is a li-po 4s 14.8v, the CHEAPEST WAY to jump start a car (with dead battery) ??

realista

Enlightened
Joined
Aug 16, 2014
Messages
210
i was wondering if it could be the CHEAPEST way to jump start a car. What do you think?

I 've recently seen that most of jump starter are using these kind of battery configuration inside:
A) li-po 3s with nominal voltage 11.1, real MAX voltage on fully charged (4,2v x3) 12,6v
B) li-po 4s with nominal voltage 14.8, real MAX voltage on fully charged (4,2v x4) 16,8v
C) lifepo4 4s with nominal voltage 12.8 (3,2v x4), real MAX voltage on fully charged (3,6v x4) 14,4v

For what i know.......C option is the "best" possible because the final voltage is the neares and otimal to be SIMILAR to a lead acid battery, in fact a fully charged car battery is at about 14.4/14/7v with the alternator of the car. But the BIG CON is the big price of this kind of batteries.......that push me out the idea of using them.

But i've found that on most chinese jump starters, it's used a 4s li-po configuration.
I've read a lot of pages about the best thing to use.. and some people howewer think that even if the maxvoltage of a fully charged 4s battery is too much high, in a real life scenario it's not a REAL problem because the dead lead acid battery, when you connect the 4 s battery, can bring DOWN the EFFECTIVE voltage of the battery.... that going down coud perfectly fit the optimal voltage range of about 12.5v to 15v needed to be SAFE for the car input circuit.

so, if my considerations are quite reasonable..... to jump start a car only need the purchase of a lipo that can generate AT LEAST 100Amps of power, better if over 130/150amps , and built yourself (or buy) a connection with 2 alligator clips, and solder them with THICK diameter electric cables, terminating with a classic RC connector like EC5 or XT60...the same used by li-po battery you have.

or buy something already built like this
200A-Emergency-Boot-Car-power-battery-font-b-Alligator-b-font-font-b-clips-b-font.jpg

.....and of course, you should have a hobby charger to be able to charge and/or test the battery quality and capacity, BEFORE to use it as a "car battery safer..."

what do you think about it? i think there is no need to buy a low quality chinese power bank with jump start function, they generally use INFERIOR QUALITY internal li-po batteries...and i think the lifesman of that battery is going to be quite LOW, being mistreated with hiigh loads over 100/150A by the car. Instead using a battery of known quality/brand there is more chance it's going to be safe(not exploding) and to save the lifespan of battery over some years..... all IMHO naturally :)
 

wrcsixeight

Newly Enlightened
Joined
Nov 10, 2013
Messages
116
A fully charged lead acid car battery rests at 12.8v or so, not the 14.4 to 14.7 as you state. A few AGMS will be 13v. My Northstar is 13.06v.

14.4 to 14.7 is the absorption charge voltage that hopefully the cars voltage regulator allows for the correct time. Usually they will drop to the 13.7v range. Some GM vehicle voltage regulators in cold weather will allow 15.3v or more.

The second that 16v of Lipo is hooked to 11.5v of depleted car battery, their combined voltage will be closer to 11.5 than 16.
 

mattheww50

Flashlight Enthusiast
Joined
Jun 24, 2003
Messages
1,048
Location
SW Pennsylvania
Keep in mind that there are two ways to jump start a vehicle. The hard way is to simply connect the second battery and turn the key. The second battery has to put out at least 100 amps (and much more for a large displacement V8 for example) to turn it over. The far easier way is to let the second battery charge the original battery in the vehicle. Delivering 30 amps for to the battery in the vehicle for a couple minutes is often enough, and between the charge you have put in the battery, and the 30 amps your boost can delivery, there is a high probability the engine can be started. If you want to delivery 100 amps or more, you better be using pretty heavy wires. I know my jumper cables are 6 gauge copper. That is easily good for 150 amps.


