The fist thing to be aware of is that a huge number of units are available on the market – some great and some terrible.
A quality power pack used properly will give many years of good service. Identify the frequency of its likely use before purchasing and find out the weight of the battery packs you are considering.
Do not go with just what the label says – if the battery pack is heavy – that suggests that it is good quality.
Seek recommendations – who else is using that battery pack? Understand the pedigree of the product and where is it made.
Check the quality of the cables; check the clamp size for suitability of application. Make sure that it is made from quality materials – what is the durability of the clamps? Ideally they should be made of bronze or steel. Bronze is by far the best. Are the clamps connected down both sides of the jaw – is power provided down both sides of the jaw?
The thicker the cable diameter, the more conductive power can be released from the power pack.
Consider if you want built-in charging or external? Quite often an external charger is something else to get lost – if it is integrated then it is all in one place.
Can the unit be repaired? Nobody expects a piece of equipment to fail, but if it does go wrong, can it be repaired, are the leads replaceable – these are the most fragile and most used components of any tester. Make sure that these elements can be replaced. Make sure that the repair turnaround is speedy and to your expectations – you don’t want it to be out of your hands when you need it most.