Disabling DC/DC Converter Fuse Removal

These are things not normally done. What are you trying to accomplish?

 

To what end? Hopefully this isn't a bizarre attempt to improve mileage.

Tom

 

Hi Calle, it sounds like an interesting project. A 10% to 20% increase in fuel mileage would be a great result, but I suspect that might be a bit optimistic and the added weight could negate much of your gains.

Each 105 AH deep cycle lead acid battery would typically weigh about 30 kg, so it looks like you'll be adding add least 60 kg. That about 4% of the cars weight.

In any case I'd be interested to know how it turns out.

 

Swag the Prius' average power output as 50 HP = 37000 W; 600/37300 = 1.6%, not "10% - 20%". Allowing for conversion efficiencies the added batteries might could increase MPGs as much as 3%, if they were weightless, but as uart suggests their weight will probably cause a net drop in MPGs.

 

The OP's calculations are off. The 12V system doesn't come anywhere close to consuming that percentage of total Prius power. Just guessing from a back of the envelope calculation, but I suspect this modification is going to cost mileage, not improve it.

Tom

 

300W sounds about right in the steady state, given the HV battery quiescent current of 1.x amps at 2xxV. Not all of that is DC converter. 210 amp-hours of deep cycle lead acid and your downconverter will add, what 150 lbs?

Before doing anything, stick a clamp ammeter over the DC converter line and see what the current is after the car has been in Ready for 10 minutes.

 

My "back of the envelope" calculations give much the same result as yours Tom. A lot depends upon what speed you're traveling, the energy savings are more significant at low speed pulse and glide, but I'm figuring about 3% gain due supplied power and about 3% loss due to the increased weight.

Additionally the energy to charge the 12V batteries is not generally free, and the charge/discharge cycle efficiency of lead acid not that great. For a typical battery charger I'm figuring about 0.8 KW hr at the wall (power outlet) for each hour of driving. Costing this at the average US rate and converting this cost into gasoline I get an effective cost of about 1.8 to 2 miles range. This corresponds to perhaps a further 3% loss.

So, even if you ignore the upfront costs of the conversion, I'm estimating about +3% -3% -3%, so about a 3% negative impact on the overall economy. Maybe if your drive is mostly low speed pulse and glide you'd do somewhat better, break even or come out slightly ahead.

 

I do not see the point of charging one 12V 35AH battery with two 12V 105AH batteries connected in series to give 24V then regulating this back to 12V. Why not put them in parallel and save the weight and losses of the regulator. If my calculations are correct 2x105+35 divided by 4hours gives a 12V load of a constant 60amps witch is wildly over estimated. I have not checked the average 12V load on the Prius but would be surprised if it was over 20a and in the configuration above would give 12hours of operation.

 

My 2005 Prius draws about 240W from the HV battery in READY mode with all accessories off. While stopped in drive with the brakes, HID lights, radio, and the cabin fan on low it draws about 400W. When gliding below 41 mph and not commanding any power into or out of MG1 or MG2 I often see currents out of the battery near 800W.

I'm not sure where that power is going, or even if those values are right, but given that the 12V system powers many critical parts of the car I suggest being careful about severing the HV-12V link. The car is known to do weird things when the 12V line sags, and there are several large and bursty power draws that can come on suddenly.

List of major subsytems that use the 12V:
http://priuschat.com/forums/gen-iii...at-does-really-feed-directly.html#post1342183

Analyzing off-grid solar-battery systems a few years ago I was surprised to discover how expensive batteries really are. When I calculated in the wear-and-tear of use I found that most deep-cycle lead-acid batteries cost about $0.40 per kWh of throughput over their lifetime. This means that if you can get grid power at $0.15 / kWh and you store it in lead-acid batteries it already costs $0.55 / kWh. You may want to consider this revised cost in your estimate. At the moment each gallon of gas costs me about $3.20 and yields 11.6-12.2 kWh. Thus each kWh costs about $0.26-0.28. This makes gasoline more expensive than grid power but cheaper than grid power stored in lead-acid batteries. Note that this only applies to lead-acids, other types of batteries have very different characteristics

