MINI CAR JUMP STARTER

[xchcui]

Hi.

I saw in the net,that there are a mini car jump starters(very small gadget-relative to the old fashion ones)that can start the car several times with battery capacity of 8000mAh,14000mAh etc..
My question is not related directly to flashlights,but related to the 12V lithium ion polymer battery that this jump starter has(BTW:this gadget include,also,led flashlight).
If i will buy the 8000mAh jump starter(it is enought to cars less that 2000CC engines) and i will charge it only to 40% capacity,will it be enough to start the car with totally dead battery(at least one time)?
If not,what is the minimum capacity that the battery should have in order to be able to start the car at the wrost case(as i mentioned-tottalt dead battery)?
I know that 40% capacity at 3.7V LI-ION battery is about 3.75V-3.8V(open circuit voltage),What is the OCV value of the battery of that 12V gadget when it has 40% capacity?
My question is based on the fact that while keeping the li-ion batteries in storage the best capacity value for it,is 40%,while more than that increase to the side of the 100% capacity will speed the degradation of the battery and the day i will need it,the battery might be degrade in such way that it will not be able to start the car(failed battery).

Thanks in advance.

 

[Gauss163]

That's impossible to answer unless you have the unit at hand for testing. The answer will depend on many parameters, e.g. temperature, chemistry, health (IR) and the design of the unit. The best you can do is ask someone to run some tests - someone who has a (relatively) new version of the unit that you plan to purchase.

 

[texas cop]

Not knowing the climate of where you live, I would as an rule take what you think you need then double it. The day you truly need it will be the coldest day possible and all other variables will be at their worst. Your double margin will help over come all those other issues. As far as keeping those cells at 40% capacity over 100% to reduce degradation, try a compromise how about 4.0 volt storage less life more power. Or how about this http://www.a123systems.com/lithium-starter-battery.htm

 

[xchcui]

... The best you can do is ask someone to run some tests - someone who has a (relatively) new version of the unit that you plan to purchase.

I don't familiar anyone that has anykind of those jump starters,so i can't test that.

... try a compromise how about 4.0 volt storage less life more power....

The mini start jumper has a polymer li-ion battery of 12V not the 3.7V,
so what is considered 40% capacity(relatively in voltage)for that 12V battery?
(40% capacity from full battery of 8000mAh is 3200mAh).
And what is the compromise storage voltage for the 12V battery at the same idea that you mention as 4V(that i assume you were talking about the 3.7V battery)?

 

[texas cop]

The mini start jumper has a polymer li-ion battery of 12V not the 3.7V,
so what is considered 40% capacity(relatively in voltage)for that 12V battery?
(40% capacity from full battery of 8000mAh is 3200mAh).
And what is the compromise storage voltage for the 12V battery at the same idea that you mention as 4V(that i assume you were talking about the 3.7V battery)?[/QUOTE]

I was suggesting a compromise. Personally I keep my batteries fully or near fully charged so I can use them when I need them. I consider batteries a disposable item that needs occasional checking, charging or replacing.

 

[xchcui]

Does anyone know what is the open circuit voltage of 12V li-ion battery,when it is 40% capacity?

 

[TinderBox (UK)]

I found this it may be of use, it does not give exactly 40% but you can work it out.

http://www.powerstream.com/lithium-ion-charge-voltage.htm

John.

Does anyone know what is the open circuit voltage of 12V li-ion battery,when it is 40% capacity?

 

[Gauss163]

I found this it may be of use, it does not give exactly 40% but you can work it out.

http://www.powerstream.com/lithium-ion-charge-voltage.htm

That will not help here. For jump starting one needs to know how the cells perform under very high burst current loads. There are no standards for testing or reporting such performance. These are extreme conditions pushing the limits of the cells (probably beyond the design limits in many cases). In these extreme conditions there will be large variations in performance even in cells from the same manufacturer, let alone those from different manufacturers. Moreover, at such high currents many new factors come into large play, e.g. losses from cable resistance and Schottky diodes on the cable, etc.

Due to these dependencies - as well as dependencies on the specific vehicle - the only practical way to know how many jumps you will obtain from a particular SOC is to perform real-world tests under exact target conditions. There is too little information known about the components and too many factors that affect performance to have any hope to make even very rough theoretical predictions.

 

[xchcui]

Thanks for your help.
The link that you gave me"schottky diodes on the cable"​,also, help alot.

 

[lumen aeternum]

It seems to me that the performance criteria is that it be able to jump start the car once, with 100% reliability. Twice would be better.

So it needs to retain enough charge to do this for a conveniently long enough period before the device needs to be put on the charger and topped off.

So you need some CCA value at some worst case cold temperature.

The user should be able to disable the safety system that protects the cells from over-discharge, in order to start the car at the cost of ruining the cells. Cheaper than a tow.

Also the device should have a push-button status display that shows the present CCA, with a table on a label to convert to actual CCA at various cold temperatures.

 

[TinderBox (UK)]

In Europe new car`s and motorbikes have the headlight turned on permanently when the ignition is on for safety, but it make it a pig to get started with a flat battery, keep on pushing.

John.