Hello everyone,
This week I built my first battery using a JK BMS B1A20S15P and 8S EVE MB31 in series for my 24V system. There are A LOT of settings to change and I’ve had a hard time finding the optimal values. I care a lot about longevity so I don’t want to exceed the safe limits but more capacity and charging speed are also important. I have two ledvance 660W panels and a Victron MPPT 100 | 50. I used off grid garage as my source for the settings while also reading the datasheet that can be found here.
So, the settings page of my MPPT looks like that now
Cell 1 is bumping your upper voltage limit, throttling back current to prevent a cell over-voltage. Looks like your balance voltage is above the cell limit, it will never balance. The BMS over protection is lower than that cell limit too. Looks like the serial driver is fighting against your BMS.
I have about the same system, but with a JDB controller. Tune your Cerbo & serial driver to get how you want to use the system, use the BMS as a backstop to head off any problems.
That’s really up to you and how hard you want to push the cells. I used 3.45V & 2.90V, 3.375V float, which makes the pack limits 27.60V & 23.20V. Mine is in a coach, I won’t live long enough to get a few thousand cycles out of it.
Your battery targets then drive other parameters in the serial driver, Cerbo DVCC, start of balancing, etc. Also the solar charge controllers and inverter/charger need to be adjusted independently so nothing is hurt if the communications breaks down. Then set safe hard limits in the BMS to disconnect as last line of defense.
I’m still tweaking mine to taper the charge near the top to allow balancing, it takes a while to get it right.
BTW, your equalization value is enough to hurt your cells. Are you using DVCC?
I want the best values for longevity. My panels are 660w and I only have 2 so with low values I’m afraid they might not charge especially when cloudy/winter and cause more harm so if you can give me the best values for longevity I’d really appreciate it.
Do you also have MB31?
Equalization is off I think
EVE 304, pretty close. I have my battery controller limit charge voltage, that’s a great benefit of the serial driver. See the Cell Voltage Limitation section in the serial driver config.ini. I don’t do any shared sensing, I let the BMS handle it.
I really don’t have any magic numbers. I used the serial driver default values and had a first discharge test of 306Ah, I was good with that. Your datasheet should have max/min for your cells, stay on the safe side of those for BMS setting.
Your BMS should balance the cells as soon as one cell reaches 3.45V. Are you sure that all your cells have 330 Ah? AFAIK these cells have a nominal capacity of 314 Ah. And I would use this value in my BMS. And I would set the Cell OVP value a little bit smaller than 3.65V, and cell UVPR a bit higher than 2.55V. I have set these values to 3.62 and 3.1.
Since my cells came almost perfectly balanced and hardly ever reach a cell difference of 0.010V at 3.45V, I did set the balance trigger voltage to 0.005V, and the start balance voltage to 3.44V.
I have the same cells, but 16 of them, and also a JK BMS. I’m using the inverter BMS though, and let it drive the voltages. My start Balance V is 3.45, same for the Cell RCV. Once one of the cells hits 3.45 it starts balancing. I only balance for 0.3 hours each time, 18 minutes, and my cells balance to 1 or 2mV. I’ve set my Cell UVP higher, at 2.7V.
A couple of things:
That equalization voltage in the MPPT looks dangerously high. I know, it shouldn’t use that function, and the BMS should protect the cells anyway, but I’d set that no higher than 3.6V per cell, just to play it safe. So 28.8V.
You should be able to watch in the JK BMS app when the battery is getting full, and see when the BMS is balancing.
No problem leaving the 330Ah capacity. It does not affect the charging in any way, it is only used to calculate the battery SOC. For my cells I have the test reports from EVE, and they all exceeded 330Ah.
Oh hello, what difference does the inverter make over the one I have?
I did change some settings a few days ago over the ones I posted.
First and foremost, you CAN disable equalization in MPPT settings but to do that you have to use the LiFePO4 profile as a base. It would make sense if you could just do it. So, here are my current MPPT settings:
The only issue that I’m having is that I’m getting Low Battery Voltage notifications from the inverter every now and then, I suspect dbus serialbattery.
I wonder if bumping my target from 3.45 to 3.5 or 3.52 will have any benefit, also if I should drop the absorption time lower.
The inverter BMS communicates to the Cerbo via CAN. It works out of the box, doesn’t need to install dbus serialbattery. So I don’t know anything about that driver.
Since you have LFP cells, why not use the LiFePO4 profile? Makes sense, stick with it
BMS: I’d still go a tad higher with the low voltage cut-offs. But that’s just personal preference. Regarding balancing: still worth looking at the cell voltages when they are over 3.4, that’s when the voltage difference becomes a lot bigger.
My battery is perfectly happy at 3.45V target voltage. I’ve only once increased to 3.55V, just to see how much capacity I might be wasting. It wasn’t much at all. Maybe 0.2%. Not worth it for me. I stick with Andy’s 3.45.
Absorption time: I try to keep it as short as possible, although there probably is no harm leaving it at 1 hour. For me 0.3h (18 minutes) gets to: a tail current that drops to basically zero. And a fully balanced battery (within 2 or 3mV). If either of those was not the case, I’d do a longer absorption.
Regarding absorption:
I guess it also makes a difference how often you get to 100%.
For my battery, not reaching 100% is the exception. Even in winter that happens only once every 10 days, and the the next day it goes back to 100%.
Over winter Andy seems to go weeks if not months between hitting 100%, makes sense to do a longer absorption (and balance) then, compared to a battery that reaches 100% almost every day.
It’s summer now, plenty of sun, I hit 100 every day, haven’t dropped bellow 85% so far. I want to see how winter will go. Perhaps I can drop absorption down to 30 minutes or even 15, we’ll see.
Thanks for the info regarding 3.55, I didn’t know the discharge curve was this steep up there.
