One Year of Driving Our BMS+ Plug-In Prius

July, 2009: This month completes the first year of driving our 2007 Prius using the BMS+ System, invented and supplied by Norm Dick at hybridinterfaces-dot-ca. I enthusiastically recommend the BMS+ System to any qualified owner who wants to enjoy a Plug-in Prius. It requires significant work and prior knowledge, but for many owners it will result in an excellent and uniquely economical way to reduce gasoline usage. An unanticipated bonus for me has been the pure pleasure of driving smoothly and quietly around town in an electric vehicle. To some extent, it is similar to riding a bike, but easier to carry the groceries.


Mileage with the BMS+

The suitability of a given PHEV conversion option (or the viability of using a PHEV system in the first place) depends on your particular driving conditions—including type of usage, distance driven, speeds, and driver motivation. The BMS+ System will suit many different situations, and I would hope that eventually other BMS+ drivers will provide some of the details on how the system performs in their circumstances, but this is how it has worked for us.

We have found the BMS+, with two salvaged Prius battery packs added to the original pack, is especially well matched to our needs. We live in a small town where our round-trips average 6 miles (maximum 10 miles, which is about as far as I like to go on a single charge), and can be safely done in all ev mode, normally at 25 mph on side streets, and all nearly flat. To keep the Depth of Discharge (DOD) as small as possible for longer battery life, I recharge between trips, 2 or 3 times a day if necessary, and try to stay between approximately 30% and 80% State Of Charge (SOC). Only my 3 day-a-week commute is longer—7 miles each way, requiring 2.5 miles of 55 mph ICE-on operation in the middle of the trip, with a charge before returning home.

CAN-view Screen After 7-Mile Morning Commute
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Before the conversion, we struggled with poor mileage on these short trips (despite using a block heater), and even my commute was only around 65 mpg at best. Our overall average was 50.6 mpg for our first year of Prius driving. With the BMS+, we are able to do most of our local driving in ev mode (normally, the ICE only ends up running about 15 miles per week, plus a few warm-ups), and our overall average since the first of this year has been 153 mpg equivalent.

(Details: We use .25 gallons of $3.00 gasoline in a typical week of local driving, and also 21.7 kwh of electricity at 9 cents/kwh. We actually pay a little under 6.5 cents/kwh, but I am adding 40% to cover the estimated charging and conversion losses, based on the read-outs from my Kill-a-Watt Meter. Since we average 138 miles/week, this works out to .54 cents/mile for gasoline and 1.42 cents/mile for electricity, or a total of 1.96 cents/mile. Divided into $3.00, this works out to 153 mpg gasoline-cost equivalent.)

We almost never take a trip over 10 miles that doesn’t put us on a certain wild freeway for 200 miles or more, so for these drives I temporarily remove the two add-on battery packs and unplug the BMS+ computer. This takes me about 10 minutes, and then as a standard Prius we get our usual 52 mpg or so at 65 mph (I don’t even try to hypermile on this freeway, it takes all my ability to concentrate on safety). Although I know other converters have permanently bolted down their add-on packs, since our local driving situation allows us to get plenty of use from the BMS+ conversion, and because I worry about the additional weight behind the axle while on the freeway with my family, I prefer to leave my extra packs behind. I don't often get to make mid-distance back-road highway trips, but when I do and choose to leave in the BMS+ System, I get around 65-70 mpg, depending on hills, traffic, and opportunities for pulse and glide. Having the extra battery capacity with its much lower internal resistance allows for more efficient regeneration, as well as easy access to "super-highway-mode" because of the high SOC.

We average about 60/40 highway to local miles, and our overall average for the current year comes to 81 mpg (gasoline only, measured by tank fill-ups). I do not have a complete record for electricity use over this period, but based on my Kill-a-Watt Meter, I would estimate it to be 58 wh/mile combined, which works out to a combined total of 71 mpg equivalent, again figuring with gasoline at $3.00/ gallon and electricity at 9 cents/kwh.

To calculate green house gas emissions and carbon efficiency, I went to Google's PHEV calculator page (google dot org/recharge/dashboard/calculator), where they discuss the results of their Plug-in Prius fleet. Strangely for a Plug-In vehicle site, they have no way to calculate CO2equivalent for a plug-in car, only for pure gasoline powered vehicles! Luckily, my 10-year old son, our resident programmer, was able to add to their java script the missing input for electricity usage, and our overall result is .310 CO2e lbs./mile overall, or .083 CO2e lbs./mile for the local BMS+ miles. This is a lot better than the rather dismal Google fleet averages, due to their habit of driving their (relatively) expensive Hymotion conversions without charging the battery pack. Too bad their carelessness, along with King County's, has given PHEV bashers so much fodder, but I digress.


