• Power Stations (PS hereafter) generally perform to spec.
• most “problems” result from unrealistic expectations, which result from
  • failure to assess actual requirements before purchase
  • failure to consider how one will recharge before purchase
  • failure to read online specs/manuals before purchase

No, you can use any panel that meets spec. Consult the power station's solar ("PV") input specs and buy panel[s] whose output specs are within the the stated limits of the PS' internal solar charge controller. For the purposes of this section we will use these actual specs for reference:

12v-45v, 10A

In this case

• panel¹⁾ Vmp must be ≥12.0v
• panel Voc must be ≤45.0v
• the controller will allow ≤10A of input current²⁾

Sometimes the spec will also state the voltage range where the panels can be run at maximum power:

MPPT range 15v-38v

This tells us that Vmp can be anywhere from 15v to 38v. Outside that range (but within the absolute limits above) the panels will make something but not maximum power.

Power station solar input docs give a Max Wattage spec with a voltage range and usually a current limit. In the case of our example that limit is 10A.

Watts = Volts x Amps, so the Vmp of our panel³⁾ will dictate what we can get at that 10A limit:

• 18Vmp x 10A = 180w
• 36Vmp x 10A = 360w
• 40Vmp x 10A = 400w

Sometimes no current limit is stated and we have to back into it. In our case the 500w will be attainable at at 45v, suggesting the current limit is 11.11A (500w / 45v). Remember the panel's Voc cannot exceed 45v so we cannot really run a 45Vmp panel.⁴⁾

Overpaneling (“too much” panel for the controller) is common with standalone MPPT solar charge controllers. The controller simply “trims” the excess current by manipulating the solar panels.

In theory, overpaneling the MPPT in a power station should work the same way. In practice they are “black boxes” with relatively little documentation from the manufacturers. Overpanel at your own risk (and share the results if you try it).

you can run the DC output to a bus or fuse box to split the connection. The total current must be within the PS' DC ouput limit; see below.

There is no easy/cheap way around this. Buy a PS that has sufficient specs to meet your needs.

1. bad connection - 12v ciggy ports are substandard even on their best behavior.
2. the chassis voltage is varying, so the 12v volt outlet voltage is varying, so the charge rate is varying (I=V/R)
3. the power station is throttling charge rate

and if theres a way to have it always charge at 100w?

That depends on the cause, the power station, and one's ability/willingness to DIY a solution. And it can't be always, just more of the time.⁵⁾

… then fix the connection. Clean/adjust/replace as needed. Anderson PowerPoles are a common upgrade.

If the wall charger output is 100w you could even run a cheap inverter from the ciggy port and plug the wall charger into that. It would be inefficient but we don't really care because the numbers are small and alternator output relative massive. This approach would work because the wall charger's output is usually at the higher end of the SG's DC input voltage limit.

If the wall charger >100w⁶⁾ you could get more consistent charging by boosting/regulating voltage up as described above. So if the wall charger runs at 15v one might boost the outlet's voltage to 15v.

If it's a larger PS that can accept 100s of watts it might be easier to run a semi-DIY relay/inverter setup.

Maybe. There are several factors involved:

1. some DC-DC chargers need to see the target battery's voltage before charging will start. These are often called “smart” chargers. If the DC-DC needs to see the voltage before charging but the SG does not expose that voltage to the DC charging input then the charger will not start.
2. if the above condition is met, then the DC-DC's voltage output needs to be within the PS's voltage input range. If the input requires ≥20v then it will not charge from a nominal 12v DC-DC charger.
3. if the above conditions are met the PS input current may still be quite limited. See the maxxing section to get the most from the charging run.