Capacitors as batteries?

[svetz]

Not new stuff, you can already buy a super capacitor battery to replace a lead acid battery as a drop-in replacement.

Pros Long life high current loads and rapid response Low energy transfer losses 120 million cycles (take that TiO) Extremely light High Power density: up 6700w/kg 10s to recharge 95% Good temperature Performance

Cons Low specific Enengy (a coke-can size cap can hold the power of an AA) Linear discharge voltage (liFePo4 is near constant V during discharge) High self discharge (~70%/week) High Cost per Wh

Check out Maxwell products: www.maxwell.com/products/ultracapacitors/modules. This datasheet shows the stats on a battery being tested by buses; caps are great for recapturing braking energy and they recharge in seconds!

 

[svetz]

These guys are using a capacitor to act as an intermediary between the battery side and the inverter side. I think the idea is to increase the longetivity of the batteries; but I'd like to hear what you all think the pros and cons are.

To me, seems like the high self-discharge would be bad overall, it's not like a big battery bank can't supply more amps than the inverter will ever use.

 

[svetz]

These guys are using AC capacitor to start motors when the inverter can't provide enough starting amps. But, it heats up the inverter!

 

[gnubie]

Seller cites 6700 watts per kg (note I'm not saying Maxwell doesn't do really good things with capacitors). A watt is a unit of rate of consumption, that spec is nonsensical. It certainly isn't 6700 watt hours. The most optimistic projection of graphene based capacitors is 100 watt hours per kilogram.

Also note that it is not intended to be used by itself as a primary source of power. It's there to store up energy from a lower output device and to be able to then deliver it at a very high rate of amps. You can buy starter packs for cars that do the same thing. They hold no charge themselves. You keep it in the boot and if you find your battery is too discharged to successfully start your car, but not completely dead, you hook the unit up to the battery and it slowly draws power to squeeze every last drop out of the battery, and then be able to provide that in one big whack to start the engine.

 

[svetz]

I didn't get that bit either, but it's common: https://www.researchgate.net/post/I...energy_density_of_a_battery_or_supercapacitor

 

[gnubie]

Nope not common. That link is about Wh, ie watt hours. The seller is talking watts per kg and stating it as a measure of power density, ie storage. Watts without time can not be used to express storage capacity. It's a rate of flow. 1 watt is 1 joule per second.

Is the seller saying the unit can discharge at a rate of 6700 watts for each kg of materials, ie the effective impedance of the unit? I hope not, because Maxwell's ESMs can push power out at around 5 to 6kW per kg for their 24v units. Go down in voltage and you start to halve those ratings.

The bus unit you linked to in the opening post is rated at 1.7kW / kg discharge rate, with a storage capacity of 2.3 Wh / kg (a total unit capacity of 140Wh). The k is not missing from those two figures. lifepo4 cells are up over 200 Wh / kg.

 

[diannne]

So a supercap cant store the amount of energy(joules) needed for sustained power. They can be used(in parallel to batteries) to power bursting loads, like a fridge compressor or engine ignition and will help with battery life supplying amps for those bursts. From what I've read, wondering if you need a small resistor in series with the supercap to imitate the internal resistance of a battery, otherwise the HIGH currents in a supercap could damage your batteries, alternator or inverter, depending on hookup..A supercap will charge up instantly, sucking up as much power as you send it as fast as hell because there's very little internal resistance. So then its sucking that power through your charge controller or alternator(which is looking for a battery's internal resistance) I saw a comment on amazon that truckers were getting rid of the Maxwell ESMs after a year because it was damaging other components.

 

[gnubie]

I can believe that. The sheer current that the ESM can sustain in the short term would be an issue for switchgear and contacts that are built to work with higher impedance batteries.

 

[diannne]

These guys are using AC capacitor to start motors when the inverter can't provide enough starting amps. But, it heats up the inverter!

a starting cap is not a supercap. a regular cap can be used to correct the power factor in motors(inductive) if the cap is sized properly for that to keep the voltage and current in phase(in series with the motor). if you are running with an inverter on thereand connecting that cap in parallel, it might be reacting to the inverter output transformer secondary which is also inductive.

 

[bulrush]

The high self-discharge rate of a super cap is why we don't use them as batteries. That would would be great if we had a super cap that would only self-discharge 10% per week, but it would likely be much more expensive than any battery on the market anyway. Supercaps can be fun to work with, though, to make a nightlight that lasts just a few hours but can be recharged every day.

