Recommendations for 48V LFP charger from AC?

[PeterWA]

I am in the process of adding an off-grid system to my grid-connected property. Multiplus II 48/5000 and a 16kWh LFP battery (48V nominal, 314Ah). Charging the battery will mostly happen using PV via MPPT charge controllers.

On a few days a year, when PV output is not sufficient, I want the option to top up the battery from the grid. Due to regulations here I can not connect the grid to the input of the Multiplus (since it is an inverter and could theoretically feed back into the grid, even if software settings specify zero feed-in).

I plan to get a separate charger that plugs into a socket in the grid connected part of the house (230V) and charges the battery. Between 1 and 2 kW output, for 20 to 40A DC.

Given its infrequent use I'd like to go as cheap as possible without risking to burn down the house. I'm not worried about the battery, I will only turn on the charger at low SOC and won't go over 80% SOC while on that charger.

Any recommendations?

I found a couple of options on Aliexpress. Opinions on those?

This one allows to set voltage and current:

https://www.aliexpress.com/item/1005006047872389.html

This one gets more power at a lower price and it waterproof, but not adjustable:

https://www.aliexpress.com/item/1005002848617131.html

Edit:
Or maybe this power supply, in the 60V 20A version? Allowing to set voltages all the way down to zero would allow me to top-balance cells with it before assembling them into a pack:

https://www.aliexpress.com/item/1005007429889384.html

I quite like the idea of getting the last one, as a multi-purpose tool. Any downsides I might not be aware of? Any better brands you'd recommend at similar good value?

 

[sunshine_eggo]

Disable powerassist, and it's impossible to feedback to the grid. Even with powerassist enabled, the chances of feeding back to the grid are VERY low.

Do you plan to configure it to feedback to the grid? If so, you are correct.

I suspect you are misinterpreting the regulations, or your regulations are beyond moronic, or the person thumping the regulation bible doesn't know the regulations, or is a moron (or both). I mean, it's like saying, you can't use tires on your car because their atoms might spontaneously split producing a nuclear fission explosion.

Your MP-II has no more chance of backfeeding to the grid than any of the chargers you've proposed. In fact, I would say it's less likely because Victron is decades-in-refinement quality hardware and Chinese-made Aliexpress sourced devices are just a lesser class of product.

Lemme help you understand... When charging, the inverter is using the circuitry it uses to convert DC to AC to convert AC to DC. It can't do both at the same time. No inverter/charger can.

The only risk is with PowerAssist. With PowerAssist, the inverter senses when the loads exceed the AC input current limit you specify. To prevent overloading the source, it stops charging, syncs to the incoming signal and then boost the input just enough to keep from overloading the source. Once the load drops back under the AC input limit, PowerAssist disengages and charging resumes. The risk is if the load drops from a near overload to near 0, there can be a very very brief period where you could backfeed. Again, if PowerAssist is disabled, ZERO chance. The inverter/charger in charger mode presents nothing but a load to the grid. No potential exists for backfeeding because nothing is producing power that can be fed back to the grid.

You already have a great AC charger. Double check the regs.

 

[PeterWA]

You already have a great AC charger. Double check the regs.

There are both technical reasons and legal ones.

Technical: It took Victron years to get some of the Multiplus II units (still not across the range!) standards compliant to get approval to be used in grid-connect systems in Australia. That has only happened about a month ago.

No one outside of Victron seems to know at the moment whether the new compliance has been achieved by software alone, or whether it did require a hardware revision.

In any case it is not as clear cut as you make it out to be.

Legal: I am on a grandfathered feed-in tariff. Adding anything to the on-grid installation will lead to cancellation of that feed-in-tariff. Wiring the Multiplus to draw from the grid would definitely do that, because I would be connecting an extra inverter to the grid (even if it is configured not to feed back).

Plugging in a charger to top up the batteries is both technically and legally safe.

 

[sunshine_eggo]

Fine... I guess the 3rd possibility was that the regulator bodies are morons...