Walmart used to sell some Jumper cables with 10 gauge copper. That is a bad joke. 10 gauge copper is good for about 30 amps, no way it can crank the engine, but if you leave it connected for a couple minutes before trying, you can put enough charge into the original battery so that the combined battery output can start the engine.
 

Gauss163

Flashlight Enthusiast
Joined
Oct 20, 2013
Messages
1,604
Location
USA
[...] Walmart used to sell some Jumper cables with 10 gauge copper. That is a bad joke. 10 gauge copper is good for about 30 amps, no way it can crank the engine, .

10AWG is (conservatively) rated at 55A in free air (unbundled). As for how conservative this is, the same table rates 12AWG at 41A, and I've put 50A through my 12AWG cables during SMPS load tests, and they were only slightly warm after 1 hour.

And those ratings are for continuous loads. For jump-starting one should consult pulse ratings, which are much higher.
 

m4a1usr

Enlightened
Joined
May 4, 2013
Messages
884
Location
Washington State
WOW! I'm thinking some of you need to go back to basic electronics and study some obvious failures from the past! I mean the internet is full of videos from the past. EC4 and XT60 connectors? Huh? Do me a simple favor and google RC world or maybe Fast Electirc boats or even emergency car jumpers. The lessons you seek have already been learned. A thousand times over. 4mm connectors will not work for 600 to 1000 amp loads! Who gives a %^$ about 10ga wires. Get a grip on the amount of required!

http://batteryuniversity.com/learn/article/how_to_measure_cca_cold_cranking_amp
 
Last edited:

Gauss163

Flashlight Enthusiast
Joined
Oct 20, 2013
Messages
1,604
Location
USA
WOW! I'm thinking some of you need to go back to basic electronics and study some obvious failures from the past! [...]

That's not a constructive reply. If you have a specific critique of something written above then please state it explicitly.
 

StorminMatt

Flashlight Enthusiast
Joined
Oct 30, 2012
Messages
2,263
Location
Norcal
10AWG is (conservatively) rated at 55A in free air (unbundled). As for how conservative this is, the same table rates 12AWG at 41A, and I've put 50A through my 12AWG cables during SMPS load tests, and they were only slightly warm after 1 hour.

And those ratings are for continuous loads. For jump-starting one should consult pulse ratings, which are much higher.

Very true. Wiring codes like the National Electric Code tend to give VERY conservative ampacity ratings for wires for the sake of fire safety. This is not an issue for jumper cables, which only need to carry high current for short periods of time. Also, residential wiring is often in conduits, and would get excessively hot with even modest current. That DOESN'T mean that a wire of a given size can carry no more than what the National Electric Code states. On the other hand, voltage drop may become excessive at higher currents.
 
Last edited:

CuriousOne

Enlightened
Joined
Oct 14, 2012
Messages
809
Guys, cable might get slightly warm, but voltage drop on it will be significant enough, to not allow alternator to crank up. So this is why thick wires are required.
 

Gauss163

Flashlight Enthusiast
Joined
Oct 20, 2013
Messages
1,604
Location
USA
Guys, cable might get slightly warm, but voltage drop on it will be significant enough, to not allow alternator to crank up. So this is why thick wires are required.

Yes, of course there will be higher resistance with higher gauge wire, but my original remark addressed only the rating. Whether or nor that is serious enough to affect the success of the jump depends on many other factors (e.g. length of wires, contact resistance, etc).
 

savvyjuan

Newly Enlightened
Joined
Dec 30, 2021
Messages
1
Location
canadian
Keep in mind that there are two ways to jump start a vehicle. The hard way is to simply connect the second battery and turn the key. The second battery has to put out at least 100 amps (and much more for a large displacement V8 for example) to turn it over. The far easier way is to let the second battery charge the original battery in the vehicle. Delivering 30 amps for to the battery in the vehicle for a couple minutes is often enough, and between the charge you have put in the battery, and the 30 amps your boost can delivery, there is a high probability the engine can be started. If you want to delivery 100 amps or more, you better be using pretty heavy wires. I know my jumper cables are 6 gauge copper. That is easily good for 150 amps.


