This is a request for suggestions / comments - I've a few specific questions but really I'd love to know what y'all think of my thinking. If you get me.
I've been running our domestic lighting and music from a couple of truck batteries in the attic, with 400 watts of solar panels on the back roof feeding through a solar charger to keep them charged.
I desperately need more storage capacity, but I know that you can only connect identical batteries together like this:
You can't connect up dissimilar batteries (whether capacity or age differs) because they'll tend to discharge into each other rather than sitting in happy charged equilibrium. So this is a no-no:
This is a problem, because I don't want to chuck out my existing batteries every time I want to add capacity. I want a solution that'll let me hook up as many old used car batteries I can get hold of.
One way might be diodes:
This'll stop the batteries discharging into each other, but am I right in thinking that this would mean that the weakest / smallest batteries would never get discharged to a lower voltage than the big ones? So, in other words, they'll all flatten evenly (at least as far as voltage goes), so I'd never end up with one battery at 10.5 volts - dangerously flat - until the others have discharged that much as well?
It's a shame to lose a bit of energy warming the diodes both on the way in and the way out, but it's a relatively small amount.
It got me wondering whether it'd be better to have the batteries charge and discharge separately. Instead of using diodes to stop inter-battery shenanigans, I could use relays instead:
Note that there's no need for one on the final battery. With this setup, you can see that the first battery is feeding my house, while the third one is being charged at the moment. No matter what state the relays are in, there's no way for the batteries to end up connected to each other, which seems nice:
But there's the issue of how to control the relays automatically. It occurred to me that I could use a single DPDT relay for each battery, controlled by the battery voltage so that when each one was fully charged it would automatically switch from "charging" to "feeding".
This looks like it could work, though I'd probably need a slightly more sophisticated circuit controlling the relay coils, so that there was a nicely presettable "switch onto CHARGE when less than 12.0 volts, switch off when greater than 13.5 volts" thing going on. All getting a bit complicated now, mind.
Well - that's where I've got so far. Any thoughts? Diodes seem like the simplest way to set up an expandable battery bank, but I'm not sure I've got my head around the way stronger and weaker batteries will interact. Obviously, a key aim of all this is to share the load, so that I don't run any one of the batteries down too low (= battery sulphation death).
Don't want to reinvent the wheel, if there's an established way of doing this…