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Discussion Starter · #1 · (Edited)
There is a lot of chat in car forums and groups about the mysterious 'IMA' battery and it's characteristics, capacity, longevity etc etc.

Enormous amounts of work has been done by enthusiasts in the last twenty years, and most of the details and secrets have been revealed.
For the start of this hopefully informative thread let's talk about the actual Nimh (Nickel Metal Hydride) IMA battery capacity using some simple maths and numbers....

63119


For now we will use a hypothetical perfect battery for our discussion and calculations to keep it simple.
Ignoring the Peukert effect, internal resistance and other weird battery things.

Battery capacity is usually measured in Ah (Ampere Hours) or Wh (Watt hours)
The instantaneous power W (Watts) a battery can deliver is calculated from it's voltage under load multiplied by the amps flowing in the circuit.
Ohm's Law is your friend here..

Your ancient (but heavy and reliable) technology 12V Pb (Lead Acid) battery at the front might be 40Ah. (12V x 40Ah = 480Wh Capacity)
When new and fully charged it can theoretically supply a current of 40 amps for one hour, 10 amps for 4 hours or 1 amp for 40 hours.
(Unfortunately the IMA cars don't keep them very well charged but that's another story!)

It can also usually supply a high current say 300 amps for 30 seconds or so to crank the car using the 12V backup starter when needed.
That's a peak power of 12V x 300A = 3.6kw (3600W)

The 84 x 1.2V cell IMA Battery in the back is a much higher voltage (100V) nominal in the CRZ, or about 8 x higher than the 12V battery.
The IMA Nimh cells have a nominal capacity when new of 5.75Ah, so we have a battery with a theoretical 100V x 5.75Ah capacity = 575 Wh.

The car when new allows a useable 75% of the 5.75Ah IMA capacity which equals 4.31Ah.
So our actual useable IMA Wh capacity is ~100V x 4.31Ah = 431Wh!
(That's less actual capacity than the 12V battery before we have even started :eek: )

But the special IMA battery cells are very good at delivering and accepting high power and current, which is how we get powerful assist and regeneration.
The CRZ Nimh IMA battery can deliver a peak power of around 100V x 125A = 12.5kw (12500w), but it can't do it for long as it only has a tiny capacity.

If your IMA battery is old and it's useable capacity is down to say 25% of the original we only have 1.44Ah available..
That's a paltry 144Wh when the battery is fully charged and gauge is full showing 8 bars :(

It doesn't take long to burn through that capacity when assisting or in autostop, especially with some auxiliary 12v loads operating.
If you enter autostop at say 50% SOC (Call it 4 bars) you have 0.72Ah or 72Wh available..

The DC-DC converter load on the IMA battery might well be 400W or more,
so you get about 10 minutes tops before the car restarts to charge itself up.

Throw in a few IMA starts and short bursts of assist as you move a few feet in a broiling summer traffic queue,
and your 72Wh of IMA battery capacity to quote the famous Nic Cage film title is......

63120


I'll post more as IMA stuff crops up or occurs to me.

Back to the Supercaps project now which actually have a pretty small capacity!! 150V x 1.5 Ah = 225Wh....
But they do have the ability to deliver enormous peak power and have a very high cycle life.
 

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There is a lot of chat in car forums and groups about the mysterious 'IMA' battery and it's characteristics, capacity, longevity etc etc.

Enormous amounts of work has been done by enthusiasts in the last twenty years, and most of the details and secrets have been revealed.
For the start of this hopefully informative thread let's talk about the actual Nimh (Nickel Metal Hydride) IMA battery capacity using some simple maths and numbers....

View attachment 63119

For now we will use a hypothetical perfect battery for our discussion and calculations to keep it simple.
Ignoring the Peukert effect, internal resistance and other weird battery things.

Battery capacity is usually measured in Ah (Ampere Hours) or Wh (Watt hours)
The instantaneous power W (Watts) a battery can deliver is calculated from it's voltage under load multiplied by the amps flowing in the circuit.
Ohm's Law is your friend here..