However, the argument becomes you are not proposing a "grid connect" system. Do you need permission to attached the proposed chargers or any other loads? If yes, then I'll just go away. If no, then there's no reason you'd need approval to attach a load to the grid. There are morons in the states that can't get past this too.

anyway:

This one allows to set voltage and current:

https://www.aliexpress.com/item/1005006047872389.html

This one gets more power at a lower price and it waterproof, but not adjustable:

https://www.aliexpress.com/item/1005002848617131.html

For either of those, I would try to confirm that the units are not also power limited on the output. I would be most concerned about the first due to its adjustability.

Edit:
Or maybe this power supply, in the 60V 20A version? Allowing to set voltages all the way down to zero would allow me to top-balance cells with it before assembling them into a pack:

https://www.aliexpress.com/item/1005007429889384.html

I quite like the idea of getting the last one, as a multi-purpose tool. Any downsides I might not be aware of? Any better brands you'd recommend at similar good value?

I agree. I have a 30V/10A supply that I use for all kinds of things including saving Christmas....

https://diysolarforum.com/threads/diy-solar-forum-saves-christmas.32450/

 

[robbob2112]

Disable powerassist, and it's impossible to feedback to the grid. Even with powerassist enabled, the chances of feeding back to the grid are VERY low.

Do you plan to configure it to feedback to the grid? If so, you are correct.

I suspect you are misinterpreting the regulations, or your regulations are beyond moronic, or the person thumping the regulation bible doesn't know the regulations, or is a moron (or both). I mean, it's like saying, you can't use tires on your car because their atoms might spontaneously split producing a nuclear fission explosion.

Your MP-II has no more chance of backfeeding to the grid than any of the chargers you've proposed. In fact, I would say it's less likely because Victron is decades-in-refinement quality hardware and Chinese-made Aliexpress sourced devices are just a lesser class of product.

Lemme help you understand... When charging, the inverter is using the circuitry it uses to convert DC to AC to convert AC to DC. It can't do both at the same time. No inverter/charger can.

The only risk is with PowerAssist. With PowerAssist, the inverter senses when the loads exceed the AC input current limit you specify. To prevent overloading the source, it stops charging, syncs to the incoming signal and then boost the input just enough to keep from overloading the source. Once the load drops back under the AC input limit, PowerAssist disengages and charging resumes. The risk is if the load drops from a near overload to near 0, there can be a very very brief period where you could backfeed. Again, if PowerAssist is disabled, ZERO chance. The inverter/charger in charger mode presents nothing but a load to the grid. No potential exists for backfeeding because nothing is producing power that can be fed back to the grid.

You already have a great AC charger. Double check the regs.

Lemme ask you @sunshine_eggo a question on that - assume what you describe happens and a tiny amount of power is backfed into the panel where the inverter/charger is plugged into. Wouldn't that be absorbed by anything in the house consuming power? i.e. a hottub that runs all the time, a fridge if it is on, a fan that always runs, etc?

 

[sunshine_eggo]

Lemme ask you @sunshine_eggo a question on that - assume what you describe happens and a tiny amount of power is backfed into the panel where the inverter/charger is plugged into. Wouldn't that be absorbed by anything in the house consuming power? i.e. a hottub that runs all the time, a fridge if it is on, a fan that always runs, etc?

Yep. The backfeed amount would have to exceed the loads to actually backfeed.

 

[42OhmsPA]

Just order a cheap rectifier. For 16S LiFePO4 I'd go with the 57V variant unless you want to reprogram them.

There's a link in the quoted message of this post and a plethora of information in the entire thread.
https://diysolarforum.com/threads/diy-chargenectifier.56329/post-1134200

 

[PeterWA]

Just order a cheap rectifier. For 16S LiFePO4 I'd go with the 57V variant unless you want to reprogram them.

Thank you so much, I've read up a little and found this a fantastic value for money solution. Seems a very solid unit, and the secondhand one will do me just fine, given how rarely I plan on using it.

Got one on order now AUD 127 delivered (EUR 78, USD 84).

I love this forum, so quick to get to such a good outcome.

 

[42OhmsPA]

Thank you so much, I've read up a little and found this a fantastic value for money solution. Seems a very solid unit, and the secondhand one will do me just fine, given how rarely I plan on using it.

Got one on order now AUD 127 delivered.

I love this forum, so quick to get to such a good outcome.

You're welcome, glad it was helpful.