You may think that you need to replace your battery because the electronic control panel lights up only briefly and then turns off with only brief flickerings thereafter. Many people on ebay purchase another battery and it solves their problem. The real issue is that most likely the battery has been completely depleted of it’s charge and you need to run the bike for a while (2-3 minutes), let the flywheel come to complete stop, then restart the pedaling and there may be enough charge on the battery to allow the control panel to stay light, if not keep pedaling a little longer since the battery may need more of a charge. Once you get the control panel to light up, do a good workout for a considerable amount of time as it will charge the battery even longer and it will most likely retain enough of a charge for the next time you need to exercise. If on the other hand you fail to get the display to stay on ever even after doing the start, pedal a while, wait for it to stop spinning several times, it may very well be time change the battery.
Also, most of the offerings on EBay are very expensive. Most sell for $40.
Sportsmith has them for a very reasonable $20 ea.
Also on Amazon for a more reasonable price of $20, free shipping.