Questions regarding 12 volt loads on a 24v system? Ultracapacitors?

PNW_Steve · December 2016

Hello Everyone,

As I am tweaking the design for my bus electrical system I have begun to consider going 24v on my house batteries. My single largest (by far) DC load will be the inverter. My lighting will be LED and I won't have any major DC appliances. So, I expect my 12v loads should never exceed 30A.

I have looked at various options for supporting 12v loads from my 24v bank and am not excited about the 24/12v converters.

I did see mention of someone using a 30amp MPPT charge controller to charge a separate 12v battery from the 24v battery bank. That idea caught my eye but adding another battery opens a can of worms.

Could I follow that example but substitute something like this for the 12v battery:

coolestore® 16V 83F Ultracapacitor Engine Battery Starter Booster Car Ultra/Super Capacitor

https://www.amazon.com/coolestore®-Ultracapacitor-Battery-Starter-Capacitor/dp/B00PU4J7GY

I will not mount a battery inside the living space and I would have to build a new support & battery box on the frame rail to place one outside. From what I can see, I could mount this in my electrical cabinet where my charge controller, inverter etc are.

Any thoughts?

Thanks.

S.

mcgivor · December 2016

The simplest is the DC DC converter, have used them extensively on 24V Land Cruiser's to run stereo systems, got them from Samlex, that capacitor is just an electrolytic capacitor which dumps it's charge for a short duration to prevent voltage sags, at least that is the theroy. As I always caution, NEVER be tempted to tap off one 12V battery in a 24V string as this WILL dammage one if not both batteries

PNW_Steve · December 2016

mcgivor said:

The simplest is the DC DC converter, have used them extensively on 24V Land Cruiser's to run stereo systems, got them from Samlex, that capacitor is just an electrolytic capacitor which dumps it's charge for a short duration to prevent voltage sags, at least that is the theroy. As I always caution, NEVER be tempted to tap off one 12V battery in a 24V string as this WILL dammage one if not both batteries

Thanks for that.

Any thoughts regarding using an MPPT charge controller feeding an ultracapacitor bank to support 12v loads?

Thanks again.

BB. · December 2016

I would suggest that using an ultra capacitor on a 12 volt battery is both expensive, unnecessary, and risky.

Expensive... A super cap stores very little energy vs a true battery bank. They are suggested for cars audio systems that are crazy loud and when you have ripple current in the 100's of Hz range (bass) to provide momentary peak current. In general, a large enough bank to feed needed power to operate the inverter/loads for 2+ days an 50% maximum depth of discharge is large enough to supply needed surge current. True super caps do not have a very long life--Generally a few years for the ones I have researched.

Unnecessary... The super cap for audio is just to carry through power for a 1/2 cycle of audio or a few 1/10'ths to 1/100'ths of a second. An inverter needs to supply surge current for multiple cycles lasting upwards of several seconds--Supplying power for 1/10 second "extra" is not going to really do much to start a well pump motor.

Risky... A super cap into a dead short can do massive amount of damage. The wiring and fusing/breakers run the risk of simply vaporizing instead of stopping the current. Particularity if the system is >12 volt battery bus (~12 volts and below, sustaining an arc is not very easy--Over ~12 volts, you have an arc welder).

BB. said:

Re: Using Deep Cycle Battery as Car Starting Battery

Be very careful with "true" super caps... They usually only have a few year life and are very sensitive to over voltage. Also, a dead short with a super cap can be hugely more dangerous than with a lead acid battery (which can be scary enough).
show previous quotes

john p wrote: »

Ultra caps make great expensive toys but for supplementing a battery bank a total waste of money.
Maxwell caps are good. We have a bank of them at work with a total 100,000 farads and 24v. Just the best toy available. they are mounted on a pallet then a trolly and out the front of the trolly is a nice 3/4 inch copper rod. Now if you wheel this up to a engine block and the tip of the rod gets about 1" from it then the discharge will take large pieces off casing off the block. al in one second.

The main problem with caps is the dramatic drop in voltage as they discharge. ie a 12v lead acid battery full about 14v half full.orhalf empty?about 12v only 2v difference. Now 12v bank caps full 14v half full/empty about 8v. Useless .

