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bwilson4web · 12-05-2009, 10:48 AM

Hi,

After some PM discussions about Prius battery modules, it is time to discuss 'lessons learned' about them:

The upper module came from the NHW11 Prius, 2001-03, and the lower one from the NHW20, 2004-09, the ZVW30, 2010-current, and possibly other Toyota hybrids. Significant changes between the modules:

Substantially stronger case - in particular the terminals
Offset terminals - no mixing of modules
Lower internal resistance - the NHW20 carries more current with less loss
Identical Ahr rating
Mold release - the non-conductive, silvery material

The only Toyota report about traction battery failures indicates the NHW11 ones have been running ~1%. Given a USA vehicle salvage rate of ~3%, salvage battery packs should last many years. In contrast, the NHW20 failure rate is at least one and possibly more orders of magnitude lower, 0.1%-0.01%. I bought a failed traction battery pack in 2007 to understand what causes these failures.

Research before starting this effort:

Toyota patent on battery refurbishment - there is a well written patent that describes how the traction battery is weighted to measure electrolyte loss, a hole drilled to add a 6 mole, KOH solution, and plastic weld close the module. Then the battery is 'reactivated.'
TIS battery sealing - there was an early problem with electrolyte leakage causing a ground-fault. Toyota issued a TIS that takes the traction battery out and applies a 'glue' to help seal the terminals.

Now NiMH battery chemistry is different than ordinary battery chemistry because it stores charge as hydrogen gas trapped in a metal hydride matrix. The other side uses two forms of nickel hydroxide to hold the oxygen:

The metal hydroxide, a spongy mass, is separated by a 'plastic cloth' from the nickel hydroxide coated electrodes. The KOH electrolyte carries the charged ions between the two electrodes. The permanent failure mode is the electrolyte gets too low and spot heating melts the plastic separator, a permanent short. Given each NiMH cell generates 1.2 VDC, this difference is the signature of a failed module that can not be repaired. The battery module has to go to the recycler.

Charging NiMH batteries generates atomic hydrogen that 99% enters the metal hydride matrix until discharge. But a little of the hydrogen and oxygen escapes the metal hydride matrix and nickel hydroxide electrodes. During charging, this pressurizes the cell, which means it has to be mechanically clamped:

The rates of gas generation are fairly low, these are not great internal pressures. But overcharging has to be avoided because if the metal hydride is saturated, the modules become a high pressure, gas container and bad things can happen:

There are several approaches to limit overcharging:

pressure spike - the penultimate, a pressure switch in the cell or a compression load sensor stops charging when the pressure takes a spike. This is based upon direct measurement of excess gas generation. A load-cell would be an excellent signal source.
dV/dt - just as the NiMH cell reaches peak charge, there is spike up followed by a slight drop in voltage. The slight drop can be detected by smart chargers like RC hobby units and charging stops. However, a stack of cells becomes a problem because they all won't reach the same peak charge at the same time.
temperature rise - the excess H{2} and O{2} gas will recombine and generate heat. This temperature spike can be used to stop charging but by then, the cell is already overcharged. Cheap NiMH chargers use this and better hobby chargers use this as backup.

This survey of overcharge protection approaches is a short list of what I've learned from the RC community about NiMH batteries and Isidor Buchmann's excellent book, "Batteries in a Portable World." But I wanted to find out if I could replicate Toyota's battery refurbishment patent.

I was able to refurbish NHW11 modules BUT I do not recommend it because it does not fix the leaky terminals. A refurbish NHW11 module is just going to lose the H{2} and O{2} gas over time and have to be refurbished again and again and again. A better solution is to use NHW20 modules and given a 3% salvage rate, they should be readily available and less expensive than the NHW11 modules.

