HV Battery Repair, My Epiphany. Is It A Good Idea???

can you replace a cell without removing the battery?

 

You have to dismantle the battery (after removing it from your Prius) to replace a module since the stack of 28 modules is clamped between two pressure plates.

JeffD

 

Even with taking the battery out of the car to disassemble and reassemble, if the Grid Charger will work versus charging/discharging individual modules this will still save a lot of time. Correct ?

 

How are you going to balance individual cells when you can only charge the whole thing?

Battery fails from individual cells out of balance, especially the ones in the middle that are hotter.

 

Here's my potentially incredibly faulty logic.

If it was that easy to balance and maintain an aging HV battery?

A lot of battery reconditioning services and places would already be doing this and handling failing modules in this manner.

BUT...maybe you are on to something.

 

How? do you have leads attached to every cell?
You can balance flooded lead acid battery by overcharging the whole battery and replacing lost water. I don't believe you can do it to NiMH.

 

i do not know, eric becky says individual balancing on an old battery doesn't work that great anyway, that's why rebuilds don't seem to last very long. this might be as good and a lot faster.

 

...you're way over my head....I am still confused about why it's not possible to bring the new module to the same balance as the rest of the pack without doing the whole pack. Don't answer that question though, I don't need to know yet.

What is the cost of the new equipment $400? We could have a group purchase to share.

 

All I can say is "this is a interesting string". Hope to see it working out as this may be better than the other methods available today for the dreaded P0A80...

 

Further, my understanding is that each module is secured by two screws to the bottom of the traction battery case, so access to the bottom of the case would be required to remove modules.

It wouldn't be a very good mechanical design if the 28 modules were held in place solely by compression.

The grid charger might be useful if you really have a need to charge the entire traction battery, although force-charging would result in a similar end without having to spend $400. Force-charging (flooring the accelerator pedal while the gear selector is in D and the car is held stationary via the brake pedal) for no good reason is not recommended because of the risk of potentially overheating the transaxle MGs. Its advantage is that the battery can be charged up to 8 green bars on the MFD gauge relatively quickly, within several minutes.

A low risk alternative is simply to make the Prius READY for 30 minutes and let the car sit on the driveway, with the gear selector in P. That will gently charge the traction battery to the extent that the traction battery ECU sees fit, and there is no risk of any transaxle damage.

If you are fixated on the grid charger concept, a low cost alternative would be to construct your own using a Variac, a bridge rectifier that can handle the voltage and rated at 2A or more, and a couple of electrolytic capacitors to filter the DC voltage. Of course that setup will not have current or voltage limiting so you have to be very careful not to overcharge the battery, and there are obvious safety issues if you are not familiar with how to handle high voltages. (In that case, you shouldn't be messing with the traction battery, anyway.) A 0.5A or 1A fast-blow fuse at the rectifier output would allow you to establish the maximum current flow and protect the bridge rectifier, and the AC input to the Variac also should have fuse protection.

The grid charger is not going to be too helpful if the concern is that an individual module is not matched to the other 27. The following text comes from the FAQ section of that company's website:

"Unfortunately, in any one specific case, it is not possible to say for sure how much the charger will help. The amount of improvement depends entirely on the condition of the underlying battery pack itself."

My translation of that text is that if the traction battery is in good underlying condition and just needs a charge, then there is no need for a grid charger because you can use the Prius itself to charge the battery; while if one or more modules need help because they are not balanced with the remaining modules, the grid charger is not the appropriate tool.

 

The basis of the grid charger operation is the same as with charging individual modules. When charging modules you cannot charge individual cells as there is no external connections for these on a module.

Both systems work by slightly overcharging the modules allowing cells that are at a lower level of charge "out of balance" to catch up with those that are fully charged. If this is done with 6 cells in a string "one module" or 168 cells in a string "the complete battery" makes no difference except for the voltage required to accomplish the charging. The current required "amps" remains the same.

The original Prius had such a charger built into the car that was designed to run from a 12 volt supply. although this did not fully charge the battery, but bought it up to a usable level of charge if the battery charge was accidentally depleted. This system could easily be replicated using a 230 volt inverter, bridge rectifier, and current limiting devise. In fact a 120 volt inverter would also work with two single diodes arranged in a voltage doubler circuit to obtain sufficient voltage.

The drawback with using a whole pack charging system is the inability to check the capacity of individual modules but the capacity of the pack could still be checked, and the capacity of the pack is that of the weakest module, or in real terms the weakest cell within that module.

Patrick is correct that the modules are fastened down from the underside. So the battery needs to be removed to change modules.

Bisco. Individual cells cannot be replaced only modules of 6 cells. I do understand this is probably what you meant, but it can be confusing to new members and those with less battery knowledge if we do not keep the terminology correct.

John (Britprius)

 

Ok guys, there are a lot of thoughts and theories going on in this thread. I'm a bit overwhelmed by all the questions and I likely am not going to do a good job of replying to all of the items raised.

I plan on being at PriusChat's "ToGoBefore" event in Boston this September and will have a technical session about this very subject. I highly encourage you to show up. There will be plenty of discussion time where we can beat this subject to death.

There are people who are doing what usnavytgc proposes with varying degrees of success.

Working on the pack while it is still in the car
If you are planning on taking the lid off the pack, don't do it in the car.
As you already noted, it would be very awkward to remove a cell with the pack still in the car. Especially with those bolts on the bottom. I've never bothered doing it in the car since once you get it far enough to take the cover off, you basically are ready to take it out of the car. Plus working on it the car there is lots of exposed metal. Waaaay to easy to short something out in my opinion.

If you were just going to add a Hybrid Battery Charger/Balancer, but not removing any modules, then you would work on that in the car.

Hybrid Battery Charger/Balancer
Britprius explained the overall idea behind a Hybrid Battery Charger/Balancer.
Can you build your own? Sure. If you have the skills and feel confident in your ability. Guys in the Honda world have been doing it for years. But most people are not comfortable with that, so that is why the market has arisen for a user-friendly version that consumers can install on their own, and not have to build it.

Shops doing individual module replacement.
There are some shops that do individual module replacement, but the still tend to charge customer ~$1,500 or more. Plus their warranties are generally a year or less. There are many issues from a shop's perspective that they have to deal with including cost of quality equipment, cost of warranty work, cost of labor, technician training and safety, access to quality and quantity of materials, excessive length of time in the bay for the service, etc.

Using the Prius to charge/balance the pack
The Prius cannot access the upper or lower regions of state of charge.
Hybrid Battery Charger/Balancers take the pack outside the normal range in order to completely fill (or drain) the pack.

Preventive Maintenance Charging to avoid a P0A80
Overall it would be better to avoid getting a P0A80 in the first place. I think this is where the Hybrid Battery Charger/Balancer works best.

I have more to reply about, but I'll leave it there for now.

It'd almost be easier if someone else just interviewed me and they wrote down the answers for me.

 

I thought P0A80 meant increased internal resistance of one of the modules (loss of voltage under load) rather than simply uneven charge across the modules?