It’s summer in Buenos Aires, and of course that means we have regular power outages, during which not even the electric fans work. I now have an apartment with a two-meter balcony; I could definitely put a bench on it. And the bench could have marine deep-cycle batteries in it. To be comfortable to sit on, it should have 500 mm from the floor to the seat height; it could be, say, 1750 mm long; and it could probably productively be 500 mm wide, too. (I could add backrests if that’s too wide for comfortable sitting, but I think it may actually be a bit scanty.)
A properly designed bench could conceal a lead-acid battery bank within it, and its placement on the balcony could provide it with good ventilation to prevent hydrogen buildup, as well as safe drainage in the case of a containment failure. If we have 50 mm on each face of the cuboid devoted to supports and the like, we still have 400 mm × 400 mm × 1650 mm. A 1650 mm × 400 mm bottom area is ⅔m². A typical car battery (BCI group 58, 58R, or 59) might be 255 mm × 183 mm × 177 mm high, occupying a base area of 0.0467 m², 14 times smaller. In practice I think you’d have to line the batteries up inside a bench of this size in two rows of 6 batteries, with their long axis parallel to the bench’s, so you’d only get 12 of them in there.
Let’s be more specific, though: there is a large D-BAT DB180N 12V deep-cycle nautical battery which claims 12 V, 235 mm × 235 mm × 520 mm, and 180 amp-hours (7.8 MJ), for sale for AR$6500 (US$176). If you made the bench just a little wider, you could fit seven of these motherfuckers (US$1232, 54 MJ at US$23/MJ) in there: 520 mm × 1645 mm long.
Or, take a more typical car battery like a Bosch S4 55 D. It sells for AR$2799 (US$76) and is 12 V, 170 mm × 170 mm × 240 mm, and 65 amp-hours (2.8 MJ). If you put the batteries at right angles to the bench, you could get 18 of them (US$1368, 50.4 MJ at US$27/MJ) into a 480 mm × 1530 mm floor area. You’d probably be better off with a deep-cycle type and with fewer electrical connections to go wrong, but at least this shows that the pricing and energy density on the other battery are not too far off.
Amazon has a 35-amp-hour (1.51 MJ) deep cycle battery for US$110 (US$73/MJ) which is 127 mm × 167 mm × 196 mm and weighs, in archaic units, “25 pounds”, which means 11.3 kg. This works out to an energy density (probably fairly invariant for lead-acid batteries) of 133 kJ/kg, so a 50-MJ setup like those described above should weigh about 370 kg.
So with 50 megajoules in your pocket, so to speak, how long could you run things?
This laptop is using 9 watts. It would drain the battery pack in 1500 hours. It would drain a single 2.8 MJ car battery in 86 hours. That’s a pretty long power outage.
There are little 3-watt USB fans, but a strong electric fan might be 75 watts. It would drain the large battery pack in 185 hours (almost 8 days) or a single 2.8 MJ car battery in 10 hours.
If you were willing to limit your total power drain to, say, 250 watts, you could hook up the batteries internally with fuse wire to get some added safety against electrical faults. If you put the seven deep-cycle batteries in series to get 84 volts, 250 watts would be just over three amps; you could use four-amp or five-amp fuse wire, which would be about AWG28 (0.32 mm) for iron wire, AWG24 (0.51 mm) for tin wire, AWG33 (0.180 mm; hard to find!) for aluminum wire (also hard to find), or AWG35 (0.142 mm) for copper wire.
For the limited but extremely important use of cooling things to 0° or above, a potentially better approach is to bank cool in the form of ice rather than batteries. 370 kg of batteries holds 50 MJ of energy, which can remove about 100 MJ of heat from your house. By contrast, 370 kg of ice with its enthalpy of fusion of 333 kJ/kg can remove about 123 MJ. Moreover, water costs several orders of magnitude less than batteries do, kilogram for kilogram; it’s just a matter of the refrigerative apparatus to freeze it with and then, possibly, some kind of automated ice-cube handling apparatus to allow you to handle large quantities of ice without running delicate pipes through all of it. But the crossover in cost probably isn’t until a GJ or so.
(With 2 kW of sun streaming in through the window, 100 MJ is only 14 hours of cooling; surviving a week-long heat wave would require several times that.)
Shanghai and London, December 3, 2019 – Battery prices, which were above $1,100 per kilowatt-hour in 2010, have fallen 87% in real terms to $156/kWh in 2019. By 2023, average prices will be close to $100/kWh, according to the latest forecast from research company BloombergNEF (BNEF).
If we remove the Sumerian units, that works out to US$305/MJ in 2010, US$43.3/MJ in 2019, and US$27.8/MJ in 2023. Why are these prices so much higher than the prices I observe in the retail market here in Argentina in 2019? Because Bloomberg is specifically talking about battery packs for electric vehicles, although they don't mention this until later in the article, and lead-acid batteries are far too heavy for any but short-range or aquatic electric vehicles.
But it's very interesting that BNEF is predicting that lithium-ion batteries will get into the lead-acid price range.