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Just a few thoughts on M cell size NiMH batteries in relation to the Gen-1 Honda Insight.
The M cell size is the size one full step larger than the F ...
AAAA ( the size in '9v' batteries )
AAA
AA
A
C
D
F
M
Like the other sizes there can be slight variation between different manufactures even in the same size and same chemistry... different Capacity , size details , weights, power rating, etc.
The sample of 2 cells I picked up a few years ago to test out as a potential candidate for future use in my Gen-1 Insight were.
~400grams each
~42mm Outside Diameter
~91mm Long
Rated at 26Ah
I pulled them out the other day ... to consider the pros and cons vs the A123 20Ah rated pouch cells I bought more recently ( with the group buy we had here in InsightCentral ).
In theory they could be a near drop in replacement ... would still need a SoC fooling device similar to Peter's ... sense it was discovered by others here in Insight Central that OEM the car will cap you to a limited amount of useable battery capacity no matter how much you actually have.
They are significantly larger than the D cell size ... but due to all the extra space open space in a OEM Battery box one should be able to nest 120 cells into that space with little modification... which on that front would put them near the the A123 in terms of space occupied in the vehicle.
Charging is simpler with NiMH chemistry because they are more tolerant of abuse with things like over charging and over discharging... the OEM had rated these M cells to sustain 3A over charge for 26 days without cell rupter or venting ... just heating up ... and as long as the heat produced was dissipated they would not be toasted or significantly harmed from just an event ... of course such events on a regular basis could potentially reduce the usable cycle life into the 300 to 500 cycle range ... but not a death sentence like such a 26 day 3 Amp over charge would be to a A123 20 Ah pouch cell... slower rates at even lower amps will over charge less , are less stressful , and have less negative effects ... and of course like Panasonic did for Honda in the Insight ... narrowing the SoC / DoD usage band closer to the middle ... 90-10 , 80-20 , 70-30 ... etc ... will greatly increase the cycle life of the cells.
They are also very tolerant of over discharging and even the occasional cell reversal ... although of course best to avoid if one can.
So one could potentially save a bit of money $ and complexity in a far simpler electronics for a M cell based battery pack.
Also there is very slight total capacity benefit ... the 120 M 26Ah cells vs the 50 A123 20Ah cells ... the NiMH M cells would have about ~17% more total capacity in that same vehicle space.
Those are all good things ... but there are significant cons as well.
The best pricing I know of for NiMH M cells like the 26 Ah kind I got ... would cost about 3x as much for the 120 M 26Ah cells compared to 50 of the A123 20Ah pouch cells... that's a major negative.
Another is that while the volume used in the vehicle may be virtually the same between the two options ... the NiMH 26Ah M cell's lower energy density would mean that it would weight almost 2x as much ( ~25 more kg ) ... compared to the 50 A123 20Ah pouch cells... which is another major negative for such a option.
As far as I know ... none of the Low Self Discharge manufactures of NiMH are yet producing in the M size ... largest I've seen is the D size ... which means that such a 120 cell build will have significantly more self discharge than the A123 cells ... this is another negative as long as the LSD-NiMH are not used ... LSD-NiMH would be fairly similar self discharge compared to the A123.
Both are able to have cycle efficiencies in the mid to upper 90% range ... but to use a trickle over charging method to balance the NiMH will waste a lot of energy and can bring that down a great deal... of course using a bleeding resistor based BMS to top end balance the A123 would be no better than the NiMH top end wasted heat either.... but there is a few slight difference to consider there as well.
There is also something to be said about the difference in Peukert effects ... meaning ... the faster one discharges a given battery cell ... the less % of Ah or Wh you get from it ... the chemical reactions are slower than the electronics ... you could let the fast discharged cell rest for a while and come back to get a bit more out and so forth to narrow the gap ... but the effect is due to the chemical reactions dispersion in the medium in the cell being much slower than the rate of the electronics pulling energy from the cell.
Tests of the M cells showed that at fairly lower Amp rates ~10A ... they still maintained more total usable pack energy than a A123 pack would have had.
However there are many people who have tested the A123 20Ah Pouch cells at very high amp rates ... and shown very little Peukert usable energy loss ... Although to be fair I haven't done it yet ... eventually on my list to do is to run a higher amp series of tests on the M cells to see how well they hold up ... but until I do that ... not all NiMH cells , but most NiMH cells have significant Peukert effects from higher power rates ... which is something to consider in a vehicle application with the potential for higher power rates.
So while there are some pros ... over all I think the cons would out weigh the pros for most people ... ~2x weight , and ~3x $ ... seeming to be the 2 biggest cons of the 26Ah M cell.
