HV battery balancing

I've replied to your message and bumped the two older threads that detail the process I went through to build a balanced battery pack.

Without more information I don't know what you mean by "balance/charge" the battery. In all likelihood, you have a module or two that needs replacement, which will require removal and dissassembly of the battery pack. That's not hard. But determining which modules are weaker than others is time consuming. It's relatively easy to replace obviously failed modules. Then balance charge the pack by arranging them so that all the positive terminals are on one side, and all the negative terminals are on the other side, with a wire on each side connecting the modules into one massively parallel battery pack. Then you can apply a charge across the entire battery pack with a single charger, which saves a lot of time, and doesn't require a high voltage charger. There's no guaranteeing that the modules are properly matched, but it will probably get you going.

I have a spare MRC 989 battery charger, if you're interested.

 

First off, it would be good to ask, how technically proficient are you? Are you comfortable taking the battery out and taking it apart?

I should have mentioned safety earlier, but there are several things you want to be aware of while working on the battery. First of all, the electricity itself. The battery can generate about 300 volts, which is more than enough to kill you. Before you remove the battery, there is an orange plug in the case that splits the battery in two banks, which cuts the voltage in half, making it a lot safer, but you still need to be careful, so be aware that you should wear rubber electricians gloves. Once you get the battery out and the cover off, you should remove the buss bars on one side of the battery first. The side without all the little sense wires is easier. Once you get the nuts off, you'll have to pry the orange buss bar holders off the side, they are held on with sealant. Once they are removed, the highest voltage you should be able to find is about 18 volts, which is much safer. Once you remove the buss bars on both sides the highest voltage you can see is about 8.5 volts.

Also, there is the electrolyte, which leaks from the battery terminals. It's nasty stuff. You'll undoubtedly find corrosion on your terminals and buss bars. Again, the rubber gloves are a good idea, and safety glasses as well. But you should keep some boric acid eye wash solution on hand (from a pharmacy), to neutralize the caustic electrolyte if you get it in your eye. But don't get it in your eye in the first place, it's really nasty stuff. Or on your skin. Since you're taking the battery all the way apart, I would go ahead and wash the modules and buss bars in vinegar and rinse with water. It's cheap, and once you have them washed you no longer have to worry about electrolyte (much). Soak them in a basin filled with vinegar and then brush off the electrolyte and corrosion with a toothbrush. Also, once they're washed, it will reduce future corrosion on the buss bars, and prevent p3009 codes from electricity following the path of the electrolyte leak. That alone can save some headaches.

The electrolyte leakage is another issue that you should be aware of, since it can trigger a computer code if it drips down the side and allows current to flow to ground. There is a Toyota service that should have been done on 1st generation batteries, where they slather on some kind of sealant, and put little felt pads under the terminals to absorb the electrolyte. It doesn't help much, the stuff still leaks. I soaked the buss bars and covers in vinegar to neutralize any electrolyte, then soaked them overnight it mineral spirits to soften the sealant (stuff like glue) off. Once the sealant was soft I blew it off with compressed air.

Note, the battery is heavy, and you won't want to turn it over. But underneath, each module is bolted to the battery with two 10mm bolts. I put the battery on the bench with part of it hanging out, and removed the bolts from underneath, in order to remove the modules. You will also have to remove the temperature sensor wiring on the top, the vent tubing, the end plate, and the rods that run across the top of the battery between the two end plates that hold the battery pack together.

With a sharpie, write the number of top of each module so you can tell them apart, and what order they were in before you took them apart. Unless you do the whole charge/discharge test, you probably should put them back in the same order you took them off in. Take voltage readings on each module (post them in this thread, and the rest of us will follow along and give you advice. Take pictures and post them, especially if you don't understand something) Then you can rearrange the modules, and put on the wire to arrange them in parallel. It doesn't matter where the weaker ones go, because they are all in parallel.

To connect your modules in parallel, once you wash your modules you can just strip a piece of solid core copper house wiring, say 12 or 14 gauge will work fine. Or even a piece of coat hanger. Make sure you that all the positive terminals are on one side of the pack, and all the negative terminals are on the other. Otherwise you will be short circuiting a very powerful battery. It can throw sparks and make a lot of heat very fast. Set the wire across the terminals and squeeze it down with the buss bar nuts. Don't overtighten, because you can break the battery terminals. Just snug them. You'll need an inch pound torque wrench to put the buss bars back on later.

