BMS anomaly (or just normal)

DestroyFX

New Member
Joined
Oct 4, 2021
Messages
36
Hi, I did install the first part of my system.

I have 4 solar panels (400W ea)
I have 16 lifepo4 cells, 100AH for a 48V system
I have a 16S BMS

Here the issue or maybe leading to the issue:

The battery cells are part of two 24 volt battery (plan was to take them apart but on receiving them I noticed the bus bars where solderad directly and I could not dismantle them.

So I did wire them in serial and connected the BMS like it was a 16S battery (because that was the initial plan. And I already got the BMS)

BMS connections are fine, each contact on the BMS connector increment the voltage as expected and when I test the battery voltage and compare to voltage of the BMS black wire to battery positive, both did have the same exact voltage.

I knew the batteries did have a small voltage difference, like one pack of 8 cell had 0.1v more or less than the other. As the batteries pack are soldered by group of 8, I decided to just plug it and say, let the BMS do it's balancing job for a week or so.

Here the weird things begin

Initially battery did charge as normal, pack was like, 54.1V
when pack reached +-57.1V, the voltage from the BMS side (between BMS black wire to ground), jumped to 80V and dropping slowly (about 20 sec) until it get down to under 58 (on where my charge controller resume charging because the limit is set to 58V). Once the charger resume, the cycle append again and again and again but it eventually stabilized (after about 5ish min).

Now, when I test on the BMS side (BMS black to positive), I have a voltage at about the charger controller cutoff voltage of ~58V)
And when I test the voltage from the battery directly, it read 56.2V, so I have a difference of about 1.8V between the battery negative and the BMS negative.

Note, I have no load right now as I wanted to get the battery charged and balanced.
Is that normal, like the BMS just began it's balancing cycle and wanted to just "tell the charger to stop" by increasing the voltage on the BMS side? (Like a smart device that did a few cycle over to detect when the charger stop charging)?

Or I should get another BMS?
The one I have is like that one https://www.aliexpress.com/item/4000261414856.html?spm=a2g0o.productlist.0.0.17087e8c5HaOTj&algo_pvid=bb84ab79-fc06-443a-893d-50f5aa246376&algo_exp_id=bb84ab79-fc06-443a-893d-50f5aa246376-0&pdp_ext_f={"sku_id":"10000001059958637"}

Model Lifepo4 16S/130A

Edit : I also notice I do have a ~15w load on the charge controller
Edit 2 : Saw that post : https://diysolarforum.com/threads/voltage-overshoot-with-makeskyblue-v119-scc.20220/ . I do have a makeskyblue too, but V120, and the voltage overshoot seam to be proportional to his in that post.
 
Last edited:

DestroyFX

New Member
Joined
Oct 4, 2021
Messages
36
Ok.
The reason it was overshooting was the bms detecting a few cells at 3.7V (half) and cutting off.

Now that I have half a 3.6 and half at 3.2 (I should have rechecked as I tested the balance difference like 2 months ago and I moved to another place and just installed everything))

Anyway, each cell in the same "pack" are equal. So I just plugged both pack in parallel to balance them better that way. Current voltage across all the cells while balancing is 3.36. Will leave them like that untiil tomorrow morning them I will plug the BMS and SCC back.
 
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