24v solar / 12v battery system design help

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atomix asked Mar 7, '19

24v solar / 12v battery system design help

Hi, I'm new to solar power and am in the process of designing a modest micro off-grid system.

Is it ok to use 24v panels to charge a 12v battery via an MPPT controller?

Specifically I would like to use the following items:

2x 200w 24v Victron Panels in parallel (SPM032002400) - Rated at 5,55A max / 36V max each
Victron 100/15 MPPT Controller - Rated for 400w @ 24v / 15A max / 100V max
One or more Victron GEL 12v 60Ah batteries in series

Is this set-up suitable/recommended? I've been through the manuals and I think all these items work well together. And I believe the MPPT controller will reduce the voltage from the panels while increasing the current provided to the battery, meaning there is minimal losses/the setup is efficient.

Any advice is greatly appreciated.

offgrid system design Help

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murph answered · Mar 07, 2019 at 12:04 PM

It's not just "ok" to charge 12V from 24V via MPPT, it's essentially a minimum practical requirement. The PV input must be at least battery+5 to start and battery+1 to continue charging. A single 12V panel may struggle to hit that in less than perfect circumstances. You need at least either two 12V panels in series or a 24V panel to maximise charge time (or higher, keeping Voc*1.25 less than 100V on a 100/x MPPT).

The power ratings on the MPPT chargers are for the battery voltage, not PV voltage, so it's 200W @ 12V.

If your 12V batteries are in series, that's not a 12V system. They need to be in parallel for a 12V system. On the other hand, two 12V in series for a 24V system (which then needs 48V or higher panel configuration for best results).

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atomix commented · Mar 07, 2019 at 12:43 PM

Ok, thank you. I understand that now. We don't get allot of sun here during autumn/winter so a higher voltage will definitely help to keep the PV voltage higher than the battery. 2x 200w 24v panels are just within the specs of the 100/15 MPPT, though the short circuit current is just over when multiplied by 1.25. So perhaps a 100/20 MPPT controller would be more suitable?

This also leads me to another question: I notice the battery data sheet states "The charge current should preferably not exceed 0,2C ". Should I therefore make sure the capacity of the battery is high enough handle the current from the MPPT? That is, the 100/20 (20A) controller really needs to be paired with a 100Ah battery.

Also I apologize for the basic questions. I always seem to get series/parallel mixed up by name. I won't make that mistake in practice though.

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murph atomix commented · Mar 07, 2019 at 01:04 PM

The current rating isn't a problem as such. The MPPT will just safely limit the output to its max. Go up a size of MPPT for more or max output with peak solar conditions, or accept that the peak output will be flattened / capped. The 1.25 thing is for the open-circuit voltage (Voc), not the current. Too high a voltage (it goes above Voc at low temperatures) can kill the MPPT, but too high a current is safely limited.

Yes, stick to 0.2C max charge current for most VRLA batteries (e.g. Victron AGM & Gel). So, at least 100Ah for a 20A charger. Two 60Ah in parallel gives an effective 120Ah bank, or go for a single 100Ah or 110Ah battery, or larger (0.1C is a practical minimum for reasonable charge times, but as low as 0.05C can be useable).

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Answer 1 · 2019-03-07T13:09:31Z

atomix answered · Mar 07, 2019 at 01:09 PM

Ok brilliant. Thanks very much, I think I got it now.

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