The AGM batteries in most lead acid jumper packs are not impressive batteries.
They are usually the UB12120 or UB12180, 12Ah and 18Ah respectively, and can be purchased for 25 to 40$.
http://www.mrsolar.com/content/pdf/Universal/UB12180.pdf
Their self discharge is not very impressive, so recharging them monthly to keep them as full as possible is mandatory, or they can fail to perform their duty when required, when still relatively new.
http://batteryuniversity.com/learn/article/how_to_measure_cca_cold_cranking_amp
The chargers which come with these jumper packs are usually simple single voltage wall warts which will only seek to eventually bring battery to 13.6 or perhaps 13.8v. This can be Ok when the jumper pack is new and not depleted very much and it is plugged in for a long time.
These batteries when new, or when older, and more deeply discharged, will benefit from a higher amperage charging source seeking 14.7ish volts and holding that voltage for a few hours. Slowly getting them upto 13.6v over 12 hours is not as good for them as getting them to 14.7ish fairly quickly and holding voltage there while amps taper to 0.5% of capacity.
I know this is opposite of grandpa's trickle charge everything mentality, but it is a fact AGM'S can be tickled to death with low charge currents.
Limit amperage to 1/3 the rated capacity. When depleted the battery can easily absorb more than this.
In that PDF above of the Ub12180(18Ah capacity), it says the internal resistance is 18mohms
The Odyssey pc625 is also 18Ah capacity and has only 7 milli ohms, and 200CCA
http://www.odysseybatteries.com/batteries.htm
Huge price difference, huge difference in cranking amps, and self discharge.
As for knowing if an older jumper pack AGM will perform its task, Without a load test at low temperatures,....well your jumper pack might need a jumper pack assist.
