My inverter seems to be producing too high a voltage when solar input increases, causing the system to shut down and re-cycle many times during the day. I do not see fluctuations in the grid voltage wwhen I take the system off line. Is there a voltage regulator in the inverter that can go bad?

call the ups engineers for an answer to the problem when connected to solar power
there may be a problem in that solar panels produce various voltages during the day ( depending on the cloud coverage ) and the ups requires a reasonably steady input voltage to operate properly
I have found the eaton ups product to reliable so the suggestion to talk with the engineers to ensure that the system is compatible with the solar set up would be to your advantage

I like to ask, is the voltage from the solar panel messure with some load connected or is open circuit, normally solar panels produce high voltage than the working system, the reasson for that is to have Amperes in of to charge the batery, these is related to the size and power generated by the panel.
No need a resistor or in any case use a voltage regulator to prevent over charge.

Solar panels produce DC voltage.
some solar panels have the inverter integrated into each solar panel.
I would suggest contacting a local solar panel supplier and asking them if they could add a Battery Charging System to the Solar panels you use. An then what sort of inverter to add to the bank of batteries.
An inverter converts DC voltage to AC voltage to use in the grid.
The AC voltages and DC voltages involved have the possiblity to kill if you much around with electricity.

I know this is an older question but I would not recommend using these batteries for your inverter. While they are powerful for their size, LiOn batteries require careful metering and control of both charging and discharge cycles and do not last long if fully discharged (IE: running an inverter until they are dead). You would get much better performance from a deep cycle lead acid battery, which is designed to be discharged fully.

If you were to use the drill batteries for your project, you'd want to get someone with electronics experience to wire a connection to the Ryobi charger to accept the lower voltage input from the solar PV (instead of 120V) so the charger can regulate the charge rate. You'd need to make sure the solar panel voltage is within the same range as the charger's low voltage side (never tested, but guess around 20-24VDC). In addition, you'd need to make sure the inverter is monitored and turned off before the batteries shut down (their internal circuit protection shuts them down around 10% IIRC). Don't rely on the battery to protect itself, that is a sure fire way to ruin your batteries (no pun:-).

Hope this helps.

I highly suggest you invest in a good quality inverter. For most equipment either pure or modified sine wave is fine. I would suggest you install a line conditioner after a UPS for any computer products. The line conditioner stabilizes the voltage much more closely than a UPS will.
As far as building your own, I've taken apart a lot of UPS models which have a DC-AC converter in them and they are filled with lots of heavy heat sinks, transistors, resistors, capacitors, etc. You can usually find these very cheap since the batteries die but the converter is fine. Check the input voltage of the original unit and start from there to make one easily that has everything already assembled with minor modifications.

Two possible problems; input voltage too low, or the output load too high. If input remains at 12v or higher and it shuts off with no load, then the inverter is bad.

are your inverters rated at 60 cycle full sinusoidal wave form or 60 cycle square wave. while both will be tolerated in a transformer, the square wave tends to heat up because it isn't a sine wave. the peaks of a standard sine wave are cut off if observed on an oscilloscope.

need to replace the power supply