(4) LiTime 12V/100 AH batteries(I know). Customer provided these batteries. They are paralleled with 2/0 cable of equal lengths. All positives are on one far corner of the battery bank and the negative going to the shunt is on the other farthest side of the battery bank. I am confident they are paralleled correctly and effectively. Problem is that upon discharging at 20 amps only two of the batteries are discharging. The other two remain in standby. I can observe this in the app. It doesn’t appear to change as time goes on either. It appears the two batteries will remain in standby. I am able to go into the app and shut of discharging for the two active batteries which forces the ones in standby to begin discharging. If I then turn back on discharge for the first two all 4 will discharge evenly. Then they will also recharge evenly. However once they are fully charged and I put it in another discharge cycle the first issue occurs again. In fact sometimes a different combination of batteries altogether will discharge.
I am not sure how to resolve this. I considered originally paralleling to a common bus but there is just no room in the step compartment of this Class C. I am not sure that would fix the issue anyway.
Not really a Victron issue and a good case of why fitting multiple dumb batteries in parallel is not ideal.
If the batteries are charging and going into standby is this because your charge voltage is too high, so the BMS is hitting either max battery voltage or an individual cell voltage and stopping charging. If this is the case, ideally you should not be triggering over voltage protection (OVP) so see about reducing the charge voltage and also check to see how well balanced the cells are. If one is going high then reduce charge voltage to try and give it chance to balance rather than triggering OVP.
To come out of standby requires the voltage to drop below a threshold afaik, if a couple of batteries come out of standby they are probably holding the voltage high enough to keep the others in standby. They may eventually come out of standby as more energy is consumed. I have no idea how much tweaking you can do with the LiTime BMS, you may be able to adjust the threshold value.
Thanks for the reply. I removed the batteries, charged them all individually using a Bluesmart on lithium setting. Reinstalled and the problem persists. There is no way to check individual cells in the app. Only a cells icon that tells you they are “OK”.
Furthermore, I discharged them at 20 amps for about an hour. As is consistent with the problem only two batteries where discharging. I wanted to see if eventually the others would come out of standby and discharge as well. I brought them down to 83%(within the LiTime App) and the other two never discharged. I then plugged in the rig to allow them to recharge. Immediately all the batteries went to 100%. It seems clear to me that the BMSs are not cooperating.
Get a voltmeter and measure the voltage, during that 20A discharge, on the battery terminals, exactly where they come out from the battery case.
If the voltages are the same, then don’t worry, all is OK and the chemistry will do its thing.
The thing is, 20A discharge on even 2 x 100Ah batteries is hardly anything and will not cause much voltage drop so the batteries on standby will remain until you take a high current or discharge them further. The voltage curve is very flat at low discharge rates. I have seen other reports of this happening but only when the charge voltage drives the batteries into OVP, so do check this.
That makes sense. However this is unworkable anyway. Its an RV. Unless they turn on their air conditioner every time they use the rig they will likely be drawing 30 amps or less.
And no, @alexpescaru the chemistry is not “doing its thing”. Essentially what has been happening is two batteries discharge and the others remain at 100%. A charge cycle starts and the BMSs of the discharged batteries are somehow picking up the voltage of the other fully charged batteries and then registering as 100% charged. All chargers go off(Multiplus and MPPTS). This happens consistently to the point where the first two batteries are eventually at low enough voltage that the others finally start to discharge(my theory). When the customer brought me the rig the Smartshunt was registering as 93% SOC.(yes smartshunt settings are correct and it is installed correctly). after about 3 hours at 20 amp discharge all 4 batteries where dead(actually sleeping).
I wonder if that after sufficient time the 2 batteries in standby will start to discharge with the 20A load? It seems that it would not really matter if only 2 batteries carry the small load while the set is at ‘full charge’ as long as the other 2 batteries eventually pick up the load.
This also sounds like there is some variation in the internal BMS of 2 of the batteries, and they don’t sense the difference between the pack voltage and the load voltage correctly.
However, this issue needs to be taken up with the battery manufacturer (by the client?), rather than here.
I’ve looked into one manufacturer’s “Fuel Gauge” type BMS for small batteries, and as usual these small highly integrated chips have their quirks.
I agree. If it hasn’t been understood already, the biggest problem with these type of batteries is often consistency of quality and, a close second, lack of good customer service and tech support.
Any opinions on whether or not paralleling to a common bus would possibly change anything?
I doubt that changing the battery wiring will change anything. With Li battery charge cycle efficiency at 95+%, it’s not worth worrying about. If one battery ends up charging another, the loss is not great. As long as there is sufficient current capacity to handle peak loads - i.e. if loaded do all batteries turn on?- then there should be no worry.
Read the rest of the post and you will see the problem is more complicated. The problem is inconsistent. Sometimes when charging the batteries all “catch up”. the lower ones receiving charge and catching up to the full ones. Other times two batteries discharge and after load is removed and charging starts they immediately go to 100%. Of course the chargers shut off. Time goes on. Shunt SOC is wrong, batteries don’t charge, and continue to discharge all the while showing high SOC on the Smartshunt due to the voltage of the batteries that are not discharging, problems multiply. This is the condition it was brought to me in. If the batteries eventually in fact charged one another that would be a different story.
This could happen only if some batteries are disconnected from the pack by their BMSes.
What I wanted to say is that if, for example, there was no interference from BMS, so only the cells in series/parallel were “presented to the exterior world” and the connections between cells and/or batteries themselves properly made, the Kirchhoff’s laws and chemistry applies and at some point the cells/batteries will reach equilibrium.
But it seems that your situation is a little bit more complicated that this…
As a side note, I have 9 Pylontech batteries in a system. The batteries, during last month, were subjected to partial and incomplete charging and discharging cycles. Now, after this period, you would say that all should have the same SOC, but no, the SOC between these 9 batteries is varying between 30% and 60%.
