Charging LiFePO4 from 12V

[dwh]

Running a regulated power supply into a solar charge controller is fine. That's how you bench test a charge controller anyway.



Running an alternator's output into a solar charge controller might, or might not, not work very well.

Alternators are "regulated" power supplies, but they are not regulated to one voltage, they are regulated to a *range*, such as 13.6v - 14.5v. So, the voltage varies, though with older vehicles the voltage is often "low at idle RPM, high at higher RPM" and not much in between.

Modern computer controlled vehicle alternators are even worse...taking into account many different factors, such as temperature, engine load, braking, tire pressure, day of the week, phase of the moon and gods know what else...the voltage is all over the place.

Which is fine for keeping enough juice in a battery to start a vehicle, which is all it has to do out the door from the factory.

But running some whacky constantly variable supply-side voltage into a solar charge controller...well...

For a PWM controller it would be fine. A PWM is just a high speed chatterswitch that rapidly connects/disconnects the power supply to/from the battery based on battery voltage. It doesn't matter what happens to the supply-side voltage as long as it remains somewhere above the voltage of the battery being charged. They even make replacement voltage regulators for some vehicles which have PWM.

But with an MPPT controller...maybe not so good. An MPPT circuit varies the load/voltage on the supply-side to find the point where it gets the most watts/amps out of the supply-side. That's the "tracking". It does this by periodically "sweeping" the load/voltage on the supply-side up and down to zero in on the "maximim power point". While it's sweeping, it's not doing an optimum job of battery charging.

So feeding a constantly varying voltage into an MPPT circuit might just end up with the MPPT never being able to zero in, stop sweeping and settle down.

Different charge controllers from different manufacturers have different sweeping schemes. Most do regular sweeps on a timer. Some sweep every hour. Some every 10 minutes. Some have programmable sweep rates. Some run a timer, and also trigger a sweep when the supply-side voltage changes by X%...



Running a regulated constant voltage power supply into an MPPT solar charge controller would work fine. That's how you'd bench test it anyway. In fact you'd want one where you can adjust the voltage so you could test the MPPT function.

Running an alternator's constantly and rapidly varying voltage into the supply-side of an MPPT solar charge controller might work just fine. Or it might not charge a battery worth a damn. Just depends.


Good luck with that. Let us know how it turns out.

 

[dwh]

The panels I've been seeing say ~17Voc, but usually have a voltage regulator to bring that down to 13-15. That may just be the el-cheapo models though?

No, there's no voltage regulator in the panels.

Voc is "Voltage: open circuit". That's the max voltage the panel will produce with no load.

Vmp is "Voltage: maximum power". That's the operating voltage under full load.

Those are the voltages that apply during "standard test conditions" (STC). In the lab/factory with 1000 watts per square meter of light shining directly on the panels at 90 degrees perpendicular, with the panel at 77 degrees Fahrenheit.

Those voltages will go down as the solar panel heats up. And it will heat up in the real world, being black and facing the sun.

A solar panel with 17Voc and 13-15Vmp under STC - once it gets hot is going to end up with a real world Vmp too low to fully charge a "12v nominal" battery.

 

[loudboy]

Great info, thank you all. I'm only looking to make an cheapish Goal Zero clone, so a simple and less-than-bleeding-edge efficient setup will serve my needs for now.

 

[Rando]

Almost all MPPT charge controllers are buck topology - in that they can only take a higher voltage from the solar panel and convert it into a lower voltage/higher current for the battery. Furthermore, they generally need to see charge voltage + 5V or so at the solar input to start, then battery voltage + 1-2 volts to continue to regulate. So connecting your alternator to the solar panel input most likely will not work as the voltage won't be high enough.

It could work if you put a 12 - 24V DC-DC in between the alternator and charge controller, but you would have to switch between panels and alternator/DC-DC being connected. The varying input voltage shouldn't be an issue - if the charge controller can handle mottled shade or a partly cloudy day, it can deal with wildly varying input voltage.

 

[dwh]

Yes it can handle the varying voltage. But it will spend a lot more time in sweep mode - or will spend a lot less time operating at the supply's MPP.

 

[feltyellow]

Just to echo the folks saying you'll need to boost the voltage, probably through a dc-dc converter: I ended up with a lower-than-normal voltage 12v solar panel which usually produces around 14v. I have to add a boost converter to get it to work with my mppt charge controller, otherwise it doesn't even notice that it is connected.

That said, I ended up just connecting my 40a 12v drop-in lifepo4 battery in parallel with my starting battery (currently through a switch, though i'll add a relay if it all works out). I've only done limited testing but so far it seems to charge the battery at 13.9 volts and it doesn't blow the 30a fuse. I think this should charge just fine given the charging curve for my battery and I don't think I'll need a specialized charger, though I probably won't be able to get to 100% without solar. My only question is whether this will overcharge/float the battery -- I don't think it will since the batteries max voltage is higher than 13.9.