OK Before I start I have read that it's a really really bad idea because you'd have to constantly monitor the batteries to make sure they were still alive, kicking and healthy. Even if you're not using them as deep cycle batteries but creaming off the top. So given that starting point - "It's a really bad idea" there's no point in responding to this message to say that it's a really bad idea. I KNOW that already. But is there a way around this really really bad idea? Come at it from a different angle?
So imagine that you're in the book "The Martian" and you need electric power. You have access to 12V automotive Lead Acid batteries and unlimited electronics, well all that farnell or digikey can supply, and you need a solution. I know that if you weren't in "The Martian" but instead had just won millions on the lottery the solution would be to buy a new everything, and two of them.
So given that I'm on Mars and I'm trying to "engineer" a feasible solution... I don't have an answer, I just have another question, which might be an equally bad idea... So even if you look into using Li 18650 batteries there seems to be a serious headache of balancing batteries. So there's a problem of monitoring and balancing batteries. Could you overcome these issues with an IC that can monitor a Lead Acid battery? I'm looking at
http://www.ti.com/lit/ds/symlink/bq34z110.pdf but that says it's not suitable for new designs. I'm not sure that would even tell you that a battery was bricked and should not be connected to the load, or charged. Then it is quite an expensive IC but at least you're talking about adding that IC to a lead acid battery not an 18650. So first question how to determine the charge level and health of a 12V Lead Acid battery. Is that smaller problem solvable? Is there an alternative to the TI-BQ34Z110? There's lots of fuel gauges for LiPo but Lead acid not so much.
So I'm not sure about the usefulness of the TI-BQ34Z110 but lets assume for a moment that you had information on 10 lead acid batteries. The other thing I'm wondering is could you NOT attempt to balance the batteries (NOT use them in parallel in a 12V System) but use a number of them in series. OK it would take a uC and a lot of switches but if the uC knew the health of the batteries and could control their position in a circuit could the uC put 5 healthy batteries in series giving a nominal 60V. The uC might report that battery 'A' is toast and it's for dumping. But use the remaining batteries intelligently, possibly pulling batteries out of the circuit and replacing them with others to maintain a 60V output for as long as it could.
So each battery 'A', 'B', 'C', ... 'J' Can be switched to any position 1, 2, 3, 4, or 5 in a series circuit by a uC. Given that switching that much power fast is going to be a nightmare it might not even be possible. Still I'd imagine that UPS systems probably intelligently switch batteries it might be possible.
So the uC intelligently switches batteries into "position", in the same way when there's Solar energy to charge a battery the uC intelligently selects a battery to charge and can "switch" the charger to a battery. We know that battery 'A' in this example is 'toast' so don't even bother, but Bulk charge battery 'B'. Once 'B' is up to a certain charge individually do the same for batteries 'C', 'D', 'E'... and when they're all bulk charged connect a constant voltage across all of them in parallel to top them off, (assuming that you've got all 9 batteries up to 12V or something. They're all balanced and can be connected in parallel at this stage, I think.)
If you were going to use an off the shelf inverter, or a 12V off grid DC system the 60V has to be stepped down which is another headache and will introduce losses into the system but... You might need to raid a huge inductor from something which can handle that kind of power. Say for example you were going to power a 1KW Inverter you'd need 1,000/60 = 16.67 Amps. I'm thinking that might require the innards of a welder or something. (I know nothing about power electronics, did you guess?)
Actually that 16.67 Amps sounds a bit high so assuming that any of this is possible you could try for 20 batteries of which you connect 10 together in series to give 120V and a requirement of 8Amps, or there abouts. That 8Amps is better in terms of requiring less heavy duty Mosfets/Switches, but being able to switch 20 batteries to at least 10 positions would be a ridiculous number of switches. It's starting to feel like the BOM used to break the Enigma code during the War. Far far too many switches.
Going to buy a lottery ticket.