Stanley Fat Max 700 won't take a charge

Do you have a VOM? You can check for continuity in the wires.
And test for voltage at the battery, before, and after putting the charger on it.
What is the battery voltage?
If you don't have a meter, you can use a test light and see if it lights, and how bright it is.
Maybe the original battery was/is not bad, maybe the charger is.
Or the connector on the device. Maybe a broken wire?
Try another charger, that has a similar connection.

 

The charger for your device may be broken, and not charging the battery.
If you have another 12 volt charger, you can plug that one in and see if it charges the battery.
Continuity is where you put the meter in "ohms" and touch the leads to the ends of the
the wire (without power) and it should zero out, and probably beep. Meaning the wire is good.

If you put the meter on the battery, note the voltage, then attach the charger, the voltage should go up.
If it doesn't, either the charger is bad, or a wire is broken somewhere.

 

If you have a harbor freight store nearby, They have a very cheap VOM that they literally give away for free sometimes.

Did some searching and found this coupon, Harbor Freight Tools Coupon Database - Free coupons, 25 percent off coupons, toolbox coupons - 7 FUNCTION DIGITAL MULTIMETER

Last time they had a free one was back in October.

 

You can also get on there mailing list(email) and you'll get there ads with coupons pretty much every day....

 

If you have an android device, The app in the link i provided has most all the coupons including ones from their Inside Track Club. It is searchable as well.