curranb79 said:
So I don't want to spend a bunch of money on an emergency backup system when a pretty significant system is on the way. That said, I was contemplating the GC2 system that was going to run me around $500 (≈ $200 for batteries, ≈ $200 for inverter, ≈ $100 for charger).
Some thoughts:
Do you want to spend $300 on batteries/charger you won't use on anything else?
If you need more battery to do the backup tasks you want, you could instead invest that into a spare ebike pack similar to what you have, or better (same voltage (14s) but bigger capacity, better cells, or whatever you happen to need).
For how much battery capacity (Wh, kWh) you need, you would need to know how many Wh that all the things you're going to run will need to use during the times you'll be using them. For instance, let's just say some item needed 1000w, and needed to run for an hour out of the day (either all at once, or a few minutes here and there). That's 1000Wh, or 1kWh, each day. Let's say you have another item that needs 500Wh over the day. And another that needs 200Wh over the day. And another that needs 700Wh over the day. That's a total of 1000 + 500 + 200 + 700, or 2400Wh, 2.4kWh, per day.
Inverters are not 100% efficient, and probably run better at some loadings than others, so let's just use 75% efficient as a rough number, to be on the safe side (even though you'd probably get better than that). So it means it takes 1.33 as much power input to get however much power on the output. (1/0.75). So to get 2.4kWh out of the inverter, you'd have to put in 2.4 x 1.33 = 3.192kWh.
For those SLA batteries you started with, if they were about 1800Wh usable per pair (I didn't do the calculations, am trusting Fechter's math; he's probably better at it than I am anyway), you'd need at least four (two pairs) of them to get enough Wh to run that example system. Which means $400 for batteries, plus the charger...for something you'll only use for the power outages, if they ever happen.
If you have a 52v 13.5Ah pack, then 52 x 13 (round down with your supply, round up with your demand...it's safer
) is 676Wh, or 0.676kWh. So to meet the likely demand of almsot 3200Wh, you'd need at least 5 of those. Prices (depending on the crappiness or goodness of the cells inside, and their BMS, and the pack build quality, none of which you can see on the outside or tell until you actually have it and use it
) range from $250 to $700+ per pack. So it's a lot more expensive, but these batteries are something you can use for your ebikes...and if you don't, you could sell them to other ebikers, whcih is a lot more likely a possiblity than selling the SLA setup.
The idea was, in case of a hurricane-like disaster where power is out for multiple days, to be able to run the car with an inverter on it for a couple hours each morning to charge the battery bank and plug in the fridges for a cool down (BTW, I live in a multi-family so I'm wanting to provide some level of power for my family and my three small apartment units), and potentially run a small AC unit to cool down a room.
First, as Fechter notes, the alternator wont' run at sufficient revs with the car idling to keep the battery charged under that kind of load, most likely--you'd probably have to sit there and keep the revs up during the usage period.
Some cars don't have sufficient engine cooling to do this for long periods without actually moving to keep cooling air flowing, so that's a potential concern. Also, do you have enough gasoline around to be able to run the car at revved-up states for long periods?
If you have to run the car anyway, an efficient generator (honda, etc) capable of handling the loads you'll throw at it would be a better investment, and simpler.
Second...is the inverter rated for inductive motor loads? If it isn't, it's probably going to explode the first time the AC or fridge compressors kick on. If you have more than one hooked up at a time, it'd probably do it when more than one kicks on, if it didn't do it before. (ask me how I know
)
Then...is the inverter rated for sufficient peak power to handle the motor startup load (whcih is a LOT more than when it's running, and may not be stated anywhere)? If not....again, could poof the inverter.
Then...is the inverter rated for sufficient continous power to handle the running loads....at the same time as when a peak load occurs from something else running on it?
Then...is the inverter a true pure sinewave inverter, or a "modified sinewave", or just a basic "can't be bothered to tell you"
inverter? Some devices don't work well on anything ohter than pure sinewave input, and some will actually be damaged by non-sine inputs.
If you really have a critical load that *can't* go without power...give it it's own inverter. If there are several critical loads...give each it's own inverter. The battery power source you probably wont' have just fail. But the inverters can just die if overloaded, and if you don't have a spare, then *all* your stuff is down, for the duration of the power failure.
y bigger concern is if such a disaster were to occur in winter, when I would need a small amount of power to keep my natural-gas boiler running for the main house,
If this is a 24VAC system run from a 115VAC to 24VAC transformer (some are, some arent') then you might be able to run it on 24VDC, but you would have to check to find out how it is made. If it is, then a likely very small battery would run it for quite a while, like two little 12V SLA like many UPSs use. (or a 24v lithium battery, etc) Or a 24v DC-DC that runs off an ebike battery, or the car's 12v.
and to run the heat pumps which are the only heat source for the apartment building. Obviously everything would not run "as normal" but I'm wondering if I'd be able to run the heat pumps alternately (these are the heat pumps I have: https://www.fujitsugeneral.com/us/products/split/wall/asu15rls3hy.html).
It doesn't say how much continous or peak power it needs, so I don't know if they would work or not. I'd recommend asking the manufacturer:
--Will these run on an inverter?
--Do they need a special type of inverter, or will any inverter do? (the whole sine/non-sine/etc thing)
--What is their peak startup power demand? (worst case)
--What is their continous running power demand? (worst case)
There are always solutions to stuff like this...but first you have to know for sure what it is you need, or you might end up with stuff that doesn't do the job you want, and have to spend more money (and have wasted the other money, or worse be stuck in a power outage without the backup you thought you had).
If I were doing this, and the likelihood of a power outage was minimal but potentially long enough to be very costly in lost refrigerated items and/or health/etc problems from lack of building thermal control, and a long-term solution was already on it's way:
--I'd total up the worst-case continuous load I *had* to have on the inverter all the time, to know what it's continous output has to be. Then I'd total up the worst-case peak load it might have to provide, ever, to know what it's peak output has to be. Then I'd get an inverter that was pure-sinewave output (just in case I ended up with things that didn't tolerate less-than-perfect power input), that had a peak and continous rating at least half again what I'd determined as "minimum necessary".
Then I'd invest the battery money into more / bigger / better ebike batteries, that could easily handle the inverter's worst-case peak and continous loads, with enough capacity to run as long as necessary for any continuous loads.
Then ensure the charger for those would work on the car's inverter, so that I could charge those with the car just idling, if they were going to be needed for longer than their best-guess runtime (how many Wh (kWh) you would need them to output over a typical outage, given that inverters aren't 100% efficient, perhaps 75% guesstimate). And ensure the charger could charge them fast enough (and that the batteries can safely *be* charged fast enough) to keep up with the demands that will be made of them during the outage.
If I were doing this *as* a long term solution, I'd probably go to places like BatteryHookup, etc., and get some of the ex-EV battery modules they have (designed for high-power usage) and build a powerwall with them, and just get inverters that would handle the entire house power usage....but you'll already be doing this, most likely, with the solar system setup (though with a prebuilt powerwall rather than DIY).
(keeping in mind that if one battery alone can't handle a peak or continous load, two or three wired in parallel probably could, depending on the ratings).
