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What happens when a battery charger is starved?

fafrd

Solar Wizard
Joined
Aug 11, 2020
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I’m looking for ways to charge a battery only with available AC-coupled solar power, which has led me to wonder the following:

‘What happens to an AC-battery charger if it is only powered with a fraction of the AC power is requires?’

Let’s say, for example, I’ve got my 10A 24V LiFePO4 charger connected to my off-grid PV system currently only generating 160W of AC power.

The charger needs ~320W of AC power to push ~288W of power into the battery.

So will the charger consume that 160W of energy to charge at 5A (or less if efficiency drops below the nominal 90%) or will it fail to start charging at all?

MPPTs and DC-coupled charging do a fantastic job of tapering charging rate to match available solar power and I’m trying to understand whether there is an easy way to achieve the same thing with an AC-coupled charger…
 
AC charger will overload the source. Probably shut down the source.

Seems odd to have a PV system to only make AC power at the 160W level. What are the actual system components?

Otherwise I think you are better off with an MPPT controller to charge the battery direct and a separate inverter to run from battery power.
 
In a traditional AC Coupled system there is no charger, the HBGT H-Bridge doesn't care which way the current is going. There is no charger.
Phase shift controlls the grid tie inverters.
 
AC charger will overload the source. Probably shut down the source.
I’m not sure what you mean by that. Are you saying, for example, that a 3kW PSW will just shut down if you attempt to power a 6kW battery charger with it?
Seems odd to have a PV system to only make AC power at the 160W level. What are the actual system components?
It was just an example to illustrate the question.

The system components I am considering are an EV charger powered by a Hybrid Inverter that can be programmed for zero-export and zero import with AC-coupled solar power connected to it’s AC output.


Otherwise I think you are better off with an MPPT controller to charge the battery direct and a separate inverter to run from battery power.
I hear ‘ya (and that is exactly how I’m charging my 24V LiFePO4 battery).

Unfortunately, charging an EV with an MPPT does not seem to be an option.

I’m looking for ways to use excess AC-coupled power that otherwise would be getting exported to instead charge an EV (ideally without importing any power from the grid).
 
Using the standard L1/L2 J1772 port to charge an EV is about 720 watts minimum with 120 volts and 6 amps minimum. This is fairly inefficient as the EV computer and cooling pumps will use about 200 to 300 watts as overhead. Tesla that can be controlled by the car might go a bit lower.

The hard part is controlling the charge rate by what is available at the solar inverter on the fly. The EVSE pilot signal can be changed or stopped but will require a custom program to do this. Unless you are well over 1000 watts it may not be practical.

If you can charge a separate battery controlled by the available source the system could work. Then as the battery gets full dump 1500 watts into the car until the battery is low. Good luck.

Otherwise yes a 6kw load will shut down a 3kw inverter.
 
Using the standard L1/L2 J1772 port to charge an EV is about 720 watts minimum with 120 volts and 6 amps minimum. This is fairly inefficient as the EV computer and cooling pumps will use about 200 to 300 watts as overhead. Tesla that can be controlled by the car might go a bit lower.
Did not know that. Is that 200-300W overhead whenever the EV is ON (just driving the V2L output, for example)?
The hard part is controlling the charge rate by what is available at the solar inverter on the fly. The EVSE pilot signal can be changed or stopped but will require a custom program to do this. Unless you are well over 1000 watts it may not be practical.
I have a total of over 3kW of AC-coupled Solar out of which I’m consuming an average of under 350W during daylight hours, so I’d be charging at at least 2kW.

I have no idea what EVSE is - do you have a link to explain?
If you can charge a separate battery controlled by the available source the system could work. Then as the battery gets full dump 1500 watts into the car until the battery is low. Good luck.
Charging my separate LiFePO4 with too much charge current is exactly the problem I’m trying to avoid.

But I think one solution I have is to use the LiFePO4 battery and GTIL inverter to close and gap.

Let’s say I turn in the 2kW charger once the AC-coupled solar output exceeds 1500W.

So worst-case, the EV charger needs another 500W and the house at need an average of 350W with a peak of as much as 600W (peak load when main fridge is running).

So I may need a worst-case additional 1100W peak or 850W average which can easily be provided by my two 1000W GTIL inverters from battery power.

850W of average power from my GTILs requires an average of 1063W from my LiFePO4 battery because GTIL efficiency is only ~80%.

My DC-coupled array is currently 1/4 the size of my AC-coupled array and I’m planning to double that to 1/2 when needed.

So 2kW out of the AC-coupled array should translate to 0.5-1kW of DC-coupled charge current being supplied to the LiFePO4 battery (meaning the worst-case power draw from the battery will be an average of as little as 63W with the increased array or much as 563W from my current DC-coupled array.

It’s not very efficient, but this should mean that on any normal-production day, I can charge the EV for 4-5 hours @ 2kW during the highest-production part of the day without importing and only exporting when AC-coupled solar power exceeds total load of EV charger + house load),

Otherwise yes a 6kw load will shut down a 3kw inverter.
Thanks, that’s the answer I was looking for.

The new NEM 3.0 decision has just been issued by the CPUC and it appears I’ll have more time to architect a solution to this problem than others have been lobbying for.

Legacy NEM1.0/2.0 customers are having their grandfathering period reduced from 20 years to 15 years, but that still gives me 10 more years on my current rate plan before the onerous terms of the successor tariff get imposed on me. The utilities were lobbying for an immediate transition of all legacy customers to the successor rate plan, so this decision gives me much more time (by which point there are hopefully even better solutions available).
 
EVSE is Electric Vehicle Supply Equipment. This is the interface between the electrical supply and the vehicle charger. That cord included with the vehicle is an EVSE. You can buy aftermarket home units or that public L2 charging station is an EVSE.

Best shot to control the charging that I know is the Open EVSE. This was a home reverse engineered project that started a decade ago when the LEAF was introduced.

https://www.openevse.com/
 
OK so if the 3Kw inverter is grid tied it very well could import the deficit when faced with a 6kW load. Depends on the equipment and how it is wired. Best to review the manual.
 
EVSE is Electric Vehicle Supply Equipment. This is the interface between the electrical supply and the vehicle charger. That cord included with the vehicle is an EVSE. You can buy aftermarket home units or that public L2 charging station is an EVSE.

Best shot to control the charging that I know is the Open EVSE. This was a home reverse engineered project that started a decade ago when the LEAF was introduced.

https://www.openevse.com/
Interesting - thanks.

As I said, looks like I’ll have much more time than I thought I’d have to address this, but if an Open EVSE such as this would allow a small ‘brain’ to automatically increase and decrease EV charger current/power as available AC-coupled solar power increases and decreases, that is exactly what I’m searching for…
 
OK so if the 3Kw inverter is grid tied it very well could import the deficit when faced with a 6kW load. Depends on the equipment and how it is wired. Best to review the manual.
Yes, the 3kW is grid-tied (Microinverters).

And I’m sure any AC load including an EV charger can import any additional AC energy needed to meet power needs - that’s how I’m using the GTILs now (when my 3kW electric oven heating element turns on, the 3 GTILs max out at 850W each and 1300W is imported from the grid to supply the element.

My challenge is that (in 10 years) I’ll want to avoid any export while also minimizing import…
 
It applies for government assistance..

I'll show myself out...
 
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