Voltage question

[dwilker788]

HI,

Newbie question here, but I have 24 v system with a 3000 w inverter. I have it hooked up to the washer and dryer. Last night using the dryer it dropped to 25.9 v when it finished and now it says 26.6 v in the morning. No charge as it was overnight. Just trying to figure out why the voltage went up overnight and what charge the batteries really have?

Thanks!

 

[Steve_S]

Whenever there is a load, there will be a certain amount of SAG which recovers after the load is removed. This is one main reason that BMS' have a Delay before low volt disconnect occurs to prevent a disconnect under a surge or heavier load if batts are low. The bigger the load the greater the SAG. BTW, This is normal with any battery system in place, some chemistries handle it better than others and LFP is very good at dealing with it, as it recovers quite quickly actually.

There are ways to reduce that effect depending on your installation setup. If you have battery packs in Parallel, they divide & share both Load & Charge proportionately to their capacity. This can reduce the SAG seen by any one pack at a given time.

 

[Phantom]

Im no expert but when your using something it will show a much lower voltage then when at idle
the best way is to wait a few min. with NO load then check the batteries at their post with multimeter

 

[RCinFLA]

There are two primary pieces of voltage slump with discharge and rise during charging. Lesser significant part is cell ohmic resistance due to metal foil current collector and positive, and negative electrode resistance. The ohmic resistance does not change too much over life of LFP cell unless there is delamination of electrodes to their metal foil current collectors. Electrolyte degradation will also increase it a little. It is close to the resistance you would measure on a 1 kHz impedance meter. For a typical 280 AH LFP cell it is in the 0.18 to 0.25 milliohm range.

More significant is the ionic overpotential voltage which is overhead required to move lithium ions through the cell to support the demanded cell current. The ionic overpotential voltage slump for discharge or bump up for charging is greater the greater the cell current (proportional to log of current). For LFP cells in 90-20% state of charge range, the ionic slump/rise has an exponential time decay that takes one to three minutes at given cell current to reach equilibrium. It is the ionic overpotential impedance that degrades the most over cell aging due to poorer lithium-ion migration rate within cell. It can increase two to three times over useful lifetime of LFP cell. It also is greater the colder the temperature of cell is.

In your particular case, the high battery current due to washer load, likely dropped each cell by about ballpark 0.06 -0.1v of terminal voltage slump. Times 16 cells is about 0.6-1.6v total voltage drop for series connected cells, plus any cable/connector resistance times current additional voltage drop. When load current dropped off, the overpotential voltage will go back to unloaded equilibrium level in typically less than 3 minutes.

All batteries regardless of type exhibit this overpotential terminal voltage effect. Lead-acid battery have a much greater overpotential voltage slump due to their complex chemical process and larger molecules to migrate around. They also take 15-45 minutes to reach equilibrium after a cell current change.

Li-Ion batteries have a very low overpotential vs current compared to most other battery chemistries due to simple lithium to lithium-ion conversion and migration.