Battery Pack Installation

I bought my 2 add-on packs in August, 2007 (both 2005's with about 40,000 miles on them), just a month after buying the Prius. At the time, there was no suitable method for interfacing the additional packs and their grid-charging ability to the Prius, but I was naively optimistic that the problem would soon be solved. It was a good thing that I got such an early start, as it took me a whole year to figure out a suitable way to install and tie-in the batteries, and I also built 4 versions of my charging system. By the time I managed to solve these problems, Norm had designed and built his amazing interface and so it all came together last June. Our total cost to date is about $2200, including the batteries ($550 each), the BMS+ and CAN-view units ($400 each), the Lilliput touch screen ($180 on ebay), plus about $120 worth of connectors, fans, and charger parts (I made my own).

Typical Pack-view Screen In Park
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So far, Norm’s system of paralleling Prius battery packs to split the power and regen loads three ways seems to be working very well. The screen above shows a typical background load of 1.8 amps and only .05 volts between the modules of the original battery pack. While driving at approximately 20 amps (steady 30 mph, level road), the normal difference is about .09 volts. Although I cannot monitor the add-on packs while driving, I have checked them several times on the bench with a 2 amp load. It is a bit of a scramble (using only 2 calibrated meters), but as near as I can determine the differences are almost as good as the original pack.

In addition, the internal resistance for the three-pack setup is between .05 and .09 ohms, and during local or back-road trips, the temperature barely increases. In contrast, freeway driving with only the original pack (admittedly more demanding) will show resistance of .22 to .29 ohms, and the temperature will often rise 20-30F, all of which indicates to me that Norm was correct when he predicted that paralleled batteries would work well on the Plug-in Prius conversions, as long as proper safeguards are added to the charging system. The normal HSD controls work perfectly to protect the 3 parallel packs while driving. (Possible exception: very long downgrades in hot weather, when the main pack might need to be cooled by the Toyota fan, but the add-on packs could overheat since my fan system only operates during grid charging. It’s not very hot here, and there are no real downgrades anywhere that I drive while operating as a PHEV, so not a factor in my conversion.) Obviously, it is advisable to keep your batteries cool in the summer, warm in the winter, and to recharge as often as possible to limit the DOD. Time will tell how the Prius packs hold up, but so far so good. The supply of salvage packs seems to be increasing so the prices may actually be coming down—good news if we need replacements someday.

Batteries and Charger Fit Under the Floor
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I installed the add-on packs on a 9/16" plywood base that spans the spare tire (I made a cardboard template first). Four cooling fans are mounted to the underside where they can suck a bit of coolish air from under the spare tire and blow it up through the batteries, very important during charging. An interesting feature to note is that the 2 packs fit between the main battery case and the rear of the hatch area with exactly enough room to stuff 1/8" thick coroplast protectors fore and aft. It is a very snug fit—just a coincidence? Also, the packs are exactly the same size the other direction (2 packs make a perfect square), and could be mounted at 90 degrees, but I chose sideways to keep the tie-in cables as short as possible. The folding floor closes normally, with additional support for heavy cargo provided by the 2 black wooden spacer bars (running fore and aft in the photo).

Charging

By far the most difficult and risky part of the conversion is the charging operation, as has been proven by several documented battery "expansion" incidents. (The part that required the most knowledge, talent, and skill was, of course, the BMS+ and CAN-view interfaces, but I only had to write a check for that.) By the time my BMS+ was ready, I was on the fourth version of my charger and it was working reasonably well, but if Norm had already been offering his Charge-Safe unit before I had my system ready, I would have bought it in a heartbeat.

