Incorporating capacitor(s) into 36v solar/battery system

[solarpowergood]

Hello, new guy here. I noticed some are using a capacitor (or capacitors) in a solar/battery system for "buffering" sudden energy demand or surges from motors and the like. I like that idea. It makes sense to me.

I'm not an electrical engineer, nor an electrician, plus I suck at math. Can anyone here be so kind to tell me...
1. If using *extra* capacitance is actually useful for the purpose of buffering loads and
2. If yes, then what capacitor (or capacitors) should I use *specifically* for a 36v, 3000w pure sine wave inverter power by 3 * 12v lead acid batteries in series. My understanding (so far) is that a single "super-capacitor" simply wired on the dc side of the inverter's +/- terminals is all there is to it. Thank you.

 

[sunshine_eggo]

Adding a capacitor as you describe is also referred to as "hard start."

Hard Start Kits: What are they, and why would you need one for your HVAC?

In this article, we discuss what a hard start kit is, why you might need one for your air conditioner, and steps you can take if you need one for your system.

indoortemp.com

You install it for the device you are trying to start vs. something the entire system can use.

 

[Rich V]

I think he is asking about a capacitor directly in parallel with the batteries. The advent of super capacitors (100s-1000 farads) would allow you to have a significant amount of stored energy as a short term load equalizer. How this would interact with inverters, BMS, charge controllers etc. I have no idea.

 

[v_green57]

It would make a pre-charge scheme of some kind necessary. Something that those of us with big LF inverters already have to do as they have those caps internally (hence the big spark if you connect a cable directly).

The theory behind what the OP is proposing is sound IF the FET's in the inverter can handle the short-term surge current.

Pretty big if.

 

[BentleyJ]

The problem with capacitors is with the voltage. Electrolytics that will individually handle 40V can be purchased but their capacity is limited. Graphite super capacitors of the Maxwell Technology variety have very high capacities by comparison but are limited to 3.5V plus or minus depending on the brand and model number. Then you have to put several in series and then they technically may need a BMS to make sure none of them go over voltage and get damaged. Super Caps are not low cost items either.

If you are worried about surges then get more LA batteries or switch to Li batteries which have much less voltage sag under high current motor starting

 

[DIYrich]

I think he is asking about a capacitor directly in parallel with the batteries.

Inverter already has them. That is why you need a precharge resistor when connecting a battery.

 

[solarpowergood]

And... it looks I should skip the capacitor idea. Thanks for the feedback everyone.