AC300 charging via generator during power outage

I’m considering the AC300+B300 to use as a backup power source during power outages. My plan was to run my critical devices (refrigerator, gas furnace, some lights) directly from the AC300. I had planned to periodically charge the AC300 using a small (1600W) gasoline generator, as needed.

My generator has plenty of power to periodically charge the AC300. (2-3 hours would be plenty to charge a B300 battery.) However, my generator does not have enough surge power to start my furnace or refrigerator reliably. As such, I planned to run all my devices from the AC300 (which has plenty of power), and periodically use the gasoline generator to keep the AC300 charged.

My concern / question is regarding the UPS mode of the AC300: From what I can gather, when the AC300 is plugged into AC, it feeds that AC directly to the output, rather than use the inverter. (I suspect this is because the inverter is a “bi-directional” inverter that can either be used to charge the battery from AC, or create AC from the battery, but not both at once.) As such, I’m not sure that the built-in AC charging will work properly in my particular case.

I’ve read through the different UPS modes in the manual, and I can’t tell for sure if one of the modes will work. (I really want an “Online” UPS mode.) Naturally, I don’t want to spend close to $4,000 unless I’m sure that it will work

As such, I’m hoping to find a way that I can make the AC300 work in my case. The only thing I could think was to buy one or two Bluetti T500 / 500W AC to DC adapters, and use them to charge each of the B300 battery(s) directly, from it’s AC adapter port. It would cost slightly more, but seems like it would perhaps allow me to charge from a generator, while still using the AC300’s powerful inverter.

Does anyone have a good idea of whether this might / might not work? (In particular, can you charge the B300 directly with the T500 while it’s connected to the AC300?) Are there better / alternate approaches that might work better?

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From page 44 of the AC300 manual:

● Can it be charged and discharged at the same time?
Yes, the unit supports pass-through charging function for both AC and DC outputs.

On a side-note, it says regarding using a generator (page 20):
“…a generator with a pure sine wave output is recommended (e.g. Inverted-based Generators)”

Also, you may want to check this thread if you are going to use a transfer switch:

It seems when grid power is restored to the AC300, it automatically shuts of AC Output when using a transfer switch.

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Thank you very much for the helpful response! I appreciate it.

The generator is an inverter generator, with a pure sine wave output, so I think I am okay with that part.

The statement from the manual (“Yes, the unit supports pass-through charging function for both AC and DC outputs.”) is a bit ambiguous to me, as I think it could be interpreted two ways:

#1) The AC input charges the battery and the battery runs the inverter (online UPS) or
#2) The AC input is passed directly through to the output, and is also used to charge the battery (offline ups)

In both cases, the AC input power is passed through to the output, so it’s not clear to me which one it is. Mode #1 would work great for my application, as the battery could handle the brief surges. However, my understanding from reading the manual (which describes an offline UPS) is that it may be working as #2. If so, then the generator (which is used for both charging the battery and running the output) won’t have enough power.

I just want to be extra cautious before making a big purchase, to try to make sure all will work in my case.

As before, I appreciate the helpful response, as well as the link to the other thread about the transfer switch.

Ah, I actually didn’t understand the difference between offline and online UPS before, but now I get it – so thank you for the explanation!

I definitely agree the AC300 is offline – it explicitly says that right on the screen, so it looks like scenario #2 is where we are at.

(I notice the EP500 has both offline and online UPS. I don’t know if you’ve considered going that route, but I see with the discounts now on Bluetti’s website, the EP500 is only about $200 more than an AC300/B300, and you get an extra 2kWh for that: good deal. I preferred the AC300 for expandability, but if 5kWh is enough for you, it has the feature you want.)

I’m intrigued by your idea of charging the B300 directly. I noticed in the review on Hobotech (YouTube link) at the 19:45 mark, he charges the B300 directly and simultaneously through the AC300. So it seems like it would work. Your idea is almost like a “backdoor” online UPS, the only caveat being you are limited to 500 watts charging…

I actually think I am going to order a charger and give that a try. It may be a way for me to circumvent the problem we are having with the transfer switch. I will report back on my results…

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Thanks for the great suggestion on the EP500! I hadn’t looked at that one closely, and it does look like it has the features that I’m looking for. As well, the sale makes it very tempting. Thanks!

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I can confirm that it does work.

I hooked-up a DC charger (a simple 120V to 12V car-adapter charger – still waiting for my T500) into the DC input of the AC300, and unplugged the AC300 from the wall. The DC charger will charge the battery, and the inverter stays on. The battery can supply more output than the incoming charger when needed. The AC300 acts like a “online” UPS this way.

Excellent! That sounds like a very successful test-- seems like there is a good way to use the AC300 as an online UPS.

On a related note, I did end up ordering the EP500-- it seemed to have the features I was looking for, and the sale seemed pretty good. I do have very minor worries about the fixed capacity, but I suspect that 5kWh should hopefully be more than enough.

Thanks again for running the DC input test!

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And thank you for the idea! It also solved the problem I was having with my transfer switch mentioned in that other thread.

I actually think you could probably add B300 batteries to the EP500 using a similar technique: charge the battery directly with a T500, and hook the DC output of it B300 to the DC input of the EP500.

Sorry the EP500 does not have a DC input. AC wall or Solar.

See pages 23-24 of the EP500 manual. You can charge from AC, solar, and a third DC input like a T500 simultaneously.

After looking at the Bluetti user video, Bluetti is using the PV solar port for DC Input. So if you do not want to use solar, then the dc port can be used for a b230/b300 battery.

Bluetti is using the PV solar port for DC Input. So if you do not want to use solar, then the dc port can be used for a b230/b300 battery.

Unfortunately, I don’t think this quite works for the EP500 (non pro):

The B300 has a nominal battery voltage of 51.2V. However, the EP500’s PV input range is 55-145VDC, which is just a bit too high to charge from the B300. (The EP500 Pro has a wider PV input range-- presumably due to a better MPPT controller-- and can accept the 51.2V directly from the B300.)

Theoretically, someone could create a DC-DC converter to boost the 51.2V from the B300 to 60V or so needed by the EP500, but it would take a bit of work to do, and would add some further losses.

Just curious what size generator do you have that will not start your furnace and refrig?

Just curious what size generator do you have that will not start your furnace and refrig?

I have a small, lightweight inverter generator that can output around 1,600W continuous. I quite like this generator because it’s very small, quiet, and absolutely sips gas: I can run it around 10-12 hours on a gallon of gas, and it’s enough to run my fridge and lights. This allows me to survive a 2-3 day power outage with just a few gallons of gas.

However, when the generator is in ECO mode (low idle speed), it can take a few seconds to spin up to full speed / power. As such, if the refrigerator compressor kicks in with high surge load, it sometimes stalls the compressor before the generator can spin up.

I did end up getting the EP500, and it works just about how I wanted: The EP500 can run my fridge and lights with no issue, and I can trickle charge it from the generator as needed.

Good choice. smaller gen sets can’t be left on eco mode when connected to larger load like a refrig compressor. Heating systems don’t draw much…maybe 5 amps running