# Civic / Insight NiMH Battery packs



## IamIan (Mar 29, 2009)

Part 2:

Armed with this knowledge that many of the HEV OEM NiMH battery packs that have been labeled as "Bad" might very well just be out of balance and easily serviceable. I went out and looked for what I could find from some Salvage / Junk Yards.

I found 2 HEV Honda Civic NiMH Battery packs nearby... got them both for $500 Total ... or $250 each... Picked them up in my Honda Insight. 

This turned out to be a great deal for many reasons.

First off the 6 cell subpacks are the same subpacks used in the Honda Insight... so If I want to / need to I can use them as replacements.

The Salvage yard had removed the computer modules ... no suprise ... Honda Charges an arm and a leg for those... but the Junction board was still attached to the side of both battery packs.

This is sweet because the junction board contains among other things:
A Panasonic relay that will break a 400V 60A current flow.
High Voltage disconnect IMA switch.
IMA Fuse. ( IMA peaks at 100A and ~190V )
DC-DC Fuse 20 Amps ~190V
IMA Hall Effect Current Sensor.

Each of the 20 ( 6 cell ) subpacks has attached to it a PTC strip that changes in electrical resistance as the subpack changes in temperature... this is wired in series in the pack and works kind of nice as a simple way to know not only average pack temperature ... but also if any one cell out of 120 starts to get overly hot... here is a picture showing how the PTC strip changes with temperature.

There are also 3 other independant temperature sensors in the pack to monitor specific points.

The Subpacks themselves have each of the 6 cells wielded together and there is high current bolt connection at the end of each subpack... There is a different shape to the two terminals... a Square for the + and a Hex for the -... but the bolt connection is nice for using a buss bar to connect the batteries.

Some time after the 2005 HEV Honda Changed the Civic Battery pack slightly ... the newer arrangement wields two of the 6 cell subpacks together side by side to make a 12 cell subpack.

After only a few charge / discharge cycles ... these Civic NiMH batteries have confirmed my theory ... all 40 of these subpacks of 6 cells ... have 6.1+ Ah of usable capacity.

I am now doing some additional tests to try and figure out some other specifications for the subpacks.... each of the 6 cell subpacks weighs ~1.08 kg ... which is not the best at only ~43 Wh/kg.... They have a ~3.3cm diameter including the PTC strip & plastic wrapping... are 39.3cm long from end of bolt to end of bolt... which yields ~139 Wh/L of space.

Given the $500 cost and tested 6.1+ Ah of usable capacity... that yields 3.51+ Wh/$1.

Given the 100 Amp Discharge rates used in the Insight and the ~1.08 kg weight... that yields a power density of ~666 W/kg... and ~2,142 W/L... not bad for ~10 year old NiMH battery technology.

I am testing the self discharge rate now... but the required waiting time will slow that down a good bit.


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## IamIan (Mar 29, 2009)

Part #3

My original plan included:

#1>
Confirm civic subpacks are the same as Insight subpacks... up to ~2005 civics they are... after that they aredifferent but should still be usable to pack build ups if that is desired.

#2>
Test bad battery theory... has held up well among all the Insight batteries I've tested and now for two other Civic batteries ... and several other Insight , Civic , and Prius Batteries I have read about others doing.

#3>
Test Batteries themselves to determine more specifics.

#4>
Can they be used to make a second PHEV grid charged battery pack for my Insight... to add an additional ~1.7 kWh of battery power... bringing me to a total on board capacity of ~2.6kwh ... yes they can ... but I have some work yet to do for controls / interface and enclosure and such.

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I did allot of what I set out to do... the other bits and pieces will take a bit more time ... but hopefully it will all come together.... eventually I planned to add even more battery power ... eventually my goal is to get up to ~10kwh ... then I will make a switch over to a REEV from the PHEV ... That way I will get the EV benefits for 90% of the trips... but there are times I still make ~1,000 Mile round trip drives... and currently I can't have two vehicles... so unless something can change... I will need that occasional long distance trip vehicle ... but I still look forward to shifting more to an EV like drive for ~90% of the time.

At this point I don't know ( waffle back and forth ) if I want to invest any more in this type of NiMH batteries... they are great in allot of ways for allot of things... but I can get almost double the capacity out of the same weight with other high capacity instead of high power cells... but there are some down sides there... the high capacity NiMH cells ( like the 12 Ah NiMH D Cell at ~90Wh / kg ) will weigh less for the 10 kwh ( ~120kg ) ... and cost ~$5,500 .... but they will not like it if I try to pull more than ~20kw from them at any one time for peak loads , they might do it ... but they won't like it ... while the high power , and cheaper (~$2,900) used civic / insight NiMH batteries will weigh more ( ~240 kg ) ... but will allow for up to 150 kw peaks from a 10 kwh pack.

