Fuel Gauge Question

I've been driving my miev for 3 months now and have been wondering about the fuel gauge and how to interpret it. At what point does the bar drop to the next level, for example, if 16 bars are showing, does it drop when it hits 15.0, 15.5, or does it fall once it hits 15.9 bars? I realize that we can't visually see 15.5 bars or 15.9 bars but I'm pretty sure the miev internally can.

oahumiev, I'll start off by answering the reverse of your question: when charging and the sixteenth bar kicks in, there is still a fair amount of time before the power draw decreases and then shuts off completely. Thus, I would call that 16.9+ bars when she's "full".

To answer your question and using this logic, the transition from 16 to 15 occurs at 15.9+ bars.

Carrying this to its extreme, the transition from one bar to zero bars occurs at 0.9+ bars. This is good, as it gives additional visual warning/emphasis to get off the road and still be able to crawl somewhere. I hope to never experience this for the sake of my battery pack.

All this malarkey is the result of automakers trying to make the experience similar to a conventional fuel gauge.

I personally would simply prefer a 0-100% SOC numeric.

JoeS said:

oahumiev, I'll start off by answering the reverse of your question: when charging and the sixteenth bar kicks in, there is still a fair amount of time before the power draw decreases and then shuts off completely. Thus, I would call that 16.9+ bars when she's "full".

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Is it even possible to have 16.9 bars? Our miev has a 16kw pack so I think 16 is the max, 16.9 would assume a 17kw pack? Agree with you that a simple SOC % would be great!

Hi oahumiev - I've never even considered correlating the number of bars to the size of the battery pack: that fact that they're both sixteen I think is a coincidence.

Our usable battery size I suspect is somewhat less than the "nominal" 16kWhr, anyway. The BMS protects both ends of the battery pack and at the lower end keeps some energy in its hip pocket to keep us from murdering the pack.

For ourselves it just doesn't matter, as in real life we simply charge and drive and pay attention to our intended trip mileage vs. Range Remaining, using the fuel gauge for further reassurance.

I just assumed that's why Mitsubishi put 16 bars to represent each kw, made sense to me. Therefore, in my head I figure each kw will give approximately 4 miles range which comes out to 64 miles total range which is near the EPA figure. Of course, using the hypermiling techniques you've provided might put it closer to 5 miles per bar.

On another topic, I've never fully charged my miev. How does one know when it's fully charged, does the charging light on display panel turn off automatically?

oahumiev said:

I just assumed that's why Mitsubishi put 16 bars to represent each kw, made sense to me. Therefore, in my head I figure each kw will give approximately 4 miles range which comes out to 64 miles total range which is near the EPA figure. Of course, using the hypermiling techniques you've provided might put it closer to 5 miles per bar.

On another topic, I've never fully charged my miev. How does one know when it's fully charged, does the charging light on display panel turn off automatically?

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Yes, when it's fully charged, the charging light on display panel turns off automatically.

Pardon me if my questions have been answered, but I was unable to find the answers.

The fuel gauge reflects the charge level of the battery pack. A battery cell's empty state is a when the electrochemical reaction that produces electricity has depleted the necessary reactants, and the cell is producing 0 volts. A battery cell's full state is when the electrochemical reaction that stores electricity has depleted the necessary reactants, and the cell is producing its maximum voltage. We know that discharging or charging a cell fully can cause damage. A battery management system (BMS) prevents either situation from occurring.

Similar battery fuel gauges on Toyota and Honda hybrids show empty when the battery pack is ~20% full and full when the battery pack is ~80% full. The BMS's on these hybrids prevent the battery pack from being discharged below ~20% and charged above ~80% thus protecting the battery pack but also reducing the effective capacity of the battery pack to ~60% of the advertised capacity. What do empty and full on the iMiEV fuel gauge represent relative to the empty and full states of the battery pack?

I have read advice on this forum against charging one's battery pack fully and against discharging one's battery pack down to turtle mode. One iMiEV owner recommended keeping the battery pack between 40% and 80% for best longevity. Are these gauge or battery pack charge level percentages? I suspect that full on the gauge may be ~80% of the battery pack's full state. If so, then charging fully would be only 80% of the battery pack's full state which would be within the "good for longevity" range, so why limit charging to less that 100% on the gauge?

