ok, so now there's 40V worth of batteries in series... that's 8 batteries need charging. And, in the last post I explained many hurdles.
Check this... the DC converters require 36V-min [74 max]... so 40V gives just enough clearance such that my 12V car system can bypass and charge three at a time, leaving 5 totalling 25V + 12V from the car = 37V to keep my loads powered while charging.
That comes at a slight penalty; now there's not quite as much available maximum power [max current is 2A, but now the voltage has dropped while charging]. Probably not a big deal, really... how often is 37V*2A=74W not going to be enough?
But, more frustratingly, it means a full charge requires three cycles, 3 batteries each, twice, and two in the last cycle.
[Hmm, I haven't yet pondered whether they can all be charged simultaneously if the system's not powering a load... 5V 8*2=16A might be a bit to consider].
OK, anyhow, the idea is simply use a double-pole/double-throw relay for each battery to switch it out of series for the circuit and into parallel for charging, and a diode on the 12V source and another on the last battery in the group handles bypassing.
Now, if I use that same logic with Even Moar Batteries, we can cut it down to two charging cycles. e.g. 60V cut in half + 12V = 42V > 36V. Now I only need two diodes [less power dissipation], and can charge the whole thing faster, six batts at a time... and I happen to have two 12V->5V 6.8A 'lighter' adapters, which can each charge three of these. And, of course, more runtime, and more max power when not charging.
[maybe 10's more reasonable? 25+12=37, although that might require bigger caps, again, hmm... maybe 14! Heck, 74/5=... sure, 15, why not?]
[BTW, this is getting a bit ridiculous, originally I just planned for rechargeable power for/attached-to a few things like a portable DVD player and TV [4G is expensive, yo!] and the little computer my buddy @Starhawk built me, oh and maybe a reliable power-source for my external backup-hard-drive... and then maybe for my electronics-projects when coding [e.g. #Floppy-bird is still in the noggin'], and then my electronics-projects which needn't coding [e.g. #Incandescent RAM is bubbling up again], and then, maybe the soldering-iron... and... well, it's starting to make a heck of a lot of sense to keep all that power centralized, I see, now, rather'n having a dozen devices each with its own battery[pack], especially since most are not likely to be used simultaneously].
Now, we still have the problems when I might bump the power-cord outta the lighter-plug, of: batteries which are charging take a quarter-second to turn-on for a load, combined-with: now there's switching time for the relays to put them back in series. So, back to a big ol' cap... two, maybe. But, check this: because our charging-bypass-voltage [12V] is lower than the total voltage of the bypassed batteries, charging the capacitor to the battery-voltage from 12V will *be* the load which will turn on the batteries. So, now, we needn't wait for the capacitor voltage to drop *before* the batteries know to turn on. Thus.... faster start-up. Of course, counteracted a bit by switching relays. But ah well. Besides, now we're talking 12 [maybe one less per charging-bank] relays, anyhow... 's not like we hadn't reached ridiculous-territory long ago.
[Of course, a reasonable electronics-hacker surely would just install banana plugs in the dashboard and be done with flaky connectors altogether! But there's always portability and dead starter-batts as good reason for this project]
Relays, yep... break-before-make, yo! [I may reconsider mosfets... but *four* vs one relay?]