And of course I could always forget about NiMH and use some type of Li instead... for even higher wh/kg and or w/kg.

For now... I think I will finish up my last bit of testing ... and get the first step PHEV booster battery finished up and installed ... I'm not in a rush ... and I have several other projects ... so it might take me several months before it is done ... hopefully baring any major problems ... before the next winter comes to RI.


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## DavidDymaxion (Dec 1, 2008)

Awesome info, thanks for posting it!


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## IamIan (Mar 29, 2009)

DavidDymaxion said:


> Awesome info, thanks for posting it!


thanks 

I also don't particularly like the layout design used for the Honda OEM Insight and Civic NiMH battery Boxes.

They aren't bad , per say ... but they aren't particularly good either.

I have a design I am working on that should allow for better air flow in the battery pack between the sub-packs... this should help some with the uneven heat distribution of the current OEM design.

As a side benefit if it works out like I hope... my layout / design should also reduce the final battery box packing size ... so it will use ~20% less overall volume. 

I have a friend who works with a CNC machine ... he is going cut out some pieces for my planed design that are a bit beyond what my home tools can do... where he works is a bit busy right now... so I will have to wait a bit for that... no big deal I got other things to do in the mean time.

I wanted to pull together the raw data from my ~300 or so to data logged battery tests of Insight and civic sub-packs... I am hopping to be able to figure out the "Peukert k value" for these sub-packs ... that way we will have a better idea of how much usable kwh of energy we can get from using them at different discharge rates... and how much a given peak kw surge discharge will effect them.

In my next post I will also outline an idea I have been kicking around... that will also take some time to flush out.


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## Harold in CR (Sep 8, 2008)

Really good info you have posted. Thanks for taking the time to share.


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## IamIan (Mar 29, 2009)

Harold in CR said:


> Really good info you have posted. Thanks for taking the time to share.


Your Welcome 

I recognize the data will have a bit of a narrow number of interested persons... after all several types of Li will given better Wh/kg ... and PBA will give better Wh/$... so NiMH in general is a bit of a niche between the two.... and of course if you have a bigger budget you can always do better than these used battery packs I am examining... but I am a bit of a battery hobbest and NiMH is still my personal favorite.

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Here is the idea I've been kicking around... it started just by asking the simple question: What is the cause of limited battery service life?

In a nut shell... there are other chemical reactions that happen that are unwanted and batteries last longer when you abuse them less.

Each specific battery chemistry has different tolerances and conditions that lead to unwanted chemical reactions... each specific battery of a type will vary slightly among that type for its specific tolerances.

This means for example a battery cell must not be over charged or you risk accelerated battery cell death... and on the other side you don't want to over discharge it or you will also risk accelerated battery cell death.

Some Battery chemistry types ... such as Li will not tollerate much abuse... and you must be more careful in their care.

Other Battery types such as PbA are pretty forgiving and will suffer if abused , but are far more tollerant than many of the Li options.

But even in a chemistry type ... like PbA there will be variation from specific batteries ... some will tolerate a deep discharge far better than others... and some will tollerant over charging better than others.

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Ok now with that general overview ... let's move on to something a bit more specific.... first some ground work ...

Let's look at the NiMH Charging terms and end of charge indicators.











The question that comes up is : What causes these end of charge indicators?

The answer is: The chemical reactions in the battery cell are changing into a different chemical reaction.

This change to a different chemical reaction is one place were we can get into trouble and get some of those unwanted chemical reactions.

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Now a bit more into chemistry , sorry a necessary evil  

When you move electrons into a battery with a flow of charging current the battery stores that energy chemically by a chemical reaction... this chemical reaction is much much slower than the speed in which those speedy electrons travel through the conductor feeding current to the battery.... this is why when you take a charging current off of a battery the voltage at the terminals of the battery starts high and then drops down over time to a more steady voltage level.

The other thing about the slower chemical reaction is that energy flows from high to low... in order to get a more equalized steady state.

Each of the millions of individual molecules inside of a battery do not simultaneously react ... just like when you turn on a heater in a cold room... the energy will spread out from the high energy places to the low energy places ... first the space closest to the heater and last the places furthest away ( most insulated ) from the heater... the whole room does not heat up 100% uniformly ... the same is true inside the battery with the high energy electrical state molecules and the lower energy state molecules.

Electrical current will find the path of least electrical resistance ... which is a combination of the resistivity of the medium and the distances.... sometimes a longer distance through a low resistance material has the lowest overall resistance ... power cord miles and miles away from a power plant... and other times the path of least total resistance is to travel a short distance through a material with higher resistance.

This means that the electrical flow through the battery is not going where it needs to go to charge the battery ... it is going where it is the easiest to go.

Now once the molecules , in the low resistance path the current is traveling in, have reacted and are in a higher chemical energy state ... there is again a potential difference between them and the lower chemical energy state molecules near them.... once this potential is large enough the molecules near these higher chemical energy molecules will distribute some of there higher state chemical energy to the lower state chemical molecules in order to try and get closer to a more uniform / steady state.

As we continue to pump in more electrons ... the molecules in the path of current flow will continue to get pushed back into a higher chemical energy state ... any energy they may have given to near by molecules is replaced by the continued electrical flow.

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Now a bit into Physics ( sorry another necessary evil  

We can measure the average energy level ... in a room for temperature ... or for pressure from a wave front... etc... but looking more closely we see that it is an average ... and there is almost never a 100% equal / uniform distribution ... there will be points that are higher than the average and points that are lower than the average.

There are also minimum levels of energy that can be transferred... even if they are grossly tiny.

The faster the state of the system is changing the greater number of points that will be above or bellow the average ... and the further apart the two extremes between the highs and lows will be.

We can see this by heating up a cast iron pan... if you supply a small amount of energy the pan can distribute that energy fast enough so that there is not any large difference between individual points.... if instead you heat up a small isolated tiny part of the cast iron pan with some highly focused and extremely hot item ... you could melt or even vaporize that tiny part of it... while the rest of the pan has still not changed its temperature significantly at all.

What this means is that there will be individual molecules of that battery that are over charged at the same time that there are other molecules of the same battery that are not.... the faster we charge the battery the more molecules per second will be more likely to have unwanted reactions ... because of minimum energy levels there is a limit ... but there is always a distribution of varied current states , and a distribution of probability of each possible state.

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Bring chemistry and physics back together in normal terms:

Slower is better for the battery.

Ok so you'll say well duhhh ... the point is that it is a sliding scale of probability of how likely is it that there will be how many molecules per second having unwanted chemical reactions ... the slideing scale goes up to higher risk with faster charging ... and it also goes up to higher risk as one gets closer and closer to unwated states ... such as overcharged or over discharged...

It is not a flate wall where it is back and white of what is harmful and what is not ... it starts in the white ... and slowly one molecule at a time becomes a bit more and more grey ... until you get to the black.

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Next post... 

What this can mean to battery service life and cycle efficiency.


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## IamIan (Mar 29, 2009)

When you look at the image of the NiMH end of charge indicators one can see that the indicators come in after the chemical reaction in the battery start to change into a different chemical reaction.

This means that just about all of today 'smart' chargers do not 100% correctly charge a battery.... some are better than others and might be up to ~95% correct... but I haven't seen one yet that 100% correctly charges the battery.... like so many things ... you do the best you can ... and deal with the limitations / inaccuracies.

Basically all of the methods listed in the above picture ... look for the end of charge ... but the things they are looking for are caused by what happens from a different chemical reaction ... by the time any of these indicators happen some of the individual molecules inside the battery have already gone past being over charged and some of those unwanted chemical reactions have already happened.

How many of the molecules of a battery cell you loose each time depends on how good the charger is.... the best chargers will limit this and only a few unwanted chemical reactions will happen each time.

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This not only effects service life of a NiMH battery ... it also effects the way people report cycle effieciency.

If you put 8Ah cell because that is when your 'smart' charger turned off... it would be inaccurate to use that 8Ah number for cycle efficiency calculations ... the question is how much energy can the battery give out... if the battery when fully charged only gives you 7Ah then if you want to test the cycle efficiency ... you need to put in less than 7Ah so that you are not counting Ah that never get stored in the battery.

So to test a NiMH cell that when fully charged will give you 7Ah output ... you put in less than 7Ah ... say 6Ah or 6.5Ah ... then you discharge it... now when you count the Ah that come back out you know you are only counting against what the battery could have stored.

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NiMH Discharge:









As the picture above illustrates...

When you discharge a NiMH cell toward the end there is slope where the battery cell voltage drops off... the end voltage point ... usually 0.9V per cell always stops discharging short ... and leaves some of the batteries capacity still in the cell un-used.

The fast C rate you discharge a battery the more of this effect you see.

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This is seen in the effect of 'growing amps' as I've heard it described ... when a PbA EV pulls off the road for a little while...

You are letting the slower chemical reactions have some time to catch up a bit ... this lets you tap into a bit more of that yet un-used capacity.