If you have excess power that's "wasted" get another few batteries. or bigger ones than you have.. far cheaper and will last longer.
super caps if heavy charged and discharged every day will last you less than a year.. Our first bank of them at worl lasted less than 6 months at a replacement cost of over $120,000 . and the new ones are loosing capacity after 4 months now after heavy use..

And the actual amount of stored power (relative to a lead acid) is relatively low:
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BB. wrote: »

Super/Ultra Capacitors are not worth it for the average (and even not so average) application... For example, on your first link:

Ultra-capacitors 48.6V 165F @ $1,800 each. And assume that you run it between 38 volts to 48 volts:
E-useable = 1/2 * 165 Farad * ((48v)2 - (38v)2) = 70,950 Watt*Seconds
70,950 WS * 1/60 sec per min * 1/60 min per hour = 19.7 Watt*Hours of storage
19.7 WH / 48 volt bank = 0.4 AH @ 48 volt battery equivalent

I may have gotten something wrong--But I don't think so... A capacitor of this size just does not compare to very well to a typical storage battery.

You can get more useful energy out of the capacitor by draining it further--But that would mean more issues for the energy conversion device (operating over a wider input range, higher currents required at lower capacitor voltages, etc.).

-Bill

Looking at your numbers... Say you can crank an engine for ~1 minute at ~100 amps:

100 amps * 1 minute * 1/60 minutes per hour = 1.67 Amp*Hour (@ 12 volt) "useful" stored energy

-Bill

If you want to try an "audio system super cap"--Go for it (and take precautions). However, I would suggest not to bother.

-Bill

Estragon · December 2016

Why are you thinking of going to 24v in the first place? Do you have concerns about wire size to a load? Want bigger inverter?

mcgivor · December 2016

PNW_Steve said:

show previous quotes

mcgivor said:

The simplest is the DC DC converter, have used them extensively on 24V Land Cruiser's to run stereo systems, got them from Samlex, that capacitor is just an electrolytic capacitor which dumps it's charge for a short duration to prevent voltage sags, at least that is the theroy. As I always caution, NEVER be tempted to tap off one 12V battery in a 24V string as this WILL dammage one if not both batteries

Thanks for that.

Any thoughts regarding using an MPPT charge controller feeding an ultracapacitor bank to support 12v loads?

Thanks again.

The charge controller would probably charge the capacitor in a milisecond with no load, and the capacitor would discharge rapidly when something is turned on, prompting the controller to try and charge, after a delay (takes a little time to recognize the voltage drop) and most likely end up with a series of pulses, which would most likely damage the controller and load. This is my theroy as I wouldn't even entertain putting it to a test.
The cost of the "super capacitor " is around the price of a 12v battery which has a stable voltage, larger capacity and will work with a controller, so for this reason I would recommend the battery.

PNW_Steve · December 2016

mcgivor said:

show previous quotes

PNW_Steve said:

mcgivor said:

The simplest is the DC DC converter, have used them extensively on 24V Land Cruiser's to run stereo systems, got them from Samlex, that capacitor is just an electrolytic capacitor which dumps it's charge for a short duration to prevent voltage sags, at least that is the theroy. As I always caution, NEVER be tempted to tap off one 12V battery in a 24V string as this WILL dammage one if not both batteries

Thanks for that.

Any thoughts regarding using an MPPT charge controller feeding an ultracapacitor bank to support 12v loads?

Thanks again.

The charge controller would probably charge the capacitor in a milisecond with no load, and the capacitor would discharge rapidly when something is turned on, prompting the controller to try and charge, after a delay (takes a little time to recognize the voltage drop) and most likely end up with a series of pulses, which would most likely damage the controller and load. This is my theroy as I wouldn't even entertain putting it to a test.
The cost of the "super capacitor " is around the price of a 12v battery which has a stable voltage, larger capacity and will work with a controller, so for this reason I would recommend the battery.

Thanks for all of the replies.

I have seen a number of people replace (as an experiment) their starting battery with an ultracapacitor array with reasonable success. I have no want to support a load drawing near that much current. Primarily 12v LED lights, cell & laptop charger. If I am pulling a relatively steady 10-20A load from a source that is capable of providing enough energy to start a car I would expect at least a few minutes of run time not milliseconds or seconds.

