Mini jump start packs

[Tracker]

http://www.amazon.co.uk/LEMFOTM-Multi-Function-External-13600mAh-Camcorder/dp/B00MXRWEMA/ref=cm_cr_pr_pdt_img_top?ie=UTF8

 

One like this recently won the Auto Express best on test of it's type and whilst it would be no good for starting a heavy diesel engine I wonder would it work to partly rejuvenate a flat leisure battery enough to get a few hours use before needing to start the engine - assuming no solar or EHU is available?

 

I understand the principle of capacitor discharge which is what this seems to be but I can't seem to grasp the techy bits about power output and how that output works in the real world other than as an instant boost to start a smaller petrol engine.

 

Perhaps one of our resident electronics experts could illuminate it for us please as it or something similar would seem a potentially very cost effective way of getting more leisure battery power for the odd occasion that one might need it?

[Brambles]

Tracker, it will be 8800 mAh at 3.6 volts. So by the time you convert this to 12 volts you can get about 2.5 Ah. About as much as a gnat peeing in a washing up bowl.

You would be better buying something like a 10, 22 or 33 AGM lead acid battery and connecting it in to charge it switch off and then leave it as a reserve to switch in if required.

Something like this... https://www.tayna.co.uk/Yuasa-REC22-12-CyclicGolf-Battery-P9549.html.

[Tracker]

Thanks Jon, I suspected that it would not work - anything that seems to be too good to be generally is!

 

Nevertheless if what you say is so, and I don't doubt for a moment that it is, how do these gadgets ever provide enough gnats pee to turn over and start an engine, not once but several times according to the Auto Express tests?

 

Sorry, and pardon me for being thick, but I just don't understand how something with so little stated power can produce so much starting power, and yet not be able to transfer that power into a 12v battery?

[Alanb]

May I suggest that you are confusing energy, (The capability of doing work. e.g kWh), with power, (The rate at which work can be done. e.g. kW)

As you will be aware, engine starting requires high power, for a very short time, hence not much energy needed.

[Tracker]

You may be right Alan!

 

In spite of a physics 'O' level some 56 years ago I have never been good at the techy theory side of things but have always been good at finding practical solutions!

 

Therefore as a mere simpleton I always thought that if you had a set level of power/energy available the two extremes of using it, in simple terms, would be full power for a short duration or low power for a long duration?

 

But maybe it just ain't that simple after all!

[Brambles]

As Alan has said, engines need very little total energy to turn them over.

Say you draw around 400 amps to start the engine and it starts in 3 seconds. Then it is 400/3600 * 3 = 0.3 Ah.

Not a lot, but what we need is voltage to drive those amps into the starter motor, and this is actually quite simple. The little battery and its DC to DC boost circuit tries to charge up the battery, the additional charge stored chemically is not very much and is not going to make very much difference to the voltage, However a lot of the energy goes into surface charge of the plates, and this is what supplies the voltage and peak current to spin the engine for a few seconds. Then these little Gnats can kick ass.

[aandncaravan]

Tracker, I see you are still looking for an emergency standby habitation battery. Plus a neat little trick solution?

How about looking at it from a different angle? By using the Starter battery?

You should be able to run it down to about 12.4v and still start the Motor. That should give quite a few extra Ah, especially if your Starter battery is a Varta LFD 90 Leisure battery as it makes a great Starter battery. : )

 

When you need to start the engine, 12.4v should be enough to fire it up,

But to be on the safe side, and replace the surface charge, get the little Gnat to bite the Starter Battery before you turn the key?

 

I am not recommending this as it is flawed for many Continental built Motorhomes, but my Dad said, "run a wire from the Starter Battery to the Habitation battery with a double (biggest physical size) 30Amp fuse each end. Put the two fuses in when you need the emergency backup power. Remove the fuses before using the little Gnat".

 

When the Alternator kicks in it will now charge both batteries at the same time, with up to 20amps, that is effectively twice what you were getting before?

You can straightaway plug the little gnat into the Lighter socket to recharge it ready for the next blast?

 

You may find, depending on how the Sargent is wired that the Habitation battery now charges at a higher rate than you normally see, because it is connected to the 'run down' Starter battery which will be getting a beefy old charge?

 

 

Sorry kids, playing with electricity is dangerous, don't do this at home.

 

 

;-)

[aandncaravan]

Just looked in detail at the little Gnat and it talks of a 15v output for 'Jump starting'.

So suggest you isolate the battery and rig a voltmeter to the battery when you use the Gnat to check the output voltage is in limits?. Or better still play safe and isolate the Starter Battery before using the Gnat?

[Tracker]

Thanks Alan. Not so much looking for a leisure battery back up - more a case of looking at potential cost effective opportunities!

 

I 'graduated' from the direct cable method many years ago, although it is so cost effective that it does still have merit! I did fuse the cable but I used cheap jump leads with a battery isolator switch to engage and to disengage it!

 

Just need to remember to operate it as I am not enthusiastic about using the engine battery for anything other than the engine unless the engine is running as I'm too old to push start 3.5 tonnes of motorhome!

 

I am leaning towards a 20 amp B2B unit but not decided yet, as a solar panel still retains the valued advantage of keeping all batteries charged when not in use with no user input required. Fit and forget!