More on Ni-HM and LiIon

[Maury Markowitz]

This thread is SO informative! I've learned more on this topic in two days that the previous two weeks.

It's not quite that bad (65% to 85%) - but they are less efficient than lithium ion. The problem with them is that they have to be "overcharged" at the end of charge to reach a full SOC, and that wastes energy.

Sorry, which ones are you talking about here, Ni-MH or AGM?

And, reading between the lines at my own peril here, what is the charge efficiency in the li-ion case?

Flooded lead acid are around 80% for the same reason. Most manufacturers recommend absorption charging, and some of that energy is not stored in the battery - it is dissipated as either hydrolysis and outgassing or hydrolysis and recombination.

Got it. As I understand it this is the part of the cycle that causes problems if you hook a li-ion up to a lead-acid charger.

[Sunking]

Just to start off... is 20% DoD "20% from completely dead" or "20% from fully charged?"

No I mean 20% of the full capacity. DOD = Depth of Discharge. So when I say 20% DOD the battery has 80% of its capacity left in it.

There are two very good reasons to only discharge a FLA battery 20%/day.

1. It gives you 2.5 days of reserve capacity for cloudy days before you hit the magic 50% DOD where you have to shut down and wait for recharge.

2. It extends the battery life considerably, and give the user the most economic benefit, which is still poor, but does give you the most bang for your battery buck.

As for Lithium, I have a positive outlook, but for now they are just not economically feasible yet. To get the max life out of them for now requires then to be charged up to 80% SOC, and then discharged only to 20% SOC. In other words you never fully charge them, or fully discharge them. because if you do you whack the cycle life in half in which for most lithium chemistry is not great to start with.

As stated energy density is not important for RE applications, where as it is critical in electric vehicles.

[billvon]

Sorry, which ones are you talking about here, Ni-MH or AGM?

Ni-MH.

And, reading between the lines at my own peril here, what is the charge efficiency in the li-ion case?

Pretty high - mid 90's for a 0.8C charge and discharge for lithium cobalt chemistry (i.e. standard laptop type li-ion batteries.)

Note here: there is a difference between coulombic efficiency and energy efficiency. Coulombic is amp-hours in vs amp-hours out. Energy efficiency is watt-hours in vs. watt-hours out. For example, if you put 100 amp-hours at 15V into a lead acid battery and discharge 95 amp-hours at an average of 12 volts it has a coulombic efficiency of 95% - but a total energy efficiency of only 76%.

Got it. As I understand it this is the part of the cycle that causes problems if you hook a li-ion up to a lead-acid charger.

Yes. Li-ion batteries need three phases as well but they are simpler:

1) Constant current, where current is set to some maximum and the voltage slowly rises.
2) Constant voltage, where voltage is held steady and current is allowed to drop.
3) Termination, where charge is terminated and the battery disconnected (i.e. no float charge.) Li-ions do not do well when stored at maximum voltage/maximum SOC. Which is one problem they have with backup systems.