Despite their very small batteries LiFePO4 jump starters really do work – this article by Collyn Rivers explains how and why. Whilst a lot of power (the rate at which energy is used) is needed, the amount of energy (the ability to perform work) required is very small.
PIC: courtesy of http://www.thingsesque.com/kangaroo-jump-start
The reason why LiFePO4 jump starters really do work is that the power required from LiFEPO4 jump starters (that are typically only 18 Ah (amp hour) is 400—600 amps but rarely for more than five seconds. LiFePO4 batteries can readily be discharged at 400-600 amps so can readily start those engines.
The actual energy drawn in doing so is tiny. It is (say) 600 amps for (say) five seconds. In terms of amp hours that is 600 amps divided by 60 (minutes) is 10 and then by 12 (five seconds). This is a mere 0.8 amp. Any 18 amp hour lead acid battery thus has sufficient energy – but cannot release that energy at the power level required. This is often misunderstood – but explained at: http://caravanandmotorhomebooks.com/what-peukert-really-meant/
Everything you need to know (and more) about caravan and motor home electrics is in: Caravan & Motorhome Electrics. Collyn’s other books are the all-new Caravan & Motorhome Book, the Camper Trailer Book, Solar That Really Works (for RVs) and Solar Success (for home and property systems).