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Old 11-21-2016, 11:54 AM   This thread is in the EcoModder Project Library | #1 (permalink)
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Prius 12v battery replaced with LiFePO4

A while back I bought a "12v" LiFePO4 battery ($130 shipped) to experiment with and eventually replace the 12v battery in the Prius. It lived in my jetski all summer, faithfully starting the old boat without issue. Since my PiP is a 2012, I figured it might be a long time before the 12v battery needed to be replaced.

Due to a musty smell caused by water infiltration, my wife decided to leave the doors open while the car was parked in the garage, and I was away on business. Needless to say, after 5 days, there was no sign of life when I went to "start" it.

I don't have a proper battery charger since I have supercapacitors to supply a jump for other vehicles.



Instead I used an linear DC power supply, with the voltage set at 13, and the current maxed at 5A, connected to the "jump" terminals on the Prius.





*** SKIP THE BORING DETAILS IF YOU WANT ***

After a few minutes of charging, the voltage was at 10v, so I decided to try plugging in the EVSE charger for the traction battery, knowing that it should also charge the 12v battery. Instead, I heard relays clicking on and off as the car attempted to draw more power than could be delivered. I had to disconnect the linear power supply to get the relay clicking noise to cease.

Next, I resumed charging the 12v battery, and went to place the car in ready mode, knowing that the traction battery would charge the 12v battery once everything booted up. I forgot that opening the driver door causes the brake booster motor to run, and that was more than the battery and power supply could handle. Again, I got strange noises coming from under the hood as it continued to struggle with insufficient power. I had to disconnect the power supply again to get this to cease.

After that, I wised up and went through the passenger door to hit the power button twice, thinking that would put the car in ready mode and charge the 12v battery. The car booted up ok, so I figured it was charging. In hindsight, I don't think the traction battery actually engages in this mode (you have to press the brake pedal when hitting start), so when I came back half an hour later, I found the car unresponsive again.

Finally, I decided I wouldn't try to resurect the 12v battery, so I charged just enough to operate the doorlocks and open the hatch so I could access the battery.

*** END OF BORING DETAILS ***

Keeping the linear power supply attached to the jump terminal on the Prius (to preserve settings, trip meter, etc), I removed the nearly dead 12v battery and replaced it with the LiFePO (lithium iron phosphate). It's rated at 20 aH, which I believe is roughtly half the capacity of the OEM. Even though it's half the capacity, LiFePO4 batteries can be discharged to roughly 30% remaining capacity without causing damage, while the OEM lead-acid battery would be damaged by that deep of a discharge.

So far the LiFePO4 battery is performing just fine, and as a bonus, it's 22 lbs lighter than the original (OEM = 29 lbs, LiFEPO4 = 6.6 lbs). Venting is not required for this chemistry.

I used a wooden block to act as a spacer since the battery is physically smaller than the original. This allowed me to use the retention bracket that held the OEM battery in place. I also had to fabricate short lead wires to adapt the power cables from the car to the terminals on the battery. Here I just used 10 AWG wire and soldered ring terminals.

My ultimate plan someday is to compare the charge efficiency of lead acid batteries to LiFePO4. As lead acid batteries near full charge, their efficiency at accepting charge plummets, dropping below 50%. More than half the energy can be lost as heat. LiFePO4 has much better charge efficiency near full charge, and I want to compare them by discharging 10%, and then measuring how much energy it takes to bring them back to full charge. This might simulate what happens in our car every time we "start" it.


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Old 11-21-2016, 12:05 PM   #2 (permalink)
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Generally a lead acid is considered to be around 80% efficient at charging and LiFePO4 is typically greater than 95%.
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Old 11-21-2016, 03:22 PM   #3 (permalink)
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The 12v batteries in hybrids really don't last much longer, for whatever reason. And when they do go, it's usually with little or no warning, as there's no starter to hear the battery having a hard time. Kinda funny how a vehicle with a battery that could run your home's AC unit can be dead in the water if the 12v battery dies.

Hope this battery works out! $120 is comparable to the lead acid battery and the weight savings would be nice. My Prius had its 12v battery replaced the beginning of 2013, just a few months before I bought it. That should give me another year or two before it goes.
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Old 11-25-2016, 05:18 PM   #4 (permalink)
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Well I may have jinxed myself. Tried starting the Prius this morning and I just got the dash lights flickering and relays trying to flip. Turned the headlights off (I sometimes leave them on, since they automatically turn off when you lock the car) and it started right up. Got the 12v battery voltage showing on the UG and it shows right around 12v after sitting for a while, dropping to about 11.5v when stuff starts running. I think it was around 25F in the morning, and it'll only be getting colder.

Now I'm debating whether I should get that LiFePO4 battery or stick with lead acid...
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Old 11-25-2016, 07:05 PM   #5 (permalink)
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Oh LiFePO4 batteries suffer from numbing really bad where they loose a lot of amp out put below freeIng.
I take mine in at night.
Also charging them when the cell temperature is below 20F will damage them.
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Old 11-25-2016, 10:20 PM   #6 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
Also charging them when the cell temperature is below 20F will damage them.
That's a good point. I thought the specs said they could go lower, but that's just for discharging. Maybe I'll stick with good ol' lead.
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Old 11-26-2016, 03:45 AM   #7 (permalink)
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It should work out over the winter, as the Prius battery location is inside the cabin, and I garage my car.
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Old 11-26-2016, 02:22 PM   #8 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
...my wife decided to leave the doors open while the car was parked in the garage, and I was away on business. Needless to say, after 5 days, there was no sign of life when I went to "start" it.
Why would leaving the doors open cause the battery to go dead?
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Old 11-26-2016, 04:00 PM   #9 (permalink)
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Quote:
Originally Posted by jamesqf View Post
Why would leaving the doors open cause the battery to go dead?
Because (at least on my Gen2 Prius) the cabin lights don't turn off automatically if a door is left open. It'll turn off the headlights if they were on when the car what turned off, but it's glad to let the interior lights drain the battery. I usually leave most of mine off just in case a door doesn't get closed all the way.
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Old 11-26-2016, 05:18 PM   #10 (permalink)
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I replaced my interior lights with LEDs, so I was surprised to find the battery dead, even if it was several days. The electric brake booster runs whenever the driver's door is opened and the pressure is low. I'm wondering if leaving the door open causes it to run the brake boost motor many times over the course of several days.

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