Hi, folks. A friend of mine lives in an area with rather unreliable grid power. He has a backup system consisting of a all-in-one 2.5kW inverter slash charger controller and a 24V bank of 4 6V deep cycle batteries. The inverter device, in UPS fashion, automatically charges the batteries when grid power is available, and switches over when the grid fails. All in all, a really nice setup that I wish I had... not because I need it, but just because it's cool.
The system runs the bare essentials: night-time lighting (or occasional daytime windowless bathroom lighting), the stereo (the family can't live without music!), a ceiling fan or two (it gets hot!) and rarely, a computer. The lights are all 120V CFLs.
Well, the grid has become quite a lot more unreliable of late, often disappearing for 12 hours or more each day. In these cases, the batteries don't hold out for the entire blackout, and there's sometimes not enough time with grid power to charge them fully before the next blackout.
Because the climate is tropical (18 degrees latitude...), and 90% of the days are sunny, I'd like to help them augment their system with some kind of solar array. However, because the current inverter/charger is so "smart," I worry about a way to do it that doesn't screw up its normal behavior. That is, I'd like it to work as it does now, but in the absence of grid power, the solar should extend the battery life if the system is under load, or charge the batteries otherwise.
I assume the harder solution is to try to have the solar always "working." That is, even when there is grid power, to have the solar helping charge the batteries, or if the batteries are charged, providing some energy to reduce grid usage. This sounds difficult to me because I know if the panels are hooked right up, the charger will "see" the current, or resistance, or something but will in general not be able to charge the batteries correctly.
If there is a way to overcome this, it would be ideal. However, since grid power (when it's there) is cheap as free, it would be sufficient to simply connect the solar only when grid power has failed. For this, I assume something simple, such as a normally-closed relay powered on the grid side, would work. Grid power would hold the relay open (disconnecting the solar from the system), but once it fails, the relay would close, and connect the solar to the batteries, which should either charge the batteries, or provide some extending power to the inverter.
Would this simpler setup work as I imagine it? It certainly wastes the solar energy when the grid power is present, but probably requires less electronics, as the goal here is to simply improve availability, and not necessarily save any money.
Possibly I overestimate the difficulty of keeping the solar from confusing the charger. If that's the case, a pointer or two would be highly appreciated. I'll refer to existing threads on how to obtain solar panels, and such...
Thanks!
-Chuck