Installing a Nexcell Lithium Hybrid Battery

A module is a container you put cells inside of. A pack is container you put modules inside of. NiMH and most of it's predecessors in the over 1v range, but less than 2v. However w/lithium a cell seems to be in the over 3v range but less than 4v. If someone can add to this specific path of logic it'd be much appreciated.

 

And a single cell can be called a "battery" in English too, although in other languages they differentiate between cell and battery (like in Spanish where a single AA battery is called a "pila" but a 9V or 12V battery is called "batería").

Every cell you add to a battery (connected in series) increases it's voltage. A 12V battery has six 2V lead acid cells. A 9V battery has six 1.5V alkaline cells. a 7.2V battery has two 3.6V lithium ion cells, etc.

Anywho, electronically a Prius has leads going to blocks of cells that are around 15V-18V each. With NiMH that's 12 cells, or two modules of 6 cells each. Each cell will sit between about 1.2V and 1.5V.

1.2V x 12 = 15V
1.5V x 12 = 18V

With the replacement LiFePO4 Nexcell battery, the BMS is looking for those same blocks of 15V to 18V each. Each LiFePO4 cell is between 3V and 3.6V each. By arrange them into "modules" ("bateries", "packs") of 5 you get your 15V to 18V. So the LiFePO4 modules are twice as fat and twice the voltage as the NiMH modules, but you need half of them, and each one constitutes an electrical "block" to Toyota's BMS.

3V x 5 = 15V
3.6 x 5 = 18V

The difference is that the NiMH cells, batteries, whatever you want to call them, have a greater voltage sag than LiFePO4. When discharged down to 1.2V they still got a ways to go to reach empty at 1V or less. But when fully charged they can reach closer to 1.6V. The Prius also cuts back depending on current and voltage. So the end result is that the saggy NiMH batteries don't charge or discharge above or below their 40% to 70% state-of-charge.

With LiFePO4 they have a flatter charge curve. I'm sure somebody will jump on here and start saying how lithium titanate batteries have an even flatter charge curve, but the fact is even with LiFePO4, by charging them between 3 and 3.6V with the BMS cutting back also based on current you reach closer to a range of 10% to 90% state-of-charge with the LiFePO4 batteries.

 

Do people agree that two NiMH modules in one block is just to simplify the voltage sensor harness design?

 

Of course. Toyota could have made it have a wire going to between each and every cell. But NiMH isn't affected as much by going out-of-balance so there's not much reason to do so. If a cell goes flat the voltage for the whole block will be affected. Comparing that block to the others the BMS can see when a cell has died and the battery needs replacing.

 

I will be installing this lithium pack soon. Does a detailed instruction manual come with the packs? Or is there a specific YouTube video that everyone goes by?

 

There are some instructions. Since it is quite an investment and potentially deadly I watched a number of youtubes over and over to be sure I had every step. Especially important was the one I found on cleaning the copper busbars. However, Jack, who markets the packs may advise that you can find on eBay a set of new busbars and nuts for the batteries. I ended up buying new nuts, but I did really work over the old copper bars. New might have been easier and better in the long run, but you will want to have them on hand as nobody can be patient enough to wait when the packs arrive! Haha.. I am happy every time I drive my car that I got the new lithium! Looks like you are lucky enough to have EV mode in your car. I hacked my gen2 and added EV which I have fun using for short trips at low speed around the neighborhood. In general, my mpg way up since addition of Nexpower. Just made 200 mile trip with 55 mpg which is way better than old battery toward the end which was closer to 48mpg before the change. Just look around on youtube and you will find what you want. Also, Jack is very patient and available for questions etc although I tried to reserve those for things not covered in videos. The Dr. Prius app of course is a necessity. Good luck with your project!

 

Thank you for this very elaborate response. I did go to eBay and found some bus bars, but I don’t think I’m seeing any copper options. Do they need to be copper?

 

I didn't see any copper ones either, but all I needed was the SS nuts which I carefully sized and ordered from an eBay supplier. I didn't have any strong sulfuric acid, but as I recall I bought a toilet bowl cleaner at Lowes that was acid formula and soaked in that and then used ultra fine sandpaper and scrubbed. Again, I would contact Nexpower direct on that question as you may save yourself some work. Of course, I have no idea what shape your originals are in, but I suspect they will need some work or new ones. I prefer copper myself for the busbar but must be clean and smooth! A great youtube on that subject somewhere by Jose who has a whole bunch of videos on the subject.

 

I like this one re: busbar cleaning,

 

I went ahead and just bought a set of nickel plated copper busbars with the stainless steel nuts