Bob Wilson

12-05-2009, 10:48 AM	#1 (permalink)
bwilson4web Engineering first Join Date: Mar 2009 Location: Huntsville, AL Posts: 843 17 i3-REx - '14 BMW i3-REx Last 3: 45.67 mpg (US) Blue Bob's - '19 Tesla Std Rng Plus Thanks: 94 Thanked 248 Times in 157 Posts	Prius traction batteries Hi, After some PM discussions about Prius battery modules, it is time to discuss 'lessons learned' about them: The upper module came from the NHW11 Prius, 2001-03, and the lower one from the NHW20, 2004-09, the ZVW30, 2010-current, and possibly other Toyota hybrids. Significant changes between the modules: Substantially stronger case - in particular the terminals Offset terminals - no mixing of modules Lower internal resistance - the NHW20 carries more current with less loss Identical Ahr rating Mold release - the non-conductive, silvery material The only Toyota report about traction battery failures indicates the NHW11 ones have been running ~1%. Given a USA vehicle salvage rate of ~3%, salvage battery packs should last many years. In contrast, the NHW20 failure rate is at least one and possibly more orders of magnitude lower, 0.1%-0.01%. I bought a failed traction battery pack in 2007 to understand what causes these failures. Research before starting this effort: Toyota patent on battery refurbishment - there is a well written patent that describes how the traction battery is weighted to measure electrolyte loss, a hole drilled to add a 6 mole, KOH solution, and plastic weld close the module. Then the battery is 'reactivated.' TIS battery sealing - there was an early problem with electrolyte leakage causing a ground-fault. Toyota issued a TIS that takes the traction battery out and applies a 'glue' to help seal the terminals. Now NiMH battery chemistry is different than ordinary battery chemistry because it stores charge as hydrogen gas trapped in a metal hydride matrix. The other side uses two forms of nickel hydroxide to hold the oxygen: The metal hydroxide, a spongy mass, is separated by a 'plastic cloth' from the nickel hydroxide coated electrodes. The KOH electrolyte carries the charged ions between the two electrodes. The permanent failure mode is the electrolyte gets too low and spot heating melts the plastic separator, a permanent short. Given each NiMH cell generates 1.2 VDC, this difference is the signature of a failed module that can not be repaired. The battery module has to go to the recycler. Charging NiMH batteries generates atomic hydrogen that 99% enters the metal hydride matrix until discharge. But a little of the hydrogen and oxygen escapes the metal hydride matrix and nickel hydroxide electrodes. During charging, this pressurizes the cell, which means it has to be mechanically clamped: The rates of gas generation are fairly low, these are not great internal pressures. But overcharging has to be avoided because if the metal hydride is saturated, the modules become a high pressure, gas container and bad things can happen: There are several approaches to limit overcharging: pressure spike - the penultimate, a pressure switch in the cell or a compression load sensor stops charging when the pressure takes a spike. This is based upon direct measurement of excess gas generation. A load-cell would be an excellent signal source. dV/dt - just as the NiMH cell reaches peak charge, there is spike up followed by a slight drop in voltage. The slight drop can be detected by smart chargers like RC hobby units and charging stops. However, a stack of cells becomes a problem because they all won't reach the same peak charge at the same time. temperature rise - the excess H{2} and O{2} gas will recombine and generate heat. This temperature spike can be used to stop charging but by then, the cell is already overcharged. Cheap NiMH chargers use this and better hobby chargers use this as backup. This survey of overcharge protection approaches is a short list of what I've learned from the RC community about NiMH batteries and Isidor Buchmann's excellent book, "Batteries in a Portable World." But I wanted to find out if I could replicate Toyota's battery refurbishment patent. I was able to refurbish NHW11 modules BUT I do not recommend it because it does not fix the leaky terminals. A refurbish NHW11 module is just going to lose the H{2} and O{2} gas over time and have to be refurbished again and again and again. A better solution is to use NHW20 modules and given a 3% salvage rate, they should be readily available and less expensive than the NHW11 modules. Bob Wilson __________________ 2019 Tesla Model 3 Std. Range Plus - 215 mi EV 2017 BMW i3-REx - 106 mi EV, 88 mi mid-grade Retired engineer, Huntsville, AL Last edited by bwilson4web; 12-05-2009 at 11:04 AM..