Traction Battery/Cell Voltages

[Llecentaur]

Hi everyone,

I have been reading through this thread, including the exchange about feeding the traction batteries to extend range.

Here is what I put together, please correct me as I am definitely not an expert, just using my pre-university physics.

1) if we feed the traction batteries around the heater connection with 80% of max charge, we should be ok and not cause any issue with BMS.

2) for my 80 cell 14.5KW battery, I calculated roughly 300 V full voltage, therefore the 80% would be roughly 250v DC...

3) what happens if I feed the heater with five 48V 40Ah batteries in parallel ?

If my math is accurate, each battery pack would deliver nearly 2kw, then that would add to the car another ~10KW ?

To be switched in when the batteries drop below 250 V ?

That would of course make a real mess in the boot and demand separate chargers. But seems to be a workable solution.

Make sense ?

Thanks
Nader

 

[Don]

To 'feed' the traction pack additional amps (to extend the range) the voltage would need to be roughly the same as the pack is rated for - 340 to 350 VDC. Paralleling a lower voltage pack with the higher voltage traction battery would not add anything . . . . the higher voltage pack (the one in the car) would be trying to overcharge the lower voltage pack and your range would be less and not more. It's pretty elementary science that you do not parallel batteries of differing voltages

I think the car would be parked and not moving long before the onboard battery pack ever discharged to 250 volts

The problem with 'jury rig' modifications like this is to get the physics correct so that A.) It works, and B.) It doesn't harm anything

Don

 

[Llecentaur]

Thank you Don,

I understand I got the 80% of max voltage wrong. It would be the 80% of the operating voltage range of the battery.

Plus I see that I wrote parallel instead of series to increase the voltage which is obviously not correct...

 

[peterdambier]

The problem is, putting anything but the same kindergarden of batteries in parallel will kill one of them. There is no trickle charging Lithium batteries. There is only overcharging one cell and bleeding to death the other one. Same chemistry might work but same Lithium Iron Phosphate from another manufacturer with slightly different mix of the electrolyte will result in different idle voltages.

There have been tests for CALB cells showing both charged and discharged cells of the same production and selection for one pack, will drift to the same idle voltage within short (days or month) time.

I have been thinking about putting 104 CALBs parallel to our 88 Yuasas but I am shure that is a bad idea. It might work as long as I am driving but I'd have to disconnect them immediately when switching off. Off coarse they would need their own charger. That is what I meant with "regenerative heating" feeding them parallel to the heater.

Nice little experiment, 9 volt blocks of either 7 (8.4V) or 8 cells (9.6V) dont cost a fortune. 40 or 35 of them will get you close enough to 330V to play with and they'll be empty fast enough not to damage the main battery, but beware of short circuits. To experiment most regulations say you need an exam. A hamradio license might be the cheapest way to learn for and pass such an exam.

 

[Llecentaur]

That sourds Luke a good test to make.

On a smaller scale of EBikes, I can confirm that the following works:

Bionx kit with 36V LiMn 10A Battery
Connected just when I start the ride a 36 V LiFePO4 straight to the Battery charger connector with an XLR plug.

I can confirm that after two good years of usage, there is no damage to either Batteries, I do not have the tools to measure regen is but I have managed to gain range on the main indicator of the kit and others with amp meter have confirmed that despite different chemistry the combination works also in regen.

I guess the difference remains that we connect the two batteries only when needed, whereas in an EV this would need more caution or why not an electronic switch.

 

[Barbagris]

In a e-bike, each battery pack has its own BMS. Both overvoltage and undervoltage protects his pack, and in intermediate voltages each pack contributes with part of the current. How? 50/50? 20/80? 80/20? no way of know that, but when a pack reaches his level of undervoltage protection, it's BMS isolates and you only have a pack running (on low voltage)

In a car, I think it may be more delicate. I'm no expert, but to increase range can see perhaps three ways of approach.
- A battery pack with a voltage different than the main pack, and a DC/DC converter to recharge it, as in the kit's for the Plug-in Prius (Enginer et al). Simpler, perhaps safer, but with lower performance (converter losses)
- A battery whith a final voltage similar to the main pack, in parallel, with some type of "insulation", perhaps a "power diode" (not sure of the name) for not charge the auxilar pack with regen.
In both cases the additional pack must have its own charger and BMS (such as e-bike)
- A battery with 88 cells similar enough to the original ones, to put it fully parallel. I think is very difficult to implement.

Time will show.

 

[Don]

Ultimately though, this discussion is about whether you bought the 'right' car - If you need something to carry you more than about 75 miles, the iMiEV isn't it and prospective buyers should realize that before they even consider this car - If you need to go 100 or 150 miles on a single charge, buying a car which won't and then trying to modify it to do so isn't a very practical way to proceed, IMO

The battery and associated electronics which protect it, charge it and regulate everything is a very sophisticated 'system' which doesn't lend itself very well to being modified. I think the only thing I would ever consider if I absolutely had to add more battery power to extend range would be to go with a completely separate system utilizing a DC to DC converter to recharge the main pack . . . . and I would probably limit that process to when the car was sitting with the ignition off and not try to do it while the car was moving

But . . . . . If I knew I needed to go more than 75 miles, my first choice would still be a different car

Don

 

[jray3]

Turns out that lithim ion batteries do have a memory effect after all, :shock: but it is very small and appears to be reversible. :roll:
http://phys.org/news/2013-04-memory-effect-lithium-ion-batteries.html

 

[alohart]

Turns out that lithim ion batteries do have a memory effect after all, :shock: but it is very small and appears to be reversible. :roll:

The small memory effect was measured in a LiFeO4 battery which is not the type of battery used in our i-MiEV's. I wonder whether the LiMn2O4 (or is it LiMnO2?) used in our i-MiEV's suffers a similar memory effect.

 

[JoeS]

Turns out that lithim ion batteries do have a memory effect after all, :shock: but it is very small and appears to be reversible. :roll:

jray3, thank you for posting this. There was some follow-on discussion of this and, as someone on EVDL pointed out: "The effect is in fact tiny: the relative deviation in voltage is just a few parts per thousand." Yawn.
The end result being that this is NOT something we should be concerned about at all.

 

[JoeS]

I'm trying to keep track of all our iMiEV traction battery-relevant parameters, and thus am posting a few links here that were mentioned in unrelated threads:

Photo of an individual Cell test by Ben: http://300mpg.org/wp-content/uploads/2013/04/DSC_6387.jpg

White Paper on the GS-Yuasa modules: http://www.gs-yuasa.com/jp/technic/vol5/pdf/05_1_021.pdf

This MSDS sheet talking about our battery chemistry (Lithium-Manganese Dioxide):
http://www.ens.dk/da-DK/KlimaOgCO2/...odeller/Documents/Batteri - trillingbiler.pdf

 

[Llecentaur]

Thank you for this summary.

Then basically, in the best of worlds, we should hope to find in about 5-10 years a "LEV100" battery with similar chemistry or at least similar properties of max voltage, max discharge hoping to have a drop-in upgrade that would give us roughly double the range !

I wonder to which extend the BMS can adapt itself to a different chemistry. I assume three must be some extend of adaptability similar to the capacity of managing a cell which is not working within ideal parameters.

Then I could imagine it would also be possible to reprogram the BMS to adapt to a new cell.

Hope there will be enough iMiev sold throughout the world to justify someone to setup those new features. For the moment, we can continue enjoying this great little car.