Running on Electric Motor

[lovemyprius05]

This is a theoretical question only. I have no intention of trying to prove or disprove if I can avoid it. If I run out of gas, will my Prius continue to run on electric power only? If so, how far or how long?

[aaf709]

Yes it will run and, depending on the SOC, it may be a mile or so.

DO NOT consider this to be a "lifeboat" in case you run out of gas. This is BAD for the battery.

[eagle33199]

Yes, it will, but the length entirely depends on the SOC of the battery, current speed, and terrain conditions. The most you'll generally get is about a mile of EV.

Keep in mind, that doing something like this will drain your battery a lot, potentially lower than it would like to go, which may cause problems with longevity - the best bet is to just stop where you are when your gas runs out and call AAA.

[mr_zorg]

<div class='quotetop'>QUOTE(eagle33199 @ Oct 30 2006, 12:16 PM) [snapback]340751[/snapback]</div>

...potentially lower than it would like to go, which may cause problems with longevity...
[/b]

Ya know, I doubt this to be true. I've done a lot of research on batteries lately with an eye towards doing some battery mods. One thing I discovered is the supposed fact that the Prius (and other hybrids too, presumably) only use about 33% of their battery capacity. Doing this makes sense for several reasons (which is why I believe it):

1) Rechargeable batteries lose capacity over time, and by using such a small amount to begin with allows the batteries to "wear down" over time without you noticing because of the spare capacity.
2) Ni-MH batteries are prone to "polarity reversal" if drained completely empty. This happens when one cell in a pack drains before the others and starts drawing power from the neighboring cells to recharge itself. In doing so, the cell effectively reverses polarity, potentially damaging the entire pack. To avoid this, you never want to drain the batteries completely dead.

Using only 33% of the capacity nicely takes care of both problems. I don't think it's possible to drain them lower than that, the electronics won't let you. So, I doubt running the battery down (on the SOC anyway) will cause any long term effects, though there's only one way to be sure. :P

[seasalsa]

<div class='quotetop'>QUOTE(Mr. Zorg @ Oct 30 2006, 12:46 PM) [snapback]340825[/snapback]</div>

Ya know, I doubt this to be true. I've done a lot of research on batteries lately
[/b]

Did your research include any of the horror stories of people who actually ran their cars as far as possible on battery?

Several years of reading these "Out Of Gas" posts lead me to believe otherwise. This is not only a longevity issue, it can also affect the fuel pump. Some people have ended up with some expensive repair bills to get their Prius back on the road after running out of fuel.

It just ain't worth it, just fill up at 1 or 2 pips and you won't have to worry about it.

[efusco]

<div class='quotetop'>QUOTE(Mr. Zorg @ Oct 30 2006, 02:46 PM) [snapback]340825[/snapback]</div>

Ya know, I doubt this to be true. I've done a lot of research on batteries lately with an eye towards doing some battery mods. One thing I discovered is the supposed fact that the Prius (and other hybrids too, presumably) only use about 33% of their battery capacity. Doing this makes sense for several reasons (which is why I believe it):

1) Rechargeable batteries lose capacity over time, and by using such a small amount to begin with allows the batteries to "wear down" over time without you noticing because of the spare capacity.
2) Ni-MH batteries are prone to "polarity reversal" if drained completely empty. This happens when one cell in a pack drains before the others and starts drawing power from the neighboring cells to recharge itself. In doing so, the cell effectively reverses polarity, potentially damaging the entire pack. To avoid this, you never want to drain the batteries completely dead.

Using only 33% of the capacity nicely takes care of both problems. I don't think it's possible to drain them lower than that, the electronics won't let you. So, I doubt running the battery down (on the SOC anyway) will cause any long term effects, though there's only one way to be sure. :P
[/b]

The problem with your theory is that the electronics expect the ICE to kick in and recharge the battery when it gets near or below the 40% SOC level. This doesn't happen when you're out of gas for obvious reasons. While it does appear that there is a threshold of around 20% SOC (based on the few reports of people who've really pushed the car in an 'out of gas' condition as far as it could possibly go) where the car will totally refuse to start, that 20% mark is seriously lower than any designed margin of safety within which the car and battery were designed to function. You might get away with that once or twice, but I wouldn't recommend it.

The other situation that overrides the normal electronic built-in safety margins is when the car is in READY mode and in Neutral...this disengages the ICE from the system and allows the battery to run below normal levels.

[mr_zorg]

<div class='quotetop'>QUOTE(efusco @ Oct 30 2006, 02:37 PM) [snapback]340882[/snapback]</div>

The problem with your theory is that the electronics expect the ICE to kick in and recharge the battery when it gets near or below the 40% SOC level. This doesn't happen when you're out of gas for obvious reasons. While it does appear that there is a threshold of around 20% SOC (based on the few reports of people who've really pushed the car in an 'out of gas' condition as far as it could possibly go) where the car will totally refuse to start, that 20% mark is seriously lower than any designed margin of safety within which the car and battery were designed to function. You might get away with that once or twice, but I wouldn't recommend it.
[/b]

I would imagine that those 40% and 20% SOC levels are relative to the 33% overall capacity. If the SOC measures the "usable charge", not the full battery capacity, that would equate to thresholds of 19.8% and 26.5% overall. If that's true, that's well within any safety tolerances. Is there evidence to the contrary? I'm not sure how we could know for sure, and I'll wager Toyota won't tell us.

