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Hi,
Can anyone tell me the reason that victron state not to connect the midpoints in a 2s2p Bank (to make 2p2s)?
Im having trouble getting my bank in balance, and i think temporarily connecting the midpoints for a cycle or two would help to bring them in line with each other.
Any feedback greatly appreciated =)
*edit - of course i would fuse this connection, rather blow a small fuse than have fireworks!
Can you provide a link to the document where you read that?
It is hard to give a reason for a statement without context.
Just to clarify, I understand the risk in case of a bad cell, hence the fuse (thinking 10A initially and keep an eye on what happens). However I dont know if victron say this should not be done because of anything to do with the BMS. I dont want to compromise the ability of the BMS to function correctly!
I am having a substantially similar problem with new and matched strings of 200Ah 12v LiFePO4s in 2s2p, 24v.
I am using shore power charging with a Multiplus 24/3000/70-50 with veconfig'd default LiFePO4 settings and/or settings as low as 27.6V absorption and 26.8 float.
I have been around the houses and rearranged the strings and pairs, connected and unconnected midpoint. The cables are precisely measured, lugged and landed properly, bussed, fused and torqued.
Midpoint balance exceeds 2.0% as bulk transforms to absorption. And within a few minutes of disconnecting shore power and thereby stopping absorption, the midpoint balance goes to 0.1% or under and all batteries come within a 0.1V of each other.
Can anyone shed light on this?
Thank you! HRx
@heartmind.research The cell balancing within the battery only kicks in at something like 13.8v. Therefore if your absorption charge is not the default 28.4, your cells will not balance as the voltage is not high enough. If very unbalanced it might take a few cycles for the them to become balanced. I have read that extending the absorption phase from 2hours to something longer as long as there is only a few amps will also help balance the cells. I would say that any changes to longer absorption time should be checked with your supplier/distributer.
Also I would not connect the midpoints. The 2 batteries in series will over time balance themselves.
I have the same 4 x 200Ah in a 2S2P setup and no interconnection of the midpoint, and cells as reported by the VictronConnect App staying balanced just fine. All charging is set to Victron defaults.
I appreciate your reply. I will disconnect the midpoint as I wait for the "Battery Balancer" to arrive. When I read Victron's blog re the balancer, they were describing the problem I am having. I have returned to 28.4 absorption, 27.0 float, 99% efficiency, 1.01 Peukert, but 15 minutes absorption, on my SmartSolar and SmartShunt. I will expand that to default 2hrs. My Multiplus is returned to same but I saw no place for Peukert's on VE.CONFIG. I will look again and return to longer absorption there too. I should have the balancer by Wednesday. Allegedly, the balancer will solve the problem.
The balancer has only reduced the problem. I am still ramping to 2.80% divergence on the tail of absorption. Within minutes of switching to float, the divergence is under 0.5%. HRx
Im having trouble getting my bank in balance, and i think temporarily connecting the midpoints for a cycle or two would help to bring them in line with each other.
If that's the purpose, better connect only two batteries (in series) at a time for a few days. So only one battery string.
When you put the two strings in parallel again, make sure that they are at at the same voltage/SOC.
Thanks for the response, but I am fairly sure that with two in series I will experience exactly the same problem, at the moment with 2s2p I have the voltage of one battery in each series string rising rapidly during absorbtion. I have set a low absorption voltage of 27.6V and even so one battery in each series reaches around 14.2 while the other ends up around 13.5. I'm pretty sure that the lower voltage battery is not charging at this point
Thanks for reading =)
To balance: put all batteries in parallel, and charge with a 12V charger (see manual).
This is what I would do, except I have no suitable 12v charger, the only charging source I have available is a multiplus 24V.
If you do not have a 12 Volt Power Supply, then how did you perform the initial BALANCING of the battery bank?
@mvas - I have a very small car battery charger (3.5A max) with no lithium mode, i used this on each battery individually and I literally sat and watched the voltage until it reached 14.2V then manually disconnected and connected a small (1Amax) power supply set at 14.2V for around 8 hours per battery. I could do this again, but I think I will have the same problem when connecting them in 2s2p afterwards as everything is the same as before.
