DIY or Buy? Charging from generator on dark days or grid down. (48v system)

[Mithril]

Scope of this question/use is needing to use a generator with an existing system to supplement on dark days, bad weather, etc. (largely when the grid is down in my case). LifePo4 batteries.

Topics covered in other threads here:
- Efficiency/cost of running generators - running at "ideal" load is a huge improvement (2x or more fuel economy), generally figure ~75% load is good, exact % depends on generator. Even running at ideal you're looking at $1 or more per kWh
- Rectified 60Hz AC into an MPPT is anything from "unlikely to work" to "will damage/destroy something" (not many people seem willing to try, understandably!)
- The market for 48v chargers is not fantastic, various factors.
- Many (most?) AIO can't charge and invert at the same time "ALL AiO and ALL inverter/chargers..." (Thanks @sunshine_eggo !)
- You can't reasonable out panel bad weather (especially snow/ice)
- Going for multi-day battery capacity gets expensive fast, even a fully DIY battery.
- A lot of AIO have issues charging on non inverter generators
- You can achieve "Constant Current" charging (stage 1) by simply sizing the charger correctly
(Feel free to counter any of the above, but I'm going to take those as givens for the moment)


Given the above, here's my thinking:
Since I will not be using the generator every day I don't exactly care if the charging stops at a lower SOC, solar and/or grid power can take the batteries to the target SOC normally (including balancing concerns etc).
For infrequent use, the extra cost of an inverter generator of the same capacity doesn't seem worth it.
Given a suitable control (remote start/stop from an AIO) and a safe voltage, a simple AC-DC power supply should work. With a suitable supply the total current could be limited to fine tune it better for a specific generator.
It wouldn't be too hard to make the remote start even smarter, adding in a relay controlled by a microcontroller watching current out from the supply so the generator turns off if it drops below ~50% output (or whatever).

Advantages of dedicated AC-DC charging from generator (regardless of DIY or buy):
- quality of AC from generator largely immaterial.
- ability to set fairly constant load, for efficiency, noise, and not having to oversize the generator
- No need to oversize generator (for possible load + charging)
- Able to use a non inverter generator and save $

Disadvantages:
- Additional loss of AC-DC-AC conversion (worth it for me for the advantages, given the occasional use)
- DIY will need some control, and buy doesn't tend to communicate with battery (neither are issues for me personally, but it's good to keep them in mind)

I feel like DIY might sense here, thoughts?

 

[timselectric]

Diy
Small engine and alternator. Remote start from AIO dry contacts. (Assuming you have an AIO)

 

[sunshine_eggo]

- Many (most?) AIO can't charge and invert at the same time

substitute above lined out text with below:

"ALL AiO and ALL inverter/chargers..."

 

[Mithril]

Diy
Small engine and alternator. Remote start from AIO dry contacts. (Assuming you have an AIO)

Already have a generator, only looking at the AC-DC part also line losses at sub 60v would be a killer IMHO. Also, are there reasonable to find alternators at the right voltage? You'd need to limit the current as well to keep the generator at peak efficiency.

 

[Mithril]

DIY is always better because you are in control of your things.

Even with DIY we buy parts. The question is "if we only care about AC-DC at the right voltage, can we reasonable beat off the shelf chargers at cost, features, or both?"
And bigger picture, and I wrong on some aspect and there's more control need for charging than just CV with a cutoff controlled by something (AIO, microcontroller, whatever) assuming we don't care about getting to 95% SOC

 

[toms]

I use a commercial diesel generator linked to the AC input on my Sunny Island inverter, I doubt you’ll DIY anything simpler.

In practice, the inverter/charger powers the house as normal and sends 100a charge current to the battery when the generator is required. Someone smarter than me will explain how this is charging, not inverting, but still powering the required AC load - either way it works well and is simple.

 

[Mithril]

I use a commercial diesel generator linked to the AC input on my Sunny Island inverter, I doubt you’ll DIY anything simpler.

In practice, the inverter/charger powers the house as normal and sends 100a charge current to the battery when the generator is required. Someone smarter than me will explain how this is charging, not inverting, but still powering the required AC load - either way it works well and is simple.

Buying a fully new inverter, and a fully new generator would be quite the expense in my case, money in my case better spent elsewhere. Especially since you seem to have at least a 10Kw generator!

The simple option for me would be buying a charger. I tried to be fairly clear on the scope an intent here, per someone smarter than me no AIO or Inverter/charger does charging while inverting, they pass through generator(or grid) power while charging. This requires having a LARGE generator (you need total capacity of inverter output you are replacing, + whatever rate your AIO/Inverter-charger is set to) which is more often then not, going to operate outside of optimal efficiency (even inverter generators reach a point where reducing load barely reduces fuel consumption. Based on another thread here, figure thats around 20% capacity on average); and then having much larger cables from the generator and so on. That's a whole lot of cost for anyone who isn't fully off grid, and way more fuel cost and storage.

