Saving my unsuitable batteries, help please!

As a first time off-gridder still learning so much about batteries it's finally dawned on me that I need to change

my approach to this urgently to avoid getting sucked into a bottomless pit of expensive battery replacement cycles!

My problems are the following:

1) That I decided to go with PV in the first place not knowing it would cost far more to maintain than the power

savings would ever offset!

2) Grid-tie is not an option here as our utility doesn't offer credits or buy-back for any extra power generated.

3) Our power grid is proving to be a lot more reliable than I anticipated when I first decided I needed batteries to save me from bumping my head in the dark. There were horror stories in the news of the national grid collapsing and it taking 2 weeks to recover, but so far we have not had one power outage of even 5 minutes since all the doom mongering started 6 months ago. Very annoying!!!

What am I to do? I don't have true deep cycle batteries so I cannot carry on as usual using an off-grid system just

to make use of my solar panels when the cost of replacing these batteries is going to far outstrip any savings

made by generated power. Even if I wait and see how long they keep going before replacing them with industrial

grade deep cycle cells - the type that can go down to 80% DOD >1000 times, the economics still don't make sense.

I have a simple idea where I could win on both fronts while still keeping my current setup, but I need the brainy

boffins on here to critique it and tell me if it's worth a shot!

My batteries are probably half-way through their useful lives now, having clocked up about 100 cycles to

approximately 50% DOD, their warranties covering 250 such cycles.

My idea: The charge controller takes them up to a certain user pre-set voltage, then starts pulsing the power from

the PV to keep it from rising further. This creates a voltage difference between the + pole on the PV input and

the + pole on the battery output of the controller. This difference can be anything up to 10 volts, enough to

light up a small bulb for instance. This bulb can trigger a Day/Night sensor, the kind that can be cheaply

purchased at any DIY store. This Day/Night sensor could be used to trigger a timer that sends power from my

2500watt inverter to my hot water cylinder for lets say 5minutes and then switches off. This would discharge the

batteries by no more than 3%. The batteries would then charge using any available PV power until the voltage

reaches the set point, PV exceeds battery, bulb lights up, timer starts. Process repeats until PV production drops

off and can no-longer light the bulb. The point is to maintain the batteries in the top 10% of their SOC.

Brain Boffins - will it work???

If so, can it be done better than I explained???

Obviously in the event of a power cut I could manually switch off the how water cylinder and use the batteries for

back-up, but this is only likely to happen once in a blue moon with things as they are.