Battery voltage swing

So I got a 2nd hand system for parents running as a UPS for now, consisting of of;

2kWp panels (Not installed yet)
MPPT 150/70 (Not installed yet)
Cerbo GX
Multiplus II 5kVa
2.5kWh byd battery with canbus comms unit (Its a start :frowning: )

Will look at upgrading the panels to ~4-6kWp to be mostly self sufficient during the day, as for the battery not sure if funds will ever be enough to go new (read 5kWh 1C and up) however we will take it as it comes.

Having a larger fluctuation on the battery voltage that my current system does, where this on jumps from 55.5V to 54.5V
Set to absorption 55.2V, Float 55V, absorption time 1hr as per the manual

Do I change this under VE configure for more control or leave it. Last question the battery parameters are 56.5V so not like the battery cares on 55.5V just want to get the most out of it by not ramming in the last few amp hours for little to no return.
Voltage drops to 53V with in 10min of load shedding 100 to 200W load

55.5V vs 54.5V will give you 3.46V and 3.40V per cell which is fine for being full and not at overvoltage. So I would keep that like it is.

Thanks, it does look like a 16 series battery.

Still don’t like how the voltage jumps one volt so quickly with not changes in load.

Is there way you can verify the jumps using a multimeter? Could be a faulty BMS?

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That is the best plan I think, will check when back at the installation

It looks like the BMS is balancing the cells at the time.

I can see on the iPad on my desk how the BMS balances the cells to a Delta of 0.004-0.008v.

The interesting thing to take note, at the time that happens, the BMS shows zero amps. The BMV shows 0 to -±30w still going into the batts, hence the volts per cell going up and down as they are balanced.

The amps recorded as zero as per the screenshot, the BMS cannot measure that low, yet it still balances, the volts going up and down.

Just a thought to ponder on.

Here is a screenshot of a BMS working, the data recorded.

The top half is the Delta, the bottom half is the cell voltages.

If I had a picture per cell over a short period of time, one can trace the individual cells easier.

To note, the Delta, top half, to the left, when the Delta shot out of set parms, the system auto throttles the Volts/amps back to help the BMS balance. To the right, over time, the Delta came right beautifully, all 16 cells balanced. This bank was top-balanced before the installation.

EDIT: Data was pulled into Graphana, direct from the BMS RS485 port.

Most BMS will have a cell voltage level where if will stop allowing a charge when a cell reach that limit. It will then balance and only release that stop again once that cell has dropped below the release voltage.
That could be what you are seeing, but you a more detailed view or else this is all speculating.

So need to get back to this installation, I tested it at least 10 times by tripping Eskom and making sure the inverter took over, however this last (1st) load shedding the power went off, inverter went down for about a min.

Sadly the logs show noting, not a dip in voltage , no ve bus error etc or anything ~22:06

I know its a 50Ah BYD battery, however this was bought as a start for my parents to atleast get by with load shedding,

Will wait till load shedding tonight and :crossed_fingers:

Side note I changed DVCC to limit charge voltage to 55V and its much smoother.

Side note, any way to get the min/max voltages from this battery? Have a rs485 to usb converter just not sure beyond that.

If BYD publish the min/max cell values you can get those in the VRM graph options. It will be part of the CANbus frames.

If the option is not there, then you could potentially get that data from the battery directly using BYD software and something like a RS485 USB adapter, but I don’t have any details for that.

Regarding the 55V limit helping it does tell me that some of the cells was spiking on overvoltage which would indicate a cell imbalnce in the battery. That can be fixed by giving the balancers in the BMS more time to work by lowering the voltage and then slowly increasing it over many days until you reach to original battery voltage. I would increase it by 0.1V a day if your battery voltage was stable. If not then keep it on that setting until it does stabalize.

It is possible that your battery might work more stable at a lower voltage because it is older.

Thanks @Louisvdw I agree it does point to an imbalance causing the spikes, sadly BYD does not post the min/max values.

What voltage to you suggest I start the balance at?

Sadly pretty sure the last owner and guys that set it up did some nasty stuff to this battery.

Use the 55V you have now and leave it like that. This will give an average of 3.43V per cell but in facture it would be more likely that a few cells are at 3.60V and some at 3.39V. The longer that you can give it at this lowe voltage the better. So 2-5 days is great if you are not in a ruch. Slow it good.
If you see it spiking up don’t be afraid to lower the voltage again.

There is also nothing wrong with keeping it at 55V for your battery as standard. The less presure in the battery to work hard the longer it will last. You just want the battery voltage high enough so that it does trigger the ballancer voltage (which is normally set to start somewhere between 3.35V and 3.40V) so again 55V is fine for that as well.

My BYD, a different model, is at 55.5V, been like that since install a year ago.


Real voltage as reported by battery BMS: