Pros & Cons of Lithium-ion batteries for solar application

Re: Pros & Cons of Lithium-ion batteries for solar application

I am also interested in setting up a Lithium bank for solar power. In addition to the solar bodger’s DIY project which is quite interesting you should take a look at the following website http://liionbms.com/php/index.php this has a lot of information about BMS technologies that are available on the market.

I purchased the book on BMS systems that was written by Davide Andrea (I believe he owns Elithion BMS and is responsible for the website mentioned above) which was quite interesting; he has a section in the book that compares the advantages and disadvantages of different balancing techniques. After reading the book I would have to disagree with the previous individual that stated resistive bleed type balancing is inadequate for a large bank.

When you start out with a new battery bank you must first charge each battery individually to the same charge, there is also a scheme that the author goes into in the book in which you can hook all the batteries together in parallel to accomplish the same thing before you put the battery bank together to use in a bank in series. I would have to look at it again to remember the details. If you do not start off with the same state of charge for all batteries you are doomed to failure from the beginning. If you do however do this the periodic balancing from the BMS should not require very much and should be fine to use a resistive type bleed BMS, the author claims on pg 79 that a 100ma bleed is sufficient for most battery banks. He does not state the ah size though. I do not think the BMS is the biggest obstacle.

At his website he compares a lot of BMS systems; there are several that I think have really good potential for solar systems. The Elithion System and the Orion Systems look the best to me from what I can tell in the states. Overseas there are several other BMS’s that are equally as good, I however would prefer something local. After looking at all of these systems for a smaller battery pack (48 vdc 300 ah) I am leaning towards the Orion system. The Orion system has a wire to every single cell in the battery bank, for banks that would be used in a car with high voltage this may get fairly messy with all of the wires however for a 48 vdc solar battery in series you are only looking at 16 cells so this is quite reasonable.

For a lithium battery BMS system to be successful it needs to be able to communicate and control other equipment in the system. It needs to be able to tell an inverter to stop inverting if the battery soc is too low so you do not kill the battery. It needs to be able to stop the charger from charging if it gets too high (Lithium batteries will start on fire if they get an excessively high charge state).

After reading the article I think the biggest problem is the solar components. I am running into the following problems.

1) Most Current BMS systems must communicate with the inverter & chargers. The primary reason to control external equipment is to prevent low or high voltage. They do this in a vehicle through canbus typically. Inverter/chargers used for solar cannot be controlled by outside equipment as far as I can tell. And none of them (that I can find) except for Victron inverter chargers use canbus.
2) A work around for this is to install a contactor, basically a disconnect from the system if the battery gets to low or too high. I was looking at doing this with a Xantrex XW4548 and contacted Xantrex. They do not support this configuration and stated if the Xantrex is connected to shore power and the battery is disconnected all bets are off at what will happen next. This leaves a lot to be desired and sounds quite dangerous. They seemed to indicate a fire could be possible, not sure though.
a. Ideally the bms would work well enough that the contactor would never be used, it is only there as a last effort to prevent a serious problem (fire or dead battery)
b. As long as the voltages stay close enough the internal settings for high and low voltage in the inverter /charger (xantrex, magnum, etc.) should be able to protect the battery bank. A BMS however is not looking at the overall charge it is looking at the charge of every single battery and if any of them are in a dangerous range they will shut-off external equipment to prevent a problem. Standard solar inverter / chargers do not have the ability to monitor individual voltages which key when working with lithium batteries.

My current idea for a work around is as follows:

1) Use a Xantrex XW4548 inverter to power house circuits (Disable charge functions), this would be connected to the Battery bank with a contactor so the BMS could disconnect it if necessary.
2) Purchase an Elcon PFC-3000 Charger that has the ability to communicate through canbus with the BMS system. This is actually ideal for me because I have a Honda EU3000is that only has 120V output that I would like to use in the system. This would not be compatible with the Xantrex so it is not a big loss for me to have to purchase a separate charger.
3) The Orion BMS has 2 canbus interfaces and can have several analog outputs to control equipment that cannot communicate with canbus. I would use the analog outputs along with a Tristar MPPT (similar to what the solar bodger has done)
4) I am somewhat paranoid so I would also like to use the contactor as an ultimate fail safe to protect the bank. From overcharge from the MPPT charger, this however is already controlled by the analog signal so it should not ever be necessary.

Problems

1) I don’t know if there is a certain method that must be used when wiring a separate charger (elcon) and inverter (xantrex) to the same battery. There is a power point on the web by Genasun “Genasun-2011-SITS” that indicates you don’t want to have the charger feeding directly into the same bus the inverter is on. I am not sure exactly what he means by this. (This is specifically for solar lithium battery banks)
2) I don’t know if the inverter or mppt will have problems if disconnected from the battery bank. The inverter may have power coming into it from the Mppt controller.
3) Any other unforeseen problems, this is a lot of money to spend to find out there is a problem

Hopefully the information above is helpful. I would also appreciate it if anyone out there with a bit more installation experience with these types of components has any feedback on the best way to connect the equipment stated above. And if there are any gapping holes in the logic of what I am trying to do.

