12 Volt Lithium Battery...Possible ?

Discussion in 'Gen 3 Prius Technical Discussion' started by TruSound, Feb 24, 2012.

  1. kingnba6

    kingnba6 Active Member

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    interest topic. I have lithium batteries in all my motorcycles but i dont think i would put it in a car.
     
  2. macman408

    macman408 Electron Guidance Counselor

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    No, it won't. The engine's duty cycle when not moving is controlled by the HV battery SoC (it'll start charging around 45% SoC and charge until 50% SoC, or something like that). The SoC depends on the current in and out of the HV battery. The 12V system is powered basically entirely from the DC/DC converter while the car is in Ready mode, with the battery there to smooth out the dips and bumps. The converter maintains the 12V system at ~13-14V or so, depending on whether it thinks the 12V battery needs charging, or just needs a float charge to maintain it at full voltage. So whatever loads you have on the 12V system are not drawing power from the 12V battery at all; the 12V battery remains fully charged at all times.

    If you change the capacity of the HV battery (and make sure the computer knows about that extra capacity when calculating the SoC), THEN you would reduce the engine's duty cycle. I'm not convinced that would buy you any added efficiency, though.

    Don't tell my battery this. I drained it to 0.4V a few months after I got the car. It's still just as healthy (as determined by the voltage on the battery while at rest and installed in the car) as it was when I got it, almost 5 years later.


    As far as the whole topic goes, it'd probably be a fun mod for some people that might work. I honestly don't think it will buy you anything measurable in terms of fuel efficiency or reduced long-term cost; if it did, Toyota (or some other manufacturer, at the very least) would likely have taken advantage of it in the first place.

    There are some things to be careful of though, it seems; the charging profile is slightly different between a lead-acid and an LFP battery. It sounds like it's close enough to use, but it's probably not optimal. More difficult is discharging - from what I understand, LFP batteries will cease to function below 2 or 2.5V; I believe this is a safety cutoff like other Lithium-based rechargeables, since charging them after this point can cause them to develop a short, at which point your battery becomes more like a heater, light source, and/or fire-starter. So you'll want to install some sort of a low-voltage cutoff device to protect your battery, and make sure that you check it frequently enough that it doesn't have time to self-discharge from the low-voltage cutoff point down to the safety cutoff point.

    One reason why lead-acid and NiMH batteries are so widely used compared to other rechargeables is that they're dead simple to charge and maintain; in the simplest case, you can just apply a float voltage, and not worry about anything else. You don't even need to stop the float charge, the battery can happily sit there all the time. Lithium batteries are much more sensitive to temperature, under- and over-voltage, cell balancing, and other such issues, so they need rather intelligent circuitry to handle them properly.
     
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  3. szgabor

    szgabor Active Member

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    When the system is in Ready the 12V battery IS NOT providing much if ANY of the power .... not sure what you are talking about ... Larger battery only would extend the "idle as in the driveway" time the Prius can go without re-charging the battery ... in normal operation it would be no help (and when the larger battery is ageing you would have more issue of system trying to keep it charged hence worst effect what you say it would avoid)
     
  4. Redpoint5

    Redpoint5 Senior Member

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    I'll be measuring parasitic draw, accessory loads, and typical battery charge/discharge use this weekend, if I get time. My plan is to use 2 kWh meters and a small LiFePO4 battery and see what the usage looks like during a typical drive. A new thread will be started when I have data to share.
     
  5. CR94

    CR94 Senior Member

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    While you're doing that ... How 'bout measuring current from the battery while the power switch is in the "accessory" state with no accessories actually turned on? Reason: People keep claiming that using the accessory mode to listen to the radio for more than a short time will kill the battery. The battery should last for days running only the radio itself (At least that was true of my last car and its radio.), but we don't know how much additional battery drain results from simply being in the accessory mode.
     
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  6. Easy Rider 2

    Easy Rider 2 Senior Member

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    What WAS your last car ?
    If it was not a hybrid of some kind, then the comparison is not appropriate.
    The hybrids have MUCH smaller 12 V batteries.
     
  7. CR94

    CR94 Senior Member

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    1980s Mazda with aftermarket stereo that was several years newer than the car. While playing at a reasonable volume level, the radio took under 0.3 amperes, less than 4 watts. A healthy Prius battery should last a long time at that rate. Someone (I forget whom, or in which thread) recently reported in Priuschat that his Prius radio uses 9 watts.

    The Prius battery isn't necessarily "MUCH smaller," although saying so is common. The a-h capacity of a standard Prius battery is near the same as that of OE batteries used in some conventional American cars even long ago. The Prius battery would indeed have a lower cold cranking amperes rating, but that's not relevant. Its physical dimensions (aside from terminals) are the same as some recent cars that use Group 51 batteries.
     
    #87 CR94, Jan 30, 2015
    Last edited: Jan 30, 2015
  8. David Beale

    David Beale Senior Member

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    Remembering of course that there are, what, -5- different audio systems in a "Prius", depending on year and option package.
    Oh, and 38 A-Hr (the reported capacity of the Prius lead acid battery) is not "similar" to a "normal" or "standard" group 51 battery. They are mostly in the range of 50+ A-Hr. 20% is a fair difference to me. :)

    And after reading Easy Rider 2's post after mine remember lots of people forget the headlamps don't go out until you open and close the drivers door. I suspect -that- is the main cause of "dead battery syndrome" while sitting and listening to tunes.
     
  9. Easy Rider 2

    Easy Rider 2 Senior Member

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    OK but what if the battery isn't fully charged to begin with ?
    And what about the drain of other things that are on in the ACC position ?
    The capacity of the battery might not be the ONLY reason but it is a factor.
     
  10. Britprius

    Britprius Senior Member

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    The "standard" Prius battery is rated at 45 AH. It also has to be remembered that in accessory mode the MFD is also powered even with the radio turned off. I did some tests a couple of years back on the 12 volt current load in the various on states. I cannot remember the current draw in accessory mode "but I will check again and report" but in ignition mode "not ready" the current draw was over 20 amps with no accessories turned on.

    John (Britprius)
     
  11. Redpoint5

    Redpoint5 Senior Member

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    I have begun to measure various electrical loads on the Prius and will start a separate thread on the subject soon, but some initial measurements are as follows:

    Car off/parasitic drain - 13 mA average (spikes to 50 mA with security LED and rests at 12 mA)
    Passenger door open (interior lights on) - 2.4 A
    Rear hatch open (interior lights + cargo light) - 2.8 A
    Accessory mode on + display screen (not ready) - 2.75 A
    Accessory mode on + radio - 2.85 A
    Start on (ready mode) 9.75 A

    I gathered this info by disconnecting the OEM lead acid battery and connected a 4.2 Ah LiFePO4 battery in line with an ammeter. The small 4.2 Ah battery could comfortably maintain the cars electronics for 10 days of sitting parked. A 20 Ah battery would give the Prius 6 weeks of parked time and not be damaged by that level of discharge. Furthermore, the LiFePO4 chemistry would make more efficient use of the charge it receives from the converter.

    I'll likely be switching out the stock battery for the little 4.2 Ah battery I already have as an experiment. Perhaps I will monitor the energy input and output for a known commute and compare with the stock battery. There might be extremely small efficiency gains, and I'm curious what they might be.

    In the mean time, I have ordered LED interior and exterior bulbs, and HID low beams. I expect this will reduce night time electrical loads by about 100 watts. Of course, I will measure this too and include the data in my thread on electrical consumption.
     
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  12. Easy Rider 2

    Easy Rider 2 Senior Member

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    I think the result of your experiment will just reinforce what most people already know.......if they have any sense of logic at all and have been paying close attention:
    Most of the premature battery failures and "no start" conditions are more because of the owner's actions than because of Toyota engineering.
    Some are also because of the dealer's actions before the cars are sold.
     
  13. Redpoint5

    Redpoint5 Senior Member

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    Where is the 12v converter located (The one that steps down the traction battery voltage to 12v)? Where would be the easiest place to get a clamp meter on the output of 12v converter?

    I am ready to begin measuring various electrical loads, but I can't do it off the battery since the converter (similar to an alternator) provides most or all of this energy.

    Alternatively, does anyone know how to disable the 12v converter via fuse or wire harness disconnect? Would this prevent the car from operating? I know that in normal cars, you can disable the alternator and power the vehicle from the battery alone.

    Isolating the 12v power source is necessary to measure the loads of individual accessories.

    Also, I took out the OEM lead-acid battery and have connected my 4.2 Ah LiFePO4 battery. I'll be monitoring energy input vs energy output to see how efficient it is.

    FYI- The battery and bracket together weigh about 29 lbs or 13.2 kg
     
    #93 Redpoint5, Mar 9, 2015
    Last edited: Mar 9, 2015
  14. Britprius

    Britprius Senior Member

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    The dc to dc converter is inside the same large silver housing as the main inverters under the hood. I do not think it will be easy to disconnect as it is connected by the strip fuse under the clear plastic lid in the fuse box. Disconnection will probably issue a DTC code, and may result in the car shutting down. I am not certain on the shutting down. It is possible that it would be easiest to disconnect in the inverter housing.

    John (Britprius)
     
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  15. David Beale

    David Beale Senior Member

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    The 12V charger is in the inverter.
     
  16. Britprius

    Britprius Senior Member

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    Individual accessory loads can be measured at the battery in accessory mode as this does not turn on the converter (charging system). It is also possible to measure the output of the converter with a clamp meter at the inverter while measuring the current goining into the battery either at the battery or at the jump point with a series ammeter or another clamp meter. Subtracting one from the other would give the various loads.

    John (Britprius)
     
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  17. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    The 'white' cable exiting left marked +12 V Bus
    [​IMG]
    This video will discuss the cable connecting the Inverter to the 12 volt battery. (it is 28 minutes long) At 6 minutes the cable is mentioned, the DC to DC converter is explained at about 17 minutes.
     
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  18. CR94

    CR94 Senior Member

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    You could also measure current to some individual accessories easily by removing the relevant fuse and connecting your ammeter leads in its place---while being careful not to damage the fuse holder terminals, etc.
     
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  19. robray

    robray Junior Member

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    I just replaced my battery yesterday with one from the local parts store here in Vancouver. It is ACDelco brand part number ACDB24R. It was $164 + tax with my discount. Original quote was $210. My car is a 2010, but I would have thought the battery would last longer. My friend gave me a ride to pick up the battery and his 2001 Civic is still on the original battery.
     
  20. robray

    robray Junior Member

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    Sorry wrong thread.
     
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