Full state of charge vs 80%

One of the earliest facts shared with newbie owners is the difference between the indicated battery SOC and the actual range of battery charge. Yet even today, there is a lot of ignorance about what State of Charge means, much less, how it is measured. Yet Prius owners have been making studies almost as soon as the car arrived. For example, there is an excellent write-up in "Prius Technical Stuff" dated from 2003:

In this case, the state of charge was measured using what is called a coulomb counter.

The State of Charge is a ratio of the current energy content of the battery versus the actual Amp Hour (Ahr) capacity of the battery. As they age, the Ahr capacity decays over time and also changes as a function of temperature and the available energy is subject to charge/discharge rates.

Since we have someone who misquotes folks to post inflated claims about the indicated battery display versus the actual SOC of the traction battery, let's have some fun and throughly discuss the subject. It is time to address inflated and nonsense claims head-to-head. . . . "Shall we play a game?"

Bob Wilson

 

Exactly! DaveinOlyWA

Those who know what they are talking about are well aware of:


Given the NiMH modules are rates at 6.5 Ahr, then we also know the "C" is typically used for charge as well as discharge rates:

• 1 C = 6.5 A
• 2 C = 13 A
• 4 C = 26 A
• 8 C = 52 A
• 12 C = 78 A (maximum I've seen in my Graham scanner data)

Now the real challenge, the interesting question is:

Given our traction battery control electronics are limited to 20 kW and the battery voltage varies over time, how many C is that? Sounds like a poll question.
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A poll question, a quiz by any other name. Something along the lines of:

1. 9.6 C @320V and 63A?
2. 11.2 C @300 V and 67A?
3. 11.0 C @ 280 V and 71A?
4. 12.7 C @ 273 V and 75A?
5. 12.9 C @ 260 V and 77A?
6. All of the above?
7. None of the above?

Come' on "experts," pick and post your answers. <GRINS>

Bonus question:

Cite a credible source as to why we do not use the rates 6.5 Ahr as the traction battery module, capacity.
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Bob Wilson

 

What a sad and pathetic answer. A coulomb counter is the gold standard.

Strange but they seem to be showing up all over this forum. For example, the lame attempt to accuse Patrick Wong of something he didn't say.

Actually there was never a question but what is called a "strawman" assertion and not by Patrick Wong.

Better posting practices would start by not making stuff up as recent examples have shown.

Bob Wilson

 

It is and remains the direct charge counter. Trying to introduce something else without saying or describing what these "mystery elements" are remains, sad to say, an inadequate and misleading claim. It implies knowledge not evident in the obviously weak claim.

Bob Wilson

 

I've not tried to count the display states of the NHW11 battery indication. Still I would add, it has no quantitative numbers. The owner only had something like this to call it 'fully charged:'

(Image from YahooGroup "Prius Technical Stuff" files.)

There is a lot we haven't figured out about the NiMH batteries, both NHW11 and NHW20. State of Charge is at best going to be an estimate based upon the designer's best guess. It relies upon some engineering estimate of the traction battery Ahr capacity, which we know from the Dept. of Energy and other studies of the NHW11 modules, decreases over time.

Personally, I'm interested in a detailed study of the NHW11 and eventually the NHW20 modules. They represent an unrealized resource because we don't know enough about their characteristics. Fortunately, some of us are reverse engineering them and gaining insights. Arguing about State of Charge is a little like disputing how many angels can dance on the head of a pin (answer: As many that want to.)

I am more interested in:

• What is the most cost-effective method to restore original Ahr capacity in NHW11 modules?
• Can we come up with a better sealing solution so they last closer to NHW20 modules?
• What should be the maintenance schedule for NHW11 modules?
• How do we optimize the existing, USA inventory of NHW11 modules?
• What operator signals or insights can we provide so they minimize NHW11 module stress?
• How can we quantify the "remaining life" of an NHW11 traction battery?

These are the sorts of original contributions, the types of investigations that move us forward.

Bob Wilson

 

Hi Dave,

Thanks for the comment:

Sad to say, not really although there is an indirect method. We know that loss of capacity is associated with loss of electrolyte, the water. This has mass so we could get an indirect measure by comparing the weight of a module versus a new module. Sad to say, this is impractical. But there is another approach, the gravity well.

This was first proposed in "Prius Technical Stuff" and the methodology is:

• force charge battery - note, this will also heat the battery but it will ensure the battery is fully charged at what the controller declares to be 80%.
• slowly up hill in reverse until it stops - this runs off of just the battery until it reaches 40%.
• potential energy change ~= traction battery kWhr for 80%-40%, or roughly 40% of the battery capacity
• convert kWhr to Ahr at 7.2 V. and divide by 0.40 => best estimate of traction battery Ahr capacity

The maintenance manual discusses a forced discharge and charge cycle using car 'chocked' and voltages monitored to measure the dV, maximum of 0.3 allowed. However, it really isn't designed to measure the Ahr capacity. But with a Graham scanner recording the current over time, we could come up with a best estimate.

The problem with both approaches is the stress on the traction battery. I would not mind running it if the car were 'cold soaked' at say 40F because the exothermic heating from charging the battery would work out about right. But I would not do it in an Alabama summer day.

Bob Wilson

 

When the owner of the 2001 Prius reported, "It is fully charged," he only had this display:

Folks who have never seen this display before may not realize there are no State of Charge metrics on it. In fact the State of Charge is at best an estimate best confirmed by doing a series of charge-discharge cycles using a Coulomb counter.

This thread is a chance to educate those who don't have clue and it may yet work. It can't do anything about the willfully ignorant but like my drill instructor used to say, "Let God sort them out."

Bob Wilson

 

Source? Back up your claims with sources ... say:

• Your own experiments
• An SAE paper
• A Dept. of Energy paper

One of the best Prius Friends, Hobbit, also prefers to remain somewhat anonymous but he backs up his reports very nicely:
Hobbit's techie-rants

Understand all it takes is either a description of your own experiments or source material. But going back to SOC, the subject of this thread.

Experimental data really isn't that hard to gather and share:

The MRC 989 simply counts the charge added and drained from the module. A simple but perfectly functional Coulomb counter. The ratio of traction battery actual charge versus the charge added makes the State of Charge, a ratio. The SOC is useful to give an indication of how much energy is stored in the battery but this only works if we know how much energy the battery can store.

In contrast, consider how many SOC units are available in this display:

Since there are no units in the display, all anyone can claim is 'it is fully charged.' Without running a charge-discharge cycle, no one really knows the battery capacity.

Since we have a stretch of cold weather, I'll try to run the test Saturday morning with the Graham scanner using the "Check ... block voltage during charging/discharging" (DI-346-348) protocol. I'm coming up on my bi-annual oil change and tire rotation and alignment, this will be a nice addition.

Bob Wilson

 

What, no backup data? What a shame.

Bob Wilson