This has been the coldest winter in my 4+ years in my part of Northern California, sometimes dipping into the mid-20s at night. I have noticed that when I take my PiP out during these colder mornings, the max regeneration rate seems to be reduced about 1/3 compared to a usual commute before friction brakes kick in. From a battery chemistry/temperature standpoint, this makes sense, but was wondering how much of this is perception vs. reality. I don’t have an ODB scanner, but was wondering if any data guys here might be able to chime in with numbers.
Install the Torque app and get an OBD2 Elm237 bluetooth adapter to track regen if you want to know for sure. Keep in mind that regen is reduced when the battery is nearly full. I'm sure regen is reduced when the battery is cold, but I have no idea by how much. Since I already have Torque setup to record max regen rates, I'll keep an eye on it on these cold days and compare it to when it warms up.
On the Hybrid System Indicator screen, The "CHG" meter seems to max out with less brake pressure on a cold early morning drive. So, for a given speed at the start of braking and same distance to come to a complete stop, it feels like I've lost about 1/3 of max regen. Don't know why I haven't noticed it until now or if it's mostly in my head. Just feels like I could make the same stops before without using friction brakes, but now regen maxes out where before it was close. Still appears to do it when battery is half full or less. That would be great, thanks!
gotcha. i have never noticed it, but not really watching. i guess it makes sense, but there are so many variables regarding battery temp, 5 of charge and ambient temp, etc., i will be interested in red points observations as well.
The HSI doesn't always tell the truth about regen ability. Often the friction brakes will engage well before you max out the regen on the HSI. You'll have to feel for when this happens or use Torque to monitor regen rates. I've got a small hill directly from my driveway that forces the car to use friction brakes because my battery is already full. Carrying heavier loads also makes it necessary to apply the brakes earlier to capture all of the regen energy. Yesterday I had 5 people in the car and noticed I had to brake well in advance of when I normally begin to slow down. It's cold out this morning and I'm taking my grandpa to breakfast, so I'll keep an eye on the regen rates.
I just got a scan gauge a couple months ago and maximum regen current is definitely lower when the battery is cold, particularly at lower speeds. It also won't draw as much current in stealth mode before starting the ICE. Note that this is with a regular Prius, not a PIP. I've also just recently added battery temperature, and was surprised by how long it takes to see a meaningful increase. It also appears to be well above ambient first thing in the morning. This all points to there being a lot of thermal mass to the battery, but as I don't have much experience yet my interpretations are all subject to change.
"Shift 5" I suspect your new temp sensor is measuring battery case temp, this can explain the delay in sensing meaningful increase. I still believe the cells themselves are warming-up very quickly with the high loads as described by the O/P.
The scan gauge shows whatever the computer reports, and the regen current appears to correlate rather well with the reported temperatures. Not sure what you mean by "the cells themselves", as cells are made of several parts including the electrodes and electrolyte, which likely warm up at different rates under load. It's similar to referring to coolant temperature when talking about the internal combustion engine. Sure, the pistons warm up very quickly under high load, yet somehow the fuel mixture and thus engine efficiency is still based on the coolant temperature.
Battery charging/discharging rates relate to its cells internal resistance (i.e. internal electrolyte temp) so bad similarity IMO. I think the system limits the max rates at low and at very high temps in order to protect the battery from irreversible processes.
I did some poking around the web, and it looks like the limiting factor when charging is preventing the cells from out-gassing. From Charging Batteries at High and Low Temperatures – Battery University Low-temperature Charge Fast charging of most batteries is limited to a temperature of 5 to 45°C (41 to 113°F); for best results consider narrowing the temperature bandwidth to between 10°C and 30°C (50°F and 86°F). Nickel-based batteries are most forgiving in accepting charge at low temperatures, however, when charging below 5°C (41°F), the ability to recombine oxygen and hydrogen diminishes. If NiCd and NiMH are charged too rapidly, pressure builds up in the cell that will lead to venting. Not only do escaping gases deplete the electrolyte, the hydrogen released is highly flammable. The charge current of all nickel-based batteries should be reduced to 0.1C below freezing. Nickel-based chargers with NDV full-charge detection offer some protection when fast-charging at low temperatures; the poor charge acceptance when cold mimics a fully charged battery. This is in part caused by a high pressure buildup due to the reduced ability to recombine gases at low temperature. Pressure rise and a voltage drop at full charge appear to be synonymous. To enable fast-charging at all temperatures, some industrial batteries include a thermal blanket that heats the battery to an acceptable temperature; other chargers adjust the charge rate to prevailing temperatures. Consumer chargers do not have these provisions and the end user should only charge at room temperatures.
I havent reconnected my Torque setup since I had issues with the OBD2 adapter, but I've definitely noticed I'm not getting the same amount of regen recently with temps much lower than normal (high 30s, low 40s in the mornings). When I leave my house, I use maybe 0.2-0.3 miles of range before I start down a hill. In the summer, I can reach the bottom of the hill (a Stop sign) without the engine coming on. As it cools during the year, I'll "adjust" accordingly before reaching the bottom (braking harder or using the trusty N) In the past few weeks, with temps taking a dive, even my normal adjustments havent been enough to head-off the ICE start-up (so ICE wants to start up sooner than normal). Although this is far from scientific, I've had this car for almost 4 years.. so I have a pretty good idea of what to expect in colder weather in the winter.
I reviewed some of my old data.. and what I said above is not entirely accurate (at least not until I review more data). But in the case I illustrated above, the issue is not so much that the car cant regenerate as much when its cold.. its more the case that the battery doesnt want to take any regeneration beyond a certain SoC when its cold (or coldER) I looked at some data from 12/12/2013 (a day when the ICE did not start) and 12/17/2012 (a day when the ICE DID start) 12/12/2012 SoC before descent: 82.75% Engine Coolant Temp: 60.8F SoC after descent: 83.92% 12/17/2012 SoC before descent: 83.14% Engine Coolant Temp: 53.6F ICE started immediately after I let off of accelerator
Ooops, forgot this was a PIP thread. So I suppose I should ask, does the larger battery in the PIP allow a corresponding larger amount of regeneration? Or is there some temperature based limit in the motors and inverters going on that limits all Prii to about the same amount of regeneration?