Interestingly, one of the ideas in another thread was getting a cheap AIO to use as the charger and having a backup/spare inverter. While an interesting option thats easily another 600 bucks, and IMHO only makes sense if you already have a high capacity generator.

 

[timselectric]

Start pricing 48v lithium chargers that output more than 15 amps. And the extra AIO starts looking good.

 

[Mithril]

Start pricing 48v lithium chargers that output more than 15 amps. And the extra AIO starts looking good.

While that might be true, I see multiple people running chargers in parallel. it actually looks like the "sweet spot" is 15-20 Amps for "48v" (at which voltage in the charging range to they claim said amps, who knows!) you don't save much by going smaller it seems.

For the scenario I laid out however, is a charger *actually* needed? An example of something that is about half the price and even has adjustable current limit to perhaps fine tune it to a small quiet generator: https://www.aliexpress.us/item/3256804737029866.html if you don't care about CC, cut the price in half again.

Another perhaps even more DIY option, I have a bunch of PC power supplies in my "things I might need later" rack; I could get a bunch of DC-DC boost converters and run them from the power supplies :D

 

[time2roll]

- Many (most?) AIO can't charge and invert at the same time "ALL AiO and ALL inverter/chargers..." (Thanks @sunshine_eggo !)

OK in the moment of a high load this is true.... however some can combine power with a sine wave generator and invert only to supplement the generator to avoid overload. Once the temporary load is reduced will immediately start charging the battery and keep the generator loaded up until the battery is charged.

 

[sunshine_eggo]

OK in the moment of a high load this is true.... however some can combine power with a sine wave generator and invert only to supplement the generator to avoid overload. Once the temporary load is reduced will immediately start charging the battery and keep the generator loaded up until the battery is charged.

True. I enjoy that feature with my Victron Quattros with my puny generator.

Even with that nifty feature, it remains true that they will NOT charge and invert at the same time.

 

[timselectric]

While that might be true, I see multiple people running chargers in parallel. it actually looks like the "sweet spot" is 15-20 Amps for "48v" (at which voltage in the charging range to they claim said amps, who knows!) you don't save much by going smaller it seems.

For the scenario I laid out however, is a charger *actually* needed? An example of something that is about half the price and even has adjustable current limit to perhaps fine tune it to a small quiet generator: https://www.aliexpress.us/item/3256804737029866.html if you don't care about CC, cut the price in half again.

Another perhaps even more DIY option, I have a bunch of PC power supplies in my "things I might need later" rack; I could get a bunch of DC-DC boost converters and run them from the power supplies :D

I'm sure that there are several ways to go.
When I started on this journey, I spent a lot of time researching auxiliary charging options. Including diy'ing my own setup. In the end I decided that the extra AIO (which is the same units as I am using in my system). Provided the most cost effective solution for me.
It can charge at any amperage I choose. (Putting any size generator in peak efficiency) Monitor the battery SOC. Automatically start a generator or switch to grid source. And it's a spare unit for my system, if one of them should fail. Which I would have had sitting on a shelf anyway. Essentially not costing me any extra money.
Plus, I plan to use the extra solar input to draw power from the battery of a future EV build. As another backup option.
Not saying that it's the best option for you. Just that it was for me.

 

[time2roll]

Even a dedicated charger may not charge the battery if the loaded inverter is drawing even more power.

 

[Mattb4]

...

I feel like DIY might sense here, thoughts?

I think you are over thinking it.

Your back up generator should be sized to fit your needs and your budget. If your budget can not afford the 100% solution, but you can live with less and afford it, than that is what you choose. I know people that have DYI a small gas engine hooked up to a car alternator to charge their lead acid batteries as an off grid source. Some have even sourced these used and near free or free.

Until you try you will not know if any particular generator will work with the AIO as a suitable AC input. Running a dedicated battery charger off the generator certainly is a simpler approach. Wiring a transfer switch to choose between the AIO and a generator can be done.Trying to make your setup all automatic is complicated and adds avenues for failure.

 

[schmism]

Another option is to DIY mod some server power supplies (see link in sig) and either direct charge or run to the PV input and have them pretend to be solar. This option allows you to 1) not have to worry about frequency stablity for the AIO's to "lock on" to. 2) charge at low wattage rates which some AIO dont like if you go AC route.

