Tech Topic: Lithium Car Batteries

Credit for inspiration on this topic goes to Ron Bauer. He recently posted on Facebook about the unfortunate situation of accidentally leaving the ignition on in the car before going off to his work assignment, only to return a couple hours later to a dead battery. He has a lithium battery, and had heard stories about other people having lithium batteries explode after being jump-started. So he erred on the safe side, elected not to jump start it, and packed the car up for the day.

Obviously, that was the safest answer! But I'd like to share my experience with lithium car batteries, and share some data points that might help you salvage the day in this situation.

And of course, if you find this useful, please continue to read/listen/share!

Disclaimer

This post is an attempt to share the semi-educated knowledge and experience I have on the topic. I'm simply documenting how I behave, regarding my own car.

I'm not a battery expert. I'm also not an engineer, lawyer, doctor, politician, or priest. So whether you decide to behave like me is your responsibility. Please don't assume the content here is correct or represents expert advice. Especially if you die or your car burns to the ground or you cord your tires. Then it is definitely not my fault. You've been warned.

But if YOU are more educated than me and have factual corrections for any errant info here, please send it to me so I can update the post (and give you credit)!

tl;dr

Please read the whole article. It would make me happy! But if you don't, here are the Cliff Notes for the rules-of-thumb that I use for LiFePO4 batteries, such as the Ballistic 18-cell that I use in my CSP Miata.

There are cases where you DO NOT want to jump-start a car with a lithium battery. But most of the time, jump-starting is fine...with these points in mind:

  • If the battery is below 8v, you probably don't want to jump-start it. Anything between 8v and 10v I would personally be OK with jump-starting, but I'd probably still keep an eye on the battery as it charges for a while, and shut the car off immediately if the battery shows any signs of swelling or getting hot.

  • An 8v car battery is a very dead battery. So if the starter gets enough juice to engage and turn over the engine, it is probably fine to jump. If the starter "clicks" but doesn't turn the motor at all, I'd proceed with caution...but I might still proceed. But if the starter gives you NOTHING and/or the dash lights are dim, I'd probably pull the battery.

Can You Spell LiFePO4?

In the context of cars, "Lithium" batteries typically refer to "lithium iron phosphate", or LiFePO4, or simply LiFe. If the "Fe" throws you off, it is because Fe is the symbol for iron on the periodic table. But I don't know what the "4" is. Not an expert, remember?

But the important thing is that this chemistry is NOT the same as "lithium polymer," which is often called LiPo, Li-Poly, or Li-ion. LiPo batteries are what you typically find in your phone or laptop. You also see them used in radio control cars and airplanes because they are the most lightweight and powerful batteries available.

However, LiPos are extremely sensitive to how they're charged and discharged. They require quite a lot of smarts in the charging circuit to charge safely, as well as smarts in the discharge circuit to protect them from being discharged to fast or too far. And if you get it wrong enough, you can end up with a fireball! I've seen this happen live. Even a 8oz LiPo battery from a radio-controlled car or airplane can create a big bonfire in the middle of a parking lot. It's really impressive to watch...if everyone stays a safe distance away.

Here's what happens when the charge/discharge protection circuits on a small LiPo aren't working properly, or when there's an internal short:

If you recall the recent fuss from the FAA over laptop batteries, this would explain why.

Combine this with the fact that cars have relatively "dumb" charging and discharging systems and you see why battery chemistry matters a lot. An alternator is about the most basic form of battery charger possible. They pump a ton of current into the battery, try to charge to 14v or so, and aren't very precise or gentile in how they do it. There is also typically no discharge protection at all, so no way to prevent a battery from being drained too fast (too much current), or too far (below a certain voltage).

By comparison, the traditional lead-acid batteries we see in cars are the honey badger of the battery world (warning: language), so they have no problems at all dealing with the "dumb" charging and discharging systems on your car.

I have seen LiPo batteries in car applications in the past. They should be the lightest and most energy-dense solution. But the ones I remember seeing were very expensive, presumably because of all the extra circuitry and microcontrollers needed to keep them happy. A quick Google search came up with no obvious sources for such a thing today, however. So maybe they are no longer produced. Or I was hallucinating. Both are possible.

The Big IF With LiFe Batteries

Which brings us back to LiFe batteries.

In nominal usage, LiFe batteries can handle the "dumb" charging/discharging systems in a car with no problem. They don't mind alternator charge currents or the heavy load of the starter. They can be jump-started, too. They're pretty stout little cells.