Once it's connected in parallel, attach the charger. You can clip right onto the wires. I charged the pack at 5 amps. Slower is less likely to build up heat, and pressure. Gas forms inside the modules if you charge it too fast. You'll want to put them back on the battery tray. You don't need to bolt them down until you're putting it back together, but you should put the top rail and the end plate back on to hold the pack together and prevent them from bulging.

 

Each module is 7.2V (nominal). 2A charge would work, but would take a long time. lots of info here
Prius Battery Photos

 

Do not, under any circumstance, charge modules in parallel. When a module is fully charged, it is already heating up, and its voltage begins to drop -- the -dV/dt inflection. The fully charged module then preferentially accepts more current from the charger, and the heat begins to rise even more rapidly. The other modules also begin to discharge into the fully charged one, because it is the only way to maintain constant potential at the connection points.

Modules that are more than 1.2V lower than the rest have a dead cell and need to be replaced, unless a battery has been sitting for months or years. Due to varying rates of self discharge, a battery long out of service may have voltage readings in the 4V - 7V. At that point, it is best to recondition the modules individually, i.e. several charge-discharge cycles on a Superbrain. This is one reason that ReInvolt will not accept battery cores that were not in recent service, because it is too hard to tell from quick inspection the good modules from the bad.

 

I agree that this would be a dangerous practice.

 

That's funny, I've been holding 20 modules in charge in parallel for 5 months. Before that I assembled my best 38 cells in parallel and charge cycled them repeatedly before reassembling the battery pack and installing it in my car, where it still works fine after 5 months.

I also just recently finished a reassessment of the 20 modules I've been holding in charge; removing them from the 20 module pack one at a time and cycling them with the superbrain to see how they compare to the data I took back in the fall. All but two of them had MORE capacity than they did 5 months ago.

 

That's not even considering the case of a dead/shorted cell, a possibility for OP. Many NiMH battery explosion stories come from sophisticated users who thought they had it nailed. See this for an example: Exploding NiMH Batteries.

I agree that modules regain capacity with charge cycling. The modules and cells are quite robust, Gen II more so than Gen I. I am a strident advocate of reconditioning Gen II modules, but some processes are much safer than others even if they take longer.

 

Food for thought. So if this is a significant problem, and I haven't had a problem with it, I've got to ask my self what I'm doing right.

First off, it seems that this is only a problem when NiMH cells are fully charged, and the voltage begins to drop. Since the Prius tries to keep the battery to between 60-80% of full charge, there shouldn't be any danger in assembling a Prius battery this way unless additional charging has been done, to get at least one module up to a full charge.

Second, when I assembled my battery this way, my modules had been through a full charging cycle on the Superbrain, which was probably the closest I came to this runaway condition. But it occurs to me that when I first assembled my battery pack in parallel, I didn't attempt to charge it. I left it sitting overnight just to allow the modules to come to equilibrium. The next day I removed the wire connections, and allowed it to sit, so I could see how the voltage in each module decayed. This was information that I used to determine which modules to include in my final assembly, since I was consolidating 2 used battery packs to make a good one, and I wanted the best modules, with the most even charging characteristics.

Third, after I chose which modules to use, I assembled them in parallel, and used the charger to DISCHARGE them so they all reduced their state of charge by a similar amount. Putting a load on NiMH in parallel wouldn't bring them close to the dV/dt drop, so again, no problems there. So as I cycled the pack with the charger, it occurred to me that the charger only sees the pack as a 6 cell module. It can't tell that there are multiple modules, so without really over thinking it I didn't allow the charger to fully charge the pack. After all, I wanted to put it back in the car with less than 80% charge anyway, so there was no reason to try to get it fully charged. My only concern was that each module had been through similar charge discharge cycles so that they all had the same state of charge, which would have been impossible otherwise, no matter how much time I took to do it. So I discharged and charged the pack between 7 volts and about 8.25 volts, without letting it get to the 8.6 volts that was usually showing on the Superbrain when the individual modules completed their charge cycle.

