Reconditioning the Gen 2 Prius HV battery

The Problem

So I've had a Generation 2 Toyota Prius since 2004. Coming up on 17 years old now in Australia, and recently I finally had what turns out to be the dreaded PA080 fault code get thrown - this is a general hybrid traction battery error.

Since the battery is relatively expensive compared to the value of the car and I don't like spending money anyway, the question becomes what can we do about this?

DIY Reconditioning

Fortunately, the car is old enough that this problem has happened before. Over at https// and elsewhere on the web, people have disassembled then Prius traction battery and fixd this problem themselves.

There are basically 2 issues at play: general NiMH degration, and polarity reversal - cell failure.

Cell Failure

In general, the PA080 code (at least in my experience), happens when a battery module will suddenly drop its voltage by over 1V.

This happens due to a phenomenon in NiMH cells called "polarity reversal" - characterized by a discharge curve like this one:


It is what it sounds like: under extreme discharge conditions, the NiMH cell will go to 0, and if left in this state for too long (or in a battery pack where current continues to be pulled through the cell) it will then enter polarity reveral - positive becomes negative, negative becomes positive. This is disasterous in a normal application, and devastating in a battery pack as the cell now gets driven in this condition by regular charging to continue soaking up current producing heat.

At this point, the cell is dead. In a Prius battery module of 6 cells, a reduction in voltage of about 1V means you know you've had a cell drop into reverse polarity and its not coming back.

NiMH battery cells primer

It's important to understand NiMH cells to understand why "battery reconditioning" is possible and advisable.


Standard NiMH battery chemistry has a nominal voltages of 1.2V. This has little bearing on the real voltages you see with the cells - a fully charged cell goes up to 1.5V, considered to be the absolute top and you're evolving hydrogen at that point - and a single, standalone cell, can be take all the way to 0V (this is not safe - miss the mark and you wind up in polarity reversal).

In a battery pack of NiMH cells, these lower limits are higher for safety: pack cells all have slightly different capacities, and once you hit 0V on one, if the others don't hit 0V at the exact same time then the empty ones will get driven into polarity reversal. At roughly 0.8V you start running into a cliff of voltage decay anyway, so that's generally the stopping point.

The graph below is an excellent primer on the voltage behaviors of NiMH at different states of charge. Note that the nominal voltage is measured right before the cell is practically empty, but for most of its duration voltage is very constant - almost linear - until the cell is almost full.


Degradation Mechanisms

The above explains the behavior of NiMH cells, but not why we can recondition them in a vehicle like the Prius. To understand this, we need to understand the common NiMH battery degradation mechanisms.

NiMH chemistry is based on the following 2 chemical reactions:

Anode: $\ce{H2O + M + e^- <=> OH^- + MH}$

Cathode: $\ce{Ni(OH)2 + OH^- <=> NiO(OH) + H2O + e^-}$

Note the M: this is an intermetallic compound, rather then any specific metal is essentially where a lot of the R&D in NiMH batteries goes.

Our target of recovery is the cathodic reaction involving the Nickel. In normal operation the Prius runs the NiMH batterys between 20-80% of their rated capacity. This is, in general, the correct answer - deep discharging batteries causes degradation of the electrode materials which is a permanent killer (over the order of 500-1000 cycles though).

Crystal Formation

The problem enters with an issue known as "crystal formation" when the batteries are operated in this way over an extended period. Search around and you'll see this referenced a lot without a lot of explanation and mostly in context of Nickel-Cadmium (NiCd) batteries.

NiMH's were meant to, and were a huge improvement on, most of the "memory effect" degradation mechanisms of NiCd batteries, however some of the fundamental mechanisms involved still apply as they are still based on the same basic active materials on the cathode - the Nickel Hydroxide and Nickel oxide hydroxide.

There are many, many mechanisms of permanent and transient change in NiMH batteries, but there are 2 identified which can be treated by the deep charge-discharge cycle recommended for reconditioning.

One is that observed by Sato et. al.: nickel oxide hydroxide has 2 primary crystal structures when used in batteries - β‐NiOOH and γ‐NiOOH.

β‐NiOOH and γ‐NiOOH are generally recognized as being two in-flux crystal states of the Nickel electrodes of any nickel based battery with a (simplified) schema looking like the following:


γ‐NiOOH is the bulkier crystal form, and has more resistance to hydrogen ion diffusion - this is important because the overall ability of the battery to be recharged is entirely dependent on the accessibility of the surface to $\ce{H^+}$ ions to convert it back to $\ce{Ni(OH)2}$.

What Sato et. al. observes is that during shallow discharging and overcharging of NiCd cells, they see a voltage depression effect correllated with a rise in γ‐NiOOH peaks on XRD spectra. When they fully cycled the cells, the peaks disappeared - the γ‐NiOOH crystals over several cycles are dissolved back to $\ce{Ni(OH)2}$ during the recharge cycle.

SEM photographs captured at 10 μm of the positive plates of (a) a good battery, (b) an aged battery, and (c) a restored battery. Note: these were NiCd's, but a similar process applies to the nickel electrode of an NiMH cell.

Source |

Although the Prius works hard to avoid this sort of environment - i.e. the battery is never overcharged - it's worth remembering that the battery is not overcharged in aggregate - but it's a physical system, with a physical environment. Ions need to move around in solution, and so while in aggregate you can avoid ever overcharging a cell - on a microsopic levels through random change every now and again an overcharge-like condition can manifest. That said - it took my car 17 years to get to this point.

