Lithium-Ion Batteries for Off-Grid Systems: Page 2 of 3

Are They a Good Match?
Intermediate

Inside this Article

Prismatic type Li-ion battery
A typical prismatic type of Li-ion battery.
Cylindrical Li-ion battery
Cylindrical Li-ion batteries are often ganged into voltages appropriate for cordless tools.
Pouch types of Li-ion battery
You’ll find these pouch types of Li-ion batteries in radio-controlled hobbyist cars and other places where weight is a concern.
BMS cell-balancer board
This BMS cell-balancer board is mounted to a set of four LFP cells. It is typical of a BMS for EVs.
An overcharged LFP cell
An overcharged LFP cell can rupture, leaking caustic electrolyte.
Prismatic type Li-ion battery
Cylindrical Li-ion battery
Pouch types of Li-ion battery
BMS cell-balancer board
An overcharged LFP cell

Discharge Voltage & Impedance. LA batteries’ discharge voltage tapers significantly as state-of-charge decreases, whereas LFP batteries’ voltage remains fairly steady until they are close to being fully discharged. LFPs have about one-quarter the internal resistance (impedance) of LA batteries, which reduces battery energy lost to heat. These both combine to improve system efficiency and prevent DC voltage sags that can affect voltage-sensitive equipment.

Charge & Discharge Current. LFPs can be safely charged and discharged at a higher current than LA batteries. But the relatively low current in RE applications (compared to EV applications) makes this aspect not very useful.

Self-Discharge. At room temperature, idle (stored or disconnected) LA batteries lose 5% to 15% of their electrical capacity per month, compared to 1% to 3% for LFPs. In RE applications where energy is used only occasionally (such as pleasure boats, RVs, or vacation cabins), or during periods of low RE source, this can be a useful attribute.

Maintenance. Wet LA batteries, if not watered when needed, will have a greatly shortened life. LFPs require no additional liquid to maintain their electrolyte levels. Sealed LA batteries, which have a much lower up-front cost than LFP batteries, compare better than FLA but their lifetime cost per kWh is greater than either LFP or wet LA batteries.

Lifetime. While longevity can vary widely depending on factors such as daily depth of discharge and LA battery type (marine, golf cart, AGM, industrial, etc.), regularly used and properly maintained common deep-cycle LA batteries have an average lifespan of about five years; LFP batteries have an estimated longevity of 10 years—half the frequency of LA battery replacement. In both cases, natural aging of the battery chemicals can impair batteries before their cycle life is used if they are cycled infrequently. When used up, both types of batteries can and should be recycled by returning them to a dealer, although due to the long history of LAs, there are presently more recyclers for LAs than LFPs.

Cost. Although LA batteries are cheaper up-front than LFPs, their lifetime price per kWh can be higher. This assumes that you can use most of the lifetime capacity (usable capacity multiplied by cycle life) prior to the battery failing due to age. With a 3,000-cycle or 10-year life (whichever comes first), one would need to cycle LFP batteries nearly daily to optimize the payback. (Note: End-of-battery life is generally considered to be when the battery can maintain only 70% to 80% of its original capacity.)

The Need for Management

The biggest disadvantage of any Li-ion battery is needing a battery management system (BMS). The job of a BMS is to monitor the voltage and temperature of each individual cell and protect from excessive charging and discharging. While any battery system, whether it be LA or LFP, can be improved with a BMS, a BMS is not typically required of LA cells. As long as all the batteries in a pack are of the same model and age (ideally from the same manufactured batch) and have been treated equally, the individual cells tend to behave the same while being charged. However, LFP battery cells, even in the same manufactured batch, can have variations in capacity. When charging, cells with lower capacity can become full much sooner than cells with higher capacity, which can lead to dangerously elevated voltages on the full cells as the others continue charging.

While LA cells tolerate brief periods of overvoltage (in fact, periodically elevating charge voltage to perform an equalization charge is recommended), even a fraction of an hour at elevated voltage can damage an LFP cell. A BMS protects individual cells from overvoltage by shunting current around the full cells when they reach their recommended “full” voltage. This allows the remaining cells to continue charging. A good BMS can detect when a cell is beginning to overheat (another sign of pending danger to cells) and shut off charging to the pack to protect all cells.

Comments (10)

John Nicholson_2's picture

Rereading and wondering if this question has any answers in the making: “Is the residential RE industry ready for LFPs?” (Being that this is published in a RE mag. I figure someone will come out with something.)

Is this the product that you made a comment on in the article: http://minibms.mybigcommerce.com/pr... ?