Alternatively I could put all 4 batteries in parallel and charge with the small car charger, but I think it will take too long for me to babysit them the whole time, and I dont want to risk the charge voltage rising to the limit of the charger (i guess around 14.6V)
The imbalance in the batteries only appears at the top end of charging, once the batteries get close to absorbtion voltage.
When testing under a large-ish discharge load (2.2kw), the batteries stayed within 0.05Volts of each other.
The batteries are around 6 years old.
I received these batteries, a multiplus 24/3000/16, a color control, and the cabling that was used in their original installation.
I found the cable lugs had not been secured properly (just taped on!) with some just pulling off under very light pressure, I also found the multiplus firmware to be corrupted, so I am quite sure that these batteries have experienced some very bad conditions and probably were not initially balanced and/or maybe never equally charged in their original installation.
Because of this I think that the batteries have all aged differently, and may have differing internal resistance.
This may mean that I need to go through a balancing procedure much more regularly than usual, and temporarily connecting the midpoint to make 2p2s when required seems like a simple solution.
Thanks for reading, sorry it's such an essay but I realised maybe a thorough explanation of the situation was required =)
Any help or feedback greatly appreciated!
6 year old batteries may be showing some signs of wear and tear.
You do need to raise the voltage of the lower voltage cells, before putting them in parallel with the the higher voltage cells. You can charge the lower voltage cells ( bring them UP ), while putting a load on the higher voltage cells ( bring them down) BEFORE connecting them in parallel.
I do not agree that you can charge each cell individually and then call them "balanced".
Connect all four (4) cells in parallel and charge them, with the exact same voltage, with equal length connections, simultaneously as one large block = Balanced.
Have you considered adding a "2S Battery Balancer" across your series connected batteries? I believe that device can move, up to 1 amp, when a 50 millivolt deviation exists between the upper bank and the lower bank, while still charging at 24 Volts.
Ok, I charged them individually as that's what the user manual suggested to do before connecting in series or parallel.
I did look at battery balancers, but they seem fairly expensive - does connecting the midpoints not do exactly the same job? If I have a 10A fuse inline then I can move up to ten amps if necessary for the price of a piece of cable. Or I can use a smaller fuse if 10A is too much to be safe.....
I will buy a 2s balancer if necessary but still am unsure why connecting the midpoint is not allowed =)
Those are batteries, not cells. Each of his four LiFePO4 12V battery has 4 cells inside.
Ideally would be to charge all 16 cells in parallel, but that would require removing them from the batteries.
I wish this was an option, but until they are not usable as a pack I cant bring myself to cut them open! =)
If the BMS begins the Top Balance process at 14.2 Volts, then don't you need 28.4 Volts across the battery bank?
But, you you only have 27.8 Volts for Absorb?
Could this have allowed one or more cells to get way out of balance and that allowed the overall battery voltage to also get out of balance?
I originally had absorb set at 28.4 for their first charge after the initial balance, but found the voltage in absorbtion had risen to 14.5 in two of the batteries. I then reconfigured them so the two weaker batteries were in series with each other and discharged them to 26V after leaving them overnight. For this (second)charge I decided to try a lower voltage absorb to see if the imbalance would still occur if I try to only charge to around 90%. The batteries are old and I wont be charging them to 100% anyway once I build my system.
That's why I wrote to charge (with your 24V Multiplus) 2 batteries in series at a time and see if the cells get more balanced ("more equal").
Not as good as charging each battery alone (or all 4 in parallel), but better than charging the 2s2p bank.
When you get your MPPT, that could charge one battery at a time (or all 4 in paralel), but I don't know if you can still somehow use the VE.Bus BMS in that configuration (without a 12V Multiplus). You could wire the "Charge disconnect" to the MPPT remote turn off, but I don't know if the VE.Bus BMS works stand-alone.
I'll definitely try this once I get my mppt, I have other projects to finish first so this wont be for quite a while! At the moment I'm testing the batteries to see how they perform before I buy anything as they are old and I think have been installed badly in their previous life =)
