Thanks for bring this subject up again SamonaPower
Batteries are definitely the "weak link" of the alternative energy systems. I wish Hydrogen storage and fuel cells were more available at much lower prices.. but such is not the case. Other storage systems such as compressed air is horrendously lossy. New revolutionary battery designs are always promised just around the corner, but never show up on the market place for one of two reasons. (1) The have problems that ultimately can not be ironed out. (2) They function so well, that manufactures can't sell new batteries often enough and it cuts into the over all profit strategies. After all if you did have something that lasted twice as long, but cost the same amount to make.. which would you rather sell over the long term ?
Over the years, I have seen several different battery companies provide a specific gravity viewing gauge right in the corner of their deep cycle batteries. At a glance you can see the exact level of charge of each battery that way. As far as I know... this is absolutely the best and perhaps only way of knowing a batteries true state of charge.
Also while possibly too pricy for home owners, there are companies that provide specific gravity gauges that will install on batteries and provide telemetry information to multi channel data loggers. This would seem ideal to me. Take a look at this URL.
http://www.serveron.com/products/BM-main.asp
I wonder if a good low cost one could be engineered for people with less than a corporative budget ?
You'd have to have your little glass bulb that would float at X height in the battery acid depending on it's specific gravity. Then you'd need a very low cost low tech way of accurately measuring it's height.
One method might be to have an translucent opaque piece of plastic for the top of your floating bulb. The surface would be rough sanded. Then fire a very directional narrow beamed current regulated LED onto it's surface. Then have a photo cell pickup located next to the LED to pickup the reflectivity of the opaque plastic.
The closer the bulb rises towards the LED and the photo cell... the brighter the reflection, and the further away from the LED and the photo cell the dimmer the reflection. The photo cell would reflect this variance in light to a sequencally switched A/D converter or in other words a low cost multi channel data logger.
Even if each homemade gauge registered a different reading do to in-accuracies of home production. One could use a pressure chamber to alter the density of liquid to change the density of the fluid via pressure and map the curve of each home built gauge independently. This individual curve could be applied to that cell on your computer, and you'd have accurate readings on all of your electronic gauges.
A small cheep low energy palmtop or laptop computer could crunch the math and show you the health of each battery as well as exactly what percentage of charge each battery is at. It could red flag trouble and sound an alarm when you need to take corrective actions.
There might be better ways of measuring the specific gravity of a battery electronically. Any ideas out there ?
John II