I agree NiMH still has potential... and they have proven to be one of the most tollerant of abuse batteries on the market... They are often times over charged and over discharged in common usage ... and in Hybrids they are usually under used staying above 20% SoC and bellow 80% SoC... of course they under use them as a simple and easy method of avoiding the longevity issues that come up from the complexity of top and bottom capacity usage.
I have more experience with the Gen-I Insight & Civic NiMH Hybrid battery packs than the Prius ones... Although I guess they are comparable ... the Prius ones might have a better wh / kg ratio due to the lighter weight plastic casing of each subpack... I know the Gen-I Insight and Civic 6 cell subpack sticks
with cylindrical metal ( D cell Shape ) casing ends up weighing in at 1.085 kg including the bolts, PTC strips, and plastic sheath... given the original 6.5Ah rating of the cylindrical cells that puts them at ~43Wh/Kg .... I know all HEV NiMHs are high power cells ... The Insight for instance pulls bursts up to 100 amp discharge rates ( over 15C ) from its 1999 year ( ~10 year old now ) technology NiMH cells ... and lab tests
on the Insight battery packs have shown internal resistance ( including all the connections in a 120 cell pack ) to be as low as 0.36Ohms... which puts individual cells in the range of ~3.0 mOhms ... of course it does vary with temperature as shown in the link above and it also varies with SoC shown here
... but still this results in a power density of about ~666W/kg .... and a Volumetric energy density of about ~139Wh/L.
My own personal experience here with my original 2000 Model year Insight Battery pack now has almost 130,000 Miles on it and 9 years... When I got an IMA error code last year ... the dealer offered to replace it under the 10 year , 150,000 mile Honda warranty ... but I declined ... I was curious and I had the means to properly test the pack to determine its condition myself... so I did ... I suspected that given a NREL
study of the Insight Gen-I battery pack temperature distribution , that the 120 cells were not all being treated equally ... and that I probably didn't need a new pack so much as maybe one or two new cells.... what I found first was that the SoC
in the pack was not balanced from 6 cell subpack stick to 6 cell subpack stick... this made sense because the Insight BCM ( Battery Control Module ) like many HEVs does not charge the cells to 100% ... so it never does the top off over charge trickle charge in order to try and balance them... and it doesn't drain them down to 0% SoC so they won't balance there... and it doesn't have any mechanism for controlling charge to individual subpacks or cells ( the way modern Li packs do )... so it made sense to me that given a varying Internal Resistance with Temperature , and a varying temperature in the pack during operation , and no apparent OEM method to balance the battery pack... So I rebalanced the SoC on my pack and put it back into my car... IMA error code went away ... and still hasn't come back yet almost a year latter... I have also over time tested several other Gen-I Insight and Civic battery packs... and this has shown to be common among all the Gen-I Insights and Civics ... I have traded some data with others who also have done similar tests ... and the results are consistent ... I don't know what the % is ... but it seems like a large % of the IMA error codes that people are getting are just a unbalanced SoC between subpacks... maybe a very small % actually have even 1 or 2 bad cells out of the whole 120 cell pack... this is probably one of the reasons Honda extended the battery warranty to 10 years / 150,000 miles in the U.S.
Or the D cells on my E-Bike... I have abused the heck out of those things for ~5 years now... I've soldered , unsoldered , resoldered the pack into different shapes like 3 times ... slow over charge on every charge cycle ... I know I've pulled some of the cells into voltage reversal several times as I have over discharged the whole 30 D cell pack several times ... I let them bake in the summer sun and heat ... freeze in the winter cold ... I pulled them out a little bit ago just to see how bad they are... They still have ~7Ah of usable capacity... that's crazy levels of tolerance to abuse in my book... No other battery I know of could I treat with a much abuse and still expect them to work at all.... eventually ( but not now ) I will build another battery pack for my E-Bike ... and right now , I think I will still go with NiMH ... although maybe I won't abuse the new pack quiet so much ...
Overcharge a Li battery... it will kill it very quickly ... depending on the type of Li you might get a fire ... Over discharge or pull into voltage reversal a Li battery and it will kill it very quickly ... Li bake in the summer sun also will kill it... 100%+ DoD cycles ... even A123 cells won't tolerate that kind of abuse as well as NiMH does.
As far as large number of cell pack designs go , NiMH also has the advantage of dying as an electrical short circuit ... a large number of cell series NiMH pack has 1 cell fail , the voltage drops but it keeps on trucking ... taking one for the team so to speak... while Li fails as an open circuit... so a large number of cell series Li Pack has 1 cell fail ... the whole series string turns off like x-mas lights... or the current you force through that one bad cell causes a fire.
Li does have better Wh/kg numbers than NiMH ... that I will give Li ... and Wh/kg has always been the #1 issue for 99% of people when it comes to battery powered anything .... so right off the bat ... Li has a large and significant advantage there... but in my book Li's advantages pretty much stop there... and in just about every other way NiMH continue to have competitive if not superior battery characteristics.
just my 2 bits
posted edited to correct typo 0.3 mOhms to correct value of 3.0 mOhms