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

Batteries and Chargers Rechargeable Batteries
1

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 like this…

……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 :slight_smile:

2

In my personal opinion, I would lean more towards option B. The reason is that 16V, while higher than battery voltage, is still only a bit of a jump from the average 14.4V of the alternator output when the engine is running. The car electrical system should be able to handle that increase in voltage for a short duration with little to no side effects, and you would have a higher voltage potential to getting the car started. Depending on the size of the engine, you may not even need the full 100A of current. Most 4cyl engines don’t require too much torque to get running. If I were to build one, I’m thinking maybe 8 batteries in total, 4s2p configuration. Something good, like a 30Q, though.

3

I agree, more volts charging into a depleted 40-60ah battery is a good thing and will have little effect on the battery. On another note I tried 3s1p 30q’s it on a Suzuki 1lt car engine and it cranked it over, this was without a lead battery connected at all. I did it for 1-2sec just to see if it would, it was stuck together with magnets I am not saying that it is a good idea, it’s not what the battery was designed for. I did see a spec sheet (cant remember the cell) of a 18650 that did have a spec of 100 amp drain for less then 5 secs listed.

4

Samsung INR18650-25R SDI 2500mAh
20A max continues discharge
100A max for 1 second
50A max for 5 seconds

maybe is this battery??

i sincerely don’t know or imagine how much AMPERE can be brutally drawn out….from a good high discharge 18650 like the best 30q, 25r, vtc5 , can someone say me a number??
maybe max 100,130,150 ampere for a very limited time?
I think that to jump start a car is better a li-po, they are tested and built to go at 60C rate, 90C rate…. and after all they are ALREADY SOLDERED with a connector, so one person can just buy the alligator clip i posted in first post….without the need to DIY all welds with cables….

5

Why not use a: small, second hand, normal, lead acid car battery?
A bit more bulky, but very likely the cheapest way of getting the job done.

6

i don’t know if a small lead acid battery can handle the high burst rate needed……and, most important problem it suffers from self discharge, so you can’t count on it when you need it.
A jump starter is something a person carry into car, so shoud permit low self discharge, so you can count on it even after 6/12 months from the last charge.

7

But that’s exactly what a lead acid battery is designed to handle, that’s why we use them as car batteries. I always keep a secondhand car battery in the trunk of my car, it has saved me more than once.

8

do you think it can cause problems if you leave a lipo in the car?
the car can reach 60°C under the sun

9

it would be nice to be able to start a car with 4x18650 but i dont think its possible.

10

No you need 6 :wink:

11

in the other thread he used 3s2p configuration… but with a BAD battery like NCR18650A…… that’s not ok for high drain… i suppose rated max 5 amp.
so i am convincing that 4x18650 imr like the samsung 25r CAN jump start a car…… being rated at 50/100A pulsed.

after all ,from 2 NCR18650A in parallel the max drawn can be 15…20… 25A ? one single IMR can do better….

12

Fellows be aware that, properly speaking, good high discharge 18650 cells have ≈20mΩ of internal resistance. This means that, for every amp of current flow, output voltage will plummet ≈20mV (V = I × R) per cell in series. At very high amps current flow this has a very significant impact on the available power output, which rapidly decreases; at 100A per in-series high discharge 18650 cell group, and taking into account connectors and other possible losses, we may already be beyond maximum power transfer (load impedance equal or lower than source impedance). So, I'd at the very least go with an 4S2P config; considering how cheap are quality cells nowadays, I'd choose no less than 4S3P or 4S4P.

Oh! Let's not forget to add that, 100A per cell is staggeringly strenuous…

Cheers ^:)

13

You could try an old battery from a power tool like the 14v ones?

I just clicked the first video i found and it gave me vertigo hahaha He got 40 amps from a 12v battery.