Hybrid battery pack at 122k miles

[MeeLee]

Mpg at 30k miles while not knowing how to drive efficiently was 48mpg.

 

Mpg at 94k miles, right before the big drop, was 46mpg.

 

Mpg at 100k miles, with knowing how to drive efficiently was 44mpg.

 

Mpg at 120k miles was 42 mpg.

 

Despite me driving better, mpg gets worse. Engine is on longer, motor runs less long. Electric braking is weaker, and battery degradation happens faster and faster as the miles rack up.

 

I finally opened my battery pack, and measured each of the 72 cells. 38 cells per bank, 2 banks.

 

Lowest voltage was 3.64V, the cell closest to the inverter. Highest voltage was 3.66V. Apparently the car self balancing the cells works well. It basically overcharges the cell, until the (I suspect) internal bms shuts off the cell from charging. When the majority of the cells shut themselves off, the balancing process is finished.

 

A new cell should read between 3.77V and 3.8V.

This means I've lost between 1 and 2% of capacity, at 100k miles.

Multiply the 3.8-3.66=0.14V with 72 cells, and I've lost 10v, or 3 cells of capacity.

 

The battery at 3.80v cells x 72 cells equals to 374V.

The battery at 3.66v cells x 72 cells equals to 263.5V.

That's 10V difference.

 

Battery charge voltage is 4V/cell x 72 cells = 288V. My battery pack should not exceed this value,meaning the maximum amount of cells I can add is 8 cells of 3.2V, or 25.6V, seeing that the battery pack will further reduce in voltage over time.

 

While I can't seem to gain my previous range back by balancing the cells, I might by adding cells, as the cheapest solution without having to rely on finicky older HV batteries, or costly new batteries.

 

Right at the junction of the fuse, when you unplug it, is the feed line of the battery. One side hot (274V), the other side cold.

I'm thinking of buying 4 cells of 3.2V, for a total of 12.8V, and adding it between the fuse and the wire leading out to the motor (the cold side), to get my desired voltage and range back, without having to buy a new battery. This because all the internal cells are wired in series. All the 72 cells are in series. No central controller, temperature sensor, or voltage regulator sensor wires on any of the cells. So I guess the overall hybrid system just measures the voltage and amp draw of the entire pack.

 

The cells can easily be mounted between the high voltage battery, and the wall close to the backseat. They will be cooled by fan of the battery pack sucking in cabin air into the battery pack, causing an air draft in that area.

 

It's just a thought. Now that Lithium prices dropped, spending $200-300 for a decent 12v battery or 4 cells, should get me 100-200A rated cells.

 

The motor is 30kW. At 300V that is 100A. At 275V that's 110A. I'd be safe to get cells that can withstand 150A, so the 180-200A models as most common ones found on chinese websites, are going to be what I'm looking for.

 

I'll also need a bms to control the voltage over the cells, or just buy a 12V lifepo4 car battery which already has one built in (but has no cooling), and house it next to the high voltage battery somewhere. The latter will be easier. I can even mount it in the trunk.

 

Still thinking about this all... anyone ever tried to extend his hv battery with a 12v lithium battery?

Edited May 26, 2023 by MeeLee

[jpohlman]

I think in your posts you need to indicate if you're driving the Hybrid model or the Energi.....

[Cobra348]

If I remember correctly, he has an FFH.  can't recall year.

[Waldo]

All that deep thought and consideration without doing some basic math.

 

The difference between 42mpg and 48mpg is about 30 gallons of fuel over 10,000 miles.  At $3.50/gallon, that's about $100 every 10,000 miles.  So if you spend $300 to restore back to "full" battery health, you will need 30,000 miles just to break even.  But based on your data, in another 30,000 miles you'll have lost what you gained back, so you'll have to do it again.  Basically you'll never come out ahead, so why take the risk of damaging or ruining it?  Never mind the safety considerations, do you really think your rideshare insurance would be ok knowing you've got a homemade high-voltage battery pack sitting a foot away from your paying passengers? 

Edited June 6, 2023 by Waldo