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ezlev avatar image

Need help choosing the right components for a solar fridge project

Full disclosure: I'm pretty out of my depth when it comes to things like this, so I expect to need a lot of help. Thanks to everyone in advance.

I'm putting together a solar-powered outdoor fridge and food pantry using a Dometic CRX-80 minifridge (12V DC, ~50W, <2amp draw) and a JA Solar 395W panel (49.64 Voc, 10.27 Isc). I'm planning to purchase an AGM battery/batteries and a Victron SmartSolar charge controller, probably the 100/15 since it has the heatsink that the 75/15 lacks.

I need help figuring out the ideal battery capacity, choosing a good battery that can ship to Hawaii, confirming my choice of charge controller, and also figuring out what other things (wires/fuses/accessories/etc) I'll need to order. Please advise!

MPPT - Solar Charge Controlleroffgrid
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3 Answers
JohnC avatar image
JohnC answered ·

Hi @ezlev

Firstly 50W isn't 2A (12V), it's closer to 4-5A. But the duty cycle of the fridge may get it down to an average 2A. Don't leave the door open. :)

A /15 mppt is really a little undersized for a 395W panel, as it will only generate like up to 220W max. But it would be enough for the purpose and give good weather tolerance, like cloudy days and with the sun low in the sky.

Let's just use Ah for some rough calcs. Overnight, 2A for (say) 14hrs is 28Ah. For a lead-acid battery, never draw them down below 50% of capacity, and I find a good design point is 25% drawdown for a little safety. I wouldn't go below a 100Ah battery in this case. You'll need to charge it daily..

And when the sun's up a typical design charge rate is 0.1C - 0.2C (C=100Ah as I recommend), so your /15A fits well there.

The Victron 100/15 terminals can handle 6mm2 (core) wire,and that would be a handy size for all your wiring if you can keep the lengths down to <2m to the batt and fridge, and maybe <5m to the panel. 15A fuses would probably suffice, and a battery isolator always worthwhile.

Btw, re shipping to Hawaii. Batts are often classed as dangerous goods, and maybe easier (even cheaper?) to buy there..

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Thanks, @JohnC! Follow-up question: if I want more autonomy without charging (say it's dark and cloudy all day and I don't want the fridge to shut off) could I just size up the batteries in terms of Ah? Would it be better to wire two 100Ah batteries in parallel than to use one 200Ah, or vice versa, or is there no real difference?

@eslev

You could do that either way, but a 200Ah pb batt is very heavy. You may decide to remove one to take it away to charge it, so convenience only for 2x 100Ah.

I too was aware of the 24V capability of the fridge, but didn't mention it because 12V mains chargers are endemic, 24V much less so. Even a jumper from a running car could get you out of trouble with 12V.

More battery is always better, but even in the worst of weather a 395W panel will still provide 'something', probably enough to get by on almost all days. But Murphy rulez..

If you do go with 2x batts make them identical to help avoid preferential discharge. And to do it properly a 4-way batt isolator would be great (1, 2, Both & Off).

seb71 avatar image
seb71 answered ·
To do that, I could wire two 12v batteries in series, right?

Yes.

So two 100Ah batteries would end up as 24v 100Ah rather than 12v 200Ah like
they would if I put them in parallel?

Yes.

What's the benefit of the higher voltage?

Thinner wires. More efficient.


Also, if you have a 12V battery, the 100/15 charger can output maximum about 12Vx15A=180W.


If using a 24V battery, the same charger can output maximum about 24Vx15A=360W.


If going to use a 24V (nominal) battery, make sure that the panel (or string of panels in series) voltage is above the required minimum (check the manual). But keep it under 100V allowed by the 100/15 charger (with a safety margin).


You should also consider getting a LiFePO4 battery instead of a Lead-Acid battery. For the same available energy, it could be smaller. Also it can be recharged faster (if you have enough power). It does not need to be topped up frequently, like lead-acid needs (otherwise it will be damaged).

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@Seb71, quick follow-up question: it looks like I have a choice between a 24v 50Ah battery and a 12v 100Ah one. Both lithium, both the same brand. Based on what you've said here, even though they're the same wattage I should go for the higher voltage, right? My panel outputs more than the minimum charge controller voltage for a 24v battery, and the fridge will run in a 24v system. Pretty sure I know the answer to this one, but I want to double-check it – thanks again for your help!

Between those two, I would go with the 24V 50Ah option.

Make sure you set up the solar charger correctly for that battery (at least select the LiFePO4 default profile, but it is preferably to tweak that according to your battery specifications).

seb71 avatar image
seb71 answered ·

That fridge apparently can also be powered by 24V DC. Consider going this way.

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To do that, I could wire two 12v batteries in series, right? So two 100Ah batteries would end up as 24v 100Ah rather than 12v 200Ah like they would if I put them in parallel? What's the benefit of the higher voltage, @Seb71?