True Battery Range

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Joined
Nov 29, 2017
Messages
15
Like most of you, I watch my electric range estimated distance very closely. I assumed it was accurate and that I'm getting 125 miles per charge on my 2017 i3. The last two charges I reset the trip odometer and was pleasantly surprised. Last charge I had 133 miles on the trip odometer and supposedly 30 miles left on the battery. This gives me 157 mile per charge as the rex comes on at 6 miles left. This second charge I have 141 mile driven and 31 miles left which give me 165 miles on the battery given that the rex comes on at 6 miles. Most of my driving is in town and I do drive like a little old lady. I am going out of town soon and we'll see how I do at freeway speeds,
 
I ignore the range display, since I find it very inaccurate. The algorithm BMW uses for this display seems to be seriously flawed.

But I do like to see if I can squeeze out as many miles on a charge as I can.

Instead of monitoring the range display, what I do is reset the tripometer each time I have a 100% charge, keep the 'battery charge percentage' (state of charge) on the dashboard display and then mentally do a calculation on my estimated range. Example: if the tripometer shows 15 miles driven when the battery is at 90% charge, my total range will be 10 x 15 miles = 150 miles. If I'm at 32 miles when the battery is at 80% charge, my estimated total range is 5 x 29 miles = 145 miles. If I'm at 68 miles at 50%, then my estimated range will be 136 miles.

The multiplier (10x, for example) is calculated as 100/(100-state of charge). Since doing metal division can be challenging, some easy percentages are:
At 90%, multiply by 10
At 80% multiply by 5
At 75% multiply by 4
At 70% multiply by 3.3
At 60% multiply by 2.5
At 50% multiply by 2
At 40% multiply by 1.7
At 30% multiply by 1.4
At 25% multiply by 1.3
At 20% multiply by 1.25

IMHO, doing this calculation will give you a better range estimate than the BMW i3 display.
 
I do the same calc every day and put all results in the spreadsheet. So far my total mileage since purchase (end of May) is 1.4K, and my average estimated range is 100 miles (2015 60Ah BEV). I also use this to arrive to an estimate of the battery capacity using the daily mileage and efficiency numbers.

The variability of both numbers (estimated range and battery capacity) looks to be quite substantial, potentially linked to ambient temperature and/or efficiency, but I can't see any trends yet. The GOM in the morning is invariably more conservative in my case, by about 5-10 miles.

I also notice that the less miles I drive the higher numbers I get. E.g., usually when I deplete the battery from 100 to 60%, I get 100 miles and over 19 kWh of estimated capacity, but when on some days I had about 15-20% of the battery capacity remaining in the end of the day, the estimated range was closer to 90-95 miles and the battery estimate was in the area of 18-18.5 kWh. So it looks like either the "top of the battery" energy provides you with more miles, or the computer just overestimates things in the beginning. Also, when getting closer to the battery depletion, the estimated range and the start of the day GOM predicted range seem to converge.

These are my observations so far, I'll keep piling up the data :cool:
 
Oleksiy said:
I also notice that the less miles I drive the higher numbers I get.

Interesting observation. I'll need to keep an eye on this.

I usually go down to around 20% before I charge. I have a 2017 i3 Rex, and in the winter I was estimating 90 miles for a full charge. Now in the summer I'm getting close to 140 miles (estimated). I'll try charging more often to see if my estimated range goes up.

The formula I use to estimate range is:
Estimated range = tripometer * (100/(100-(state of charge)))
Where tripometer is reset to 0 when charge is 100%
State of charge is the battery percentage as displayed on the dashboard
 
Same formula here, only I use any start of the day SOC number (I don't necessarily charge up to 100% every day).

Also, to arrive to the estimated battery capacity I calculate the kWh used for the trip (tripometer divided by efficiency in miles/kWh), and then divide this number by the SOC beginning of period minus SOC end of period.

I decided to do all these calcs to learn how to better predict the range and understand efficiency, and to track the state of the battery on top of Batt.Kappa.Max. Additionally, I have columns with average temperature during the drives (based on dashboard reading in the beginning and in the end), and average speed. Hope some day I'd be able to see some trends.
 
I haven't been tracking temperature, but I made a 90 mile trip yesterday, where the temperature was in the mid 70's F, and I started with 100% and finished with 39% charge. There was a good amount of traffic so my average speed was moderate but there was plenty of stop and go.

So, my calculated range on electric would be:
100/(100-39) * 90 = 147 miles

I did notice that on the first leg of the trip, I had driven 45 miles and the charge was 72.5%. Using the formula, the estimated range would be 164 miles. So either I lost efficiency on the return trip (I do know my speed was significantly higher on the return trip) or the battery state of charge is not linear or not accurate.
 
Glad I found this thread!

I have just been reading the report on 'real world ranges' for 10 EV (Spanish magazine) and see that the pure i3 has around 145 miles (230kms) with 30kms remaining when the battery is supposedly at zero %. Not sure whether this 30kms in included in the 230 or is still 'extra'?!

Anyway having a Rex (as we do with a 94Ah battery) rather muddies these waters as it kicks in at 6% battery capacity - but how accurate is this reading? On our car it seems to kick in around 180-200kms which begs the question of how much actual battery capacity is left.