So you could use a inverter style 1500w generator instead of having to run a 5-8kw one on the AC side.

 

[sunshine_eggo]

Even a dedicated charger may not charge the battery if the loaded inverter is drawing even more power.

That's obfuscation of the issue. A dedicated charger will supply current to the DC system from the AC source.

If an inverter is inverting, it can in no way act as a charger or add current to the DC system.

An inverter/charger can do one or other - never both simultaneously.

 

[time2roll]

That's obfuscation of the issue. A dedicated charger will supply current to the DC system from the AC source.

If an inverter is inverting, it can in no way act as a charger or add current to the DC system.

An inverter/charger can do one or other - never both simultaneously.

Just saying if the generator is too small the battery will be discharging. The issue is running out of power. Did I misinterpret that? I think that is the point of having a backup generator.

The AIO can charge if the generator is large enough to supply the loads and charge. If the idea is to have a minimal generator, the power will eventually be limited. This is not a defect of the AIO and how it operates.

 

[Rednecktek]

nEvermind, I had my decimal point in the wrong place.

 

[Mithril]

I think you are over thinking it.

Your back up generator should be sized to fit your needs and your budget. If your budget can not afford the 100% solution, but you can live with less and afford it, than that is what you choose. I know people that have DYI a small gas engine hooked up to a car alternator to charge their lead acid batteries as an off grid source. Some have even sourced these used and near free or free.

Until you try you will not know if any particular generator will work with the AIO as a suitable AC input. Running a dedicated battery charger off the generator certainly is a simpler approach. Wiring a transfer switch to choose between the AIO and a generator can be done.Trying to make your setup all automatic is complicated and adds avenues for failure.

I disagree on over thinking, I think some of the replies are overthinking! :D

Sizing a backup generator to run all potential loads AND charge is a terrible waste size (cost) wise and fuel wise for me, and IMHO anyone who still has grid and is purely looking at generator power to cover grid down AND low solar days. Optimal fuel use is going to be from steady state (ish) which either means only charging, or something like some of the Victron's can do where you set the limit for the AC source. In my case it would also mean buying a whole new generator. And for me since I'm not yet at a whole home backup, I'd be either massively over buying and under using, or buying twice.

I think what this really boils down to is this:
Assuming this is for occasional charging and the goal is neither getting to 100% SOC or high enough to top balance (again I stress occasional use), is a constant voltage DC supply "good enough" assuming at least 3 things are true: Its total current output does not exceed 0.5C of any single battery pack in the system, it is controlled by some method to turn on and off as needed (such as an AIO genset stop/start ), the voltage is reasonably below the max safe charging voltage.
I feel like based on what I have read on lifepo4 charging profile here and elsewhere that "yes" is the answer.
If that is the case I feel like for non offgrid people it's better to spend the "bigger generator" and "fancy battery charger(s)" budget elsewhere, be it more solar panels, more battery, upsizing cable gauge, etc. (Basically why "waste" money on something you are using 1% of the year). Now, if you are already dropping or have dropped 50, 60, 70 grand on a whole house system another 5k-10k for more peace of mind isn't as bad. But for those of us who have not yet reached that level IMHO it makes sense to spend more wisely, and a small 2kw gen or so running a ~$100 AC-DC supply seems far more reasonable as a supplement to solar for "grid-down" (storm, drunk driver hits a pole, etc)

 

[Mithril]

I'm sure that there are several ways to go.
When I started on this journey, I spent a lot of time researching auxiliary charging options. Including diy'ing my own setup. In the end I decided that the extra AIO (which is the same units as I am using in my system). Provided the most cost effective solution for me.
It can charge at any amperage I choose. (Putting any size generator in peak efficiency) Monitor the battery SOC. Automatically start a generator or switch to grid source. And it's a spare unit for my system, if one of them should fail. Which I would have had sitting on a shelf anyway. Essentially not costing me any extra money.
Plus, I plan to use the extra solar input to draw power from the battery of a future EV build. As another backup option.
Not saying that it's the best option for you. Just that it was for me.

I think when my system hits a larger size this makes sense. Currently I'm at the single AIO, "oversized" battery bank for the system, and using it mostly as a UPS for critical loads while getting some power from solar. I expect my current AIO will *become* my emergency spare in the future and I may likely use it as the generator input then. However currently I'm looking to hit around 900-1200W on the DC side of the charger (regardless of solution) which puts my current owned generator within an acceptable range (assuming around 90% efficiency which seems realistic) and would allow for ~6-8 hours of charging per 24hours worst case for solar (complete overcast/rain for 0 input effectively) with critical loads and ~2.4kwh for "nice to have" calculated in.