Adding a super-capacitor in parallel with marine battery

[kmack67.km]

Hi,

I am wondering if it makes any sense to wire a capacitor in parallel with my marine battery. I am running a small 120VAC bar fridge off an inverter connected to a 100 Ah marine battery. The battery is charged by 3-100W solar panels (wired in parallel) and it all works fine when the sun shines for at least a few hours each day. A couple of times I have run into several overcast days in a row and the battery charge has dropped down a bit. When that happens the inverter complains for a second when the fridge compressor cycles on but then works fine after that. It only happens when the battery is low and can't supply the start up power required by the fridge compressor. I am wondering if adding a capacitor in parallel with the marine battery would help to provide the required start up power when the battery is running a bit low.

Any experience or thoughts on this would be greatly appreciated. I recognize that adding a capacitor increases the risk of getting shocked so any thoughts on how to manage that would also be appreciated.

Best regards,
Kevin

[mtbikernate]

seems to me that a proper 12v fridge would just be a much more efficient use of power.

[Sparksalot]

The capacitor voltage will equalize with the battery so I’m not sure it will make any difference.

[kmack67.km]

seems to me that a proper 12v fridge would just be a much more efficient use of power.

Yeah, I agree but my system has been cobbled together mostly from stuff I already owned and it works fine most of the time. Also, I can buy about half a dozen 120VAC bar fridges for the price of one 12VDC compressor fridge.

[kmack67.km]

The capacitor voltage will equalize with the battery so I’m not sure it will make any difference.

The voltage will equalize, but if I understand correctly, the capacitor has much lower internal resistance and will provide a higher current flow for a short period of time, to get the compressor running.

[gDeN]

Without getting too technical, the capacitor will help smooth voltage fluctuations (but you would need a rather large one, think 0.5 - 1.0 farad). This is commonly used in the car audio world when you get headlights dimming as the bass hits.

As for your case, I would only think it necessary if you notice power fluctuations when the fridge compressor powers on, as the inrush current is usually higher than running load. Since you state it only happens with the battery is low, it may not help you. I couldn't tell you with any certainty without taking measurements. Otherwise, it can't hurt if done properly, but is an added cost and complication.

[kmack67.km]

Without getting too technical, the capacitor will help smooth voltage fluctuations (but you would need a rather large one, think 0.5 - 1.0 farad). This is commonly used in the car audio world when you get headlights dimming as the bass hits.

As for your case, I would only think it necessary if you notice power fluctuations when the fridge compressor powers on, as the inrush current is usually higher than running load. Since you state it only happens with the battery is low, it may not help you. I couldn't tell you with any certainty without taking measurements. Otherwise, it can't hurt if done properly, but is an added cost and complication.

Thanks gDeN. It sounds like this might help in my situation. I had a quick look on Amazon and there seem to be several capacitors sold for car audio applications but they start in the 100 to 500 F range for ~$30 to $100. Is that size total overkill?

Best regards,
Kevin

[jaws_1972]

Look for an "A/C startup" capacitor, or "easy start" kit.