Sounds like two unrelated problems:
Snacks said:
It would instantly drop from 86.7v down to 79v as soon as I took it off the charger but it seemed to work alright .
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When I put it together all the cells we’re either 3.24 or 3.27. Is it possible that it’s dropping instantly to 79v because that’s where it was before installing the BMS?
Edit- I should have mentioned that as 48v they never dropped this much after charging
This will need more details, measurements, etc to understand the specific problem, but it is a battery system issue (cells or BMS or charger).
When the pack is still connected to the charger, what are alll the individual cell group voltages, vs what they are after disconnection?
What is the 72v charger's output voltage set to?
What were all the cell group voltages when they were separate 48v packs?
What were the 48v chargers' output voltages set to?
The drop as stated is 7.7v, which divided across 20 cell groups is 0.385v / cell. If the cells charge to 3.24v each, then that plus the 0.385v is 3.625v, whcih is close to the typical balancing charge voltage for LiFePO4 packs.
If 3.24/3.27v was their normal max voltage as 48v packs too, then the 7.7v drop as a whole is normal. If your 48v chargers were not set to a voltage that brought the cells up over that 3.24v (or not by as much as 0.385v/cell) then you wouldn't have seen the voltage drop like that on charger disconnect.
For aged or lower quality cells (or ones recycled from old packs without the seller telling you), they might not hold their surface charge, and the voltages you see are fairly consistent with that; it's just not usually an instantaneous drop.
If as 48v packs the cells charged higher and stayed higher, then something else is going on. You could measure the main battery positive and negative at the cells, with the charger connected, and see if it drops there, too, or if it only drops at the BMS charger terminals. If the former, it's still probably just the cells not being able to hold the surface charge (age, cell quality, etc). If the latter, it's the BMS not allowing charge any higher; most likely is programmed for a lower per-cell HVC; you'd have to check that with the software for the BMS if it has any (otherwise check with the manufacturer to see what the specs are for this specific model and variant / firmware version).
Decided to replace my motor phase cables , so flipped the bike over to remove the wheel, did the job and replaced said wheel and put the bike back on its tires. Nothing more, nothing less but now the voltage of the battery drops to .5 or 1.1v or some other ridiculous low voltage when I plug in the controller.
Pulled the battery off the bike, 20 cells are at 3.28v and 4 are at 3.41v. It seems to continue to intermittently drop to about 5-5.5v and then shoots back up to 79v again after a bit if I check from the discharge wires. If I check on the cell tabs it will still show correct voltage and if I check the discharge wires after checking on the battery tabs, it will now show the correct voltage as well.
This will also need more measurements, but this is a post-battery problem with the system, where an overload current is causing the BMS to shutdown to protect your pack.
The most likely cause is your controller's caps are large enough to cause such a high momentary current flow at plugin that it exceeds the BMS trip current. If this is the case, the leakage current out of the BMS would eventually charge them up enough to see voltage increase significantly, perhaps close to the full pack voltage.
The next mostly likely is taht the controller has failed, and is internally shorted at the FETs; this could be from a short between the new phase wires, or an accidental touch of the phase connectors of the controller with power still on during the process, etc.
If the battery was left connected during the phase wire replacement, this kind of problem can happen at any point if the phases get shorted even for an instant on a controller without protection against that (most dont' have any); it should only occur if the controller was also still "on", but if there is a phase wire short it can happen as soon as the controller is reconnected to power / turned on.
Disconnect hte motor from the controller, and retry the battery connection. If it doesnt' shutdown, there is a problem at the motor itself or the wiring up to the point it connects to the controller.
If it still shuts down then it is probably the capacitors in the controller taking too high a current during connection, and you can add a Precharge resistor system (lots of posts and threads about those) to correct this.