[hobbit]

The 20% figure comes up because the traditional thinking appears to
be that a "safe" range for NiMH was 20 - 80 % of charge range. It
is what the controllers in the Insight allow, for example. Toyota
seems to have decided to carve a bit off the bottom of that in the
interest of longer-term cell chemistry life, limiting safe discharge
to 40% instead, probably on several industry-experience cues that
maybe 20% is pushing it just a little too far. That's one reason
Insights are forever having the dreaded "recalibrations" all the
time after the battery is pushed a little bit -- the controller is
sensing a steeper voltage dropoff, possibly coming a little too
close to reversing a cell for comfort, and frantically resets the
condition to "empty" and does a full charge cycle. At the expense
of a lot of MPG.
.
_H*

[efusco]

<div class='quotetop'>QUOTE(Mr. Zorg @ Oct 30 2006, 04:05 PM) [snapback]340910[/snapback]</div>

I would imagine that those 40% and 20% SOC levels are relative to the 33% overall capacity. If the SOC measures the "usable charge", not the full battery capacity, that would equate to thresholds of 19.8% and 26.5% overall. If that's true, that's well within any safety tolerances. Is there evidence to the contrary? I'm not sure how we could know for sure, and I'll wager Toyota won't tell us. [/b]

The normal functional range for the Prius battery is 40%-80%, with very rare exceptions only will it go outside that range. Thus a 40% 'normal' operational capacity...I'm not sure what the source of your 33% number is. Now, the Prius will uncommonly go below 50% SOC or above 70% SOC...indeed it's much more common that it stay b/w 53%-63%...or just using 10% of it's total capacity.

[David Beale]

The original poster said he wouldn't allow his car to run out of gas, but as the discussion has taken that tract...

The traction battery in the Gen 2 Prius is a series connected set of 168 NiMH cells. There are 28 sets of 6 cell modules.

The cells may be matched (I would expect they are anyway), for equal capacity. The modules may be coded to indicate this.

Discharge:
Whenever you deal with a series string of NiMH or NiCad cells, you have to worry that the lowest capacity cell will be completely discharged before the others. That is why they are matched, although the matching isn't perfect, and cell capacity will change with age, and not all cells will age the same. If one cell discharges first, continued discharge then charges that cell backwards. This can happen to more than one cell at a time. If allowed to occur repeatedly, this will quickly result in a nickle "whisker" shorting out that cell. With 168 cells it would be hard to tell this is happening, just watching the battery voltage. You COULD tell by watching the pack voltage of each of the 6 cell packs. My experience with hand held two way radios is once you start to reverse charge a cell it is damaged beyond repair and must be replaced, as even if you "burn out" the whisker, it will quickly regrow.

Charge:
When charging, you will also have a problem with the lowest capacity cell(s). They will charge up first, and with 168 in series, it will be hard to detect this happening. When they do charge up before the others, they will then be overcharging. NiMH cells charge well up to about 80% capacity, converting a large percentage of the power to chemical charge, then they start to convert a lot of the charging current to heat. This is why the chargers for commercial packs will switch them to "trickle charge" at about 80% charge - most often detected with temp. sensing circuitry - the pack will start to heat up and so is switched to trickle.
Anyway, the lowest capacity cells will be overcharged and overheated. They may vent to prevent explosion and they will then loose electrolyte. They will loose further capacity, making the original problem worse (lower capacity than the other cells).

If you think about it, the more cells you have in series the worse this condition will be, and the more difficult it will be to detect, if you only have access to the terminals at the ends of the string of cells. One or even five cells lost can be "buried" in the normal battery voltage variation. NiMH cells exhibit about 1.2 V just before being emptied of charge, 1.25 V in their "operation range", and 1.5 V fully charged. The 168 cell pack can vary from 201.6V to 252 V! How could you tell if one cell was in trouble?

Because of this, Toyota engineers apparently made the reasonable decision to operate the pack between 30% and 60% charge, with emergency discharge to 20% and emergency charge to around 70% permitted. This would avoid fully discharging the pack and reverse charging a cell(s) and fully charging the pack and overcharging a cell(s).

Having said all this, if you ran out of fuel and "limped" the car as far as it would go, you will still have about 20% charge left (the system will "shut you down" at about this point). If you did this once, I doubt you would cause any problem. If you did this once a month, I suspect you would perhaps halve the life of the battery (just a guess).

At any rate, you can "limp" the car typically a mile or so, and some have actually gone a bit further. I think, based on the above, it would be prudent to limp it to a safe place and call for fuel. Be aware, from reports of others who have run out of fuel, you need at least 3 gal to get the engine going again if you run out while moving due to fuel pickup issues.
Also, running out of fuel is not a good idea. The electric fuel pump is fuel cooled, and even if it shuts off as soon as the ICE quits, it will have to dissipate any heat energy it will contain in air. Not a good situation!