Poor Wi-Fi performance with MacBook when using battery

I have a MacBook (late 2007) and I've noticed that my internet crawls when I'm using the laptops battery power. I can be downloading a file at 200 kB/s under A/C power, and as soon as it goes on to the battery it'll drop to 10 kB/s. I'm assuming some kind of power saving is being activated, but this has only happened recently. Around the same time this started happening, I updated my computer to 10.5.8 and I also turned in my computer to the Apple store to have the palm rests replaced for having cracked under the weight of the lcd. Would this problem be caused by either the update or the repair, and if not, what could be causing it?