Intermittent charging due to BMS disconnect

[ruiz.manuel94]

So I noticed my BMS is having a strange behaviour when the battery reaches full charge and there is strong sunlight.

My BMS is set to stop charge current if a cell goes above 3.6V or the battery voltage goes over 14.4V

So what happens is that when either of those things happen (it is the 3.6V single cell since my cells are slightly out of balance) the charging stops. However since the MPPT charger is effectively disconnected from the battery, suddenly the cell voltage drops below 3.6V, opening the battery for charge again. This results in a cycle in the charge MOS of my BMS.

I am wondering two things:
-Is this bad for my battery?
-Is there any way to avoid this? maybe shut off the limit in the BMS since the MPPT has protection anyway

My battery is a DIY LiFePO4 12V four cell and my charger is 40A SRNE MPPT (same as WEIZE)

 

[Ampster]

I am wondering two things:
-Is this bad for my battery?
-Is there any way to avoid this? maybe shut off the limit in the BMS since the MPPT has protection anyway

It is worse for your BMS to be the primary fall back. The best way to avoid that is to lower you charge voltage until that cell gets pulled down by your BMS. Alternately you could put a headlight or power resistor across the terminal of that cell to bring it down quicker. The MPPT protection is only voltage protection not cell protection so do not shut off the BMS protection or you could over charge that one cell and ruin it or shorten its life.

 

[John Frum]

So I noticed my BMS is having a strange behaviour when the battery reaches full charge and there is strong sunlight.

My BMS is set to stop charge current if a cell goes above 3.6V or the battery voltage goes over 14.4V

So what happens is that when either of those things happen (it is the 3.6V single cell since my cells are slightly out of balance) the charging stops. However since the MPPT charger is effectively disconnected from the battery, suddenly the cell voltage drops below 3.6V, opening the battery for charge again. This results in a cycle in the charge MOS of my BMS.

I am wondering two things:
-Is this bad for my battery?
-Is there any way to avoid this? maybe shut off the limit in the BMS since the MPPT has protection anyway

My battery is a DIY LiFePO4 12V four cell and my charger is 40A SRNE MPPT (same as WEIZE)

The BMS is not meant to function as a charge controller.
The way you have it configured your battery will likely drift more and more out of balance with every charge/discharge cycle.

Set your bms cell trigger to 3.65 and recover at 3.4.
The pack trigger should be 14.6 and recover at 13.6.

Set your mppt charge controller bulk/absorp to 14.2 volts (3.55 per).
If you still get a bms disconnect report back and we will help you bring your battery back into balance.

 

[ruiz.manuel94]

The BMS is not meant to function as a charge controller.
The way you have it configured your battery will likely drift more and more out of balance with every charge/discharge cycle.

Set your bms cell trigger to 3.65 and recover at 3.4.
The pack trigger should be 14.6 and recover at 13.6.

Set your mppt charge controller bulk/absorp to 14.2 volts (3.55 per).
If you still get a bms disconnect report back and we will help you bring your battery back into balance.

Thanks I will try this ASAP. However, I don't see any "recover" option on my BMS app (is this the voltage at which it would alow charging again?)
My BMS is a DALY with bluetooth APP

 

[John Frum]

Thanks I will try this ASAP. However, I don't see any "recover" option on my BMS app (is this the voltage at which it would alow charging again?)
My BMS is a DALY with bluetooth APP

Most BMS have a release/recover threshold for ever trigger threshold.
I don't know the specifics for the Daly.

 

[Ampster]

Having to recover from a BMS event is another good reason why you do not want to use the BMS as your primary means of controlling charging and discharging.

 

[ruiz.manuel94]

Well my BMS doesn't seem to have that option. These are the only related options I can change:

 

[John Frum]

suggest you make these changes

cell high voltage protect = 3.65
cell volt low protect = 2.5
sum volt high protect = 14.6
sum volt low protect = 10.0

 

[ruiz.manuel94]

suggest you make these changes

cell high voltage protect = 3.65
cell volt low protect = 2.5
sum volt high protect = 14.6
sum volt low protect = 10.0

I have made the changes.

On the MPPT, these are the parameters I can set:


And this is how it is set for the current mode (LI battery) (Not sure what the '-' means, also not sure how to replicate that in the User-defined profile since I have to give a value):

 

[John Frum]

I have made the changes.

