Comment by plorg | Hacker News Reader

Comment by plorg | Hacker News Reader

Only partially related, but I have a friend who carries around a comically-large 14000 mAh battery[0]. Comically large as in about the size of three or four iPhone 6s's stacked on top of each other - he wore it on a belt clip.

My other friends and I made fun of him. He always had his ridiculously large, power-guzzling phone plugged into it. But he insisted that he had it because it could jump-start a car (he's a mechanic, but with no formal training and only a high school education, and he said he had never used the device himself). I and another friend, me with a masters degree in electrical engineering, and he with a PhD in an engineering field, thought the idea preposterous: Surely one could not draw enough cranking amps from such a device to start a car, and could it even hold enough energy to do so?

I received, by way of my own carelessness, an opportunity to test this out when I found my car's battery dead one day in the middle of winter. My friend lent me his battery pack, and I set it up, leaving it connected to the car's battery for 30 seconds before attempting to start the car. Lo and behold, the car started.

I felt chagrined, but I also thought this was really cool. With the price and availability of lithium batteries as they are, why _should_ you need a 50-100 lb lead-acid battery to jump start a car?

Also, maybe I should learn a little more about cars.

[0] I couldn't find the Amazon listing back, but this is in the same product category: http://smile.amazon.com/dp/B00D42AFS8

My car runs off of a 3.3lb LiFePO4 battery. It has very little capacity compared to the 50lb lead-acid battery it replaced, but has no trouble sourcing enough current to start the car reliably. Lithium batteries have the ability to source tremendous amounts of current, which is why they're so popular in RC models. The best RC lipo batteries can source 65 times capacity continuously, and are often limited by the cell tabs, wires, and quality of solder rather than ability of the cells. LiFePO4 doesn't have quite the same power, but it's still more than enough to start a small car engine.

I think the reason why lead-acid is still common are:

1) WAY cheaper per capacity, by a TREMENDOUS margin (10x+).

2) Better low-temperature performance.

3) Better tolerance to being abused (overvolted, run dead).

On a car (at least an old one) the ridiculously robust lead-acid battery is used also to filter out spikes and to act as a voltage regulator. If you've ever wondered why the headlamps on a kick-start moped flicker while idling, that's what you get without using batteries as a filter. Li-ion wouldn't survive very long in this application AFAIK.

There are plenty of replacement LiFePO4 auto batteries available, and they work very well without needing much in the way of support circuitry. I think that it's mostly cost keeping lead-acid around (as well as operating temperature range and lifetime), not that nothing is suitable for the application.

The headlamps on a moped probably flicker because it uses a magneto instead of an alternator as its power source.

Can't comment about mopeds, but in car, when your battery nears the end of its life, when you turn the steering wheel (with electric power steering) while stationary, headlights dim for a short moment until the alternator increases its output.

If you're wondering why we still use lead-acid batteries in cars, it's because they don't have a memory effect and a high power to weight ratio.

Oh, I'm aware of what value a lead-acid battery has in cars. But after considering it for a short bit I'm not so surprised that you could jump start a car with a smaller lithium battery (as opposed to the bulky equipment that most mechanics around here will carry around to do the job).

But you are not jump starting the Car with the Lithium battery.

You are slowly recharging the existing car's battery so that it has enough charge to start the car.

If you disconnected the car's battery this wouldn't work.

The big battery (and leads) that your mechanic uses does have sufficient capacity and peak current to jump start the car, even if the car's battery was disconnected.

An old, old trick is to recharge your car battery in an emergency with a bunch of dry cells (eg torch batteries).

I was curious about whether the battery can push the current to start a vehicle or not. Going by the videos they post, it can:

http://thepowerall.com/index.php?route=information/informati...

The video at the lower right shows the Jeep starting with the vehicle battery disconnected (the booster is clamped to the disconnected positive lead).

Pretty much everyone in this comment thread besides you seems to think that I am suggesting replacing a car's main battery with a lithium battery. I do not believe I ever suggested that.

While I respect that you seem to have read a more circumscribed version of my comment, I'm not sure what your quibble is. A two-pound battery pack that can be used to jump-start your car seems like a heck of a win for most people.

Yes, you can start your car by disconnecting your battery and connecting a charged lead-acid battery. That seems pretty obvious. I can't think of a reason you'd need to do this, though, unless your car's main battery has gone bad. And if that's the case you will need to call a mechanic anyway.

Leave alone that the lithium batteries mentioned above are advertised to supply 400 amps, enough to start many cars.

I have the product you linked in the trunk of my car. So far I've had to use it twice and, just like you, it worked beautifully each time.

I wouldn't recommend it for your cell phone but it's way more convenient an emergency kit than jumper cables.

If I understand your description correctly (and it is the way it should be done, whether jumpstarting with external battery dolly or using another vehicle), you had the battery pack connected together with that old monstrosity of lead-acid battery at parallel, which still had some charge left and also acted as a buffer.

It would be completely different story having the lithium battery pack connected as the main and ONLY battery and cranking with it four times a day.

A typical car battery can begin at 55 Ah 12 V, this is for a European downsized petrol engine (larger engine requires more, diesel requires more, electric accessories require more) from before the advent of start-stop systems (start-stop requires better batteries or even two of them).

That is 660 Wh.

I guess the weight and volume of this in li-ion would be similar to or slightly better than lead-acid (imagine roughly 10x laptop battery or 75x 18650 cell), but the cost would be a multiple even at manufacturing prices.

Next, the car battery can sit in freezing cold for several days to weeks at -20 °C and still crank or it can be frying itself all day under direct sunlight. This is deadly for li-ion. Electric cars have heating and cooling systems for the battery packs even when the car sits still in a parking lot.

Also, while driving, the battery sits next to hot engine. You can move the battery elsewhere (driver seat or trunk, like the second battery for start-stop systems), but you need high-energy leads and the engine heat is actually a benefit for recharging the lead-acid in winter.

Compare all these complications to price, simplicity and reliability of nicely recyclable lead-acid box...

I think 55 Ah is a bit large for an econobox. That's about the size in my 6 cylinder volvo and my 6 cylinder pickup. I have seen quite a few small cars with batteries that look to me like they should be for a motorcycle.

I have replaced a few wet Pb batteries in my vehicles during the past couple years and the average lifetime I got from them is 9.5 years.

I once had a friend show up to help me start a car that wasn't starting–we thought it was the battery but it turned out to eventually be a flaky ignition switch.

It took us a good 20-30 cranks to eventually narrow it down to the ignition switch (when we bypassed it it started). I couldn't believe that little paperback-sized box was capable of doing that.

And it was a relatively big, high-compression engine.

I almost still don't believe it.

The engines and starter motors have been slowly getting more efficient over the years. I recently hired a car which, once up to temperature, would kill the engine when stopped and fire it up for you as soon as you put it back into gear. That feature wouldn't ship without a lot of confidence in the ability to start every single time without putting excessive wear on the starter.

You may already know this, but for those that are wondering - this is called "Start-Stop" and is built into many new cars. The feature saves about 10% in fuel usage: http://en.wikipedia.org/wiki/Start-stop_system

Yeah, this was a supercar made a couple decades ago, which makes the whole thing all the more impressive. Probably a pretty inefficient starter motor on it, and no start-stop on it, obviously.

This is very common in Thailand and Malaysia. Vendors, locals, they're often seen standing in the street holding both the power bank and the phone in the same hand, plugged in, while talking.

Clearly they're being pragmatical, it's all about battery time and a small set of features.

Reply on news.ycombinator.com