More on Ni-HM and LiIon

[Sunking]

Are these numbers believable though? Others, with long battery experience, have stated that the lifetime of the very best batteries at 80% DoD is measured on the order of months.

Well that is true for 90% of the deep cycle batteries out there on the market. Bu there are two manufactures out there that make really good batteries, and back up their claim with a 10 year warranty. One of them is the ROLLS 5000 series product line made just for RE systems. Just scroll down to the bottom of the page and you will see where the numbers came from.

The other 90% you are correct, and discharging only 20% DOD will only yield 500 to 1000 cycles. However as of today there is no known Lithium or NiMh that can do more than 1000 cycles with 3rd party verification. NiMh will never be able to compete economically with lead acid technology simply for the fact that nickel is a semi-precious metal. Lithium has some promise but economically they are not there as of yet.

[Maury Markowitz]

The other 90% you are correct, and discharging only 20% DOD will only yield 500 to 1000 cycles. However as of today there is no known Lithium or NiMh that can do more than 1000 cycles with 3rd party verification. NiMh will never be able to compete economically with lead acid technology simply for the fact that nickel is a semi-precious metal. Lithium has some promise but economically they are not there as of yet.

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

The reason I ended up here was because of that 20%/50% issue. I've seen a number of suppliers of li-ion in the 40 cent/Wh range, which is about twice as much as the C&D AGM's. But if it's true that the C&D will last 1800 cycles at 50% DoD, and the li-ions will last 2000 cycles at 20% DoD (or 80%?!) then they come out to about the same price. But 500 pounds lighter!

So in your opinion, is another 20% reduction in price possible with li-ion? I can't say I know a lot about it, but given the recent pricing trajectory it seems this will occur by the end of the year!

[Maury Markowitz]

One of them is the ROLLS 5000 series product line made just for RE systems.

Am I reading the page right? It does not appear these are sealed?

If that's correct, they wouldn't be useable in this application. At least I wouldn't recommend them. He won't have *easy* access to the compartment in use and I suspect he wouldn't maintain them.

[Maury Markowitz]

Geez, I'm sorry to reply to a single post with three follow-ups, but I keep remembering things I wanted to ask.

Charge efficiency... I believe you said that lead-acid was 80 to 90%, and NiMH was 50%? Or was it li-ion that was 50%?

Where exactly is the loss mechanism in this case?

[billvon]

Charge efficiency... I believe you said that lead-acid was 80 to 90%, and NiMH was 50%?

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. If you charge them slowly (1C rate) and only operate them within, say, a 30-70% range of charge they can be over 90%.

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.

[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.