 

[svetz]

The high self-discharge rate of a super cap is why we don't use them as batteries.

Price is the main reason IMHO, those things are very pricey $/kWh.

I agree if you're planning to store the energy for weeks self discharge is problematic.

But, in a situation where you're fully using the battery over fairly short time frames (e.g., overnight off-grid, regenerative braking), the self-discharge loss can easily be overcome by the higher-efficiency round-trip path they offer.

 

[JeepHammer]

I am by no means a capacitor expert...
Just a trial & error kind of guy that sometimes does things the 'Experts' give me a ton of crap about.

The obvious, surge capacity for hard starting appliances.
What I found was while my 220 vac well pump and welder did fine with the 'Surge' capacity of the inverter,
When I switched to LiFeP04 batteries, they 'Ramp Up' into load and the inverter would sometimes kick out.

I simply increased the number of caps between batteries & inverter until that stopped happening.

For guys with 'Surge' input issues, hydro or wind, how about catching the surges and slowly releasing them to charge controllers?

 

[Electric Night]

 

[batteryclearinghouse]

Any idea what to look for in a super capacitor to see if it would make a good battery.

I have these server memory power backup cards that have 4 super capacitor cells on each card. name is AGIGARAM PowerGEM.

Never played with super capacitors before.

 

[gnubie]

They'd be fine for memory backup, slowly clocked dram takes very little power to maintain and sram in low power standby is microamps, but for anything that needs watts you are looking at really high capacitance values. Often the high capacitance ones are low voltage, 2 to 5 volts. I had a 1 farad hockey puck 5v super cap ... more years ago then I'd like to mention ... and it could only hold up enough voltage to run TTL logic at about an amp for 30 seconds I think.

Some elsewhere on the forum was attempting to use cheapie caps to start a car. 5 x 2.7v 3000F capacitors in series gives us peak volts 13.5 with 600F of capacitance, for storage of 54567 joules. Depending on the car, a starter motor might put away 300 amps as it runs (call it 3.6kW ie 3600 joules / sec) but the voltage in the capacitor will soon fall below the point where the motor can't draw enough amps to turn the engine over. You'd get about 2 7 second long cranking attempts with quality capacitors and then the fat lady starts singing.

Capacitor Energy Calculator

This is a capacitor energy Calculator. A user inputs the capacitance of the capacitor and the voltage across the capacitor. The calculator then calculates the energy stored in the capacitor.

www.learningaboutelectronics.com

 

[FilterGuy]

Nope not common. That link is about Wh, ie watt hours. The seller is talking watts per kg and stating it as a measure of power density, ie storage. Watts without time can not be used to express storage capacity. It's a rate of flow. 1 watt is 1 joule per second.

Is the seller saying the unit can discharge at a rate of 6700 watts for each kg of materials, ie the effective impedance of the unit? I hope not, because Maxwell's ESMs can push power out at around 5 to 6kW per kg for their 24v units. Go down in voltage and you start to halve those ratings.

The bus unit you linked to in the opening post is rated at 1.7kW / kg discharge rate, with a storage capacity of 2.3 Wh / kg (a total unit capacity of 140Wh). The k is not missing from those two figures. lifepo4 cells are up over 200 Wh / kg.

In the electric car world they talk about power density separate from energy density.

Energy Density: How many watt-hours can be stored in a Kg of battery. For a car, this is important for range.
Power Density: How many watts of power can be driven out of a Kg of battery. For a car this is important for acceleration.
(In this forum we arn't typically thinking about weight, so the closest equivelent is the C-rate Discharge amperage.)

 

[gnubie]

Yeah, but that seller wasn't doing that. The best fit was that they were stating rate of discharge as capacity to puff up their sales, and even getting that wrong.

 

[batteryclearinghouse]

The ones for server memory backup i was talking about look like they are 33 farad... so ~110C depending on it'a voltage. That's the only place I have seen super capacitors from reuse source.

So... if i did my math correctly, it would take more than 1,000 of them to power a submarine.

Years ago when I was messing around with Guitar Tube amps, you leanerd to keep an eye out for charge capacitors. I want to say they could have a 360 to 500 Volt DC on them.