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Corpze, you never mentioned the electric motor brakes, are they removed?posting pictures of your application would also be helpful.Tommy
If all the connections are right, then inductive spikes is the most probably cause.To tame that problem, you can:1) Add a 220 uF to 1000 uF electrolytic capacitor between + and - right at the connection to the Sabretooth.2) Add a 1 kW or higher TVS diode with a standoff voltage of 25V or more in parallel to that capacitor.3) Add a similar TVS, but bi-directional, across the output to each motor, also on the Sabretooth.4) Perhaps add a 60V or better ceramic capacitor across the output to each motor -- I don't know how much capacitance -- I'd try a 1 uF first and look at the scope to see what that does.
so from your explanation, the sabertooth got fried when turning the power on, not trying to move the motors, correct?do you know what stall current your motors run at?this is a strange issue because the sabertooth does have overcurrent limiting protection, overheating protection, and auto shutdown.did you try the sabertooth again with the previous 12V SLA battery to see if it was completely fried?with those motors and batteries i would recomend going for a 2x60A sabertooth, or possibly a high current roboteq motor controller (which are very expensive)
Just turning on power may cause an inductive spike. See for example: http://www.pololu.com/docs/0J16/allBuy a capacitor rated from -40 C to +85 C and you'll be fine down to -40 C :-) Aluminum electrolytics don't do great in cold weather, so you may need a fancier kind -- again, you may have to trade off some capacitance.
Typically I believe you will use between 1k and 10k Ohms. The point of pre-loading is largely to avoid arcing in the switch, but it will also help some with capacitors. Make sure the device on the other end is okay with this kind of pre-loading, though, as some devices are very unhappy when they are fed too weak a current source.