Your ancient (but heavy and reliable) technology 12V Pb (Lead Acid) battery at the front might be 40Ah. (12V x 40Ah = 480Wh Capacity)
When new and fully charged it can theoretically supply a current of 40 amps for one hour, 10 amps for 4 hours or 1 amp for 40 hours.
(Unfortunately the IMA cars don't keep them very well charged but that's another story!)

It can also usually supply a high current say 300 amps for 30 seconds or so to crank the car using the 12V backup starter when needed.
That's a peak power of 12V x 300A = 3.6kw (3600W)

The 84 x 1.2V cell IMA Battery in the back is a much higher voltage (100V) nominal in the CRZ, or about 8 x higher than the 12V battery.
The IMA Nimh cells have a nominal capacity when new of 5.75Ah, so we have a battery with a theoretical 100V x 5.75Ah capacity = 575 Wh.

The car when new allows a useable 75% of the 5.75Ah IMA capacity which equals 4.31Ah.
So our actual useable IMA Wh capacity is ~100V x 4.31Ah = 431Wh!
(That's less actual capacity than the 12V battery before we have even started :eek: )

But the special IMA battery cells are very good at delivering and accepting high power and current, which is how we get powerful assist and regeneration.
The CRZ Nimh IMA battery can deliver a peak power of around 100V x 125A = 12.5kw (12500w), but it can't do it for long as it only has a tiny capacity.

If your IMA battery is old and it's useable capacity is down to say 25% of the original we only have 1.44Ah available..
That's a paltry 144Wh when the battery is fully charged and gauge is full showing 8 bars :(

It doesn't take long to burn through that capacity when assisting or in autostop, especially with some auxiliary 12v loads operating.
If you enter autostop at say 50% SOC (Call it 4 bars) you have 0.72Ah or 72Wh available..

The DC-DC converter load on the IMA battery might well be 400W or more,
so you get about 10 minutes tops before the car restarts to charge itself up.

Throw in a few IMA starts and short bursts of assist as you move a few feet in a broiling summer traffic queue,
and your 72Wh of IMA battery capacity to quote the famous Nic Cage film title is......

View attachment 63120

I'll post more as IMA stuff crops up or occurs to me.

Back to the Supercaps project now which actually have a pretty small capacity!! 150V x 1.5 Ah = 225Wh....
But they do have the ability to deliver enormous peak power and have a very high cycle life.
Are you working on a battery swap? What about the software that controls and delivers the power?
 

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Discussion Starter · #4 · (Edited)
Nimh versus Lithium Part 1..

Note superior calendar longevity of Lithium versus Nimh cells has not yet been proven.


Some early IMA cars are happily trundling around with original Nimh packs that work and are over 20 years old.
We have at least another ten/twelve years to go with Lithium IMA cars before they catch up to that long a real world test.

On paper and looking at the chemistry as they age, I would predict that lithium will..
not last as long as the OEM Nimh sticks in terms of calendar ageing and average years of service.

For practical purposes of course Lithium is lighter and more powerful than Nimh.
So there is a big trade off / penalty / compromise between performance and expected lifetime.

This interesting article looks at calendar aged Lithium battery safety.

https://iopscience.iop.org/article/10.1149/1945-7111/ab89bf/pdf

Our old faithful OEM Nimh sticks def don't become unsafe as they age,
but they do lose capacity, develop higher IR, etc.
 

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Discussion Starter · #5 · (Edited)
Nimh versus Lithium Part 2.. (Following on from the above post which was written before I got my hands on a Lithium OEM pack) :unsure:

I've been lucky enough recently to drive my Nimh and Lithium pack CR-Z on the UK roads a fair bit.
Directly comparing the power levels, voltage, amps, SOC etc. I can say that at the moment.......

A 'good' Nimh pack might be better than an ageing Lithium one.

