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-Am i correct on this: if the original plug had 300mA listed on itself, I can go HIGHER, in this case use the 500mA; but cannot use a charger that has a LOWER mA listed on it..
All other things being equal, yes you could. It would take longer to charge-up a discharged battery, however. Once a SLA battery reaches full-charge, it only takes a few tens of mA to replace the charge lost due to self-discharge.
I charge SLA batteries all the time with a lab power supply. It has dual meters, voltage and current. If you set the supply to 14.2Vdc (the recommended charge voltage for 12V SLAs) depending on state of discharge and battery capacity, the initial charging current could be several A. As the battery charges, the voltage remains at 14.2V, but the charging current tapers to less than 50mA. If then you set the power supply voltage to 13.8V, and come back a few hours later to check the current, it will be 20 to 30mA in cool temperatures, and as much as 100mA at high temperatures.
To charge a badly discharged 50Ah battery, you must put in ~20% more than that, so it would 70 hours at 1A, 7 hours at 10A, or 140 hours at 500mA.
Your jump starter rarely gets discharged to less than 90% of its capacity, so recharge time is much shorter.
A word of warning. If you let a SLA battery sit around for more than a few months
without being on a float charger, you might as well dump it. At 100degF, a lead-acid battery looses 10% of its charge per month. If you let a battery sit around with less than 90% of its full charge for more than a month or two, it sulphates, and looses capacity. That capacity loss is permanent, and cannot be recovered (regardless of what the Pulse Charger snake-oil salesmen pitch). The only way to prevent this is to keep the battery on a float charging regimen.