Nate Tempest

My battery is on the way out, so I've been looking into lightweight lithium iron options. In an ideal world, I'd like something that will save weight, but I can treat like a regular battery—leave the car for weeks with no fear of draining it too much, that kind of thing.

So, to determine the feasibility of that, I measured the parasitic drain on my 2006 S2k. I don't have an alarm or any other significant draws when the car is off, so it measured about 7.4mA, which is great. I did discover something interesting though: when something is plugged into the OBD reader, even when the car is off, it draws power. The flashpro adds about 10mA, and the OBDLink MX (bluetooth reader) draws over 50mA when off!

The largest battery available right now from Shorai has a "lead acid equivalent" capacity of 36Ah. These "equivalent" ratings theoretically mean it has the same current capacity of a lead acid battery with a 36Ah capacity—so it's not terribly meaningful when what you're interested in is actually capacity. Generally the conversion factor is just a multiple of three though, so a 36Ah "equivalent" will have an actual capacity of about 12Ah. Shorai says their batteries can be drained down to about 20% capacity, but ideally will be kept above 50%—so ideally we would not want to draw more than 6Ah from this battery. 6Ah / 7.4mA = 810 hours, or about 34 days.

So, if your car has no accessories or other extra drains when it's parked—you'll want to measure to make sure*—then you should be able to park it for a month with one of the Shorai '36' batteries and have it start no problem. As far as cranking amps, Shorai recommends choosing a battery that has four times the CCA rating of the expected starter draw, but at least two times, at a minimum. The S2000 starter draws about 150A, so you want at least 300A rating—ideally 600. The LFX36 battery has a CCA rating of 540, which should be great. If you wanted to push it (and/or you're not expecting any cold weather starts), you could probably go as low as an LFX21, obviously with a proportional decrease in capacity. (Even lower would likely "work", but wouldn't be recommended.)

One other thing I checked. Shorai gives these requirements for alternator voltage:
My S2000 alternator put out a consistent 14.5-14.6V from 2000RPM and up. (I haven't measured below 2000 since the car was cold, so it was idling at that level. I'm sure it's sufficient though.) So this should be fine.

I've focused on Shorai batteries since they seem like the best bang for the buck right now, especially if you leave your car parked for long periods and don't want to have to keep it on a tender. If you don't mind keeping it plugged in and want to go a little lighter, Antigravity batteries look like a good option. Their 12 cell 12V battery should be (barely) sufficient and is only 2lb. To be safe you might want to go with the 16V though. But for my purposes, it's going to be Shorai. (Although I'm sure the extra 2lb will destroy my lap times... )

*To measure your parasitic drain with a standard multimeter (like this $20 Canadian Tire special I picked up decades ago), you'll need to disconnect the negative terminal as shown here. Start with the highest current measurement setting, which is likely 10A. This is just in case there is a significant drain; you don't want to blow the multimeter fuse by running too much through the low current measurement circuit. Press the positive lead to the cable and the negative lead to the negative terminal on the battery, as shown here. Once you've confirmed that the draw is less than the limit of the precise measurement circuit (200mA —or 0.2A—on this multimeter), you can switch to that and measure again, to get a precise reading.

Fokker

I was in the same boat last August when my 3 year old battery decided to fall on its face. I've been running the Deltran/Battery Tender BTL35A480C LFE battery for 5 months now. It's 480 LCA battery that weighs 3.75lbs; MSRP is $299.95, but it can be had for around $200 new (hence why I decided to try it). After 5 months of use, I've notice virtually zero difference between it and the Group 51 battery; however, the temperature operating range is spot on when it comes to cold temps. My car also has virtually zero parasitic draw, so I have not noticed any issue when leaving it off the charger for multiple days, though I don't leave the car idle in the garage for more than a 3-4 days at most. I have the OBDLink LX and have noticed the same effect as you did in regards to parasitic draw. Supposedly the unit is in sleep mode when the car is off according to the LED indicator (flashing every 3 seconds), but I've taken to leaving it unplugged unless actively using it.

Chuck S

If you want to run a puny lead-acid car battery you need a battery cutoff switch that severs the negative cable right at the battery. Only consideration then is the natural self discharge all batteries go thru. Faster in the heat of summer. They practically hibernate in cold weather. A trickle charger may be needed in the summer if the car sits too long.

Lead-acid automobile batteries are not designed for deep discharge and rarely survive more than a very few. Deep discharge batteries excel at deep discharge but don't have the interior structure for high amp discharges as when starting high compression engines. There is no free lunch.

Lithiums have great potential. Problem is their cost.

-- Chuck

Nate Tempest

I don't see much reason to get an undersized lead acid battery at this point, given where lithium iron batteries are at. If you want to save money, you could get a minimally sized lithium one and use a cutoff. Maybe a bit more expensive, but longer lasting and more reliable. For me though, it's worth it to get one with capacity to spare and not have to worry about it. I'll put a quick disconnect in for winter when it'll be parked for a long time, but I want something I generally won't have to put much thought into.

anorexicpoodle

Ive been running a Shorai 2.5lb battery for 2 years now.

