SMITTY wrote:So, I've got that Davis Vantage Pro 2 weather station outside here that uses a solar panel to power the wireless system. The panel charges a capacitor during the day, and is SUPPOSED to use that stored energy at night. Once the capacitor is drained, a 3v battery takes over. When you start going through batteries every 4-6 months (should last 3 or more years), us Davis owners know it's time for a new capacitor.
The stock, factory installed capacitor is 2.7v & 10uF capacitance. That 2.7v rating in a 3v system could be the reason why this is such a failure-prone component of the Davis systems. A search will find hundreds of posts all over the net on this problem.
I don't think you have a correct understanding of how the solar cell, battery charging circuitry likely in intended to work. I know nothing about the Davis products and a quick look at their web site didn't didn't help much. No schematics at all. When the solar panel is producing max output, it usually more than the battery can accept, that's why you need a regulator. A well designed regulator also can increase the voltage so the battery can still be charged at low light levels.
Typically the solar cell charges the rechargeable battery, via some control circuit. It's important not to overcharge or over temperature the battery. Doing so would reduce the battery life, or in the case of Lithium batteries cause a fire. Not a possibility here because the available power is to low.
It's likely the capacitor is used in the regulator circuit. There are two many different designs to guess what's used. The capacitor is almost surely a polarized DC capacitor. Higher than rated voltage is OK and somewhat desirable. More important is the type, electrolytic, film or tantalum and the temperature/environment rating. There is also the new, only about a decade old ultracapacitor. These actually store considerable charge. They are expensive and are low voltage, around a few series battery cell volts .
Get some more info on the circuit and/or the OEM capacitor specs and type. I'll be able to make a recommendation. A photograph of the capacitor will help. Except for the ultracapactor the price will be inexpensive when brought from an electronics components distributor. Shipping will cost more than the capacitor.