(As to the prior comment and reply, the link also has a contact with the photos. I don't know the the details that he keep.)

John Nicholson_2's picture

Copy.

mark roberts 2's picture

I'm using rebuilt Edison storage batteries, the cores are from the years 1907 and 1908. KOH electrolyte. They work great.. My Nephews grandchildren will be using the same batteries, decades down the road.

Mark Roberts

Randy Richmond's picture

Nickel-Iron batteries (which use potassium hydroxide electrolyte) like the Edison type have their place, and are very hard to beat for longevity. But their specific energy (energy/weight) and specific power (power/weight) is about 1/3, their energy density (energy/volume) is about 1/7, and their price (new) per Whr is about 3 times that of LiFePO4 (LFP). Their charge & discharge rate are much lower than LFP or LA (lead acid) and their charge profile (V-A vs time) can be tricky if you want to avoid thermal runaway. If weight, space, and instantaneous power are not a concern, and if one can afford the 3x up-front cost (new), their lifetime cost/Whr could be less than LFP. If used batteries with less than 30 years on them can be found (or if lucky, in the 100 year range like yours), the price could be very attractive. I understand that Exide took over the Edison Nickel-Iron batteries in 1972 and stopped making them in 1975.

mark roberts 2's picture

I've never had a problem with run away thermal and they get overcharged on a regular basis. My inverter and charge controller will only charge at about what is considered mid-range charging voltage for the batteries, 32 or 33 volts ( magnum inverter and midnite 250 controller. I force float at 31.5 volts. Its a 24 volt 500amp battery bank. Run them down to 19.5 volts. I dont really worry about over charging or undercharging too much.

My guess is as more and more farms and rural homes found utility power available after WW2, the market for Edison storage batteries slowly dried up. Excide bought them out and buried the technology. Its kinda amazing how much information is out there now compared to 10 yr.s ago when I 1st started researching nickle iron batteries.

10yr.s ago one would've thought the battery industry didn't really get started until the 1940's for the lack of information available..

Randy Richmond's picture

Hi John, I looked at the link and saw what appears to be LA paralleled with LFP batteries (I don't know how the capacitors are wired in). In general, mixing battery chemistries is not recommended because the voltage for the various charge stages are hard to match-up between LA and and other chemistries. Fortunately, 6 cells of LA (at 2.2V each) is about the same as 4 cells of LFP (at 3.3V each), so paralleling them may be OK for discharging, but I would still be concerned that during charge, the differing charge characteristics might prevent one or the other of the chemistries from being properly charged. Also, one can expect the LA batteries to reach end of life much sooner than the LFPs, and when they do, the LA batteries will be a drag on the LFPs, reducing the efficiency of the whole setup. If the guy who did this conversion is doing data logging or recoding his experience (e.g. the actual usable watt-hour capacity, how many cycles, etc.), it would be interesting to have the hard data to see how this actually plays out for him. Regards, Randy (author)

John Nicholson_2's picture

A friend of mine in the North Texas Electric Auto Association www.nteaa.org/ made a car which used both LA and LFP. The idea is to use LA as a buffer for the LFP as to balance and charge them. I have moved and so it has been a while since I check on the setup, but it might be something to think and write about. Here are some photos of it: http://www.flickr.com/photos/mbarkl...

Randy Richmond's picture

Richard, thanks for the question. In the next issue of HP, there will be a companion article about LiIon batteries in EVs. The article will have a table with the specs for the large format LFP batteries. You will see that the rated discharge low temp is -4F, and the rated charge low temp is about 30F. Unless you can keep LFPs above -4F they would not be a good solution for you. In the contiguous 48, even in Minnesota, many EVers find that insulating their battery box, and putting a heater mat under them (powered while plugged in), can keep their batteries warm enough, but at -75F even those measures may not work. Unfortunately, I've not explored battery options for such extreem low temps. You might browse through www.batteryuniversity.com and see about other battery chemistries that might work at those temps. But its likely that you will be trading low temp performance for some other aspect (such as lower power or energy density). Best regards, Randy, Author.

Scott Russell's picture

Thanks for weighing in, Randy. Your new article on Li-Ion batteries is now available on the site at https://homepower.com/articles/lith...

Richard Kemper_2's picture

Just how cold are LiFePo batteries able to withstand? Here in Tok, AK, we have a problem with even fully charged flooded lead-acid batteries slushing up at -75F or so, and we get that low in most years. Will this kind of cold damage these very expensive items, and what kind of performance (if any) could I expect? FLA are pretty much useless for EV power in winter, but would LFP be viable?

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