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Question about charging through a BMS

LVLAaron

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Jul 16, 2021
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I'm working on my first "real" battery pack. 400ah 12v pack. All of my experience is with small 5-25ah packs that I have always used a hobby charger to re-charge as they see limited use for a very specific task.


The pack is together after being top balanced. I've got a small Daly bms attached. I'm charging with the AIMS charger that will recommends from current connected.

The aims charges to 14.4 - so I programmed the BMS to 3.6v per cell.

I have the current turned all the way down on the charger (20amp)


At the end of the charge cycle, the unit doesn't ramp down amperage as the voltage slowly rises like I normally see with hobby chargers. It's 20 amps full steam ahead until the first cell hits 3.6 and the BMS cuts it off. Normal?

Thanks for entertaining my stupid questions!
 
It's 20 amps full steam ahead until the first cell hits 3.6 and the BMS cuts it off. Normal?
The charge current is small compared to the battery capacity, thus the battery is full before the current starts to fall.

Its considered that the battery is fully charge when the cell volts reach 3.65 and the current fall to 0.05C. For your 400Ah pack that's 20amps.

Thus with the low charge volts the battery is full when the voltage reaches target volts of the charger.

The BMS should not be used for charge control, its intended to protect against out of specification voltages or currents.

Setting the charge voltage to 13.8 to 14 volts will still fully charge the battery, terminate the charge when the charger hits target volts.

Mike
 
Thanks. That makes sense. Appreciate the sanity check.

The charger (On the LifePo4 setting) is a fixed 14.4v volts.

I guess I could switch it over the "Gel" which bulks to 14.0 and floats at 13.7
 
Change the bms values, cell limit to 3.65 and the pack limit to 14.6

I prefer to use chargers that have user programmable settings. I suggest not to use GEL , the absorption time will be too long.

Mike
 
  1. Basic Inverter and Charge Controller Charge Settings​

Below are the typical settings when using an inverter or charge controller with LiFePO4 batteries.​

  • LiFePO4 batteries do not require Equalizing / De-Sulphation.​
  • LiFePO4 batteries do not require Temperature Compensation for voltage.​
CHARGE PARAMETERS
12V
24V
48V
Bulk Voltage​
14.0 - 14.6​
28.0 - 29.2​
56.0 - 58.4​
Absorption Voltage​
14.0 - 14.6​
28.0 - 29.2​
56.0 - 58.4​
Absorption Time​
depends on charging profile being used​
Float Voltage​
13.3 - 13.8​
26.6 - 27.6​
53.2 - 55.2​
equalize(not used) set to lowest time
13.3
26.6
53.2
Low Voltage Cutoff​
11.0 - 12.0​
22.0 - 24.0​
44.0 - 48.0​
High Voltage Cutoff​
14.6​
29.2​
58.4​
Termination Current *​
≤0.05C​
≤0.05C​
≤0.05C​
Termination Current Example: 100AH charge max 0.5C/50A, termination current = 0.05C/5A

* when using multiple batteries in a battery bank, reading current from the bank as a whole is ineffective.

LiFePO4 can be safely charged with either of these modes:

1-stage profile (constant current (CC) aka Bulk Stage) profile will charge the battery ~95%. The 1-stage profile is sufficient, since LiFePO4 batteries do not need to be fully charged, they will settle to 95% after charging.

2-stage profile (constant current, constant voltage (CC-CV) profile aka Bulk and Absorption Stages). The 2-stage profile will charge the battery 100%. This may also have the effect of triggering a BMS HVD (High Voltage Disconnect), therefore take appropriate precautions by using conservative charge settings to begin with.​

  • Optimal Charging will occur at 0.5C charge rate per battery. The number of batteries multiplies the amps required to meet 0.5C charge rate.​
  • The ability for any single battery within a bank of batteries should be capable of handling the full charge & discharge potential of the system.​
NB: Some BMS' (Battery Management Systems) can interact with Inverters, Chargers & Solar Controllers which can improve overall performance, reliability and longevity of the battery systems. These capabilities are dependent on the equipment being used.

* Equalize: Some Solar Charge Controllers / Chargers / 3-Stage chargers have this. Disable or Set to lowest time allowed and at float voltage equivalent. LFP does not require this.​
SOURCE: ➡️ Luyuan Tech Basic Lifepo4 Assembly Guide
 
Ah, makes sense. I'm used to using CCCV "gadgets" and the LifePo4 type on this charger is probably a single stage.

Not sure what the charge curves look like on the Generic, Gel, and AGM settings... I could try and set up some type of monitor and charge up some of my AGM cells.
 
I'm working on my first "real" battery pack. 400ah 12v pack. All of my experience is with small 5-25ah packs that I have always used a hobby charger to re-charge as they see limited use for a very specific task.


The pack is together after being top balanced. I've got a small Daly bms attached. I'm charging with the AIMS charger that will recommends from current connected.

The aims charges to 14.4 - so I programmed the BMS to 3.6v per cell.

I have the current turned all the way down on the charger (20amp)


At the end of the charge cycle, the unit doesn't ramp down amperage as the voltage slowly rises like I normally see with hobby chargers. It's 20 amps full steam ahead until the first cell hits 3.6 and the BMS cuts it off. Normal?

Thanks for entertaining my stupid questions!
The current begins to taper when the total battery pack voltage reaches the charger's voltage setting, in this case 14.4. What i suspect happened is the first cell reached 3.6 far enough ahead of the other cells and the total pack voltage hadnt hit 14.4 to begin cv mode.
 
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