Lithium-ion 12v Battery Replacement

[redpoint5]

Originally Posted by Ecky

Keep the car plugged in overnight, or get a much larger battery, a heating element and go out to the car an hour or two before you leave (ever) and start warming it. I'm not sure if it's really even safe to freeze them at all. Maybe take the battery inside every time you park? A big enough battery to always have a heating element running?

My plan is to use 2 cheap lithium ion batteries, and swap out the one that had been sitting with a freshly charged one. I'd have quick connectors in the cabin to make the process quick.

[Stubby79]

Not that I'm about to be a guinea-pig for potential rip-off products, but I like the look/concept of this...




http://www.ebay.com/itm/MAC-BAT-SUPE...A/173575698959

[Ecky]

At $159 it doesn't seem a bad deal. Nice enclosure, looks like it has a bank of 6x 950 farad caps, though the source is unknown and they may or may not have a balance circuit. Paired with what looks like a 6-7ah sealed lead acid battery like we use in our alarm panels at work. Maybe you could do a lot cheaper if you assembled yourself with parts from China, but not US sourced.

[hayden55]

Originally Posted by Ecky

If the goal is weight savings, I don't think a lithium is going to achieve it in cold climates.

Yeah purely weight savings I don't think you will gain much. And personally, I think spending $200 on a battery system to save a couple pounds would never pay off. Cool factor yes. But I'm not here to tell you that! I'm a nerd too and wanna do cool stuff with my internet friends as well. haha
But I have no doubt 18650 cells could start a starter motor in cold conditions. The correct way to me to do this would be to rig up your battery on a test bench, throw it in the freezer, and see if it will crank the car when cold. If it can't start the motor you can clamp it, interpolate up, and then redesign you battery to get your ccas.
Then rig it up again in the freezer, (you can even take a freezer you don't care about and throw on an aftermarket switch and turn it into a fridge and hit whatever temp your heart desires) and throw on your battery heater and see what it takes to hit 65F. A lot of these systems designed to keep a value like this will bounce back and forth effortely and keep target like magic. We did something similar with a self driving robot, and were afraid since the wheels weren't squared correctly it would pull in a direction. Nope. It compensated the whole time like magic and you couldn't tell it wasn't perfect. Its just SOCs process thing so much better than humans.
Final point, Its just what sort of daisy chain of parallel and series will get you the correct cca, reserve capacity, and voltage.
I'd say the main benefit would be is if you buy panasonic/lg/sony etc.. legit cells and stay between 40-70% discharge the rated cycle life is something on the relm of 40,000 cycles.
Rated capacity on deep cycle lead acid batteries (not starter batteries rated for 20% dod.) is 400-500 cycles at 80% dod, and only 1500 cycles at 50% dod.
Reference: depth of discharge (dod): rated safe discharge from 100 to x %.
cold cranking amp (cca or ccas): when battery chemistry slows and resistance increases internally, this is the amps the cell will output when cold at whatever temp.
But saying all of this I have experience designing these things for indoor facilities, not the outdoors. I haven't even seen below 28F here in AR this year. I think it was 58F today.

Oh, also the lithium cells are something on the order of 5-6x more power dense. For our autonomous robot project we had we built a pack of 18650 batteries that weighed 0.7 pounds, and everyone else had some config of lead acid batteries weighing 12 pounds. Extreme case, but it was hilarious to see our cargo capacity vs vehicle weight compared to everyone elses.

[redpoint5]

My interest in lithium replacements is to reduce the amount I spend on Pb batteries. It's something like $250 to purchase the 2 batteries my truck requires to get sufficient CCAs.

In the TSX, I want to experiment with the "virtual" alternator delete idea. It too can be very inexpensive.

[Ecky]

Any recommendations on a 12v warmer with a thermostat? So far the only systems I've found are 120v.

[Ecky]

When I got home today I brought the lithium battery back inside. I'll let it thaw and examine its condition. Might just be frozen. It's been consistenty below 10F at night for the last few weeks.

After an hour I measured the parasitic draw with just the capacitors in the car: 25ma. There's probably optimistically ~1 amp hour of energy that can be lost before the capacitors will no longer have the voltage to start the car. This doesn't account for the trickle balancing circuit and natural self-discharge.

So, 40 hours to deplete the caps just keeping the clock and radio presets in the car. Or in other words, chances are good that if I park the car on a Friday and try to start it on a Monday, voltage will be a little too low to crank. The idea behind the lithium battery was to keep the caps topped up for 20-40x this long, but it looks like that just isn't going to happen in this weather. If I can't find a heater solution I'll probably snag one of the tiny 7Ah SLA batteries from work and run it until the weather warms back up.

EDIT: I wonder if the energy needed to keep the lithium battery warm and run the controller for this would reduce its standby time to less than that of the capacitors. I'll keep digging.

[Ecky]

I wonder if this would be big enough?

https://www.amazon.com/Turnigy-Progr.../dp/B00USQVMKS

Unfortunately its lowest setting appears to be 25c. Might be ok if its well-enough insulated.

[hayden55]

For how small I would imagine it to be. I would get a 12v thermal probe kit with relay off of amazon, and a 12v jacket heater kit. Then any sort of insulation material around it would be good. Even an r-10 foam box would work from lowes foam board. I would look into how the EV manufacturers implement theres. I know the standard ones just flick on when the car starts, not running it 24/7.

[hayden55]

Actually a makeshift thermoelectric cooler would be cool. They can heat and cool.