The Envoy 3*AA and Console 3*C are pretty much useless when running a WLIP providing only a few hours.
I'm not convinced that the WLIP will run reliably on battery power at all. Whenever I've had the power go down on a WLIP system, it's always required a power recycle to reset the logger even though the Envoy/console had batteries in. Maybe I just wasn't lucky enough to catch it in the first few hours of outage or maybe the consoles batteries were already partially depeted - must admit that I haven't run any rigorous tests of WLIP on battery power alone. But I do also wonder whether the WLIP actually does benefit from being powered from 5v and not the 4.0-4.5v of a partially depleted battery.
But maybe you have done some tests?
So how can we make use of the battery backup curcuit to improve backup times? Thoughts?
I'd suggest that the only simple solution is to wire the battery contacts to an external battery box using primary cells. If you had a 6-cell box of D cells wired as two parallel banks of 3 cells then that would give you a respectable current reserve. (But this is obviously on the assumption that 4.5v is indeed sufficient to power the WLIP reliably.) BTW I'm not sure how you can have a 4.5v SLA battery: two LA cells gives you a voltage of around 4.0v (maybe 4.2v @ literally 100% charge and no/minimal load) and three LA cells would be 6v nominal. Are you sure it wasn't say a 4-cell nicad pack that might give you eg 4.8v?
Also the max recommended voltage to apply to a VP2 console is 6v (personal communication from Davis).
Thinking more of trying to eliminate DC/DC convertors (like 6 to 4.5v or 12 to 4.5v) from the system as this only introduces something else in the system that could fail and run direct from battery.
Hmm, I'm not sure that as soon as you move from a simple external battery box (which could I guess be an offsite-rechargeable pack in principle) then any of the solutions is swapping one type of added complexity for another.
For example, it is possible to imagine the console being powered from an efficient 5v or 6v UPS that is fed 12v DC from a solar PSU. I think the circuitry for this sort of device is fairly straightforward and it could use any size of 6v SLA that you cared to specify. But I haven't come across anything like this that is readily commercially available (I dare say that they do exist but where do you get one from and at a reasonable price?)
Obviously, another option is to use a separate 6v solar panel/controller/SLA just to power the console. Although 6v components are not as easy to track down as 12/24v, you can with a little effort find 5W 6v panels and regulators, which are potentially powerful enough to be useful if they're just powering the console/WLIP. (At some point in the design process you have to bite the bullet and decide just how much current/power reserve you want to build in. So a 17Ah 6v battery, to take a size at random, would give you around 170 hours - roughly a week - WLIP run-time. )
But I'm not convinced that putting in eg a separate 6v PSU for the console/WLIP or using a 6v PSU (if you can find a suitable part) is any simpler than the solution I use which is to reserve the last 20% or so of the capacity of a 12v/100Ah battery purely for use by the console/WLIP. Yes there's an electronic voltage switch involved and also a 5v DCDC converter, but both parts seem pretty reliable and are relatively inexpensive to buy.