Internet speed drops on battery power but improves with generator—why is this?

So here's my latest anomaly I've noticed my internet speed (high speed broadband dsl) being very slow and not letting pages load. It worked normal for the first three months since off grid living started. I've contacted my service provider and they have trouble shooted a few things made some repairs gave me a new modem and still slow speed. Now, the other day I decided to fire up the generator and voila speeds up on the internet. Wake up the next morning hmmm slow speed but it's morning I'm on batteries hmmm go out fire up the genny and voila speed is back up again hmmmm. Tried and proven for some reason in the past two weeks my t105's are effecting my modem. Any thoughts greatly appreciated thanks for reading. :)

Generator 5500 watt
Batteries 6 t105's in series parallel = 12 volt= 705 amp hours
Inverter Zantrex 1000 watt 2500 max surge
Wire 10/3 25' run to panel

leopardtron pcb

I also experienced the same few days ago..
When modem was powered by the ac supply its download speeds are very low...
and when the modem was powered by ups its download speeds were normal and working fine..
I also wish to know the reason behind it..

OK, these are just "thoughts", I have very little experience in these matters:

1. Are there two AC sources? In other words, is there an inverter on the batteries, converting the 12 Volts to high voltage AC AND an a separate AC source from the generator? And are these two sources, somehow, alternately switched in and out of the "house wiring"? If so, perhaps it has something to do with the quality of the AC. The AC produced by an inverter/generator can be as bad as a square wave, or it can be an approximation of a sinewave but still have a lot of harmonic content [i.e. a series of "square" steps], all the way to a pure sine wave. If your "genny" is basically a motor that turns a generator/alternator, then it very likely produces a pure sinewave, so when it's running, the power is clean enough to not cause interference in the network equipment.
2. Some kind of strange ground loop. As much as possible, all power commons should be connected together at or near the same point. And, at a point as close to the power source(s) as possible.
3. A bad [noisy] ground connection on the AC output of the battery array.
4. Have you measured the AC voltage being supplied to each piece of network equipment [such as DSL modem, router(s), etc.]? Measure the voltage when running on battery power alone, and when the generator is running. If there is a significant difference, that might be your problem. To troubleshoot, also measure the voltage at the sources. If the difference is no longer significant, then suspect voltage loss in the wiring [i.e. is there a longer current path from the batteries to your network equipment, than the path from the generator to the network equipment? Is part of the path from the batteries through smaller gauge wires, etc. If the voltages at the two sources are still significantly different, then perhaps there is a problem at one of the sources [i.e. not producing sufficient voltage].
5. If there is a difference in AC voltages, and if one or more pieces of network equipment has a broad voltage input [such as 100VAC to 240VAC], and if the voltage, when running on only batteries, is near one of those limits, perhaps, being on the edge of that range is causing problems in the power supply. Check the voltage requirements on each piece of equipment to make sure they comply with your setup.
6. Is there WiFi involved? Perhaps the battery inverter is generating RF noise that is interfering with the WiFi signal [rather unlikely, but who knows].
7. What about the AC frequencies? Are you sure the generator and the battery inverter, both produce the proper AC frequency for your area (and for your network equipment?).
8. Have an electrician look at your setup.

That's all I can think of from my limited knowledge.

Here's another, WAY OUT THERE, thought: When you run on only batteries, is something powered that isn't powered when running the generator? Perhaps that something is causing some kind of interference. Especially if WiFi is involved. For example, a compact florescent light. A faulty florescent light might produce enough RF interference to scramble a WiFi signal, or even generate noise on the AC line. Extrapolate from there

Steve has given a very thorough analysis, but I don't know if your personal knowledge of electronics is on that level, so I would like to summarise his main points.

1) Can you check the output voltage and frequency of both the generator and the inverter, to see if the batteries/inverter system is providing a lower voltage or different frequency ? Also check the voltage from each source at the input point to your router, and that the output/input voltages and frequencies of the equipments are compatible.

2) Is your router connected to your computer modem by a wifi (radio) signal that might be receiving interference from the inverter or some other equipment ? (Some electrical equipment produces unintended RF (radio frequency) interference within a certain range, like old cars interfering with TV reception.)

As you are "off-grid", how does your router receive its signal from your ISP ? Phone line, dedicated or optical cable ? Can you provide a general circuit diagram of your overall setup ?