I recently got my hands on eight 12V 9AH lead acid batteries. Could I simply place them in parallel and charge them with a battery tender without any special measures? Are there any risks involved? Could it damage the batteries?
Most lead-acid batteries charge at a constant 14 4 volts, so charging several in parallel is really just a charge-current issue. If the charger cannot supply enough current it will likely lower the charge voltage to protect itself. As the batteries charge up the voltage will rise, but should NOT go over 14.4 volts, or you could "cook" the batteries, releasing deadly vapors and ruin the batteries ability to hold a charge.
In a nutshell, you can do this but stay away from "wimpy" chargers. Ideally it should have charge current equal to the sum of the AH values of all connected batteries. This means their is a limit to how many you can charge in parallel, as the charge time gets to be untenable. A good 20 amp charger should be fine.
Note that many lead-acid batteries may have a 13.8 volt idle charge rating, so be sure before you invest in a charger.
Batteries don't often fail low voltage or short circuit, but if they do, then a 'very parallel' arrangement could be bad as all the good batteries gang up on the bad one to force a high current through it. Protect each battery with a fuse in series.
Before connecting them in parallel, make sure one or more of them aren't duds, check each individually into a load. You may have to charge them individually to do this.
Before connecting them directly in parallel, roughly equalise the voltages. This is more important with lithium than with lead or nickel chemistries, but it's still worth making sure that you don't have large imbalances in the voltage, even with lead batteries.
Once you've connected them in parallel, you can treat them like a high capacity battery.
Parallel connection of lead-acid batteries is done routinely in a lot of cases - including almost all UPS devices, small boats, offroad cars, etc...
The more identical batteries are, the better.
They ABSOLUTELY must be the same voltage. They MUST be of the same type (flooded/gel/AGM, starter/traction/standby), it is good if they are the same brand and even better if they are all brand new.
It is also good for them to be in the same state of charge (i.e. fully charged) when you connect them first.
You get a battery pack having the same voltage as each of them, the sum of capacities and if the connections are symmetric - the sum of their CCA.
In most cases, if one of the batteries fails and you don't notice, the rest of them follow quickly.