HV Battery Headway 38120HP LiFePo4 70S pack

3.65V is "fully charged" but LiFePO4's can handle a constant voltage up to 4.0V.

70 is a multiple of 14 with 5 per block. 5 x 4V = 20V, more than the 19.5V per each group.

 

4.0v per cell is fine as long as you have a balancing system that can move the high high regen current from the cells that have already hit 4.0v to the cells that haven't reached that voltage ....... otherwise the 4.0v cell will very rapidly reach 5v per cell and vent all the electrolyte in cloud of vapour that isn't too flash on the senses after beathing it for a few mins ......
Once a cell passes 3.8v, the voltage run away is almost vertical when plotted on a volt/time graph. The cell voltage rested at anything above 3.4v means the cell is saturation charged, so any current will cause a rapid voltage rise .....
You need to be very careful, just reading specs without actually doing the hands on research will lead to some very expensive lessons ....

76 cells works well as far as cell voltages in general matched to the Prius module sensing wires, it is just the high regen current that will cause issues with LFP cells if they are only small capacity, the rapid voltage rise at even 5CA and this will cause serious cell heating and electrolyte boiling and loss if the cell remains at high voltage and/or isn't cooled below 45*C.

T1 Terry

 

agree with your opinions and thank you for your detailed explanation.
I think that if you want to install it in a car, you need to ensure that it has high reliability. Unlike in the laboratory, the extremes of the battery can be used. It is important to keep the battery in a comfortable state, which is related to safety and life. Performance is the second factor, of course it is very important.
Lithium-iron battery has a shortcoming, its consistent performance is relatively ordinary, self-discharge, capacity will vary from individual to individual, and lithium battery charging is more sensitive to voltage, it is best to have a balanced charging device, if the configuration of balanced charging will make the system very complicated . Without a balance charging device, after a period of time, due to differences in self-discharge and capacity, the power of the modules may be different. Some modules may run beyond the limit. If you have strong hands-on ability, you can balance and maintain the battery at intervals.

 

The thing is that with the many LiFePO4 replacement projects going on, I haven't heard of any that have had "serious cell heating" or "electrolyte boiling" or anything of the sort.

 

FWiW.
There is a technology called PCM which is short for Phase Change Material energy management developed by the Illinois Institute of Technology more than a decade ago that employs a cheap high heat capacity material matrix surrounding the cells that stabiles the temperature excursions in LiION batteries during charge and discharge cycles. Very simple passive technology. Relies on latent heat capacity phenomenon. The technology was never adopted by auto manufacturers largely due to the Not Invented Here mindset of such companies. Easy to Google it. AllCell is company name.

 

This is the principle I was talking about earlier on in this thread, around 18th of last mth. The E Fluids developed for the F1 electric vehicle racing uses this "phase change" effect where a lot of heat energy is required to change the fluid from a liquid to a vapour, resulting in actual temperature at the phase change surface remaining fairly constant, even though a lot of heat energy is being absorbed by the fluid.

I typed a very detailed explanation of how all this works with easy to see examples of the process in action and how this could be used for battery cell cooling .... but after re-reading it, I started to send myself to sleep, so I deleted it :lol:

T1 Terry

 

Time will tell, the cell damage becomes apparent after 12 mths to 2 yrs, but can be even less time than that. The capacity drops so the problem is only obvious when testing the range on EV only and comparing it to when the new battery was first installed. The only other give away is the smell ....... that sickly sweat smell of the electrolyte being released through the pressure valve.

T1 Terry

 

@OBJUAN you are a great fellow, technical progress is taking place on such people!

All over the world, there are experiments in the introduction of lithium in the Prius. It is not easy to select elements with the required volt-ampere characteristic. LTO is capable of accepting more current than LiFePO4 and continues to operate when it is very cold.

I would like to share the experience of the author Romka-ork from Moscow, I will process it, I hope Google will correctly convey the thought:

Installed in prius-30 90pcs toshiba scib 2.9Ah. During the tests, I connected them in parallel to the main VVB. The voltage range fits into the normal operating mode of VVB, while the results are only positive, the consumption fell by 1 liter (native VVB is half-dead), the temperature of the LTO battery for a trip of 100 km in the heat rose by only 4 degrees, the main VVB did not heat up above 35 degrees.

90 pieces of Toshiba were not chosen in vain - an additional 50% charged additional VVB has a voltage of 220v, just like the native one. Maximum charged 243V, discharged - 200V.