I recommend anyone planning a conversion using a charger with less than adequate safety shutoff controls (temperature and timer being absolute necessities) should install the Charge-Safe or equivalent. The prismatic NiMh modules in the Prius battery are very tightly assembled into a relatively large mass with limited external surface area and cannot release heat quickly enough unless they are seriously fan-cooled. They are definitely prone to thermal runaway if too many ampere hours are inserted during the charging process. It doesn’t matter how low the charge rate is, or (to an extent) what the final voltage is. Even charging at very low rates (a few hundred milliamps) can eventually cause the battery to go into its exothermic mode where excess oxygen evolves at the positive plate, and then diffuses through to the negative plate, where it combines with hydrogen to make water and lots of heat. The hotter the cell becomes, the faster this reaction occurs, and it can (and does) lead to thermal runaway. In fact, just charging to a very high SOC with only the beginning of a temperature rise can still lead to overheated battery packs as these cells can further heat up even after the charge is terminated because the oxygen already produced continues to diffuse and combine with hydrogen (producing more heat and lowering the voltage). That is why a cut-off timer to limit the total ampere hours is so important, as is a temperature shut-off (I usually set mine to allow no more than a 5-8 F rise).

My control system is not as elegant or automatic as Norm's, but it meets my basic safety requirements. I have a 20 amp GFCI at the wall (on a dedicated 20 amp circuit), then 2 Intermatic "sauna" one hour timers (also rated at 20 amps) plus a 20 amp fuse, all in series on the way into the charger. Plus, a comparator circuit will shut the whole unit off if the battery temps rise by more than 5 or 10 degrees (shut-off temp is set manually, referring to the starting temperature). So the charger will shut off in one hour (even if one of the timers fails), or if the battery temps start to rise. All three batteries are fan cooled, by powerful computer case fans (full 1 amp at 12 volts, Vantec TD9238H 92mm Case Fan, 119 CFM). These are noisy, but powerful, and continue to run even after the charger shuts off, until I check the battery temps before physically unplug.

In addition to the 4 fans under the add-on packs, the fifth—and most important—fan is on the output side of the main pack cooling plenum. I didn't tie into the Toyota fan for charge cooling, as I wanted to have the least connections to the OEM setup as possible. Instead, I removed the final section of ducting from the output plenum and slipped on my fan, reversing the process when pulling the packs for freeway trips. (Note: If I lived in a hot climate, I think I would worry more about cooling the main pack than extra connections to the Prius system. I would probably send 6.5 volts @ 2.7 amps through a diode to the Prius fan, which is its medium speed, according to the manual.)

These 5 fans only run during the charge sessions—while driving with the BMS+ in our typical low speed, low power-draw manner, the batteries have never risen more than a few degrees above ambient and I see no need for extra airflow. If there were, it would almost certainly be the tightly encased OEM battery that would overheat first, and it has the high-capacity Toyota fan and controller to cool it while driving. I have remote temperature readouts on all 3 packs in the center armrest (plus the CAN-view temperature of the hottest of the Toyota sensors under the main battery), so I can keep an eye on things, especially on hot days.

The charger itself supplies 4.5 amps to the three packs, or about 1.5 amps to each (split nearly evenly between the 3 packs, +/- 10%), dropping to 3.8 amps at 238 volts. After a drive, as I turn off the Prius, I check the CAN-view to see how many Ah's were expended, look at the voltage, and set the timers accordingly (I have a simple chart on the charger). If I expect to leave again soon, I try to charge up to 236-238 volts, which may require me to reset another 20 minutes or so on the timers. If I am not leaving until the next morning (or several hours later the same day), I usually only charge to 232 volts and then top up just before driving, so as to avoid the waste of the incoming grid power that would be lost as self discharge—which I also understand may shorten the battery life a tiny bit each time. This sounds tricky, but it quickly becomes a routine and is no bother at all.


Additional Modifications

Because I like to drive with day-time running lights on, I replaced the 55-watt running lights I had originally installed with a pair of led driving lights that draw 4 watts instead. Every little bit helps.

For safety, when driving in areas with pedestrians, bikes, and skateboarders (who often do not hear us approaching slowly from behind), I have installed a little sound system—a pair of plastic-coned old computer speakers hidden in the OEM fog-light recessed housings behind the grill, combined with a white-noise source and the 12 volt amplifier from the same computer sound system. I can push a button to turn the sound on when needed. I am aware that some regular Prius drivers think this is a silly thing to worry about, but we have had many instance where pedestrians try to jay-walk without looking over their shoulder, or bikers just about jump out of their skins as we sneak up on them.

