit Well-Known Member

    I did some measurements this morning...
    The 12V Battery current was measured directly with clamp-on meter.
    Nothing intentionally operating, except for the Infotainment system (radio on).

    Vehicle OFF:
    12V Battery current starts at 2A when door is opened / closed, but fairly quickly drops to 1A, then 0.16A. OBD operation is not possible.
    This 160 mA seems to be the parasitic current. I did have the Vgate adapter attached which draws around 50 mA, so I think the parasitic current from the vehicle is probably closer to 100 mA. Maybe this would drop even more, but I was not patient enough to wait.

    Vehicle in "Accessory" mode:
    12V battery current starts at around 5A, then settles in at more like 4.5A. OBD operation is not possible.

    Vehicle in "ON" mode:
    12V Battery current is 14.7A. The 12V Battery voltage ("OBD Module") is 11.5V. No charging is taking place. OBD is operational.
    The HV Battery voltage was 342 for me, and the HV Battery current was 0 (occasionally reading would dither a little < 0.2 amps, but mostly said 0).

    Vehicle in "Ready" mode:
    12V Battery current is -20 Amps (negative indicates charging). The 12V Battery voltage ("OBD Module") is 14.4V. HV Current was indicating 1.62A.
    Over time (~5 minutes), the 12V charging current tapered off (-20A, -16A, -15A,... -6A) as the battery was charging. The HV current correspondingly dropped from 1.62 amps down to 1.0A.

    In summary, the 12V battery drain is 100 mA (1.2W) when OFF, 4.5A (50W) when in "Accessory", and 14.7A (175W) when "ON".
    The HV battery isn't really used until in "Ready" mode, in which case it provides 1 Amp (340 Watts). In "Ready" mode, part of the power provided by the HV battery is operating the vehicle (likely the 175W+ for "ON"), and part is charging the 12V battery. I think the negative current that is charging the 12V battery would continue to drop as the system determines it reached full charge.
  21. Did you mean to say Ready mode in the above sentence?
  22. MrFixit

    MrFixit Well-Known Member

    NO - I meant what I said... A good battery should support accessory mode for a LONG time without the need for charging from the HV battery. Perhaps "forever" is an overstatement, but for all practical purposes, I would not worry about using the radio in accessory mode for as long as I needed.

    As you can see in my above post, when you are in "Ready", you are burning quite a lot of power (200-300 Watts) vs. the ~50 Watts in Accessory. Of course the HV battery is much more capable, but you are being much more 'efficient' in Accessory.

    "ON" mode is triple the consumption of "Accessory", so I would steer clear of it.

    What is the Amp-hour specification for the Honda 12V Battery? If it is 100 Ah, then at 5 amps (Accessory mode), it should be good for 20 hours (well, thats not 'forever'), but pretty good.
    Last edited: May 1, 2021
  23. I’ve seen Reserve Capacity ratings for a 51R battery of 85 minutes. Converted to aH that would be 35.42aH. So, about 1/3 of forever.

    Personally, I’d put the car in Ready mode if I intended to sit for more than a few minutes and enjoy the audio system. Efficiency be damned. And, I carry a lithium jump pack for those unforeseen calamities.
    Last edited: May 1, 2021
    MrFixit likes this.

Share This Page