DIY 12V Lithium Battery Low Voltage while under light load

[Luthj]

If you get to 14.4v, 3.6V measured at the cells. Then the battery is charged.

 

[wlfpck1]

Interesting scenario here. I think the battery might be full charged. The charge controller shut off. Battery is ~13.9V, however even with a BMS the cells are not balanced. Cells are reading 3.73, 3.39, 3.37, 3.37.
Cell that is reading 3.73 is the B- cell with the negative lead of the BMS attached.
Maybe try putting a load on the inverter and see if the voltage drops?

 

[Bob B]

I'd try to get it top balanced ..... I don't think that Daly BMS has much balance current. Do a search on here for balancing with a resistor .... but you probably don't have one. Maybe connect them in parallel and let them sit a while.

 

[wlfpck1]

I'd try to get it top balanced ..... I don't think that Daly BMS has much balance current. Do a search on here for balancing with a resistor .... but you probably don't have one. Maybe connect them in parallel and let them sit a while.

I'll order a benchtop power supply and get them top balanced. They were balanced from the manufacturer when they arrived but I know LiFePo4 voltage may not indicate the cells are truly balanced.

 

[wlfpck1]

I'll order a benchtop power supply and get them top balanced. They were balanced from the manufacturer when they arrived but I know LiFePo4 voltage may not indicate the cells are truly balanced.

Update: got the cells top balanced and they're charging up to the full 14.4V which is good.
Still having an issue of voltage drop under load. Resting with no load the battery voltage is 13.0V, but when pulling 1,000W it'll drop down to 11.6V. It'll even drop .3V when pulling 25W. All connections are tight. I think the pack is charged but as we know voltage on Lifepo4 isn't a great indicator of charge. Also this voltage drop is while using at night so the pack isn't charging.

 

[Luthj]

Are your bus bars dropping lots of voltage.

 

[wlfpck1]

Are your bus bars dropping lots of voltage.

How would I test this? I did double up the bus bars but they are pretty thin.

 

[Luthj]

With a load, measure the voltage between both ends of the bus bar. A good place is the stud/bolt head. For example my 2/0 bus bars drop around 2 mV at 100A.

 

[Oldtoad]

My buss bars are squashed copper tubing.

 

[MisterSandals]

How would I test this? I did double up the bus bars but they are pretty thin.

If you are getting a considerable voltage drop your bus bars will be very warm to the touch. All the loss is in the form of heat so VERY easy to check.

 

[wlfpck1]

If you are getting a considerable voltage drop your bus bars will be very warm to the touch. All the loss is in the form of heat so VERY easy to check.

Wow I just confirmed the bus bars are warm to the touch when pulling ~400W. Battery dropped from 13.2V under no load to 12.3V with load.
Wouldn't any bus bar get warm though? How do I fix this issue? I have already doubled up the junky bus bars they came with. I am using a DIY 12V battery with prismatic cells like these: https://www.aliexpress.com/item/4001269432774.html

 

[wlfpck1]

Any advice on how to fix bus bars getting warm? I would think all bus bars get warm when under high loads.

 

[snoobler]

I don't know if it's been discussed, but some bolts provided are too long, and they don't provide good contact with the bus bars.

If your bus bars are getting warm it's because they're too thin, or they are only lightly held in place. If it's because they are too thin, replace with thicker or add more thin ones. If it's because the bolts are too long, add washers to the bolts.

 

[wlfpck1]

I don't know if it's been discussed, but some bolts provided are too long, and they don't provide good contact with the bus bars.

If your bus bars are getting warm it's because they're too thin, or they are only lightly held in place. If it's because they are too thin, replace with thicker or add more thin ones. If it's because the bolts are too long, add washers to the bolts.

Thanks so much for your help! I think I will build my own bus bars out of copper tube since it seems there aren't good quality bus bars available. I am still seeing voltage drop under small loads like 40 watts. Even though the bus bars aren't warm at that wattage can I be sure the bus bars are the issue?

 

[snoobler]

From my original response:

1) battery is not fully charged.
2) you have a loose connection somewhere.
3) you have high resistance in a cell or component (fuse/breaker)
4) you have bad cells.

This thread was getting tedious, so I haven't hung in there for every post. I still don't see any evidence that you've narrowed it down beyond minimizing #1.

You need to stop looking at total voltage.

What are the cell voltages before and during the load? Fill out the following information:

At rest, record:
Cell 1 volts:
Cell 2 volts:
Cell 3 volts:
Cell 4 volts:

Apply load

Load, W:

Approximately 15 seconds into the load, without removing the load, record:
Cell 1 volts:
Cell 2 volts:
Cell 3 volts:
Cell 4 volts:

Remove load

wait 5 minutes

At rest, record:
Cell 1 volts:
Cell 2 volts:
Cell 3 volts:
Cell 4 volts:

 

[wlfpck1]

From my original response:



This thread was getting tedious, so I haven't hung in there for every post. I still don't see any evidence that you've narrowed it down beyond minimizing #1.

You need to stop looking at total voltage.

What are the cell voltages before and during the load? Fill out the following information:

At rest, record:
Cell 1 volts:
Cell 2 volts:
Cell 3 volts:
Cell 4 volts:

Apply load

Load, W:

Approximately 15 seconds into the load, without removing the load, record:
Cell 1 volts:
Cell 2 volts:
Cell 3 volts:
Cell 4 volts:

Remove load

wait 5 minutes

At rest, record:
Cell 1 volts:
Cell 2 volts:
Cell 3 volts:
Cell 4 volts:

Thanks snoobler for your patience as I am just a novice. I will fill out the above info when it's dark and the panels aren't producing any output.