The Gen II Prius burns about 230 grams of gasoline per kWh in its most efficient range of operation. Assuming we are operating in a less efficient regime lets say that we are burning 240 g/kWh. A US gallon of 87 octane gasoline is about 3785mL * 0.739 g/mL = ~2800g so our Prius will extract about 2800 / 240 = 11.6 kWh per gallon. At 50 MPG that works out to 11.6 / 50 = 232 Whr/mile. With the Torque app on my phone altering my driving habits I have been able to get about 57 MPG sustained over multiple tanks. Assuming I manage 230 g/kWh this works out to 214 Whr/mile.

If you are driving at 60 mph it takes you 1 minute to drive a mile. In that minute the electronics have used somewhere between 240 and 800W thus 240 / 60 to 800 / 60 = 4 to 13 W*hr/mile. The 12V electrical load is 4 / 230 to 13 / 230 = 1.7% to 5.6% of the power used by the car. If you replaced that power without adding any weight the largest improvement you could expect at 60 mph is somewhere between 2 and 5%, or 1-2.5 mpg. If you spend a lot of time in stop-and-go traffic that depletes your HV pack and makes your engine run inefficiently this mod could make a bigger difference but I don't know how to quantify that.

If the majority of your time is spent at speed altering driving habits, training yourself to use Torque, getting an engine block heater, or getting LRR tires hold more promise than supplying the 12V system with grid power stored in lead-acid.

 

You would have to remove the clear plastic cover over the fusible link box, then physically break the 100A DC/DC fusible link.

After you decide that breaking that fusible link was a bad idea, then you would have to remove the main relay/fuse box so that you will have access to replace the fusible link assembly. Other posters have written in detail about how to do this.

Usually the DC/DC fusible link is broken as the result of a botched jump start. You are the first poster that I have seen who wants to proactively do this to a car that is working fine.

 

Thatguy When the Prius is stopped in drive if you look at the MFD you will find that it is putting power out to the wheels, witch are not driven by the 12v supply. The HID's use 70watts plus about 30watts for the front and rear side lights, or 8amps the fan and radio another 5amps. The extra wattage when gliding cannot be being used by the 12v battery that has know other function than running the computers and other electric pumps on an intermittent basis hence my estimate of 20amps as an average load, but as Patrick stated the idea does not stand.

 

I agree, some things you can reduce 12v power consumption from like replacing lamps for LEDs as I have and fitting a little trickle charger for the 12v battery while I am charging the plugin kit, I am sure this helped FE but remember one of the highest current consuming items on the list is the brake fluid compressor, not something you might want to save power from. Just a thought would the system give a fault code if the fuse was pulled?

 

To put it simply without breaking out the calculator your project is flawed by
one major Prius peculiarity. And that's sooner or later at the end of the Inverter substitute battery charge that 12 volt line will sag due to various super current draws it see's.

The Prius hates that B+ sag and see its as a major issue and throws every CEL it has including breaking & drive ability issues. Your super reliable car becomes instantly very testy. One of those events on a very busy highway at speed and that extra mileage will quickly become not worth it not to mention scare the crap out of you.

Nice try though. Maybe a bank of car stereo supercaps to switch in if it bottoms out but if it see's that sag all bets are off.

 

If you see this, it means you are not pressing firmly on the brake. Press firmly and the Prius will stop simulating automatic transmission creep. The MFD will then indicate zero power to the wheels.

Tom

 

Someone in another forum did this kind of "mild plug-in" with a 14V deep cycle battery and claimed good results. YMMV of course.

 

Another way might be to spoof the sense wire voltage to say 14.2V that might shut down the 12V inverter and let the 12v systems run on the batteries.