The M cell size is the size one full step larger than the F ...
AAAA ( the size in '9v' batteries )
AAA
AA
A
C
D
F
M
Like the other sizes there can be slight variation between different manufactures even in the same size and same chemistry... different Capacity , size details , weights, power rating, etc.
The sample of 2 cells I picked up a few years ago to test out as a potential candidate for future use in my Gen-1 Insight were.
~400grams each
~42mm Outside Diameter
~91mm Long
Rated at 26Ah
I pulled them out the other day ... to consider the pros and cons vs the A123 20Ah rated pouch cells I bought more recently ( with the group buy we had here in InsightCentral ).
In theory they could be a near drop in replacement ... would still need a SoC fooling device similar to Peter's ... sense it was discovered by others here in Insight Central that OEM the car will cap you to a limited amount of useable battery capacity no matter how much you actually have.
They are significantly larger than the D cell size ... but due to all the extra space open space in a OEM Battery box one should be able to nest 120 cells into that space with little modification... which on that front would put them near the the A123 in terms of space occupied in the vehicle.
Charging is simpler with NiMH chemistry because they are more tolerant of abuse with things like over charging and over discharging... the OEM had rated these M cells to sustain 3A over charge for 26 days without cell rupter or venting ... just heating up ... and as long as the heat produced was dissipated they would not be toasted or significantly harmed from just an event ... of course such events on a regular basis could potentially reduce the usable cycle life into the 300 to 500 cycle range ... but not a death sentence like such a 26 day 3 Amp over charge would be to a A123 20 Ah pouch cell... slower rates at even lower amps will over charge less , are less stressful , and have less negative effects ... and of course like Panasonic did for Honda in the Insight ... narrowing the SoC / DoD usage band closer to the middle ... 90-10 , 80-20 , 70-30 ... etc ... will greatly increase the cycle life of the cells.
They are also very tolerant of over discharging and even the occasional cell reversal ... although of course best to avoid if one can.
So one could potentially save a bit of money $ and complexity in a far simpler electronics for a M cell based battery pack.
Also there is very slight total capacity benefit ... the 120 M 26Ah cells vs the 50 A123 20Ah cells ... the NiMH M cells would have about ~17% more total capacity in that same vehicle space.
Those are all good things ... but there are significant cons as well.
The best pricing I know of for NiMH M cells like the 26 Ah kind I got ... would cost about 3x as much for the 120 M 26Ah cells compared to 50 of the A123 20Ah pouch cells... that's a major negative.
Another is that while the volume used in the vehicle may be virtually the same between the two options ... the NiMH 26Ah M cell's lower energy density would mean that it would weight almost 2x as much ( ~25 more kg ) ... compared to the 50 A123 20Ah pouch cells... which is another major negative for such a option.
As far as I know ... none of the Low Self Discharge manufactures of NiMH are yet producing in the M size ... largest I've seen is the D size ... which means that such a 120 cell build will have significantly more self discharge than the A123 cells ... this is another negative as long as the LSD-NiMH are not used ... LSD-NiMH would be fairly similar self discharge compared to the A123.
Both are able to have cycle efficiencies in the mid to upper 90% range ... but to use a trickle over charging method to balance the NiMH will waste a lot of energy and can bring that down a great deal... of course using a bleeding resistor based BMS to top end balance the A123 would be no better than the NiMH top end wasted heat either.... but there is a few slight difference to consider there as well.
There is also something to be said about the difference in Peukert effects ... meaning ... the faster one discharges a given battery cell ... the less % of Ah or Wh you get from it ... the chemical reactions are slower than the electronics ... you could let the fast discharged cell rest for a while and come back to get a bit more out and so forth to narrow the gap ... but the effect is due to the chemical reactions dispersion in the medium in the cell being much slower than the rate of the electronics pulling energy from the cell.
Tests of the M cells showed that at fairly lower Amp rates ~10A ... they still maintained more total usable pack energy than a A123 pack would have had.
However there are many people who have tested the A123 20Ah Pouch cells at very high amp rates ... and shown very little Peukert usable energy loss ... Although to be fair I haven't done it yet ... eventually on my list to do is to run a higher amp series of tests on the M cells to see how well they hold up ... but until I do that ... not all NiMH cells , but most NiMH cells have significant Peukert effects from higher power rates ... which is something to consider in a vehicle application with the potential for higher power rates.
So while there are some pros ... over all I think the cons would out weigh the pros for most people ... ~2x weight , and ~3x $ ... seeming to be the 2 biggest cons of the 26Ah M cell.