Lithium-ion 12v Battery Replacement

[hayden55]

I was definitely just clamping the car in my driveway. I had the AC on full blast low auto and the radio on extremely high. Forgot the the lights and all that. But I even made sure to kill the hv battery down to two bars and have the engine flick on too.
The key with the Prius though is that it tends to use the hv converter for mostly everything when the car is in ready mode. Not everything but close.
Guys on Priuschat swear the car only uses the 12v for when the car is not in ready mode... But i did see a little bit of draw.
There won't be any difference in fe besides what you might get from dropping battery weight (on the prius), and the life cycles are extremely long on lithium batteries if you set it up correctly.
The big kicker here too is you don't need to power a starter from the 12v battery. The starter for my mustang was a 250a fuse.

[Ecky]

I'd love to use lithium but I'm concerned about damaging them during Vermont's winters. We regularly see a few weeks with highs below 0F and sometimes a night that gets as cold as -30F or so in the Champlain valley, and -40F in the rest of the state. To my knowledge, attempting to charge most lithium chemistries below 32F requires at best greatly reduced current. Charging too quickly (or at all, depending on the temperature) causes metalic lithium to electroplate the anode, which can result in the battery exploding or catching fire randomly. So, for 4+ months of the year I'd need either a battery heater, or to disable the alternator until the battery has warmed up.

I was reading about A123 Systems' LiFePO4 automotive batteries, and they rate their batteries down to (if I remember correctly, their site isn't loading right now) -30C, or ~-22F. If I were to speculate, they might have some combination of heating elements, BMS, insulation, or special chemistry that makes them able to accept charging at such low temperatures.

[redpoint5]

Lead acid batteries are less than 50% efficient at accepting a charge when near full capacity, which is where they need to spend most of their time to get a long service life out of them.

I haven't tested this yet, but there might be some small fuel savings to be had by not wasting energy trying to charge a Pb battery to full constantly. Lithium based batteries have much higher charge efficiencies at near full capacity.

To be sure, the fuel savings would be small, because the amount of energy contained by a typical automotive battery is small in comparison to the fuel tank.

Originally Posted by Ecky

I'd love to use lithium but I'm concerned about damaging them during Vermont's winters.

There is a 4.2 Ah LiFePO4 pack available from HobbyKing for under $50. You could test with that to see how it fares. Would be valuable info to the community too. Test capacity before installation, and then after a VT winter. Since the Prius battery lives in the cabin, I suspect it would be fine, especially since it will hardly discharge, and therefore hardly need a charge.

[Ecky]

Mine will, unfortunately, be used to start a 2.4L engine on days where it's below -20F, and then be expected to be charged immediately by the alternator at full current. I've seen a lot of pictures of these batteries which have caught fire and mine also lives in the cabin, and that makes me very leery indeed.

[redpoint5]

LiFePO4 doesn't catch fire, just puffs up.

You could do as I was planning, to put a supercap under the hood for starting, and place a current limiting resistor in series with the lithium battery that lives in the cabin. That would limit both the power it uses to start the vehicle, and the rate of recharge, while keeping the supercaps topped up when the vehicle is parked.

[Ecky]

I was under that impression as well. Hard to know what to make of all of the Amazon customer images of LiFePO4 batteries which caught on fire when they're not supposed to be able to.

https://www.amazon.com/Battery-Tende...70_&dpSrc=srch

Edit: my electrical theory is a bit weak but isn't it the case that since capacitors self-discharge, leaving one connected to the battery all the time would eventually drain it?

[redpoint5]

Originally Posted by Ecky

Edit: my electrical theory is a bit weak but isn't it the case that since capacitors self-discharge, leaving one connected to the battery all the time would eventually drain it?

The self-discharge is minor, and rapidly decreases as you back away from the peak rated voltage. 6-series connected BCAP model capacitors are rated to 16.2 volts. Assuming your LiFePO4 is maintained around 14v by the alternator, that works out to 2.33v per 2.7v rated capacitor. The drain will be practically nothing. I bet your alternator would shoot for an even lower voltage; somewhere around 13.5v.

Your biggest concern is the electrical draw when the vehicle sits unused. Pretty easy to measure your phantom power load and then divide your battery mAh capacity by that number to determine how many days of sitting parked the battery would sustain.

I had put together a very crude spreadsheet back when I had more time to work on this stuff. I'll try to update it and make it more user friendly sometime.

Check out the TSX tab on the spreadsheet below. My car draws 21 mA when parked. On a 4200 mAh battery, I can park for 6 days before needing to start and charge. On a 20,000 mAh battery, I can go a month.

https://docs.google.com/spreadsheets...it?usp=sharing

[hayden55]

I would consider something like the lithium battery cell format (18650, 21700, 20650, 20700) with something of a rating of 30A+ of max burst discharge. The starting of most vehicles and things is more of a momentary max so you just need to be able to survive this without overheating your cells and aim for a low c of discharge. Typically, you will be able to charge at a much higher c than you can discharge (which is good). A lot of people don’t like 18650 because they remember the old cells which were only rated at 6A max discharge… meanwhile at the time Nimh cells of the similar format were rated at 60A max discharge. So, if you throw a couple of 3.6V cells in series you’ll hit ~14.5V nominal and this “S” will be able to handle 30A per “S”. Then you can parallel a couple more “P” ‘s to get your max amperage capacity and your total usable capacity you will need.
Typically, deep cycle car batteries are rated for 80% dod with a 400-cycle life, while lithium cells at will have a 92% dod capacity with a 500-750 life cycle. For reference of what can be achieved if you stay between 40-70% depth of discharge the Panasonic cells in the Tesla are rated for 40,000 cycles. Of course, with car batteries if you don’t sit and drain your car battery with the engine off they will last much longer so life cycles depend on C draw and dod. I would look more into building one, but I’m in college and can’t afford to fix something that isn’t broke. So, say if your car has a 200a starter you would take 7 P’s in a 4s config. Then just see if you’re capacity is large enough and add from there. Also, you would need to protect from overheat, overvolt, and over discharge which isn’t super hard with what is available now. At least the overcharge won't happen since the pack will be large, and the overdischarge feature is already a feature on the Prius unless you override it. You could even integrate a battery warmer and a cooler. Still going to be like a 300$ battery, but at least it will weigh nothing and be super cool. You could also sub in lithium iron cells in the aluminum box format from EV power. They are cheaper for capacity but less power dense. So, I would say super capacitors aren’t necessary unless you want to use them.

[Ecky]

I'm usually game for trying something, anyone have a recommendation for some supercaps? Or, at least some specs to look for? I don't have a sense for the capacity I'd need. I've also never set up a BMS or balancing circuit.

[teoman]

There are a bunch of no name or random name cap sets out there.

But personally I will buy the same as Redpoint5 has.

Maxwell is a very reputable brand.