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tdcampbe asked

SmartSolar MPPT 75/15 operation

Hi! First post here so bare with me.

I have a SmartSolar MPPT 75/15 controller connected to a 100w panel mounted on top of my offroad rig that powers a 12v fridge and inverter. I use a group 27DC 104ah battery in the system to carry me through the night. The fridge pulls about 65W when running in max mode and about 45W in eco mode. My panel uses SunPower Maxien cells and can easily produce 65w, even on cloudy days.

My question is this, if I don't run my fridge or inverter for a day or so, my battery is easily charged and the controller will be in float mode (1-5W production). If I turn on my fridge and start to draw 4-5A, the controller will remain in float mode and let this load be carried by the battery until the voltage is low enough to start to allow the bulk cycle. Is there any way to program the charge to allow the panel to carry this load and leave the battery at full capacity until it is determined that it is actually needed (no sun or shade, ie no power being produced from the panel)?

MPPT Controllers
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JohnC answered ·

Hi @tdcampbe

When your mppt is in Float mode it will continue to target Float V, even if there's a load on. So it will compensate for the load providing there's enough solar to do it.

Same with Bulk mode, except there it's targetting the higher Absorb V. If you see V tapering from either setpoint then the load is too high for the available solar.

If you have a specific issue with this, please say..

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tdcampbe avatar image tdcampbe commented ·
Thanks for the response. I think I have other issues happening. When the compressor would kick on yesterday when the batteries were in float, the mppt still would not allow any charge from the panels even though they were in full sun and the battery would drop to 12.5-12.6 when the compressor was running.


I have ran my fridge all night and this morning. My battery is down to ~12.5v in an idle state and the charger is still "stuck" in float mode with 19.8v and 0a from the panels. Granted it is not a full sun morning but I should be getting 15-20w. When the compressor kicks on, the battery drops to about 12.3 and still no charge from the mppt. From the research I've been doing this morning, it seems like I might have a connection between the battery and the mppt possibly?

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tdcampbe avatar image tdcampbe commented ·
Looking into it further, it looks like a voltage drop issue. With my fluke, I am getting 12.99v at the battery terminals. At the terminals on the MPPT, I am getting 12.78v. However, the app is showing 12.5v for the battery voltage. Not sure why the app voltage doesn't match the mppt batt terminal voltage but that appears to be my issue. Battery is fully charged and should be in float mode as it is. The differences in voltage between the actual and the app reported led me to believe it should be in bulk.

Any ideas why the mppt batt terminal voltage wouldnt match the app readout?

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