But there's always a catch, right? So let's amend our statement:

LiFe batteries are very robust and handle car applications just fine IF the battery isn't too dead.

LiFe batteries can be charged really fast and they do handle things like heat and over-charging really well, relatively speaking. But they do have limits. On every horror-story I've personally heard regarding a LiFe battery in a car, the battery was REALLY dead, and the owner jump-started it. So it seems clear that it was either charged too quickly for too long by the dumb alternator, or perhaps that the cells somehow get damaged in the process, causing an internal short in the battery that triggers a catastrophic failure. To summarize, I don't know why the bad things happen...just that they seem to follow a clear pattern: Jump-starting a car with a really dead battery.

The "good" news is that LiFe batteries are still considered NOWHERE NEAR as dangerous as LiPo batteries. Some seemingly-credible sources on the web seem to indicate that they're just about bullet-proof. Unfortunately, we wouldn't be discussing this topic here if that were universally true. In any case, I do believe these statements to be reasonable:

  • LiFe batteries will take quite a lot more abuse than LiPo batteries before they fail.
  • If a LiFe fails, it typically will just stop working
  • If a catastrophic failure happens, the battery can overheat/smoke/burn/melt/etc. That can still result in damage to your car, burnt paint, melted wires, etc. I have heard of at least one case where it actually started the car on fire. However I've not seen or heard of one exploding into a giant fireball as a LiPo can. (And at car battery sizes, an exploding LiPo would be a BIG problem!)

Get a LiFe?

LiFe batteries are pretty stout, but perhaps not totally bullet-proof. Meanwhile, you can get a small lead-acid AGM battery pretty cheaply, these days. So why take any risk at all on a LiFe battery?

  • One obvious answer is weight. These things weigh SO much less, you think they shipped you an empty box! The general quote around the interwebs is in the neighborhood of 65% less than an equivalent lead acid battery. Though the definition of "equivalent" is a bit murky, due to the nature of the chemistry differences.

  • Deep discharging and high current loads tend to permanently hurt the performance of lead acid batteries. But not so much for LiFe.

  • LiFe batteries typically offer 3x the number of charge/discharge cycles in their lifespan.

There are a few tips I'd recommend when buying a LiFe battery for your car:

First, go with a brand you trust. There are only a few actual battery factories out there in the world, so most of the individual cells come from the same places. But, still, some of the suppliers are better than others. More importantly, there will still be differences in how the cells are assembled into packs and packaged into something you can use as a car battery. So cheap housings and low-quality internal wiring can still cause problems.

Second, buy the "big" one. I happen to have a battery from Ballistic. They offer an 8-cell battery that weighs 1.5 pounds. Seriously, 1.5 pounds!!! That should have plenty of power to start my Miata in a perfect world. But when the world isn't perfect, it will be dead. Fast. They also have a 3.5 pound, 16-cell battery that has roughly the same capabilities as the stock 25+ pound Mazda battery (500 CCA and 28 A/H-equivalent). For TWO POUNDS, you get tons of insurance against a dead battery. That's worth it, in my book! And it still weighs less than half the weight of the tiniest lead acid AGM battery, which is still usually over 8 pounds.

Third, get one with balance ports. To learn what this is and why, keep reading.

Avoiding Problems

I personally have no concerns about running a LiFe battery in my own car. However, I absolutely do pay attention to the voltage. The only real rule is that I won't try to jump start the car if the voltage is too low.

In terms of specific numbers: The official minimum voltage for a LiFe cell is 2.0v per cell. Nominal voltage is 3.2v. So that means you'll typically see cells in your car battery arranged in packs of four, making 12.8v the nominal battery voltage, and 8.0v the minimum charge voltage.

(If you're confused about the 8-cell vs. 16-cell batteries referred to above, that's how many individual cells there are in the battery. But they're all arranged in sub-packs of four. The individual cells in the sub-packs are wired in parallel, to determine capacity. Then the sub-packs are wired in series, to determine voltage. So in the 8-cell battery, you've got four sub-packs of two cells each. And for the 16-cell, you'd see four sub-packs of four cells each. So they both make the same 12.8v, but the 16-cell has double the capacity.)

But if you measure your battery at 8v, you have to remember that you're measuring the AVERAGE of four packs of cells that have been wired in series. So it is entirely possible for one of those packs to be below 2.0v. Thus, I usually start to get a little nervous about jumping a car if the battery is close to 8v or so. I'll do it, but I'll watch it like a hawk while it recharges.

