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Depleted Li battery pack- UPDATED and ok


hemsteadc
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I went and did it  - I ran my 48v 100ah Relion battery pack almost empty - 90% depletion according to my gauge.  

I know all the admonishments so please spare me, I feel bad enough already.  I have a call into Relion but so far no help.  I don't know if there's a recovery procedure or if this very expensive battery is now a very large paperweight.  When I tried recharging, it wouldn't accept all the amperage it's supposed to, and the "Ah available" stayed at zero for almost an hour.  So I gave up.  Anxiously awaiting that call before I remove it from the coach.

In the meantime, anybody have any positve suggestions?  Thanks

Edited by hemsteadc
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You should have two large relays. Mine are round. If it I low charge, it will cut off load but will let it charge. If it over charged, it will cut off charge but allow load to be used. One will be on positive side, and one on the negative side. Your battery wires go from the bank to the relay then to the buss bar. There are more wires coming from the relays too. If they are wired wrong, it will let you use the load when they are low but will cut off the charge voltage. 

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Ok, I just got a call from the Relion tech.  The low voltage cutoff is around 40vdc.  Pretty low.  The voltage when I discovered the problem was around 48.5.  So we're good.  

After many hours of charging the battery is behaving well and coming up on a full charge.  Whew.. feel like I dodged a bullet there. 

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You should have a battery management system supplied with the battery. The BMS protects the Lithium battery form things it does not like....Most Lithium Chemistry system s have the BMS included with them. But in some cases it is not. It is my opinion that ALL Lithium chemistry systems should have a BMS. You can also use an inverter with low voltage cutoff, but that is not the same as a BMS that is right on the battery bank.

It is possible you purchased a system that had the BMS as an option. If you designed this system yourself you should have included it. Or an installer should have at least consulted with you about it.

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1 hour ago, hemsteadc said:

You mean I should have 2 relays that came supplied with the battery?

You may or may not have the relays Ronbo described.  It depends on your battery supplier (in this case Relion).  Not all Lithium Ion battery suppliers supply the necessary battery monitoring controls with their batteries.  I went to Relion's web site and after a quick scan, I do not see anything about battery monitoring equipment.  It looks like they just supply batteries set up like a replacement drop in battery without any built in monitoring, so you probably do not have any of the equipment Ronbo and Jack were referring to unless you added it on your own.

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One of the things we need to clarify for future discussions is the meaning of BMS.    The meaning of the term ranges simple from hi and low voltage cut offs by relay to systems which actively keep the cells in balance. I like the former  but remain skeptical of the latter. 

 

Hemsteadc I would think about setting up a low voltage cut off at around 46 or so. If you Lithiums are LifePO4 than 40 is irrevocably dead (It unfortunately is not unusual for lithium manufacturers to do things like this) . The way LiFePO4 works is that the voltage will hold relatively stable and then have a steep sudden drop. The voltage drop is not linear so it is best to keep the margins quite safe as you still get the majorty of the stored Ahs but with much less risk.  As Jack said if your inverter has a low voltage cutoff then that is good protection especially for us fulltimers that can catch the situation but  redundancy is your friend. In the last two years I caught  my batteries going too low twice. It should not have happened but in both situations a combination of human error (that being me)   an unusual sequence of events almost got me in trouble. 

 

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I'm also skeptical of the "balancing act" of a BMS. I'm able to monitor that aspect myself, and frankly, reports from the field do not indicate balancing is a high priority "feature". But the voltage cutoff is critical. And as stated, redundancy is your friend. Having absolute (or as absolute as possible) protection for the bank is critical with lithium chemistries. Either that, or open up the checkbook.

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http://cdn2.hubspot.net/hubfs/441554/lithium-spec-sheets/RB48V100_Spec_Sheet.pdf?t=1495820355718

That's my battery.  It came well-equipped with BMS.  No DIY here.  And it certainly opened up my credit card, hopefully for the last time with batteries.

That spec sheet is the only owner's manual I got.

Edited by hemsteadc
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Hemstead I looked at the specs they provided and I do not see the 40 V cut off they told you about. This is what makes me so nervous about the drop in packs. Their numbers are all over the place but the LiFePO4 chemistry is constant. The chemical properties are not going to vary so when I see numbers that vary among these drop ins, I just wonder what the hell is going on. Maybe they have some special components. I just wish they would explain it. I am NOT picking on your batteries. This is my concern with all drop ins and that when you go to their sites they provide varying charge recommendations etc.   I have attached a discharge curve for LiFePO4 chemistry to follow up on what I was trying to explain earlier. The typical RVer would be discharging at the top blue line. Each LiFePO4 cell has 3.6 volts. You can see by far the most capacity is between 3.3 and 2.8. They refer to the sharp upward above 3.2 and downward below 2.9 (approx) as the legs and you can see these are extremely steep. This is why voltage meters are only of some benefit in measuring capacity of LiFePo4 batteries or in establishing cut offs. Packs will sit most of the time between 3.2 and 3.0 and then things move very fast. For this reason measuring amp hours is really the most accurate way to measure remaining capacity. But since we need to establish some voltage on and off numbers, we need to examine the appropriate place on the curve for cut in and cut off.  Based on these curves you can see why most pack builders charge up to 3.4 and have a cut off at 2.8 or 2.9.  Get on the other sides of those numbers and you have nothing but risk with very little reward. Trying to cut off on those sharp legs is like trying to catch a knife so stay away. There are 4 of these 3.6 cells in a 12 volt battery and 16 of them in your 48 volt battery. For you this means recommended voltages between  54.4 and 46.4.   When your pack was at 48, it still was at about 3.0 volts per cell (assuming the cells were balanced which again is why you want to avoid extremes).  So you were still quite safe and the 90% depleted may well have been accurate.   BUT if the battery cut out is 40 volts as they said then that means you could go down to 2.5 volts per cell before cut out.  I don't think your cells could recover from that. That is dead. So something is wrong there and I would make sure that you have your inverter or other mechanism set up to cut out and or alarm well before your batteries get that low. Dave

discharge.jpg

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13 hours ago, Daveh said:

Jack. It sounds like you are getting close. Are you going to build your own pack or buy a drop in?

We will be building our own pack from an electric vehicle. a quite large pack, as these go. More details in the next couple of months. You will see one at the HDT Rally, BTW.  Ours will be a little larger.

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