It does not seem like that long ago my Prius was modified in the best kind of way. One year ago I had my Prius modified to be a plug-in by installing a Hymotion battery. I have learned a lot over the past year and have not had any major trouble with the extra battery. I will run through some of the stuff I have learned over the past year. Learning how to drive all over again. In order to get the most out of the Hymotion battery a manual EV button is a must for city drivers. Letting the Hymotion battery do all the work is OK for the average user but if you have a short commute or terrain that is hilly or unpredictable manual control of EV will get you higher MPGs. Because I'm driving in EV mode more than using gas the coolant temperature will be colder longer and may not be optimized for EV driving. To get around this you have to fake the coolant temp. This is more for advance users that can monitor the coolant temp. If you can fake the coolant temp to above 159F you can enter EV mode and warp stealth a lot sooner. With the combination of the manual EV button, the coolant temp hack and a good mostly low speed route I am able to achieve 300-500MPG according to my ScanGauge on my trip to work and back. If I can maintain a normal commute for the length of a tank of gas I believe I can achieve 3000-5000 miles without stopping at a gas station to fill up. I have estimated with my CAN-View that the pack holds about 16.5Ah and on my commute I use about 12.5-13Ah. Thats about .59Ah per mile. From the wall I measure that the car uses about 250Wh per mile all electric. You do have to be careful not to draw too many amps for long periods of time or you will start to drain the stock battery. I have found a continuous draw of 30A seems to be about the point at witch you will not draw the stock battery down but the Hymotion wont charge it either. All electric driving on flat roads. To sustain speeds these are the least about of amps you can draw. MPH -- AMPS 25 -- 15 30 -- 20 35 -- 20-25 40 -- 30-35 41-50 -- 40-50 55 -- 55 55+ not really possible because 60A is the most before the engine starts to burn gas. When the engine is on at highway speeds the car will draw about 30A and you will get about 150MPG at 55MPH. Small issues / problems. I have not had any major problems with the battery so far. My battery was upgraded with the latest firmware a while back. I have had the flashing lights a few times for a low 12v battery. I have had flashing lights for a current sensor malfunction but that cleared its self and only happened once. I have only had a few partial charges. I think it was due to very hot weather. The day before Hybridfest 2009 I had my battery checked for a small manufacturing oversight. The problem was the first few batteries that were made had the fan positive and negative wiring reversed. This was a quick fix and hopefully helped the partial charging issue in hot weather. When I had the firmware upgraded one of the wires must have come loose from the pin on the wiring harness. Instead of having to drive 100's of miles to get it fixed I soldered the wire back on to the pin and reinserted the pin back into the wire harness. That was an easy fix for me. Mileage stats. Total miles: 9594.6 Gas used: 75.6 gallons Total gas cost: $181.92 Total kWh: 1380.4 Total kWh cost: $110.43 Total fuel cost: $292.35 Avg. Wh per mile: 143.87 Avg. MPG: 126.9 Avg. CPM: 3.0 Best tank 2502 Miles 299 MPG Since its been about a year since I have done my first mileage tests I will try to perform an all electric drive and all highway drive to see if the battery capacity has changed. I will try to do these tests on my next days off that are nice and sunny. So far I love my Prius PHEV and I don't know what I would do with out it. I will never buy a regular gas car again and I will never buy a regular hybrid again. My next car will have to be a plug-in or an all electric vehicle.