the newer XL-5 speedos have a sticker saying they have LVC (low voltage cutoff). There is also one on the chassis in some cases.

If you dont have cutoff, you can still use the battery. You just have to be careful to stop running it before it gets below 6V.

They make cheap sensors that plug into the balance port and tell you the voltage, and beep when it gets to 6.4V. They sell for about $5 on ebay