Additional Information

Victron looks like a good solution, I however cannot afford their battery bank. I contacted their dealer in Colorado to see if the canbus could communicate with Orion BMS. The answer I got was basically if it isn’t Victron you are on your own.

Re: Pros & Cons of Lithium-ion batteries for solar application

BB,

Thanks for the input, I will see what the Guys at Midnit Solar can come up with.

Just to clarify, from what I can tell Lithium batteries are only prone to fire if the voltage runs to high. They would not start on fire if the voltage was low. Low voltage however could ruin the battery cell and make it no longer usable.

The idea with using a charger from Elcon with canbus communication would be to iliminate overcharge from a charger either from my generator or from the power line. I am most concerned about overcharge due to a fire risk. EV converters are already using this charger for cars so I don't think there would be much problem with doing this.

The tristar MPPT could hopefully communicate with the analog signals from the BMS to shut down charge current from solor panels if the battery voltage gets too high.

For protection from draining the batteries I think the Low voltage cutoff built into the inverter should be fine. To maximize battery life I don't plan on running the batteries past 20% SOC. Due to this if the batteries are all being balanced periodically I don't think there is much risk at any one cell having a voltage that is different from the rest of the pack that would damage the cell. The BMS can also run the contactor so this would be a back up protection.

Re: Pros & Cons of Lithium-ion batteries for solar application

mtdoc wrote: »

One of the things that a BMS does - at least the ones I have experience with - is prevent over charging and over discharging. Using a lithium battery bank with BMS in a typical solar installation with quality charge controller and inverter would mean that now both the BMS and the inverter and charger would be preventing over charging and discharging. Redundancy is good.;)

The other important function of a BMS, as I understand it, is to maintain balance between each individual battery cell. This is something that an inverter or solar charge controller cannot do. The BMS does not need to communicate with any other electronics (inverter or CC) to do it's job.

Many electric bike enthusiasts run high voltage, high amp, Lithium ion battery packs with no BMS. They use switch mode power supplies , often stacked, to charge their batteries. But they also use equipment to monitor individual cells and balance cells when needed. They closely monitor their battery packs and act as human BMS's if you will.

Lithium Iron Phosphate chemistry is inherently much safer than other lithium ion chemistries and the risk of fire, though not zero, - is not a real concern in any reasonable RE system with over and undervoltage protection. Thermal runaway and resultant fire is a possibility with traditional lead acid chemistry as well. And of course with FLA batteries there's the dangers that come with Hydrogen gas production...

I would not be surprised if LiFePO4 batteries become the standard for RE systems in the not distant future. I think the main hurdles right now is higher cost and even more, the fact that most Solar installers, equipment manufacturers and retailers have little or no experience with them.

The BMS protects the battery by telling other equipment to either stop producing power (voltage too high) or stop using power (voltage too low). It also balances the cells by either transferring power between cells or bleeds power off of high cells to allow other cells to catch up (bleeding power is the more common approach). The main idea behind the balancing is too balance frequently to prevent cells from ever getting too far appart. BMS's do not have the ability to blead enough power if they are being feed excessive amounts of power. (ie. if all cells are already at 4 volts and charging must stop but the charger keeps producing power the BMS will not be able to keep up.) The typical method of stopping a charger that you cannot communicate with is for the BMS will disconnect the battery bank from the charging source through a contactor. From what I can gather from online and from Xantrex technical service most solar inverter/chargers do not like to be disconnected from the Battery bank. (I don't know why this is just not a supported configuration, power comming into AC1 from line power with no battery to act as a buffer between the house and AC1. Not sure what will happen but it seems that this is definetly not good with Xantrex XW inverters and from what I can gather on line probably other inverter / chargers have a similar problem. If anyone knows more about this that would be great)

Another possible method to prevent overcharge would be to just have the BMS send everything to a dump load. Not sure if this would cause additional problems or not. Seems like it should work.

Typically if all the cells are balanced properly the Low or High voltage cut off built into the inverter/charger should make it so the contactor would never be used. With Lithium cells though if you get an unbalanced bank for any reason a pack voltage is not good enough way of measuring voltage for the low or high voltage cut offs. (possible reason for unbalanced bank with BMS: BMS's typically balance at a specific region they do not usually balance at all voltages. If you have a BMS that Balances at 3.7 volts and you rarely get your batteries to this high of a voltage they might get out of balance over time due to the BMS never kicking in the balancing function. If this happens for too long of a period of time (months) if you ever get to 3.7 volts the difference maybe large enough that the BMS cannot handle it.