How should I charge this battery?

[dentaku]

My father has a horribly cheap looking FIXIT Tools rechargeable Halogen flashlight but we can't find the plug anywhere.
I has a standard barrel jack with no labels at all so I don't know what it needs. Supposedly this thing also comes with a 12V car adapter so that's another clue.
I opened it up and the connector is center positive. There's not much inside it other than one of those cement 10ohm resistors, a switch and a power jack.

What kind of adapter should I use to see if I can charge this thing?

The 6V sealed lead acid battery looks like this.

[helius]

3-cell SLA battery should be charged from a regulated supply set to CC @2A, CV @6.9V.

[Zero999]

Just to clarify the above: C.C. means Constant Current and C.V. constant voltage.

You need a power supply which will limit the current to 2A, when the resistance is low and have an open circuit voltage of 6.9V.

A simple LM317 regulator circuit connected to a 3A 9V transformer, bridge rectifier and filter capacitor would do the job.

For 6.9V:
R1 = 150R
RV1 = fixed 680R

[jitter]

My father has a horribly cheap looking FIXIT Tools rechargeable Halogen flashlight but we can't find the plug anywhere.
I has a standard barrel jack with no labels at all so I don't know what it needs. Supposedly this thing also comes with a 12V car adapter so that's another clue.
I opened it up and the connector is center positive. There's not much inside it other than one of those cement 10ohm resistors, a switch and a power jack.

What kind of adapter should I use to see if I can charge this thing?

The 6V sealed lead acid battery looks like this.

If you don't mind charging the battery outside the flashlight, then a whole load of options are available, one of which would be an automatic SLA battery charger, another DIY.
But if you want to charge it inside the flashlight,  then you'd be looking at an adapter of about 7 V and no more than 2 A.

An SLA (Sealed Lead Acid) battery regulates its own charge current, depending on the state of charge, hence the need for a constant voltage source as stated on the battery. However, initial current (whenever the battery is empty or near empty) will need to be limited to a certain amount, depending on the capacity. In this case it's 2.0 A. This is to prevent build up of pressure (if you listen to one of these charging, then you'll hear faint noises of bubbles moving around in the gel).

Edit: I used to have a power pack with a big 12 V SLA battery. It was intended as a booster for starting cars with low (but not dead) batteries. This was supplied with an unregulated 12 V/ 1 A wall wart. The old fashioned type with a big transformer (not the switch mode stuff). This would do 12 V when loaded, but with less load it would go a bit higher. Exactly what a 12 V nominal SLA battery needs for charging.

[dentaku]

We're going to have to look around some more and see if there's a 6.9-ishV 2A adapter somewhere in the house.

[Gyro]

The LM317 datasheet includes an application circuit to reduce the peak charging current by the addition of a low value resistor between the regulator output and the adjustment network.

R1 and R2 values would be the same as Hero999's post for a 6V SLA.