Has anyone ever run the Rex petrol empty and does the i3 then struggle on with any remaining battery capacity (or is that finished too)? I did notice that the performance on the Rex seemed no different and as the Rex is only 30Kw (way below the electric motor) I presume that the battery is not actually flat but still has 'secret' capacity to augment the Rex. When does this run out and the i3 revert to an under-powered petrol/electric car??!

Any speculation/empirical evidence out there?
 
Well, I don't have a Rex, so no real world experience. But you could check your battery capacity at Rex launch yourself. Next time the engine kicks in, just pull over, get into the hidden menu and check the Batt.Ladung reading in kWh, that's it. It's down in the list following the Batt.Kapa.max.

BTW, you could divide this number by the current SOC and arrive to the estimated battery capacity - this is just one more way to check it up on top of the well publicized Batt.Kapa.Max. In my case the capacity calculated using this approach is consistently 2-4% lower than the Kapa.Max number, very unfortunate...

As to the Spanish test - they seem to have messed up the numbers for BMW i3, so I don't trust them much over other vehicles either. Their 231 km corresponds to about 34.2 kWh usable capacity under the efficiency they included in the rankings (14.8 kWh/100 km or 6.76 km/kWh). If calculated using the real usable i3's pack capacity of 27.2 kWh, the efficiency was supposed to be at 8.49 km/kWh or 11.77 kWh/100 km. In other words BMW i3 should have become the leader of the test! Now go figure where numbers went wrong, in the efficiency numbers or in the mileage they recorded :roll:
 
Sorry to be so clueless (new to an i3) but where is the 'hidden' menu?? Hidden from whom?!
 
GuyD said:
I have just been reading the report on 'real world ranges' for 10 EV (Spanish magazine) and see that the pure i3 has around 145 miles (230kms) with 30kms remaining when the battery is supposedly at zero %.

Anyway having a Rex (as we do with a 94Ah battery) rather muddies these waters as it kicks in at 6% battery capacity - but how accurate is this reading? On our car it seems to kick in around 180-200kms which begs the question of how much actual battery capacity is left.

The remaining capacity is a calculated value which isn't totally accurate. As an i3 battery pack ages, the capacities of its 96 individual cells begin to deviate, so the remaining capacity calculation could become less accurate. In extreme cases, several i3 owners have reported running out of propulsive power before the indicated charge level was >0%. So it's certainly not a good idea to routinely run an i3's battery pack down to a very low indicated charge level if one doesn't want to risk becoming stranded.

GuyD said:
Has anyone ever run the Rex petrol empty and does the i3 then struggle on with any remaining battery capacity (or is that finished too)?
An i3 is always propelled by battery power whether the REx engine is running or not. An i3 REx is a BEV with a generator that kicks in to charge the battery pack. So when a REx engine runs out of fuel, an i3 continues being propelled by battery power with no struggle involved as long as the indicated battery capacity is >~3%. A REx out of fuel performs just like a heavier BEV. When the indicated charge level drops below ~3%, the power delivered to consumers (e.g., climate control, propulsion motor) is gradually cut back to extend the remaining range.
 
ICE cars, at least some of them, have range to empty calculations as well, and they are often wrong, too. The computer does NOT know how you will drive in the future or the weather, so it is an educated guess. You'll find that your estimate is better IF you're running the nav system with a destination in it, as then, the car DOES know your intended route, speed limits, and terrain, so its estimate is more educated.

AFter awhile, you'll get an idea of your typical miles/kwHr, and can estimate your range if you can do a bit of math in your head. Given the moving average the i3 uses, that may be more accurate if your driving is going to be different and/or in different environmental conditions.

Everyone's driving technique is somewhat unique. The computer does a decent job, just like the gas gauge on an ICE about how much is left, but how far you'll get, only you can figure that out. In general, the car's estimate is a bit pessimistic, and you can eke out a few more unless your changing your techniques.
 
OK so we have now completed our own real world range test on our new-ish i3.

Drove 215kms setting off on full charge. Weather pretty cold (for here!) - 8-14deg C; gentle breeze; mostly highway at average 90kph. Bit of up and down but no real hillclimbing.

Rex kicked in at about 208 kms and the i3 ended up home with 10% range left (having been presumably topped up a bit from 6% by the Rex) Did, however, seem to use about 15kms petrol range to do 7-8kms (gentle uphill) and to charge the batteries a little.

This is certainly the longest trip we would do normally from home so that is useful info and relaxes us a bit. The car does seem to get more range with our normal 20+ deg C days.
 
Speed and temperature are the biggest factors in range available. The drag goes up radically as the speed goes up. Temperature is not as much of an issue if you set a departure time and don't take a lot of intermediate stops where things cool off in between segments. Especially on the REx, heating can be a factor...the BEV has a heat pump that provides most of the heat unless really frigid, then it would become like the REx. The heat pump can provide maybe 3-4W of heat from the use of 1W of energy to run the heat pump. When resistance heating is called for, that drops to 1W of heat from 1W of battery capacity...a major difference. Two reasons the REx isn't as efficient as the BEV: extra weight and no heat pump. You don't get something for nothing.
 
That's very interesting - I did not know that the Rex and Bev have different heating and a heat pump is definitely the way to go.

The South African i3 does not have the facility to pre-heat the car (seldom so cold here - no heated seats or steering wheel either..) but I'm thinking it may marginally increase the range on cold days (less than 12deg C) if I turn on the car and leave it standing for a while still plugged (if this is possible?!)
 
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