'Good' in this context means a new/low Nimh mileage pack or an older one which has been cycled a few times and with the useable capacity reset.
I am very pleasantly surprised at how good a warm rejuvenated CR-Z Nimh pack is compared to the weak ~60K miles Lithium one I have.

The OEM Lithium one is limited in its capacity, and def down on maximum IMA output....
It has much higher internal resistance than I was expecting, and the voltage sags a lot (~20V) when its cold.
It can't maintain full assist as long as my good Nimh pack and cannot be rejuvenated by cycling or grid charging etc.

With Lithium it's an inexorable downward path of cell chemical degradation, lost capacity and higher internal resistance. :cry:
With Nimh it's similar of course, but a lot slowerrrrrr, and you can periodically reactivate and rebalance the cells to a degree with cheap equipment.

Nimh also withstands gross abuse much better than Lithium and is intrinsically safer.. (It's less reactive..)

So my advice for now is don't all rush out and buy a wrecker OEM Lithium pack to drop into your Nimh car just yet. o_O
More testing and evaluation is definitely needed.

(PS I do need another UK OEM Lithium pack for parts if anyone spots one in a breakers) Thanks. (y)
 

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They are very interesting findings.. I wonder if it's a one-off with that particular cars, of if this will be the trend going forwards?
 

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Discussion Starter · #7 · (Edited)
Note. I am not saying Lithium is bad per se.. :geek:

A Lithium pack is a lot lighter than Nimh watt for watt, and in good condition can deliver tons of power. See Tesla 'Insane Mode' 0-60 mph stats for details!!!:eek:

Lithium is the way I will be heading due course for the 30kw+ Frankenstein IMA project simply because it's lighter and much more compact etc!

I do not expect my super pack project to last or be economically sound, but I will be expecting excellent performance for a few years...
 

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Oh I agree; technology wise it is a no-brainer.

But if you took two stock cars, an early Lithium and a Nimh, the chances are that in average condition there is little between the two. Going forwards it's likely to become beneficial running a Nimh as you can perform mains charging and reset commands that you have highlighted.

With that in mind, If I were going to buy a car to run for ~5 years I'd know which one I would pick.
 

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Discussion Starter · #9 · (Edited)
With that in mind, If I were going to buy a car to run for ~5 years I'd know which one I would pick.
Nimh is well understood and easy to fix and fiddle with.
New aftermarket cells are available for a relatively simple overhaul.

Lithium is much more difficult due to the scarcity of parts (Not many cars, no new cells apart from complete OEM packs $$$$$).
Lack of available technical information (No hard copy workshop manuals, only online very expensive subscription access).
More complicated cell block/pack assembly/construction.
 

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Discussion Starter · #10 · (Edited)
Nimh longevity.

Having experience of Honda Nimh IMA packs across several models and nearly 20 years,
I wanted to say something about the reasons for the longevity and reliability of the CR-Z Nimh packs. (y)

Note the same Nimh cells and pack/ima setup are used in the Insight G2 and Jazz Hybrid as well as the CR-Z.

1) The Cells.

The sticks/cells themselves are 3rd or 4th generation premium grade top quality 5.75ah Nimh types made by Panasonic (Japan) in huge quantities.
The construction and assembly of the sticks is first rate and they are well capable of 125A discharge and 75A charge rates.
That's impressive, especially for what is basically a glorified D Cell torch battery....
There are several detailed Honda research papers online about the design and evolution of these sticks from one generation to the next.

2) Thermal Management.

The cell sticks are assembled into rugged plastic ventilated cases for safety and security..
There has been a variety of case designs with varied airflow patterns, orientation in the vehicle, and fan cooling setups.
The CR-Z Case is a 5th generation good spacious design with plenty of gap between sticks and large airflow passages. :)

It was preceded by the G1 Insight, HCH1 Civic, HCH2 Civic & the Accord. These earlier designs had significant hot spots and grossly uneven air distribution.
This gave rise to unequal stick/cell ageing as the charging/discharging efficiency and chemical degradation depends a lot on temperature.
Unequal ageing leads to charge imbalance, which exacerbates the above, and they get into a catch 22 circle of deterioration.

