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Author Topic: Battery jumpstart pack.  (Read 2109 times)

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Teejwano

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Re: Battery jumpstart pack.
« Reply #15 on: September 12, 2018, 04:07:39 pm »

G'day all,  while I agree that the Li-ion jumpstarts are the ducks guts, just thought I'd share a recent project that works a treat...

I stripped some 6 or 8mm2 cable from scrap at site I was working on, typically used for ovens etc and got other parts from Jaycar.  Soldered on jump cables and crimped connections for 12Ahr SLA (Sealed Lead Acid) Battery, which I can use for other applications and am able to float charge with 5W Solar/ Charge Controller get-up.

Just another option, I mainly did to learn some new theory (in researching what would work) and skills.  Used it twice, so cheerin...also heard of similar thing being done with cordless tool batteries, which these days are getting pretty awesome.  Hopefully the compressed jpgs attach if anyone wants a squiz.   
« Last Edit: September 12, 2018, 04:12:14 pm by Teejwano »
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Arkane

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Re: Battery jumpstart pack.
« Reply #16 on: September 13, 2018, 10:15:34 am »

What were you intending to use this setup for?
The battery  (SLA deep cycle )  can not supply much in the way of amperage to crank much at all!
the small li-ions can though!
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Teejwano

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Re: Battery jumpstart pack.
« Reply #17 on: September 13, 2018, 10:38:25 am »

It has worked twice with flat car battery which on its way out.

This battery is effectively an uninterruptible power source (UPS) battery.  What's important is the Amp-hour (Ahr) rating, regardless of chemical composition, Li-ion, SLA, NiCd etc.  My 12Ahr SLA is sufficient. Anywhere between 7-20Ahr is good, so next time I'm gonna rig up my 7Ahr Li-on battery from my toolbag.

Below is some info I went off... can't remember source but it's on the money

"What happens when the Car Starts? A fully charged fresh Lead Acid Car battery has a terminal voltage of 13.8V. When we start the car, this voltage drops to 11.5V or less for few seconds. At this time, the battery delivers around 300 Ampere current to crank the engine and the car starts. So the power used at that time is

11.5V x 300 Amps = 3450 watts. When we convert this power into Horse Power (HP) we get 4.62 HP (used online calculator). So to start a car normally, around 4 HP power is necessary and for that, the battery should deliver 300 Amps current and its terminal voltage should not drop less than 11 V.

Now what happens when we start the car with a flat battery? Even though, its terminal voltage is around 12V in Meter reading, the voltage drops suddenly to 9V or less when the engine cranks. So at this time, the flat battery delivers only 200 Amps or less to crank the engine. So the Power is

9 V x 200 Amps= 1800 watts. So its Horse Power is 2.41 HP.

That means, the power is only 50% than the required power to start the car. Car will not start and the engine crank with a sound only. How this happens in a Flat battery? When the battery is aged or kept for long time without charging, “Self discharge” takes place. This is due to the difference in potential between the top and bottom of the battery cells. Similarly, the specific gravity of the Electrolyte (Sulphuric acid) also differs in the top and bottom. This results in the flow of an internal current through the battery and self discharge takes place. As a result of this self discharge, the battery voltage drops considerably below 9V when we attempt to start the car. The battery fails to deliver high current because the electrolyte is not capable to carry high current and a potential develops across the electrolyte.

So what the Jump starter do?

The Jump starter has a new 12 volt battery. It is the “Booster battery”. Its Ah can be between 7 Ah to 20 Ah. When it is connected to the flat battery of the car, the terminal voltage of the flat battery increases to 12.6V within 2-5 minutes. This is due to the transfer of charge from the Booster battery of the Jump starter to the Flat battery. The Booster battery gives few Amps current for this. When this happens, the efficiency of the electrolyte in the flat battery increases considerably and the terminal voltage of the flat battery increases to 12.6V or more quickly. So this increase in the terminal voltage is the secret of the Jumper starting. When the terminal voltage increases, the flat battery delivers around 200 Amps and the booster battery in the Jump starter gives around 100 Amps. So this combination gives 300 Amps for the quick starting.

The voltage of the Booster battery must be 12V to match with the Car battery. But its capacity can be between 7 Ah to 20 Ah or higher. The booster battery delivers only a some power to the flat battery. So its Ah will not affect considerably. A 12V 7 Ah UPS battery is ideal. If you feel weight in the Jump starter unit, use a Lithium ion battery rated 12V, 12,000 mAh to 30,000 mAh. Then the unit becomes light weight and portable. But the cost will increase considerably."
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Teejwano

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Re: Battery jumpstart pack.
« Reply #18 on: September 13, 2018, 10:45:17 am »

Oh I get ya now Arkane... nah SLAs come in varied cycles for different uses.  Mine isn't Deep-cycle.
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Arkane

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Re: Battery jumpstart pack.
« Reply #19 on: September 14, 2018, 09:16:28 pm »

Is that flat battery really flat or just flat enough to fail to start?

The trick with the li-ion battery is that it maintains voltage better than the SLA battery and thus the starter solenoid engages and the residual energy in the SLA battery can be utilised.
That clicking you hear with a nearly flat battery is the solenoid failing to engage properly and this is where most light duty jump-start packs excel !
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