More on Ni-HM and LiIon

[Maury Markowitz]

I ended up on this forum because Google led me here when I went looking for information on Ni-MH batteries for solar off-grid uses. There's very little out there, a discussion from two years ago ended up on the first page of Google results! The only manufacturer of large Ni-MH batteries that came up was Saft, but my attempts to talk to their sales department about the NHE line has failed - bounced around departments, disconnects and no returned calls.

I'd like to pick up on that thread though, at least as a starting point. The first issue was raised by Mike90250, who noted:

"I think AGM has recharge efficency of about 95%, Flooded is 80% and NiMh is about 50%, so you would need a lot more solar PV to recharge them with."

This may sound naive, but is this still true? And what is the recharge efficiency for li-ion.

The rest of my questions relate to a post by Sunking that followed Mike's:

" So if you need 1 Kwh per day

[Naptown]

The 5x number is based on the DoD for lead acid batteries. This will give the longest life for the battery. look at the manufacturers life cycles for various depth's of discharge. The deeper you discharge it daily the shorter the battery life.
Also most threads here refer to off grid solar systems. The x5 is to give a 2 day autonomy to account for cloudy weather and still keep the batteries above the 50% DoD. If this is mounted in a truck that operates daily you will not need that much capacity as the truck alternator will recharge the batteries daily. I would assume the freezer is not in operation on weekends and is shut down. If that is not the case then a larger battery capacity is needed as the sit time over the weekend without the benefit of daily recharging during the course of operations will drain the battery further.

[Maury Markowitz]

The x5 is to give a 2 day autonomy to account for cloudy weather and still keep the batteries above the 50% DoD.

Ahhh, that *may* explain it. However, when you look at the original thread that's not the setup. But perhaps the original poster simply used 5x because he was used to it?

[tandrews]

Not knowing what the freezer is used for, or how much of it's internal capacity is utilized, I offer this:
Have you considered double insulating it and freezing some water as thermal ballast to carry it through to the next day?
Drop thermostat temp to offset off grid times?

At least this would drop your battery backup requirement weight.
just sayin.

[Sunking]

Murray the manufactures are telling you not to discharge more than 50% because beyond that point lead sulfate crystal growth rapidly accelerates and starts to harden on the plates. Inj other words you are really destroying the battery. If you take a look at some of the better deep cycle batteries discharge/cycle life curves of say Rolls RE 5000 Series look something like this:

20% DOD = 5000 cycles
30% DOD = 4200
40% DOD = 3700
50% DOD = 3200
60% DOD = 2800
70% DOD = 2400
80% DOD = 2100

Regardless what battery chemistry you use, you would never want 2 day reserve. You would be dark for months out of the year. If you get just one cloudy day you have to shut down, and wait for a full day of sunshine to recharge, or run a generator an awful lot of time during the year. Even if you use Lithium or NiMh the minimum would be 3 day reserve and you had better have a generator.

[LucMan]

This is old school technology but it works exceptionally well.
Take a look at cold plate refrigeration technology. Just another option that may fit your requirements without using batteries. 12 v refrigeration systems are available to operate when the truck is running, and the cold plates can maintain temperature for extended periods.

http://www.e-zkold.com/faq.htm

[Mike90250]

What's the cost of a block of Dry Ice (frozen Co2) in your area - that may be the best yet.

If you are going to be driving with this weight, maybe you need to look at Li style batteries, but they don't have easy BMS to keep them healthy.

[Maury Markowitz]

Rolls RE 5000 Series look something like this:

20% DOD = 5000 cycles
30% DOD = 4200
40% DOD = 3700
50% DOD = 3200
60% DOD = 2800
70% DOD = 2400
80% DOD = 2100

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. The chart above suggests such a battery would last the better part of a decade.

Regardless what battery chemistry you use, you would never want 2 day reserve. [snip] Even if you use Lithium or NiMh the minimum would be 3 day reserve and you had better have a generator.

This particular application will work like this

[Maury Markowitz]

If you are going to be driving with this weight, maybe you need to look at Li style batteries, but they don't have easy BMS to keep them healthy.

This is why I'm here actually. Is anyone here using Ni-MH or li-ion? If so, how did they get the charging to work? And who supplies them?

[Maury Markowitz]

This is old school technology but it works exceptionally well.
Take a look at cold plate refrigeration technology.

I had never heard of this, and it sounds like a perfect solution! Thanks LucMan!

[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.