An overview of the findings of the PriusPlus group who are attempting something similar:
(BTW, most people on here may have picked up on the fact that I am a big fan of lithium, so here is my favourite quote from this review!
"Given today’s cell availability, NiMH technology is just as hard and expensive to implement as LiIon, but with 1/2 the value." :wink: )
1. The state of CalCars’ NiMH battery pack
1.1. Procurement status
1.1.1. With the exception of the Saft NHP10-340, which is not in
volume production, and possible Chinese cells of unknown quality,
we have found no appropriate cells
1.1.2. Even huge battery companies like Saft and Cobasys produce
only one or two sizes of high-rate-discharge cells (Cobasys’ are
too large and heavy for a PRIUS+).
1.1.3. A custom D cell pack has been our fallback possibility
1.1.3.1. We have found a promising technology (anodized aluminum
tubes) for building a thermally acceptable battery pack from D
cells, but . . .
1.1.3.2. 3-4 parallel strings of (176-182) cells are required, and:
1.1.3.2.1. Lots of electronics are necessary to assure no
problems occur due to paralleling NiMH strings, to assure that
no cells are reverse-voltaged due to capacity and charge
balance variations, to detect and minimize charge imbalances,
and to detect end-of-charge. This discipline is of the same
order of complexity and expense as that required for a LiIon pack.
1.1.3.2.2. All NiMH D cells that I’ve tested have double their
expected internal resistance. This means that to better the
Prius’ OEM battery in this respect – required to avoid an
increase in hybrid gasoline consumption with the PHEVs
additional weight – will require four parallel strings of one
of the three best types of D cells tested, plus at least one
string of Maxwell D cell supercapacitors in parallel.
This solution will cost over $10,000 plus the control/monitoring
electronics, as two of the three usable types of D cells cost $10.71
each, the third has 15% less capacity than the others, and each string
of supercapacitors costs $2400.
1.1.3.2.3. Charge imbalance solutions require careful
monitoring of substrings of 8-13 cells. If individual 1.2V
cells have low capacity or get out of balance with their
peers, little can be done beyond reducing the imbalance via
additional overcharging (very limited without shortening the
battery’s cycle life) and/or reducing the effective capacity
of the pack.
1.1.3.2.4. Thermal management of the pack is critical, as
1.1.3.2.4.1. Charge imbalance is greatly increased by
temperature differentials between cells.
1.1.3.2.4.2. Cycle life is halved by every 10 degrees C
above room temperature during operation.
1.1.3.2.4.3. Double the expected internal resistance means
double the pack’s creation of heat.
1.1.3.2.5. Supercapacitors may be a partial solution to the
internal resistance limitations, but . . .
1.1.3.2.5.1. They have not been tested yet for effectiveness
or how many strings are required in real-world driving
situations
1.1.3.2.5.2. They add cost, weight, and volume: per string,
$2400, 25 lb, and half the volume of a string of NiMH D cells.
1.1.3.2.6. After all the work and expense of designing and
custom building such a pack and its electronics, there is no
guarantee that it would work well and have a decent cycle life.
1.2. The auto companies may be moving toward LiIon packs for their
hybrid vehicles
1.2.1. Why I think so
1.2.1.1. LiIon cells have around twice the specific energy of
NiMH cells, and triple the cell voltage.
1.2.1.2. There are now LiIon cells capable of high rate
discharge and high rate intermittent (regen braking) charge
1.2.1.3. Thermal runaway problems and propensity to explode have
been solved for small, quality battery packs.
1.2.1.4. There are companies touting their LiIon systems as
hybrid batteries.
1.2.1.5. Existing hybrid vehicles are, due to the automotive
development cycle, from designs at least six years old
1.2.1.6. There are rumors of LiIon-based hybrids in the pipeline.
1.2.2. What it means
1.2.2.1. The NiMH chemistry is more of a lame duck technology
than we imagined
1.2.2.2. At best, only NiMH cells that are already in automated
volume production can be expected be available in the future.
1.2.2.3. Future price reductions or product improvements are
unlikely.
1.2.2.4. Product discontinuations must be anticipated.
1.2.3. What it means for CalCars
1.2.3.1. Given today’s cell availability, NiMH technology is
just as hard and expensive to implement as LiIon, but with 1/2 the
value.
1.2.3.2. LiIon safety issues for vehicular battery packs either
have been or are soon to be solved, probably in various
industry-acknowledged ways.
1.2.3.3. By the time CalCars has tested an NiMH PRIUS+, auto
companies will most likely be viewing LiIon technology as
mainstream, too.
1.2.3.4. At this point, it apprears to make most sense to skip
NiMH altogether.