On the MPPT, these are the parameters I can set:
View attachment 116226

And this is how it is set for the current mode (LI battery) (Not sure what the '-' means, also not sure how to replicate that in the User-defined profile since I have to give a value):
View attachment 116227

I guessed a bit on some of these

type of bat = user

equalizing voltage = 9
boost voltage = 14.2
floating voltage = 13.4
boost restoring voltage = 13.3
low voltage disconnect restoring = 12.8
under voltage alarming voltage = 12.1
low voltage disconnect voltage = 12.0
discharge limit voltage = 11.9 (exactly sure on this one)
over discharge delay time = 5s
equalization duration time = 0
equalization charging interval = 0
Boost duration time = 30 minutes (this is probably absorption time)

Does your charge controller have load ports?
If yes, are you using them?

 

[ruiz.manuel94]

I guessed a bit on some of these

type of bat = user

equalizing voltage = 9
boost voltage = 14.2
floating voltage = 13.4
boost restoring voltage = 13.3
low voltage disconnect restoring = 12.8
under voltage alarming voltage = 12.1
low voltage disconnect voltage = 12.0
discharge limit voltage = 11.9 (exactly sure on this one)
over discharge delay time = 5s
equalization duration time = 0
equalization charging interval = 0
Boost duration time = 30 minutes (this is probably absorption time)

Does your charge controller have load ports?
If yes, are you using them?

Your guess is better than mine LOL. I will be setting these tonight and see the behaviour tomorrow (It is quite hard to set since I have to crawl inside a cabinet in my van)

My controller has load ports, I don't use them. Should I?

Why is your "equalizing voltage" so low? I take it that it won't even be used since the time is at 0, is that right?

Thank you so much for your help John, you are a life saver

 

[John Frum]

Your guess is better than mine LOL. I will be setting these tonight and see the behaviour tomorrow (It is quite hard to set since I have to crawl inside a cabinet in my van)

My controller has load ports, I don't use them. Should I?

Not usually useful.

Why is your "equalizing voltage" so low? I take it that it won't even be used since the time is at 0, is that right?

We don't ever want to do equalization on LFP batteries.

Thank you so much for your help John, you are a life saver

I'm glad to help.

 

[ruiz.manuel94]

So after a few days, I can report back on this. (We didn't have much sun so couldn´t really reproduce until now).

The new settings seem to have made the BMS less prone to switching charge off (only saw it happen once, and probably because my cells are out of balance). Now it charges the battery more.

However, upon trying to change the "user" charge profile, I noticed that I dont have control over the entire set of parameters I previously thought. Here is a list of what I can actually set (and the values I set):

Battery type = "User"
System voltage = 12 V (can be 12 or 24 only)
Equalizing charging voltage = 9 V
Boost charging voltage = 14.2 V
Floating charging voltage = 13.4 V
Over-discharge recovery voltage = 12.8 V
Over-discharge voltage = 12.0 V

Today I noticed a srange behaviour: with full sun there was no charge going into the battery. Checking the LED indicator I saw that the charging was at the "Equalizing phase" which is set at 9 V (lower than the battery voltage). The charger has protection against feeding current back to the panels, so I guess it was actually doing nothing. I stopped charging for prevention.

My take is that this is the case because I have no way of setting the "equalization duration time = 0".

What should I do? Go back to the "Lithium" predefined profile?
Maybe I should get my cells balanced, but I can't get the battery out of its place to parallel the cells as it would require enormous amount of effort (disconnecting lots of stuff, moving fixed furniture, and most importantly having no power for a few days).
The BMS has a balancing feature I have not used and I also have an Active load balancer in place but disconnected since it didn't seem to be doing much.

 

[mikefitz]

Comments,
The controller may get upset with zero in the duration times for boost and equalisation, set both to 15 minutes.
Set equalisation volts the same as boost volts, a lower volts than boost may cause confusion.
Set boost to a lower value if the BMS is detecting over volts, even 13.8 will be OK.
Set the the BMS to balance on charging, threshold 3.4 volts with a 20mv delta.

You don't need to pull the battery apart to improve balance. If you have access to the cells apply a load, car headlamp bulb, across the high cell for several seconds. Carry out this process under charge as the cell is approaching the high value, whilst monitoring all the cell volts.
You should not need the active ballancer to achieve a usable balance.

Mike