Cell has high internal resistance = cell gets hot during use = chemical degradation speeds up = higher internal resistance = goto start of line!

The cooling fans used have varied from the anemic 12v computer PSU type on the G1 Insight to the leaf blower on the HCH1 Civic onwards.
The later 3 phase pwm controlled fans offer high airflow and positive pressure at the expense of fan noise and power consumption.

The fan control mechanism has changed drastically, and the way the battery control module (BCM) energises and controls the fan has significantly improved.
In the early cars the fan would not try and warm the pack with ambient air unless it was extremely cold. Below freezing..
Also it would often not engage early or fast enough at higher temperatures to prevent the battery overheating. :cry:

The CR-Z on start up will immediately try to warm up the pack if the temperature is low and there is enough differential between the pack and the ambient air in the car.
The running of the fan on slow a lot of time avoids hot or cold spots developing, and brings the pack quickly into it's optimal temperature range of between about 15-30C

The fan has a big reserve of power, and on high shifts some serious air which can cool a hot pack or IPU pretty quickly.
The fact the blower fan is used at varied output a lot more than the earlier ones may go some way to explain why they fail more often.
Plus the fan electronic drive design might be a little weak itself.

Watching the temperature sensors in the battery the CR-Z fan is able to keep the three sensors to within a couple of degrees of each other. :)
In the earlier cars there could easily be 5/10C difference between cells/sticks in the top and bottom of a pack. :rolleyes:

3) The Honda Fudge Factor.

The battery control module is constantly assessing the state of the battery balance etc and adjusts a figure called the useable capacity as it varies.
It does this by detecting when a cell is full or when a cell is empty as it charges and discharges during use.
The Ah capacity between the empty and full readings gives the computer the maximum pack capacity available..
It then graciously allows us to use a part of this capacity for our SOC window.

As the pack ages this window gets smaller and the OEM bar display re-scales itself to this smaller and smaller capacity window.
It goes up and down faster, but most owners don't notice as the change is so gradual.
By not allowing the pack to be pushed hard it can operate with a slowly dwindling capacity for many years.

Luckily we know good old Nimh can be rejuvenated with some (cycling) low current charge/discharging to reactivate dormant/lazy cell material and a system reset. .

Conclusion. A strong Nimh pack is surprisingly good for day to day use in our cars.

63904


The attached pdf is an expert evaluation of a 160,000 mile Insight G2 Nimh pack.. Impressive..... (y)
 

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My 2012 crz has 267583km never had a check engine or ima fault (knock on wood)
But I lost my job at the start of this pandemic so she got parked and sat for a year. Now that I'm not living inside it I can afford to get her back on the highway.
The battery has been pretty weak though, I don't make it halfway up a gradual hill before I'm out of assist and she starts charging. I pulled the module out and checked each pair of cells and they seem to all be around 16.1v.
So should I try to revive these cells? Would I even notice an improvement? Or should I just order a new pack?

-gromyjoe
 

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If car has not been driven for a while IMO all it needs is a bit of exercise for the cells( A few long continuous drives in all modes) and maybe an under hood battery. If it does not respond to that then follow the advice by @PeterPerkins on the process to revive the cells. Only 3 members here have had to replace the IMA battery.

My 2012 crz has 267583km never had a check engine or ima fault (knock on wood)
But I lost my job at the start of this pandemic so she got parked and sat for a year. Now that I'm not living inside it I can afford to get her back on the highway.
The battery has been pretty weak though, I don't make it halfway up a gradual hill before I'm out of assist and she starts charging. I pulled the module out and checked each pair of cells and they seem to all be around 16.1v.
So should I try to revive these cells? Would I even notice an improvement? Or should I just order a new pack?

-gromyjoe
 
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