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different batteries react differently ... but 100% DoD where 0 capacity is left in the cell pretty much damages just about any type of battery cell.

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If you want to test the cycle efficiency of a Battery the way to do it properly would be to first determine via 3 to 5 charge / discharge cycles exactly howe much Ah the battery will give out.... then you charge it up to a number 10% or so bellow this Ah amount ... you discharge it to the safe cut off point ... and in order to account for the unused capacity you are not discharging you charge it up again with the same amount of Ah ... you do this limited Ah charge and proper voltage cut off 3 to 5 times ... and in the end you will see a pattern emerging.

from the Civic and Insight NiMH cells I have been able to get the cycle efficiency to level off at ~95% when charging & discharging at 2 Amps or C/3.... which is significantly above the normally reported cycle efficiency number for NiMH ... and I think that is because most of the time people do not do the cycle efficiency test correctly... and they add in Ah that the battery could never hold... and they don't account for the unused Ah left after discharge.

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I think we can avoid the wasted Ah from incorrect charging ... which increases cycle efficiency ... and at the same time prolong the service life of a NiMH.

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next time ...

now that you have the back ground ... my proposal ... idea I've been kicking around ... and still have to do some testing to verify it and then figure out a way to implement it.


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## IamIan (Mar 29, 2009)

Battery charging, simple ... for complicated reasons.

CL & CV
Current Limited & Constant Voltage

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This result is for several reasons.

A Constant Voltage ... that is no higher than the voltage the batteries will be at rest once 100% fully charged.... for NiMH this is about 1.39V per cell.

Allot of people like to use a CV that is higher than the batteries rest voltage ... this is useful for charging the batteries faster... but a CV at or slightly under the batteries 100% SoC rest voltage is safer for the battery... as you get the battery more and more charged and closer and closer to 100% SoC ... the rate of charging continues to slow down... which is good because the probability of unwanted reactions that cause perminate damage to a NiMH battery are related to the combination of how fast you are charging and how close you are to the unwanted state.

We see this kind of charge rate in capacitors and Li batteries ... where it starts chargeing at one level ... that might have to be current limited to prevent damage... I would recomend setting the CL will depend on the battery ... but in general ... about C/2 is a good rate... and then after not too long it becomes CV and the current slowly begins to drop off in a logarithmic fassion.

NiMH will complicate this slightly of course... over time a NiMH can get a voltage depression .... this can be treated with 3 to 5 full charge / discharge cycles at a slow CC rate of C/10 or less.... but it does mean that the 100% SoC rest voltage will change for the battery over time... and an ideal NiMH charger will have to adjust to this change ... perhaps with an occasional calibration cycle.

So like many other types of maintenence I recomend multiple battery cell battery packs do some battery pack maintenance every now and then... how often depends on your BMS ... but at least once or twice a year.

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Battery Discharging... Simple Theory , complicated implimentation.

CL & CV
Current Limited & Constant Voltage

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It isn't a typo ... the issues we want to address / avoid on discharge are the same kind of things we want to avoid on charging ... so the stradegy is the same.

First it starts as CL to what ever is safe for the batteries you are using... the NiMH batteries in the Insight and Civics get used for up to 100Amp rates ... so we don't want to pull anything more than that.

Then in short order it would change to CV discharge ... so that none of the NiMH cells are pulled bellow 0.9V.

As the capacity of the packs drops down the amps drop down with it.

Same reasons as charging ... the danger is the probablitlity of damage goes up realted to both discharge rate and proximity to the unwanted state.

Again it is also very common for people who want to tap that yet un-used capacity faster to want to discharge the voltage bellow this point knowing that it rebounds when the load is taken off.

Just like the charging the discharging current will become logarithmicly decreasing.

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Ideally each invidiaul cell can be monitored ... This is practicle for large formate cells ... but is not practicle for smaller format cells.

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Eventually I would like to implement a testing rig for this theory ... I want to quanitify some parts of it... 

like what kind of logarithmic function will be seen for charging and discharging ... My suspicion is that the logarithmic effect while nice for the batteries will have too large of a performance impact on a PHEV or EV... charging time is less important as long as a significant majority is done in 6 to 8 hours... discharge time though could be a deal breaker.

But maybe a long time from now when I have been able to test this theory out ... I will also want to know how much of an effect / improvement it will have ont he service life of the NiMH batteries... from what I've been reading ... if my theory holds true ... it would prolong the service life of the NiMH batteries for the effective lifetime of the driver.

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It is unlikely this theory will be tested before I go ahead with my PHEV battery pack plans ... and it is unlikely to effect it much... but... it is an idea I've been kicking around ... wondering where the data numbers will end up falling.


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