BUT…
The voltages of the 135 cells inside all batteries are pretty much the same, the difference between the min and max voltage of all these 135 cells is only 2mV.
So I know that it’s the calculated SOC among BMS’es that is wrong and not the cells themselves. For sure, even on the flat zone of the voltage curve, an only 2mV delta means, for sure, that SOC is pretty much the same. A 2mV delta couldn’t be the difference between 30% and 60% SOC…
So, you see, the chemistry - cell voltages - is doing its thing…
In that case, I would bench test the batteries individually log the cell voltages at 90 - 100% on charging to the point of disconnection, and discharging from 100% to 0%, again particularly at disconnection. Then look at the inconsistencies and possibly lodge a warranty claim if the figures indicate.
This takes time, and client will have to pay the price for buying ‘cheap’ batteries.
similar to Alex, I have 4 sets of 26V lithium in parallel, and with varying age and SOH of the cell packs. However, I do have a unified BMS system for all 32 cell banks. This also controls the charging, and I can usually manage to keep the cells balanced to <50mV in discharge and <150mV at full charge. the BMS cannot disconnect any individual bank. Battery charge current distribution is still pretty well balanced, so I’m not worried.
If the internal BMS’ are linked, have you tried unlinking them or running the batteries dumb? If that works, you may have to get a single external BMS as a replacement.
You have to check that with the BMS-Manufacturer. Not all BMS support to operate “in parallel” with other BMS. MOST actually require the individual Battery-Moduls to be linked with some sort of Bus (Can / RS232) to “orchestrate” their behaviour when acting in parallel. Sometimes manufacturer specific cables are required for this. They look like ethernet, but have a different pin-assignment, so regular network cable does not necessarily work.
Also check the Wiring Unlimited guide for physical connections: Even with paralleld modules it is “best practice” to have the systems +/- connected to individual ends of the paralleld units, else there will be a voltage(drop) difference between each module, leading to uneven discharging. (3.3 Parallel battery bank wiring)
Thanks for all the input. I have not come to any conclusions at this point. I also notice that although my chargers are set at the manufacturers specs of 14.4 and 13.6. Tail current at 2.0% and charged voltage at 14.2 the batteries still achieve “fully charged” status in the app and stop receiving charge way before this. ALL the BMSs say 13.7 volts but my multi meter at the battery terminals(along with all the victron components) says +/- 12.7V. I am leaning towards bad BMS functionality at this point.
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
16
I have two comments for what either are worth. First LiTime batteries have terrible BMS’s, so keep your expectations low. Second, I recommend you set absorption to 14.0V, charged voltage at 13.8 and float at 13.5. Then keep an eye on the cell voltage deviation and if they don’t balance fully then try 14.2V. With these crappy batteries I have found a lower charge voltage and longer absorption time works better. Did the OEM happen to tell you at what pack voltage the cell balancer kicks in?
You are all awesome and I so much appreciate your knowledge.
I have come to realize that the access to the BMS through the app is severely dumbed down. It only shows you what it feels like a person of low level of expertise can handle. Once the batteries reach around 13.6 V it no longer registers charging current or voltage. It just say “fully charged”. It may be absorption charging at 14.4V/2.5V but the app wont tell you. This has been part of the confusion. If it isn’t clear to all already. Use these type of batteries with discretion.
This is helpful input and goes along with what I have been noticing. I think the BMSes are not working in the same world as the Smartshunt. My Smartshunt settings, although may be good for many LFPs are not working with these batteries. I am often observing significant inconsistencies between what my Smartshunt is picking up and what the LiTime app is telling me about the batteries. I am coming to the conclusion that part of the problem here is I have been assuming the info in the LiTime app is accurate. It is not. Even when the Smartshunt shows charging voltage/amperage and my Multiplus is showing its in absorption charge at 14.4V/2.5A the battery app happily declares 13.7V and “Fully Charged”. If I meter the terminals directly and put amp clamp on the battery cable I get readings consistent with the Victron products. The problem with performance that your suggestion may address is that it seems the BMSes arent allowing the batteries to fully charge. I will try extending the absorption time and lowering the charging voltages to see if it helps. In the end I am not sure the app will give me specific enough data to be helpful anyway.
OGPS
(Ed @ Off-Grid Power Systems - offgridps.com)
19
It’s a BMS issue. These cheap BMSs do not accurately measure low currents. I don’t know what the threshold is for a LiTime BMS, but low currents spread across multiple BMSs means each BMS sees a small load, and errors can be exacerbated.
Your experience with customers bringing their own batteries mirrors our own. A few years ago we decided that we would not install anything we didn’t design or sell. By far, the most common problems were Amazon batteries.
You will know which batteries are reliable and which aren’t. Stick to your guns and in the end, the customers you work with will be happy. There is almost never a good ending when customers want you to install stuff they purchased elsewhere - even good stuff.
I can’t promise that connecting each battery individually to the DC buss will solve this particular problem, it’s a good practice to always design your systems this way. You aren’t dependent on every BMS performing flawlessly and affecting any other BMS directly attached.
Are these the “Smart” batteries from LiTime? I have seen similar issues reported involving various brands of the newer batteries that are designated “Smart”. Will Prowse on YouTube did a video recently describing the issue. One of the manufacturers he spoke of (not LiTime) was able to provide an update to the BMS programming that apparently solved the problem.
I use (12) of the LiTime 12v, 460ah batteries in my system, 4S/3P configuration, for approximately 70kwh of storage. These are the earlier versions without the “Smart “ BMS, and have had no issues. I occasionally check the current draw on each series string under both charge and discharge conditions, and find that they are always within a couple of amps of each other.