I don't pay that much attention to the fuel gauge - The computed mileage remaining is a much better indication of where you are range wise, or so it seems to me

I always charge mine fully. I gotta believe that if there was a marked difference in the packs longevity between charging it at 90% or 100%, the engineers would have set it up where the charger kicks out at 90% - They probably did . . . . likely what we're calling a 'full charge' is actually what they determined is best for the pack. They are after all giving an 8 year warranty on the battery pack

I do try not to recharge until it's down to about a third remaining though - Hopefully we'll gain enough confidence in the range meter to be able to take it down to 10 or 15% remaining once every couple months . . . . I *think* doing so periodically is good for it, at least from everything I've read

I'm also a firm believer in a slow charge . . . . though I think I'm going to pay the $300 and have my factory level 1 charger upgraded to 13 amps - I may also use it on 240 at 13 amps occasionally, should the need arise, but anytime I can, I'll continue to use the 120 charger

Don

My owner's manual has a bright orange insert that states every few months you should run the battery down below two bars and then fully charge. It states the purpose of doing this is to cause the computer to re-calibrate the charge level indicator. My car (and owner's manual), came off the boat in April.

dumemama said:

My owner's manual has a bright orange insert that states every few months you should run the battery down below two bars and then fully charge. It states the purpose of doing this is to cause the computer to re-calibrate the charge level indicator. My car (and owner's manual), came off the boat in April.

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I read that also, but didn't it say every 24 months?

Other thoughts on battery depletion:

Is it linear? pulling off the top end of the battery may not be the same as when your near the bottom. So I would be careful of estimates of mileage based on how quickly one or two bars disappear.

As I recall, my orange insert says to take the battery level down to two bars and fully recharge every 24 months.

The manual also says to fully charge the vehicle every two weeks. (it didn't say it had to be discharged to any special level before this though.) Also, it does say not to "top it off" all of the time if it already has a near full charge.

So, if it is not good to fully charge the battery every time, why does the manual say to charge it full every two weeks? This is a little bit confusing in my opinion.

Are 16 bars really 100% charge or have the engineers programmed all of this into the BMS so that the battery life is maximized and the batteries are not really charged to the max when the gauge says full? The batteries seem to have a pretty warranty on them after all and the manual says to fully charge it every two weeks.

SteveB said:

....
Are 16 bars really 100% charge or have the engineers programmed all of this into the BMS so that the battery life is maximized and the batteries are not really charged to the max when the gauge says full? The batteries seem to have a pretty warranty on them after all and the manual says to fully charge it every two weeks.

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Would love to hear directly from MM about this.

The insert says to do the calibration procedure once in the first year and then at least once every 24 mo. It does not caution against doing it otherwise. I don't know if depletion is linear. But charging is stated as not linear, the system automatically slows charging at the top end and prevents overcharging. Given that management, and instructions to fully charge every coupla weeks, it would seem odd that fully charging regularly and more frequently would be a significant problem.

I hit turtle mode in my first 24 hours and fully charge at every opportunity, since I only have level 1 at home. I end up my day with 2 bars once a week or so. I'll let everyone know if my battery craps out early.

SteveB said:

So, if it is not good to fully charge the battery every time, why does the manual say to charge it full every two weeks? This is a little bit confusing in my opinion.

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I believe they're thinking that once every two weeks is once every 8 or 10 recharges . . . .

Don

I think you need a nearly full charge every so often to keep the RR indicator accurate.

down to 2-3 bars almost every day and full charge every night.

We will see how she does in the long term.

Based on the technology of the SciB battery, we should be way ahead of the LEAF folks. Time will tell.

I've thought of all these questions. However, we can't really produce any meaningful answers without knowing how the battery charge has been programmed. It's too bad Mitsubishi doesn't make this stuff readily available. I suspect they are are trying to make the system relatively fool proof for the casual driver. From my own experience, charging with a graphing kill-a-watt meter, charging is almost perfectly linear (which makes no sense from my understanding of electricity, capacitors, batteries. It should be exponential) until about the last .3 kwh that trickle charge. I assume the BMS is doing this. I consider full when the orange light on the stock charger goes out and the amp draw is near zero.
As far as the use of bars vs. an analog gauge, it may be to make it look like a gas gauge, as previuosly mentioned. However, it may also be to emphasize that it really is an estimate. As bad as some gas gauges are, measuring the true remaining capacity of a battery is a much more difficult task and there probably is some inaccuracy.

However, this stuff is great. I don't have the technical background to wire my own car, but I'm absorbing every bit of data from you guys who do. Please keep posting the data.

I don't know if the battery pack has a usable (DOD) capacity of 16Wh, or if the maximum is 16kWh, and you can use some factory set portion of that - like 93% of it or ~14.8kWh. If it is the former, then the actual maximum capacity needs to somewhat greater, like ~17.2kWh. My best guess is that the latter is correct - they would want to quote the largest capacity battery they could.

Does anybody know the DOD (depth of discharge) rating on the Yuasa pack in the i MiEV?