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Comment by westurner | Hacker News Reader

> please correct your usage of power/energy density. Power density is measured in W/kg, energy density is measured in Wh/kg. Supercapacitors tend to excel in the former but be poor in the latter.

I'd update the units; good call. You may have that confused? Traditional supercapacitors have had lower power density and faster charging/discharging. Graphene and hemp somewhat change the game, AFAIU.

It makes sense to put supercapacitors in front of the battery banks because they last so many cycles and because they charge and discharge so quickly (a very helpful capability for handling spiky wind and solar loads).

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I think you may still be a little confused. Power density is the rate at which energy can be added to or drawn from the the cell per unit mass. So faster charging and discharging means high power density. Energy density is the total amount of energy that can be stored per unit mass. Supercapacitors are typically higher in power density and lower in energy density than batteries[1].

You're right that it makes sense to put supercapacitors in front of the battery banks for the reasons you said.

[1] http://berc.berkeley.edu/storage-wars-batteries-vs-supercapa...

I must have logically assumed that rate of charge and discharge include time (hours) in the unit: Wh/kg.

My understanding is that there's usually a curve over time t that represents the charging rate from empty through full.

[edit]

"C rate"

Battery_(electricity)#C_rate https://en.wikipedia.org/wiki/Battery_(electricity)#C_rate

Battery_charger#C-rates https://en.wikipedia.org/wiki/Battery_charger#C-rates

> Charge and discharge rates are often denoted as C or C-rate, which is a measure of the rate at which a battery is charged or discharged relative to its capacity. As such the C-rate is defined as the charge or discharge current divided by the battery's capacity to store an electrical charge. While rarely stated explicitly, the unit of the C-rate is [h^−1], equivalent to stating the battery's capacity to store an electrical charge in unit hour times current in the same unit as the charge or discharge current.

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