The ultracapacitor "battery replacements" that I have been looking at cost about 1.5 times what a decent group 27 battery. Battery lifespan= 2-3 years.

If I add a battery I will need to build a carrier attached to a frame rail and put an access door in to get to it. I will NOT place a vented lead acid battery in my living space. The cost of the longer battery cables and the fabrication of the mount etc. far exceeds the cost of the ultracapacitor that CAN be mounted in the living space.

As far as the instantaneous current availability, I would think that fusing both legs (12v+/12v-) with a 100A fast acting fuse would provide adequate protection for my application. Am I mistaken? Obviously a catastrophic short in a piece of gear could allow damage to occur to the gear before the fuse blows with either the battery or the ultracapacitor.

Lastly, If the MPPT charger won't play well with the ultracapacitor the the idea is useless....

BB. · December 2016

You pretty much need a "minimum size" battery bank with a typical MPPT type charge controller. If you have too small of bank, it is possible for the MPPT controller to over voltage the capacitor/small bank (part of the MPPT cycle for many controllers is to pull as much power from the solar array and pump it into the battery bank to figure out the Vmp*Imp=Pmp equation.

Solar charge controllers (and even car alternators) really use the battery bank to "buffer" and hold the DC bus voltage within nominal range.

-Bill

bill von novak · December 2016

PNW_Steve said:

I have seen a number of people replace (as an experiment) their starting battery with an ultracapacitor array with reasonable success. I have no want to support a load drawing near that much current. Primarily 12v LED lights, cell & laptop charger. If I am pulling a relatively steady 10-20A load from a source that is capable of providing enough energy to start a car I would expect at least a few minutes of run time not milliseconds or seconds.

Well, just do the math. The energy (in joules, or watt-seconds) you can get is 1/2*C*(Vhi) - 1/2*C*(Vlo) where C is the capacitance in farads, Vhi is the voltage you charge to and Vlo is the voltage at which your inverter (or load) drops out. If you end up with a number like 1200 that's 100 watts for 12 seconds.

Lastly, If the MPPT charger won't play well with the ultracapacitor the the idea is useless....

Once the capacitor is charged it will work fine. If you try to start into a discharged ultracap the MPPT won't even turn on; you'd have to charge it manually to some reasonable voltage first.

mike95490 · December 2016

For this application (charging a supercap), I would NOT use a MPPT controller as a down converter. I'd use a 3 terminal switching regulator or DC-DC supply http://www.mouser.com/Power/DC-DC-Converters/Non-Isolated-DC-DC-Converters
some 3A for $25 or whatever you need to maintain your load requirements. No worries about the 12V side going so low the MPPT shuts off.
You may need to pre-charge the 12v super cap via a automotive light bulb and voltmeter to not have the supply current limit for the initial powerup.

PNW_Steve · December 2016

mike95490 said:

For this application (charging a supercap), I would NOT use a MPPT controller as a down converter. I'd use a 3 terminal switching regulator or DC-DC supply http://www.mouser.com/Power/DC-DC-Converters/Non-Isolated-DC-DC-Converters
some 3A for $25 or whatever you need to maintain your load requirements. No worries about the 12V side going so low the MPPT shuts off.
You may need to pre-charge the 12v super cap via a automotive light bulb and voltmeter to not have the supply current limit for the initial powerup.

Thanks for the link. I had already looked at the dc/dc converters and had trouble finding one that was suitable for the occasional peak load that could exceed 30 Amps that was not extremely expensive. If I have overlooked one I would welcome a recommendation.

mike95490 · December 2016

If you are concerned about several loads piling on at the same time, split it up and use several smaller DC-DC converters

nubbs · May 2019

I use a 1.o farad capacitor in my solar system hasent blown up yet takes the load of my inverter and batterys but im going to try a car audio battery on my primary system think the car audio battery will be safe to use with deep cycle battery and 4000watt inverter i got a 12000 watt on order

nubbs · May 2019

I us a 60amp pwm controler and marine rv deepcycle battery with 3 100 watt panels but my 1farad capaciter handels startup loads on microwave and stuff 1000 watts and below but im going to try a car audio battery capaciter

Questions regarding 12 volt loads on a 24v system? Ultracapacitors?

coolestore® 16V 83F Ultracapacitor Engine Battery Starter Booster Car Ultra/Super Capacitor

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