There's more detail to this story - a lot more - and pulling a complete picture out of the literature is tricky. For example the γ‐NiOOH phase isn't considered true γ‐NiOOH but rather γ'‐NiOOH - the product of Nickel intercalating into γ‐NiOOH, rather then potassium ions (from the potassium - $\ce{K^+}$ used as electrolyte in the cell). It's also a product of rest time on the battery - the phase grows when the battery is resting in a partly charged state.

The punchline of all of this is the reason Prius battery reconditioning works though: the Prius is exceptionally good at managing its NiMH cells, and mostly fights known memory effects while driving. However, it can't fight them all the time and with time and age you wind up with capacity degradation due to crystal formation in this ~50% state-of-charge (SOC) range. And importantly: it's experimentally shown that several normal cycles is highly effective at restoring it by dissolving away the unwanted phase.


There's a secondary degradation mechanism that's worth noting for those who have seemingly unrecoverable cells in a Prius: dehydration.

Looking again at the NiMH battery chemistry -

Anode: $\ce{H2O + M + e^- <=> OH^- + MH}$

Cathode: $\ce{Ni(OH)2 + OH^- <=> NiO(OH) + H2O + e^-}$

you can see that water - $\ce{H2O}$ - is involved but not consumed in the reactions. This is also kind of transparently obvious: you need an electrolyte for ion exchange. What is not obvious though is that the situation under battery charging is technically a competitive with a straight electrolytic water-splitting reaction:

$\ce{2H2O <=> 2H^2 + O^2}$

This is a known problem - though largely resolved from normal recombinative processes in the battery (having a shared gas headspace allows the H2 and O2 to recombine back into water) and can be assisted by adding specific recombination chemistry and normally just resembles a loss function on charging the cells, simply producing heat.

This is a tradeoff in battery design: a sealed cell doesn't leak gas, which ensures it can eventually recombine. But a sealed cell can overpressure and rupture, at which point the cell is destroyed. The Prius cells are not sealed - a one-way overpressure blow off valve is present which vents at 80-120 psi - 550-828 kPa (this is substantial) - and the cells themselves depend on being clamped to prevent gas pressure from damaging them during charging.

But the result is the same: failed seals or overheated cells over a long duration may have lost water through either electrolysis processes.

There are ways to fix this sort of failure - and the results are spectacular - but this is definitely into "last resort for experimentalists" sort of intervention. Typical NiMH design uses a 20-40% w/v KOH solution in water. LiOH is added to improve low temperature performance, and NaOH is substituted partially or fully for reduced corrosion in high temperature applications.

Per this link 30% w/v KOH and 1.5 g/L LiOH is suggested. For the purposes of cell rehydration, an exact match is probably not important as a "dried out cell" will still contain all its salt components (though depending on redissolving them may not be the best option). A starting point for other mixes might be this paper which concludes a 6M KOH solution is optimal.

The big results reported over by this PriusChat member for anyone considering this are here - where he notes he used 20% KOH. Of note: getting deionized water, and a suitably un-metal contaminated salt, is probably key to success here (as well as sealing up the cells properly - the trickiest part by all accounts). That said - various metal dopants are used in NiMH cells to contribute all sorts of properties, so this may be a small effect. It is worth worrying about polymeric impurities in salts - you can eliminate these by "roasting" the salt to turn the into carbon ash.

It is noted in the literature that 6-8M KOH is the sweet spot for discharge capacity - however the use of a 1M solution for total cycle life has also been noted here.

One key parameter for anyone considering this is a rule of thumb figure for electrolyte volume of 1.5 - 2.5 mL A/h. For Prius cells this corresponds to 9.75 - 16.25 mL per cell, or 58.5 - 97.5 mL per module (each module has 6 cells).

Doing the Work

You'll need to dismantle your battery out of your car to do this. This can be done quickly once you know what you're doing, but follow a YouTube tutorial and take a lot of photos while you do it. Also read the following section and understand what we're dealing with.


This is part in the story where we include the big high voltages can kill warning, but let me add some explanatory detail here: the Prius HV battery is 201.6V nominal - in Australia this is lower then the voltage you use at an electrical outlet every day. But it is a battery - it has no shutoff, and it's DC power (so being shocked will trigger muscle contraction that will prevent you letting go).

Before you do anything to get the battery out of the car, make sure you pull the high voltage service plug, and then take a multimeter and always verify anything you're about to touch is showing 0V between the battery and car chassis.

Now the tempering factor to this is, handled properly, this battery is quite safe to work with once disassembled. High voltage is only present between the end terminals when the bus bars are connected - broken down into the individual modules the highest voltage is 9V from the individual NiMH modules.

Specific Advice

What does the High Voltage disconnector do?

The big orange plug you pull out of the battery does two things: it breaks the the circuit between positive and negative inside the battery, which makes the voltage at the battery terminals in the car go to 0V. This makes the battery safe to handle with the cover on.

It does this specifically by sitting between the 2 battery modules in block 10, and breaking the connection there. Because the battery output is wired from the last module positive, to the first module negative, this breaks the circuit.

There's a secondary benefit to this once the battery is open: breaking the battery wire here limits the total possible voltage inside the battery to ~130V (from block 1 to block 10). This is still a lethal voltage though.