No special treatment, no tender, starts and runs like a normal car battery. Sometimes i go weeks without driving it, it seems fine. Only issue is if you spin on track, and have to hot-start it it doesnt work out so well.

flanders

Nice writeup, I just like to ad a few things.
Our starter easily draws over 400A on cold starts and the alternator outputs up to 105A.
The max charge rate on the bigger Shorai batteries is 24A.
Shorai states that all LFX batteries are for ATV's and motorcycles only, not any car.
They are not going to get an easy life in an S2000 even though they seem to work pretty good for lots of people.
On this forum I think I only seen one that went up in smoke, so far

Nate Tempest

The starter draw was the one piece I haven't personally verified. If it does indeed draw 400A on cold starts, that would obviously be more of a stress (although theoretically the larger Shorai batteries should still handle it, based on their specs).

Regarding alternator current though, it's important to note that just because the alternator can supply up to 105A, that doesn't mean it will. The current supplied by the alternator is equal to the difference in voltage between the alternator and battery, divided by the battery's internal resistance—which will be about 1/3 that of a comparable lead-acid battery. This is why it's important not to let the battery drain too far while the car is off. The further the battery is drained, the higher the current will be. I've read about two fires in lithium car batteries (at least one was Shorai; can't recall if both were, but it's the same principle). Both involved severely discharging the battery then either jump-starting the car, or placing it on a charger. Obviously it's still not ideal that it failed catastrophically, but it's important to understand why it happened. Given that the S2000 alternator puts out less than 14.6V, which is less than the maximum specified by Shorai, then as long as the battery voltage is kept above the minimum level that they recommend (and ideally above the 50% capacity level, approximately 13.1V), that type of failure should not happen. (Given that they would have specified the maximum alternator voltage and minimum battery voltage based on the charging current the battery can handle.)

It's true that these batteries aren't yet recommended for cars, and they won't be great for cars with larger engines or significant parasitic loads. They also require you to follow the instructions and not let their voltage drop too low. But if you're willing to do that, IMO we're at the point where they can be used without much hassle. That said, I'm not recommending them to you either; use your own judgement! If you'd rather get something sanctioned, Shorai is apparently coming out with a (larger, heavier) battery designed for cars later this year. Or you could pick up one of these from Antigravity. Pricy and not as light, but will definitely have all the power you ever need, and has a cool low voltage shutoff feature.

flanders

I've measured my starter and it peaked at 460A, outside temp was around 12°C/54°F.

I'm well aware of all that you mentioned but it's a good explanation for others, I was to lazy to write all that
I've been looking to get a Lithium battery for a few years but I think they are still to expensive for what you get, there's also still way to much marketing and shady claims.
The antigravity looks nice but the price is kind of ridiculous.
This battery is sold in lots of places with different names and it's the closest I found to be worth a try GBS 12V (4-Cell) 20Ah LiFeMnPO4
Specs are still a bit to low for safe long time reliability (10C 5s max discharge and 1C max charge rate) but a true 20Ah capacity is great though.

sweetdill

I've used a shorai for years now, when they are drained too far they are irreparable, so if you leave your lights on just once or something similar, you're out a battery.
My brother had a Ballistic Battery that got too low and we decided to bump start the car like you would with a lead acid battery, and just let the alternator charge it, it went full smoky melt down within 1/4 mile (no flames though).
Now we both have the Shorai 36's and I disconnect the negative cable on the battery between events, and every few months run it on the Shorai charger to balance the cells.
So long as the battery never drains too far, great battery and super light, I can't think of a cheaper way to drop 20lbs from an already gutted track car.

djsilverjohnson

If you want to run a very small battery you can add a bank of capacitors to aid starting. They work great as long as the car starts easily, but won't let you grind away at the starter like a 40lb DieHard. Here's one of many Youtube videos on the subject;
If you want to run a small lithium battery, the safest way to do it is to wire in a battery charge controller with it to protect it from overcharging. Lead acid batteries handle it better but lithium batteries will best-case die and worst-case explode if overcharged. I think some of the high end lithium car batteries have the charge controllers integral to the battery.

For troubleshooting battery drain, look for a clamp-on ammeter with DC capability. When you first turn the car off or first disconnect/reconnect the battery the current will be higher due to stuff like security systems, radio controls, etc. Let it sit for a few minutes until everything settles down, and anything left over is true leakage.

It's also a good idea to check the health of the alternator periodically. Check the DC charging voltage, but also check for any AC voltage. Anything over 200mV AC is a sign the diode pack is failing. AC voltage will kill a battery and isn't good for your ECU/ABS/Radio either.