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First off, this is pretty much just a thought-experiment! I don't plan on actually going through with this as I'm sure it would void any warranty I have on my installation. Also, I'm totally unqualified as an EE; some basic schooling on the subject but not much more. Doesn't change the fact that it can be fascinating at times =)

Anyway, that said, I was randomly clicking through product pages on Ali Express this afternoon and found quite a few of MPPT controllers capable of charging (banks of) Lead-Acid batteries.

Currently, my (SAJ) solar-inverter is connected to its full capacity with panels cells and on a clear and sunny day it will deliver almost 30kWh solar energy to 220V AC. Part of it is used up directly in the house, part of it is sent to the grid. When there is no sun, I can recuperate that excess back from the grid. As there is no such thing as a free lunch, I'm "taxed" for the privilege of using the grid as a battery by means of the (theoretical) capacity of the inverter. Thus, adding more panels would result in me having to add an extra (or bigger) inverter and thus pay extra too. That got me thinking: what if I installed another -- independent -- string of PV panels, had it store the resulting energy until the sun is down and then somehow push that energy through the inverter towards the grid. Barring any losses this would then result in me putting twice the amount of kWh on the grid without the need of a bigger inverter.

Apart from avoiding extra taxation, this might also be useful to charge an electric vehicle at night (usually at work by day, at home by night) without the need for a separate AC circuit. Initially drawing from the Lead-Acid batteries and seamlessly switching to grid power as the former go empty (1).

Naively it would come down to:

  • By day, business as usual, the inverter is connected to the PV panels
  • By day, the 2nd array stores energy into say 24 batteries using 6 strings of 4 batteries (48V) each)
  • By night, the PV panels are disconnected from the system
  • By night the batteries are 'rewired' in series (288V (2)) and connected to the inverter whom then empties the batteries into the grid.

Things I wonder about:

  • Would the inverter be able to handle the surge of incoming energy? The batteries can easily 'flood' the electronics with hundreds of Amperes but I would assume the latter are capable to limit the incoming flow (3)
  • What device would be able to pre-limit the current to say 15A without too many losses; if such a thing exists?

Thank you for any insights!

(1: even MORE electronics needed I guess to avoid the batteries being drained all the way to the bottom, which as far as I know is "a bad thing (tm)", even for deep-cycle batteries)

(2: the DC voltage required is 80V-600V, so 288V should work fine. To get 30kWh like this each battery should have 1.25kWh or shy over 100Ah which sounds feasable although not cheap =)

(3: in fact, I've already got 20% over-capacity PV wise but the panels are oriented partly South, partly East so I only get more than 100% in rather exceptional situations. In those cases you can see the inverter top off the curve to its max capacity. From what I read it does this by upping the required voltage on DC side thus limiting the panels output current as it goes out of the 'ideal' V/I curve; I guess the same would be true with the batteries?!?)

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  • \$\begingroup\$ There are completely off-line photovoltaic/storage battery systems... they will charge the battery and automatically switch over to draw power as needed. However, batteries make the system vastly more expensive than just solar panel(s) and inverter, and life expectancy of the battery is inverse to cost (e.g. a 20 kW-hr battery made to last 30 years might cost 5 times the price of a 7-year battery). \$\endgroup\$ – DrMoishe Pippik Jul 28 at 21:42
  • \$\begingroup\$ It is possible to store energy and use it later. Generally, when you amortise the cost of the batteries out over their useful life, it is not cost effective to use batteries if grid power is available to you. It is cost effective for off-grid installations. Batteries are also good for short-term backup power. Once you have multiple energy sources, all the switching and control becomes complicated and you really have to have a clear idea of what you are trying to accomplish. The grid operator wants to make sure line workers do not get electrocuted by your generator or inverter. \$\endgroup\$ – mkeith Jul 28 at 23:47
  • \$\begingroup\$ So you need grid approved interconnect from any power source that supplies power to the grid. If you don't want to supply power to the grid, then you still need a transfer switch to change over from grid to your other power source. \$\endgroup\$ – mkeith Jul 28 at 23:49
  • \$\begingroup\$ hmm.. yeah, my quick lookup of (car) batteries on Amazon let me conclude quite quickly that it wouldn't be cost effective. Didn't stop me from wondering what kind of setup I would need for that and if in case of e.g. a zombie attack you would be able to jury-rig a string of batteries to the PV Inverter to create 220V so you could watch the emergency broadcast on TV … or reruns of The Walking Death for survival tips. \$\endgroup\$ – deroby Jul 29 at 19:20
  • \$\begingroup\$ Propane generator is your best bet for TEOTWAWKI. Maybe you can run it on methane (which you can make yourself somehow after The End). Some modification would be required to run on methane. Also, propane fuel can be stored for a very long time unlike any liquid fuel. \$\endgroup\$ – mkeith Jul 29 at 21:02

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