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agm asked

MPPT 75/10 basic setup not satisfactory

I bought over a dozen MPPT 75/10s because I thought their deployment was straightforward. The longer I play with them and read the forum, the more confused I get. This shouldn't be difficult, so let me start with what my goal is:

On a ranch, our goal is to have several mini-solar power stations to power units that don't require much power. At a first stage, I'm deploying a wifi mesh system; without going into too much detail, the typical amp drawn is about 0.2A.

The ranch is in California with lots of sun. I figured I'll get some practical experience how large a battery I'll eventually need to get the system to run 24/7 when the weather is 'normal'. As a starting point, I grabbed a standard 12v 7Ah battery, such as https://www.amazon.com/12-VOLT-7Ah-BATTERY-Battery-Electronics/dp/B000R9PYEQ

My goal was never to end up with this small a battery, but I wanted to test some basics in my garden before taking it to the ranch.

I have a variety of small solar panels, but would like to work with a simple panel, 20W-40W, here's a 20W panel I use: https://www.amazon.com/gp/product/B01IFJ73X4

Here's the problem: a new battery, once discharged, things don't' work as hoped for. From what I can tell, the battery simply doesn't charge properly. It charges a tad. If the setting is to 'always on', it will indeed relay the power sometimes

I believe the wiring is correct. I've played with various settings, but before diving into them, maybe someone can talk some sense into me what I'm obviously doing wrong. This shouldn't be complicated. @Guy Stewart (Victron Community Manager) or any, pls help.

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klim8skeptic avatar image klim8skeptic ♦ commented ·

Hi agm,

Your margins are too thin,

I imagine for your latitude, in winter you would have 10 hour days, and 14 hour nights?

In those 10 hours you have to harvest enough Ah to power your device, and recharge the battery.

Your load might only be 0.2a, but that adds up to 4.8Ah a day. Lead batteries are not very efficient, so add another 25% on top of the 4.8Ah. Total harvest would have to be at least 6.0Ah just to recharge every day.

Lead batteries also need some time in their absorb phase to fully charge each day, this is going to be hard to achieve on 10 hour days.

Agm batteries don't really like to be charged at more than 10% of their rated Ah capacity, keep an eye on the charge rate.

Your solar panel is way to small to achieve decent charge.

You might average 8Ah harvest a day, averaged over the year from a 25w panel. But that will drop down badly during your winter months.

You need a much larger solar panel.

You need a bigger battery.

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2 Answers
JohnC avatar image
JohnC answered ·

Hi @agm

Those small panels are likely '12V' rated, designed for pwm cc's. For mppt to work properly, a higher V is better. Try stringing two of them in series to give you a '24V' panel.

There's only so far you can go with this, and the limit there is expressed in the 75/ rating of your units.

There's more to the whole story, so ask if you need. Just *never* exceed panel open-circuit V of 75V.

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agm avatar image
agm answered ·

Most appreciated, both answers I received above make a great deal of sense. I'll be baaack when I've had time to explore based on this input.

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