[Atomic-S (OP)]

I have been considering getting, for my vehicle, one of these portable power packs that can start the vehicle in the event of battery failure and can provide power for other purposes, and which are typically powered by a lithium-ion battery. I have no experience with them, and the information available from certain vendors is sketchy, leaving unanswered questions. Among these: Prices vary, but it is unclear just what all one gives up for a lower price.  Such as longevity. How long can these units  be expected to last until they would have to be replaced? 

[chris]

I have been quite sceptical of these gadgets prior to now, although admittedly I have never actually done the maths to work out what might or might not be achieveable with them.

My understanding is that they plug into the cigarette lighter / power socket in the car and top up the battery.

The max rated current through the power socket (roughly) is 10 amps. P=VI, so 12 x 10 = 120 J/s or 120W.

The starter motor of a car engine pulls 100-200 amps. Let's assume the engine needs a 5 second crank in the cold to make it start: that's 200 amps x 12 volts x 5 seconds = 12000 joules.

At 120W power delivery from the top-up charger, that's 12000 joules /120 joules/second = 100 seconds to return the energy from a single starting crank, which sounds reasonable.

But these products are usually marketed as rescue packs to get you going when you've left the lights on on and pancaked the battery. They often say they can "restore a flat battery in 15 minutes". So let's examine that:

A car battery is about 150 Ah (amp-hours). This means it can supply a current of 1 amp at about 12 volts for 150 hours; that's 1 x 12 x (150 x 3600) = 6,480,000 joules of energy.

To deliver this amount of energy back into the car battery via the cigarette lighter - at 120W - would therefore take 6,480,000 / 120 = 54,000 seconds or (divide by 3600 seconds per hour) = 15 hours!

After 15 minutes at 120W you could have transferred 120W x 15m x 60seconds per minute = 360,000 joules.

These are extreme case numbers that I have used, but all the same, you can see that one of these "get you started" rescue packs is not capable to restoring a car battery to the fully charged state from a fully discharged state.

Also, the current draw of 10A or higher from the recharge pack is quite high and likely to, ultimately, degrade the cells; so they are not going to survive too many discharge cycles like that.

I'd suggest, therefore, investing in a pair of jump leads, a mobile phone and an AA or RAC subscription as a more viable alternative.

[Can someone please check my reasoning / maths above, just in case my early-morning mental fog has caused me to make a mistake]

[Colin2B]

Chris, your maths are correct but a vehicle battery may not have discharged below 50% so the starting point is higher. Also, I would never use one of these devices to recharge a battery, but putting the 2nd in parallel may give enough amps to get the vehicle started, then the alternator will soon get the battery back.
I have used one of these as an emergency system and they work when the car battery gives a slight growl but won't turn the engine over eg v cold weather. One problem is remembering to keep them charged up  []
If the weather is really cold or you have doubts about a battery, better to put on trickle charge when not in use or spend the money on a new battery, or even a spare, small, car battery and jump leads can be cheaper (many emergency starter packs only have a 17Ah battery). Remember as well that only deep discharge batteries will survive a discharge of 50% without serious loss of capacity.

Edit: I should have mentioned, important to look at CCA (Cold Cranking Amps) of a battery, for a short duration, say 15 sec, even a small battery will deliver a few 100 amps, enough to start a car. Also, if you do get one get it with jump leads as the cigarette lighter is really only good enough for charging when engine running, not starting.

[mathreal]

I have a lithium ion emergency vehicle starter which also dubs for a battery pack to recharge cell phones and other mobile devices on long trips.  I've had to rely on it recently as I've discovered the 12V battery for my car is almost dead but the replacement battery is easily available for retail so I had to order it online.