Walmart used to sell some Jumper cables with 10 gauge copper. That is a bad joke. 10 gauge copper is good for about 30 amps, no way it can crank the engine, but if you leave it connected for a couple minutes before trying, you can put enough charge into the original battery so that the combined battery output can start the engine.
Correction 10 gauge is good for so many amps up to so many feet so if you have a very short say 1 foot length of 10 gauge you can put quite a bit of amperage through it without it burning up. The longer the length the bigger the gauge you go! That is why it’s a good idea! if you’re running a tablesaw or something that draws say 12 amps, you have a long long extension cord it should be thick conductors that way the conductors do not heat up like a curling iron when higher amperage draw comes through
 

Timothybil

Flashlight Enthusiast
Joined
Nov 9, 2007
Messages
3,665
Location
The great state of Misery (Missouri)
My question here is can four lithium-ion (in whatever chemistry) support a 100A drain more than once or twice, if at all? The highest output current rating I have seen for a cell was 20A. Granted, that load will be only for a few seconds, as any modern internal combustion engine with electronic ignitions is going to start in a few seconds if it will start at all. But still, what will repeated tries or even tries separated by days or weeks do to those cells.
 

Lynx_Arc

Flashaholic
Joined
Oct 1, 2004
Messages
11,096
Location
Tulsa,OK
Likely you could get closer to 50A for short bursts from these cells and if the voltage wasn't dropped too low the higher voltage could translate into higher power output overall. What sometimes happens with these packs is they can help charge up the weak battery so instead of it competing with the starter motor for power it can actually help in the effort to start the car. I've shorted a nimh AA and got something like 12A from it and I know few people actually want to draw more than 2-5A from them in normal use. In other words it may be possible to get 150A or more for a short burst out of high discharge 18650s.
 

DIWdiver

Flashlight Enthusiast
Joined
Jan 27, 2010
Messages
2,718
Location
Connecticut, USA
For 18650 cells, I haven't seen super high discharge rates. But for LiPo pouch cells, I've seen continuous discharge rates as high as 85C, with pulse rates as high as 170C. If you had just a 3A-H cell, that would be over 500A! These cells would be suitable for automobile starting even if the main battery is disconnected. Maybe for some big engines you'd need somewhat higher rating, but it would not be hard to build a 6 A-H 4S pack capable of starting heavy pickups without assistance from the main battery.

But something nobody has mentioned yet is trying to understand what is going to happen between the time you attach the booster and the time you try to start the engine. This is going to depend on a lot of things, including but not limited to:

Number of cells in the booster
Impedance of cells in the booster
State of charge of the booster
Wire size
Wire length
Clip strength and resistance
State of charge in the main battery
Size of the main battery
Condition of the main battery

If you were to combine a large vehicle battery that's deeply discharged but otherwise in good condition with short, thick wires, solid clips and a poorly designed booster, that's a recipe for overloading the booster cells before you even touch the ignition key.

On the other hand, a well-designed booster with good cables could in many cases start a vehicle immediately after connecting it without overloading the cells.

Or you could expect to connect the booster, wait a few minutes, then start the vehicle.

A professional who is regularly called to rescue vehicles with dead batteries would certainly want a large booster that could start most vehicles with minimal delay, assuming that jumping from their vehicle was for some reason not an option. I think that with increasing frequency, we will be seeing boosters as the first option before jumping.

But a homeowner, off-roader, camper, etc. might want a less expensive option than that.

So there's not a simple answer. It's about design choices, price tradeoffs, and expected conditions to be encountered in use.

But I do think that in most cases, high-discharge LiPo pouch cells are the right choice for this application.

And really pay attention to what you want vs. what you are buying.
 

Latest posts

Top