Mppt inverter as dcdc charger

[GoneDiving]

I'm looking for a step down dcdc charger to run off my 48v Lifepo4 house bank to charge a infrequently used 12v SLA battery.

Can I use a 20a 12v mppt solar charge controller with my 48v house bank replacing the PV input? Is the input/output of the controller governed to its rated capacity or will the house bank supply essentially unlimited current and fry it?

Thanks.

[Emmalina]

You will fry your multi point power tracking solar controllers. Bad Idea

[markcouz]

I have been using an MPPT (WRND 12/24-30amp) charge controller to charge my 400ah 12v LiFePO4 from my 100ah 48v LiFePO4 bank for over a year with no issues.

The 48v bank is charged via PV using a separate MPPT charge controller (also WRND).

It works well because the voltage and amps is fully programmable (via built in LCD, not external App) so i adjust the 12v bank voltage lower when not on the boat to extend battery life.

WRND is a Saffer product that you may struggle to get, so not sure you will have same result with your product, but I am very happy.

[markcouz]

Edit: that model (12/24) is 40amp, not 30amp.

[markcouz]

Additional note...


The product that I am using has a Vmp limit of 75V - this is because it is a 12v/24v auto select system. A standard 12v MPPT controller may have a Vmp of 50V which puts you at serious risk of frying it.


The point being that you will need a product with a Vmp higher than your maximum expected voltage of the Lithium bank.

[GoneDiving]

Thanks to both of you. I'll have a good look at the programmable settings. If I can see max output current then that's positive. If the mppt will feed all available input current then I see a ball of flames resulting. ��

Cheers.

[markcouz]

I think you will find that the MPPT will control the output current to within its capabilities - you should concentrate on input voltage which is where damage could occur.

If you were trying to charge a 12v lithium bank then I would suggest a programmable unit, but your 12v bank is lead acid so less critical.

[Sailing Ohm]

It don’t think the controllers are limiting to that current as much as they are telling you that’s the max allowable and you size based on that.
Our midnite solar MPPT controller has a max allowable input wattage. Not sure what happens if you exceed that rating but I do know they make models that handle higher wattage input.

What’s the infrequently used battery used for?

[markcouz]

I have often installed land based over spec arrays (eg 5kWp on 4kW MPPT) with no ill effects, the result is a flatter charge curve with more power produced in am and pm with no increase in power at midday where the MPPT is limiting the power accepted from the array.

The above using Voltronic Axpert, WRND, and (another local brand) Microcare - NOT every brand out there so your results may vary.

If in doubt climb on the internet and order the WRND 12/24-40, I can 100% confirm that it will work in your case as I am currently using in exactly the same situation.

[BjarneK]

I use my Victron 75/15 MPPT regulators (75 V and 15 A limit) with 300 W solar panels, one for each panel. The flat mounted panels are large enough to produce a higher current than the 15 A that the regulator can provide. I have been around 20 degrees north all summer and no problems are evident. To me, this indicates that the regulators just limit the output at 15 A. This is also consistent with what I see on my battery monitor. My situations is a little different from the original question, but still related, so I thought I would let you know.

[mitiempo]

Originally Posted by BjarneK

I use my Victron 75/15 MPPT regulators (75 V and 15 A limit) with 300 W solar panels, one for each panel. The flat mounted panels are large enough to produce a higher current than the 15 A that the regulator can provide. I have been around 20 degrees north all summer and no problems are evident. To me, this indicates that the regulators just limit the output at 15 A. This is also consistent with what I see on my battery monitor. My situations is a little different from the original question, but still related, so I thought I would let you know.

All of the Victron MPPT controllers are unlimited as far as panel wattage is concerned. The output is limited to the rating - 15 amps on a 75/15 for example.

You are leaving about 25% of your panel output on the table though with 75/15 controllers.

[BjarneK]

Originally Posted by mitiempo

All of the Victron MPPT controllers are unlimited as far as panel wattage is concerned. The output is limited to the rating - 15 amps on a 75/15 for example.

Thanks for the confirmation of that.

Originally Posted by mitiempo

You are leaving about 25% of your panel output on the table though with 75/15 controllers.

In higher latitudes, the sun will typically not get high enough for this to even happen on my flat mounted panels. Where I am now, I do max out the regulator sometimes, but only when the sun is very high and there are no clouds. Those are the days when I have plenty of power anyway. So for me this is a good solution with small inexpensive regulators that provide the full power from the panels when needed, but does sometimes miss out on a bit of power when not really needed.

[GILow]

Originally Posted by Emmalina

You will fry your multi point power tracking solar controllers. Bad Idea

Really? How?

[mitiempo]

Originally Posted by BjarneK

Thanks for the confirmation of that.

In higher latitudes, the sun will typically not get high enough for this to even happen on my flat mounted panels. Where I am now, I do max out the regulator sometimes, but only when the sun is very high and there are no clouds. Those are the days when I have plenty of power anyway. So for me this is a good solution with small inexpensive regulators that provide the full power from the panels when needed, but does sometimes miss out on a bit of power when not really needed.

It's a personal choice.

My preference after deciding on panels is to make sure the controller does not work at maximum all or most of the time. Runs cooler and likely will last longer.

This summer I installed several 320 watt panel systems with 100/30 controllers which can handle 440 watts.

[CapnBazza]

Originally Posted by GoneDiving

I'm looking for a step down dcdc charger to run off my 48v Lifepo4 house bank to charge a infrequently used 12v SLA battery.

Can I use a 20a 12v mppt solar charge controller with my 48v house bank replacing the PV input? Is the input/output of the controller governed to its rated capacity or will the house bank supply essentially unlimited current and fry it?

Thanks.

Thank you for this question. Using my redundant Blue Sky Solarboost 2000e as a dc-dc charger to keep my standalone start battery topped up from my LiFePO4 house bank to avoid a flat start battery after prolonged non use, had not occurred to me. I thank you for the suggestion. Thinking about it, I can’t see why it wouldn’t work. The Solarboost just sits there doing nothing, using it like this means it earns the space it occupies.