# Battery energy efficiency and Coulombic efficiency - what are they?

I'm looking at two different batteries for home energy storage. Both batteries have a coulombic efficiency in the upper nineties, say 96%. One battery has an energy efficiency of 90%, and the other has an energy efficiency of 60%. I believe both ratings represent loses of usable energy, but I don't understand entirely.

If I understand correctly, a coulombic efficiency of 96% means that if I charge a battery with 100 Wh of energy, I'll only get 96 Wh of usable energy on discharge..? Now, energy efficiency confuses me the most. I've read that it has to do with the charge and discharge C-rate. I'm assuming greater C-rates lead to greater potential for energy loss. But, how can a battery have for example a 60% efficiency rating if you don't know the C-rate? It would seem energy efficiency ratings would look like this instead: 60% @ 0.5C. Unless, there's a standard C-rate when one isn't shown..?

Lastly, a 60% energy efficiency rating seems awfully low, am I really losing 40% of energy?

Thanks for any help, and sorry for my lack of education.

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anorlunda
Staff Emeritus
I think you mean charge rate when you say C-rate.

Think of resistive losses when charging a battery. At constant voltage, energy accumulates in proportion to current I, but losses occur proportional to I2.

If someone gives an energy efficiency without specifying charge rate, they are presuming a standard charge rate without saying so.

The possible range of energy efficiencies is 0% to almost 100%.

For home use, cost, size, weight, lifetime, storage duration, and fire hazard may be more important to you than energy efficiency.

berkeman and Rich76
From the internet, coulombic efficiency or Faraday efficiency is the energy lost when electrons or ions participate in unwanted reactions, or if the electrolytic products are allowed to recombine.

The other losses in batteries are I2R (current dependent, ie charge/discharge rate) and voltage losses, ie to charge a cell you need to apply more voltage than the cell voltage, then to discharge the load voltage must be less than the cell voltage, this voltage differential is a loss in the system.

So energy efficiency is all of the losses put together and is heavily dependent on charge/discharge rate.

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Rich76