It turns out that the VVB control computer calculates the percentage of the charge not by the voltage on it, but calculates how many ampere-hours are flooded and how many ampere-hours are drained. Because Normally, the VVB operates in the range of 40% ... 75%, the working capacity is about 2A * h, which is well within the additional VVB. Due to the lower internal resistance (~ 100mOhm, versus 350mOhm for the native one) most of the currents are taken over by titanate.

 

And this is another project by -DMITRY- Prius C car. Implementation date 05/08/2021
The battery was assembled into the body of the native VVB. Elements used: LiFePO4 GBS 20Ah. 52pcs. And also two balancers - active and passive.
Two cells are additionally hung after the ecu. ECU controls 50 cells. And the balancer controls all 52. There are no problems with currents.

 

Interesting, how do you divide 52 by 14, the number of modules monitored by thevoltage sense wires? 56 cells makes some sense, 4 LFP cells to match the 2 x NiMh modules it is paired with or replaces.
Next, from personal experience, GBS prismatic cells are a poor choice, at least the square 4 screw terminal units I have in 40Ah configuration. They leak electrolyte vapour something fierce requiring a lot of ventilation to avoid the smell making the cabin occupants feeling nauseous after a very short period.

T1 Terry

 

mechanical stability is paramount importance for battery safety, that photo has literally bungee cords holding the thing down.

 

50/14 +2. ECU controls 50 cells. And the balancer controls all 52.

 

This is a temporary solution, test

 

I'd be a tad more nervous about all those NiMh modules with open vents potentially venting two parts hydrogen and one part oxygen into the passenger compartment ...... the right concentration of that gas mix and a stray spark would turn a Prius into a VW Beetle shape and require replacement driver and passengers I did get to witness the results of that as mix igniting in a very small concentration ..... let's just say, the tin ware will never fit back on that battery pack again

T1 Terry

 

Ummm....... The computer is looking for between 12v and 16v per two modules, so how do you fool the voltage sense wires into seeing the voltage they are expecting to see?

T1 Terry

 

I was wrong, the Prius C has 10 channels.
50/10 +2
ECU controls 50 cells. And the balancer controls all 52.

 

This is a bit different purpose. The IIT AllCell Phase Change Material is meant to act as a heat sink and release mechanism to protect the battery from temperature excursions which are major reasons for early cell death. Phase changes store and release a lot of energy and is a passive device. Why car manufacturers don’t try it, is a great mystery of the universe. Then again, maybe the New Prius Battery guy will take a look at it some day?
https://www.allcelltech.com/pcc

 

I can't see where this is any different than using tank filled with an E fluid that also holds the lithium cells. There is both a heat sink effect both ways because the heat soak will also warm all the cells to the E fluids temp and a localised heat sink affect when any part of any cell increases to the point the E Fluid changes state from a liquid to a vapour.
By changing the surface pressure acting on the E Fluid, that change of state temperature point can be shifted up or down a number of degrees C without changing the primary heat soak function designed to keep all the cells at the same temp and the same temp across all of each cell.

Lithium chemistry cells change their ability to accept or release electrical energy as the electrolyte temperature changes. LFP cells for instance, will deliver a much higher discharge rate around the 60*C mark, but held at that temp could result in a shortened cycle life ...... but why? It's because the cell will rapidly climb above that 60*C mark when rapidly discharged ..... unless a method of rapid heat dissipation is employed.
This means a cell designed for rapid discharge (very thin coatings on the anode and cathode plates) can have that discharge rate increased even further, as long as the electrolyte can be kept below the separation point where the very light volatile components boil off. This causes pressure to build up within the cell and with some chemistries, start to generate oxygen creating that potential bomb requiring nothing but a spark to set it off. I'm guessing we have all seen this You Tube clip by now

There are so many clips of lithium battery fires where thermal run away caused a real problem.

The charging temperature is another area that an E Fluid can assist. Rather than heat mats under the cells, holding the E Fluid at a set min temp will do the job much better because again, that cell temp is even throughout the cell and all the cells

T1 Terry

 

I have an immature suggestion that may differ from everyone's point of view. This is just my personal opinion. I think, if possible, try to choose a lithium battery with low internal resistance and high current for modification. Can be used in parallel to share regenerative current. The advantage of this is that it can simplify the heat dissipation design, reduce the possibility of reliability degradation due to high temperature, and low internal resistance also has a positive effect on performance. Secondly, the battery works in a comfortable state and has a longer lifespan.