In "rules-of-thumb-land" (disclaimer: I can't guarantee my practice is always appropriate!), cars usually fail to start somewhere around 9-10v or so. Somewhere below that, the dash lights will be dim, and eventually the starter solenoid won't even click, much less get the motor to turn over. So, my rule of thumb is that if you hit the key, and the starter is able to turn over the motor AT ALL, you're probably fine to grab the jump pack and get it running. But if you see super-weak dash lights, or you don't hear even a click from the starter solenoid, DON'T JUMP-START THE CAR. Check the voltage before proceeding.

We installed a $10 voltmeter gauge from Amazon in the car. We got one of the round ones, and it fit almost perfectly into what used to be the cigarette lighter on the Miata dash.

Low Voltage - Now What?

So, you've found yourself in possession of a battery that has been discharged too far to jump-start. Now what?

The first thing to do is try charging it with a battery charger that has a LiFe-specific mode. Depending on the model you have, you may see some success. However, most LiFe chargers will look for a minimum voltage. If it is dedicated to automotive applications, it will probably consider that 8.0v minimum we talked about, above. Below that, they'll proclaim the battery failed and refuse to charge.

But you can SOMETIMES still bring a LiFe battery back from the dead.

We've had a small phantom current in our Miata for several years, causing a really dead battery if we forgot to disconnect the battery and left it sit for more than a month. So I've done this several times. It requires a charger that can be set to a manual mode, or sometimes you can use a setting for a NiMH battery. RC car guys often have fancier chargers like this.

The idea is to bring up the voltage slowly, at relatively low current, while watching both the temperature and the voltage very carefully. Once the voltage reaches 9v or so, stop the manual charging and put it on the LiFe charger mode.

As an example, I typically use a 4A setting or less to revive my 18-cell Ballistic. It is certainly safer to use less if you have the time to spare. It will take quite a while to charge at this rate, so plan to be hanging around the garage for the afternoon. If the voltage doesn't come up reliably, or if the battery starts getting warm, you probably have an internal failure of the battery, and you should probably give up. But if it stays cool and the volts keep going up, you may have just saved yourself some money.

This is the charger I use. It is probably at least 7-8 years old now, and I don't see this particular model for sale any more.

Venom Pro Charger

If you look around, you'll notice that MANY of the chargers out there seem to be re-branded versions of (almost) the same thing, with four buttons and a small LCD screen. Even the one Ballistic sells under their own brand is "suspiciously similar." Most of them probably are made in the same Chinese factory. But of course you never really know. In this case, I happen to know the Tenergy brand from my years as an R/C car nerd, so I'm comfortable recommending this particular one.

Balancing Internal Cells

You'll note that the charger above has what are called "balance leads". If you have a good-quality battery, you'll see a small connector that you can hook up to the balance leads to do two things.

First is simply to check the voltage. Remember we talked about the battery voltage being the average of the four sub-packs in the battery? Each of the four sub-packs is wired in series to get the 12.8v nominal voltage. The balance leads let you test each one. If one fails, it will show less than 2v, while the others are still reading 3v+.

The second benefit of a balancing charger (and a battery with balance leads) is that the charger can ensure that each of the sub-packs is charged independently to the same voltage. Over time, the independent voltages of each sub-pack will drift. Your car's alternator will bring the average for the whole pack back up to 12.8v, but that might mean some of the sub-packs are a bit overcharged while others are underchaged. Over time, this will hurt the battery and shorten its life. So using a balancing charger a couple times a year will help your battery last longer.

When To Let Go

Eventually, all batteries die. If the thing just won't hold a charge, especially if it seems to be stuck at 8-9v, it's probably done. Again, this is where having a battery with a balance port can be really helpful, because you'll see that there is obviously one cell that is just plain dead.

But no matter what: If the battery is getting hot, or if you start to see visual evidence that it is swelling or smoking, your battery is DONE. Throw it out if you see this! I've seen people successfully remove a smoking battery from their car to prevent it from damaging the car. But honestly, that sounds pretty scary to me.

Conclusion

Despite the extra caution/hassle I take to make sure that a totally-dead battery is charged properly, I still really like my LiFe battery. It is so much lighter, and it has been going strong for four years. If I don't leave the headlights on when the car is parked, I pretty much forget that it is anything unusual.

Hopefully this has been useful in helping you make up your mind whether to buy one or not, as well as armed you with some info so you can make better decisions when you're scrambling at an event.

Thanks for reading and listening to Cone Coach! Catch you next time.

Neal Tovsen

- The Cone Coach