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Lithium-Ion Batteries for Off-Grid Systems: Page 3 of 3
Are They a Good Match?
Issue Date:
February / March 2013
Last Updated:
12/16/12
Intermediate
- Intermediate
A BMS can also help during discharge by signaling for load disconnection when individual cells drop below their minimum voltage. Cells discharged too deeply can be permanently damaged or, at minimum, have their capacity or cycle life permanently reduced.
Li-Ion in RE Systems
There are no known Li-ion BMS power conversion products (inverters with chargers, and charge controllers) for the residential RE industry (although Clean Power Auto does have a solution for RVs and boats). The missing feature in standard power conversion is BMS cell monitoring and the available RE equipment can’t modify charging characteristics to handle when some cells have reached full capacity and other cells have not. Reading the voltage of the entire pack of cells is not helpful, since there’s no means of knowing whether or not some cells have already reached their capacity. Available charge controllers can’t adjust current on the fly to match the BMS’s ability to shunt current as cells start to reach 100% SOC.
If power conversion equipment with provisions for BMS become available, then LFP batteries can be useful in RE systems if:
- The depth of cycle is often more than 30% of capacity. Otherwise, LA batteries are a more cost-effective option.
- The space available for batteries is at a premium. LFPs can save about 50% of the space compared to LA for the same usable capacity. (These factors can be very important for RV and boat applications.)
- The application requires low maintenance (such as hard-to-reach sites, or a user who won’t or can’t do the work). With twice the longevity of LAs, this also means halving the battery replacement frequency.
- The application calls for frequent cycling of the batteries—with LFPs having several times the cycle life of LAs and at a deeper depth of discharge. Off-grid applications generally fit this profile.
LFPs can definitely help solve some residential RE storage problems. But perhaps a better question is, “Is the residential RE industry ready for LFPs?” At present, the lack of BMS integration in residential RE power conversion equipment is the biggest hurdle. Once LFP-compatible products become available, then LFP batteries will find a home in many RE applications, especially off-grid and mobile.
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After earning his electrical engineering degree, Randy Richmond worked for the telecom industry. In 1999, he founded his own company, RightHand Engineering LLC, which makes products for monitoring RE systems. Since then, Randy has earned his professional engineer license and also offers design, test, educational, and consulting services for Li-ion-based power systems.
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Comments (5)
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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)
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...
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.
Thanks for weighing in, Randy. Your new article on Li-Ion batteries is now available on the site at https://homepower.com/articles/lith...
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?