Get rid of your lead acid 12V battery immediately!

[MeeLee]

My 2019 12v battery was still usable, but read pretty low voltages.

 

A replacement was easily $200, and it wasn't easy to find a replacement that would fit.

 

I ended up finding a Lithium replacement that fit legth and width wise, but was a bit higher, so the strap doesn't fit on it. But there's a screw block on the side of the trunk lid, that can still screw it in place.

 

The product I used was this $200 50Ah battery, but at the time of this writing, they only seem to have the 36Ah version. https://www.amazon.com/dp/B0BGRZP5Q6

 

The 100 and 200Ah won't fit in the compartment.

 

While most lead acid have 800cca, the 50Ah had only 100-200CCA. Though this shouldn't matter, as the engine is started by the high voltage battery. 

 

The only load on the battery is the 12v climate controls, lights, radio, usb, etc...

It's entirely possible that the 36V battery is good enough.

 

Why I recommend to swap out the lead acid:

1- it's lighter

2- It's the same price as an oem 12v replacement

3- unlike the lead acid with 500 recharge cycles, lithium has 1000-3000 recharge cycles, thus lasts longer.

4- The biggest reasons however, is that the battery charges quicker at start, when the high voltage battery is still cold, and then draws less charging current for the rest of the journey when fully charged. It charges quicker.

The lead acid will put a larger constant drain on the high voltage battery, even when charged.

 

This  battety swap results in an average increase of 42.5 to roughly 45.5 mpg.

 

There is one con though,

In the beginning, the car might lose the dash dimmer settings, and a wrench icon may appear.

This is because the Li battery charged voltage is about 13.30v, and falls outside of the expected voltage. I have found enabling the parking brake while turning off the car lowers that voltage, and the wrench icon won't appear.

 

Over time, the Li battery voltage will drop and the wrench icon won't be present at some starts.

 

Still, I believe it is best to change out the 12v  battery immediately, as it will result in better mpg, and a longer lasting high voltage battery.

[MeeLee]

Ps, the 36Ah battery might actually not have the wrench icon. 36Ah equals roughly to 400Wh, still more than enough power. 

[ptek]

It might work in your climate, but it looks like the battery will cut off charging below 0C (32F) and cut off discharging at -20C (-4F).  It could leave you stranded if used in cold climates.

[MeeLee]

You only need to worry about discharging. Is it going to be -20c (-4F) in your trunk? Because if it is, your high voltage battery won't work either.

 

 

Edit: apparently the wrench icon was for the Li battery BMS going into overprotection when the voltage exceeded 14.7V.

 

Edited December 21, 2022 by MeeLee

[MeeLee]

My first problem happened with the BMS shutting off when the charger put burst voltages of over 14.6V to the battery.

 

The car does this to burn off any oxidation on lead acid batteries, but it stops lithium batteries from charging, as their BMS will disconnect. 

There is a cheap and an expensive solution for it. You either buy a voltage regulator which is in the 200s of dollars, or you make your own charge system.

 

The best way is to use high amp diodes, to discharge the battery to the car, and other diodes to charge the car, with an inline 14V buck converter. That's the best way. Total of parts will be a few tens of bucks and some solder work.

 

Or, go the really easy route, and just solder a resistor in series with the battery.

The good thing is, that I've already done some research in this area.

If done correctly, it should cost you less than $20.

 

Apparently a 3Ohms resistor will drop the voltage by 0.5 to 1V, depending on the load and battery.

 

The 8Ah battery with 10Ah burst was not powerful enough to start the fusion. I think the minimum needed would be a 20Ah battery (or more).

 

The Li battery operates at around 13.3V, while a lead acid battery operates between 12.3 and 12.8V. So a 1V drop is what we're looking for.

 

A 3ohms 50W resistor is just enough for the 8Ah battery, so for the 50Ah battery, in theory, I'll need about 5 to 6 times more. (Need about 250-300W).

For that reason I chose this 10 Ohms pack of 5 resistors, each rated for 100W.

By putting 3 in parallel, I'll have 3.3Ohms. I could also put 4 in parallel for 2.5Ohms, or put 2 rows of 3 in parallel for 6.67 ohms, for a higher voltage sag more equivalent to a lead acid battery.

 

By using the resistors (https://www.amazon.com/dp/B077ZJKYYR/), I'm hoping the voltage sag being bi-directional, will allow the battery to charge at 13.4-13.6V, while bust voltages of 15.8V will be reduced to below 14.8V, and will not knock out the bms.

 

Likewise, I'm hoping that the charge current will be reduced in the battery for longer life, as well as the discharge voltage will be closer to a 12V lead acid battery.

 

The resistors will more than likely never have to deal with more than a few volts of voltage differential, causing them to never be hotter than hand warm, unless the 12V battery is depleted to zero, at which they might become very hot. The BMS should never allow the battery to go below 10V anyway. Mine shut off around 9.2 to 9.7V, and a 10A charge current from 9.2v to 13V is about a 79W differential, enough to melt plastic with these resistors.

 

Hence I think 2 resistors of 100W in series will be about the minimum I need, but more likely it'll be 3.

 

From my research, it seems a single 100W resistor might be too little, as the charge current is 10-15A from the high voltage battery, and the resistor does need to handle more than 130W to start the vehicle.