Hold-Down Straps

Even though the batteries are very tightly wedged in place, I also use the Toyota-supplied hold-down straps (shortened enough to make them tight without the tire under them) as an additional safety measure. Although only designed for securing a flat tire, hopefully these straps will provide an additional measure of safety in the event of a low-speed accident while ev-ing around town.

Hold-Down Straps
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Finally, I have an alternative hold-down method (just kidding, but he does love to ride there):

100 Lb. Hold-Down Dog
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Anyone interested in more details regarding the actual installation can find additional information at Norm’s site, hybridinterfaces, especially on his BMS+ page.

NW BMS+ Driver

 

Thanks for the tip on the missing pictures--I put them in an album, but somehow I forgot to make it public. I think it should be working now.

If you have time, perhaps you could describe your charger and procedures. Thanks again.

NW BMS+ Driver

 

February, 2010: Six months and 4000 miles since my original post above, we continue to enjoy driving our 2007 PHEV Prius using the BMS+ System (invented and supplied by Norm Dick at hy-bridinterfaces.ca). We are now so thoroughly accustomed to it that I don’t know what we would do without it.

Since August, we have had access to a 2010 Prius for our longer trips, which means I no longer have to remove the add-on packs from our 2007 for highway driving. So last month I decided to install a third add-on pack, in a more or less permanent fashion.

Prius salvage packs now seem more plentiful (and cheaper) than they were 2 years ago when I bought the first two. This time I found a 2007 pack for only $300. It bench-tests better than the older packs (a 2004 and 2005), but all the modules in all 3 packs stay within .04 volts, under 4 amp charge/discharge loads, which is nearly as good as the Pack-view screen of the Can-view shows for the OEM pack (.07 volts difference per pair of modules).



The three add-on packs are a very tight squeeze to fit under the floor, and this time I had to push back by ¼â€a few square inches of the rear bulkhead (at the far right and left corners of the underfloor area). I also built a new charger enclosure (still double insulated), to move it from under the floor to the left cubby area, and added a more powerful battery cooling fan system.



When ready to install, these packs weigh about 70 pounds each, so to help with the extra weight of the 4th pack, I removed the spare tire (34 pounds), and also I keep only 2-3 gallons in the gas tank, thus saving another 30-40 pounds. Since this car is normally not on the highway anyway, we don’t really need a spare or much gasoline, and the spare tire well is the perfect location for the more powerful battery cooling fan system that runs while charging.

Our driving patterns and mileage is largely unchanged, but we are now able to take our normal trips while subjecting the batteries to a less depth of discharge (DOD). Since increased DOD reduces battery life exponentially, my hope is that the 4th pack will pay for itself in increased overall system lifespan (or at least that’s how I am justifying this upgrade to my very understanding wife.).

I really love driving this setup, and will probably only come to regret all the time I have put into it when I see the first factory PHEV’s start appearing on the road—they will be awesome machines, light-years ahead of all the various conversions available now. But we still have only $2500 invested, and by the time the factory PHEV arrives, we will have been driving our DIY version for nearly 3 years.

NW BMS+ Driver

PS: We also love the new 2010--it is improved in almost every way from the GenII. Too bad Toyota is getting so much bad press these days, but maybe it will help their customer service department improve to be in the same league with their world-class engineers!

 

It has been way too long since I updated this page, but as of October, 2011, we now have nearly 3.5 years and 24,000 electrically-powered ("EV" or "CD" mode) miles for our Plug-In Prius using the BMS+, created by Norman Dick at Hybrid Interfaces. The system continues to work perfectly and we love it, but there are now so many other ways to drive electrically--Leaf, Volt, the upcoming 2012 Plug-in Prius, etc.--that news about the BMS+ is a bit out of date. In fact, I think Norm has discontinued the BMS+ sales.

Speaking of the 2012 Plug-in Prius, last year we had a chance to drive one of the demonstrators for 2 months (you can read our impressions at https://sites.google.com/site/nwpriusplus/home ). While we found it to be a very impressive car, however, the production version still won't have the 20-mile range that I had hoped would have given it a better chance to convince more US drivers to buy it. The 14 miles range listed for the demo car was really more like 10-12 before ICE start, even with slow speeds and mild temperatures, so it will be interesting to see how the production model actually compares to the new 15-mile range.

NW BMS+ Driver

PS--Sorry I have never shared my DIY charger design, just too risky to make it public. But no need now, as there are inexpensive basic chargers available that will be way better, safer, and more efficient than my old design.