And finally, the 20 modules that I've been keeping charged, well, again I have no intention of trying to get a full charge on them, just enough to prevent them from discharging enough to reverse a cell. So I experimented a little, and found that about 40 mA is enough to keep them at about 8.2 volts. They stayed that way for a couple of months before I got curious to see how they were holding up, and began charge cycling them individually to see how they compared to their previous data. Again, this brought them up to full charge, which is where the danger lies, but because when I added them back into the pack they were at a higher voltage than the pack they discharged rather than charged, and thus avoided the dV/dt issue.

As to the problem at hand, I would certainly agree that the OP should not attempt to use any modules with a dead cell, which usually shows up in the voltage being about 1.5 volts lower than the rest of the pack. Which is probably why the ECU isn't allowing the car to start. But I can't see any other way to balance the charge on 38 separate modules, short of buying 38 chargers, since it would take a couple of weeks to run each module through a complete charge cycle, and by then the modules that had been charged earlier would have self discharged significantly.

 

My prius has been running for over 5 months after balancing the battery. Note I also discarded several modules and just used the best 38 modules I had between the two battery packs I was working from.

As per Seilert's warning, I'd be remiss if I didn't tell you to watch for the things I mentioned in the previous post. Don't connect the batteries in parallel until you have measured all the voltages, and post the voltages here. A module with a dead cell is generally about 1.2 volts lower than the rest of the modules. Don't connect the batteries in parallel if they are more than, say 0.7 volts difference between the lowest and highest module. And don't charge them until they've been discharged to a lower voltage than the lowest module. You can safely discharge them to 6 volts.

Do you have a way to discharge the battery? Discharging the battery is at least as important in the balancing process as charging it is. You want to discharge them FIRST all to the same voltage, and THEN charge them up again, but not to fully charged. If you fully charge them you can get the runaway charging effect that Seilert described. I used 8.25 volts as my high end, but that figure was based on the number given by the MRC, which may not be the same as your voltmeter. Probably the safest bet is to never charge them higher than the highest voltage in your original readings.

 

10.5 per module is pretty high, pack voltage would be above 380V. however, i believe this is within normal parameters for gen 1 while under/after heavy charging/heavy braking. Could someone else confirm that? I have certainly seen 270V on my HV pack in a gen3, meaning 9.6V per module.

 

When I owned my 2001, the highest voltage that I recall seeing (using the Ecrostech miniscanner) was around 320V, or ~8.4V per module.

 

Joe, have you checked the voltages again? I can't see them staying at 10.5V for that long. If you like, please post your 38 module voltages and we can give you some additional advice perhaps.

 

Yeah, when charging individual modules on the superbrain, they get to about 8.5 volts and stay there for a long time, before the charger detects a voltage drop, and ends the charge cycle. It shouldn't be possible to get a module above 9 volts.

Even if your voltages did get that high, I'd want to discharge them if possible. It can't be a good situation.

 

Wait, the 10.5 volts was reported by the ECU? Then that's a module pair, not an individual module. So each module was down to 5.25 volts, which probably means they're shot. Makes more sense that way. In any case, we really need to see all of the voltages, to see what the pattern is, not just a single reading.

 

Hello I am new to the forum and would like to ask a question relating Hybrid Battery Balancing.I have a 2002 Toyota Estima imported from Japan(where it is only sold)to Ireland and due to uncooperative Dealerships I have had to rebuild the HV Battery with a Gen 2 from a 2006 Prius, myself.I am very confident in my ability to carry out this procedure.After carrying out all the work and refitting the HV pack into the vehicle everything works ok however I still have a P3000 code which will re appear about 2 weeks,with no obvious effect on vehicle performance.Each subpack resting voltage varies between 7.82 to 7.90 on all 30 subpacks.

Q: Is the difference too large or do the cells need balancing and if so what is the best way to do this?

Qoes the P3000 code(Battery malfunction)have any relevance?.If not what does P3000 normally indicate?


Thank you.

 

This is well within the normal deviation of 0.2V or less from highest to lowest. The modules are fine.

There are usually information codes that go along with P3000, as well as other P DTCs. How are you reading the codes? Two weeks in between faults, that's a very tough problem to figure out, suggesting a non-obvious loose wire, etc. Could it be too that your battery cooling fan is not connected?

It could be the battery ECU as well. That is perhaps not the easiest/cheapest part to swap out for you, though.