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Lithium BMS


Darryl&Rita

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Open call, looking for advice on a BMS for a Lithium CALB battery bank.

I've recently acquired a set of 8 CALB batteries, that were in a previous life wired into a 24v series bank. The previous "No Name" BMS suffered a horrible death, but the batteries don't seem to have been harmed. I'd like to wire them into a series/parallel 12v bank. 2 of the batteries have voltages a little outside the rest, so I will equalize them, as per instructions I've found on the 'net.

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The BMS system by house power was the one most often used by do it yourself folks. It costs between 75 and 100 dollars. I just tried to look it up for you but looks like they have been bought out. It does say they will still sell to hobbyists so you just may need to contact them as I could not get the product to show up on their main page. http://cleanpowerauto.com/ http://cleanpowerauto.com/files/HousePower%20BMS.pdf

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For our typical applications with relatively low rate of discharge, several folks better informed than me argue for just bottom balancing the cells and leaving the BMS off. The argument is the BMS is a failure point and more likely to harm the batteries than provide any meaningful balancing. A function a BMS does perform that's useful is monitoring for and protecting against under voltage. A simple voltage monitoring circuit driving a cut out relay can accomplish same.

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Thanks, guys. I have read the boating post previously, possibly too fast. It seems lots went over my head.

I would like to use these in an RV application, with solar recharging. The measured load is well under 80% of the intended series/parallel pack 200 aHr.

A little background on these batteries, they were originally packaged as a parking lot lighting package, with solar recharging. The original seller is offering little support, and the original BMS looks to have been built in a basement shop, with Silicone used to pot the circuit board. The circuit board has no identifying marks. It had wires that look to be measuring individual cell voltage, but doesn't seem to have any means of measuring either battery temps or ambient. This was probably a bad idea in the Phoenix valley, in the summer heat.

Of the 8 batteries, 1 is sitting at the correct 3.2v, 1 is at 3.4v, while the balance are sitting at 3.8v. They may already be destroyed by overcharging. I have no way of accurately tracking the cycles these have seen already seen, so these may well be my training wheels for future Li use.

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I would do exactly what you first proposed. Strip all the excess equipment off and start anew. You can start right now just by wiring them in parallel. They will then balance between themselves passively. After they sit like that a few days pull them back apart. The cells should be close to a balance. Measure them and see how they react over a few days. If one or more drops off that is a sign you may have a bad cell. 3.8v is high but the fact that they are holding 3.8v tells me they are not bad. They may have diminished capacity but the experiment I suggest above will tell you a lot. There are different ways to balance but I find top balancing to be the easiest and I purchased a 3.6 volt charger (I can show you the model if you like). I then charged them up and let them rest. I separated them and retested as I described above. It took a few times but they all balanced and held. I then hooked them up as a 12 volt pack, set the charging parameters on my Magnum inverter/charger and have been good to go for 2 years. During the 2 years I have twice disassembled the pack and re-balanced them with the 3.6 v charger. However, there WAS very little change in the batteries balance. I just wanted to make sure each individual cell was holding up (I have 700 ah made up of 28 100ah 3.2v CALB cells). I own the BMS unit I described to you above but have never got around to installing it. I check the battery level often and have a good idea what is going on with them. This seems pretty safe IF you are a fulltimer and can be on top of them. If I was going to be gone for periods of time I would not attempt this. I am still plan on installing the BMS but right now rely on my magnum to catch any overcharging or low voltage situations

 

 

Yes, Jim, you can bottom balance or top balance. The key is to get them balanced.

 

Part of the confusion on the BMS issue is that the same initials are used to refer to a battery monitoring system or a battery management system.

 

A monitoring system is one that will measure the voltage in each individual cell and the whole pack and give you an alarm warning if high or low cut offs are met. They can also be hooked to relays shut down any charge source for a high voltage or any battery output for low voltage.

 

The battery management system actively shunts electrical current between cells to keep them in balance. These are used in automobile packs. Some form of this is also used in the ready to use "drop in" lithium batteries you now see on the market for RV use. These units are more expensive.

 

As Jim said there is an open question what is the appropriate type of BMS for RV usage lithium packs. The argument for an active management system is that you know your cells will always be in balance and you can pay less attention to them. The arguments against a full B Management S are: (1) it is not needed in RV application since the cells remain in balance (2) by introducing new circuits and active balancing you increase the risk of failure (3) these systems make the packs harder, or impossible, to break apart to test or replace individual cells and (4) expense.

 

As you can tell I am in the school of keeping things simple. You can balance your cells and assemble them as a 12 v pack and try that method with close monitoring. If it doesn't work for you--you can always add. I proposed the house power system because it is low priced, gives you both pack and cell level monitoring and has a very limited passive battery balancing or management aspect that only kicks in if you reach high voltage in one of the cells. Essentially the battery management system will never kick in unless something has gone wrong.

 

If you need straps to put your CALBS together check out this site. http://evtv.me/

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The consensus on the cruiser forums is that a bums is not required. They are on top of their batteries and have nothing else to do under sail but check and recheck their batteries. I had one cell in my system that had a higher voltage than the others. The BMS will only balance at a rate of .5 amps. So it is a slow process. After discussing this with Elite Power Solutions engineers I reduced my charge voltage by .2 volts. That took care of my problem. If I were able to buy cells cheaply I would do exactly as Dave has instructed. Get them balanced and use them. Use either solar or a programmable inverter such as Magnum to charge the batteries. If you would like I have a bunch of info I put together I can send you. If you want message me with a. Good email address.

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Thanks guys. The consensus does seem to be to minimize the failure points that a Battery Management System can introduce, as Daveh points out. Now I just need to decide on a voltage sensing relay to use for High/Low voltage cut-off. Most of the solar charge controllers I've worked with, and good solar practice, want the battery voltage connected to the controller first, and removed last. I'm leaning toward using the High voltage relay contacts in the solar panel feed lines, and the low cut-out contacts in the load lines.

Ron, I'd gladly take any info you have. Use the email address in the signature line below.

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Re Progressive Dynamics. The big companies that do well with lead acid scare me when they put out these products labeled as being for lithium. They say their high cut off is 14.6 which would absolutely fry LiFePO4s. Then they give this guidance: Lithium iron phosphate type cells, are the safest and most common type used however, they must never be charged to a voltage over 4-volts and must never be discharged below 2-volts, or they will be damaged. Note: other cell chemistries require different voltage limits.

 

So they are saying the range for LlFePO4 is between 8 volts and 16 volts. Look at the specs for any LiFePO4 battery. This is an immediate death sentence. These numbers will not shorten the life of the battery--they will kill them immediately.

 

Get a charger that allows you to set the parameters. progressive Dynamics needs to take this language down. Outrageous.

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