Lithium charging voltage at start

[Bessy765]

Victron 12/2000/80-30 inverter charger with new
2 x 280 amp Lithium pro under-seat batteries.
They ran flat yesterday as in 0% and shut off been on charge since 00:30 voltage started at 12.7 now up to 13.52 77% charged. Thought Lithium charged at a higher voltage? First picture just after turning charger on, second 09:00

 

[Kannon Fodda]

LifePo4 lithium will be full charge at about 13.6V. The charge voltage curve is quite flat, only really dropping in the bottom 20%, so 13.5 at 77% seems normal.

 

[Pausim]

The charging voltage should rise to about 14.2V as the battery approaches 100%. When charging has finished it will sink back to less than 14V.

 

[AdrianChen]

I looked at ours a couple of days ago we were getting around 17amps (solar) current - the battery voltage rose slowly/steadily from 13.2 to around 13.6v, (a few hours) then more quickly (40mins ish ) to around 13.8v, at which point the charge current dropped to a couple of amps, and the voltage within I guess a couple of minutes was 14.25v our absorption voltage, and the charge current dropped to 0.2-.0.4amps, which I guess might have been a bit of cell balancing, then within approximately 30mins it was 0.

On EHU or with more solar it would have been much quicker for the Bulk phase of course.

Seems to me to be consistent with the accepted mantra that the difference in capacity gained from pushing the voltage from 14.2 to say 14.6 would be a gain of 1-2ah capacity, on my 430ah pack.

 

[Pausim]

My understanding of LiFePO4 chemistry is that it has a very low internal resistance when charging, which means it will swallow whatever the charger feeds it (within the BMS limits). When the battery approaches fully charged the resistance increases rapidly, at this point the Voltage rises and the Amps drop off. The chemistry of the battery cells controls this process, the BMS has an over voltage limit to protect the cells but this should not be needed to control the charging process. When the battery is full the charger needs to switch to a lower voltage or switch off, some chargers do this by monitoring the rising Voltage and falling Amps (tail current).

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[Lenny HB]

I have a Multiplus 3000, I limited the charge current 90 amps.
If I charge the batteries (2 x 330 Ah) they charge at 80 to 90 amps the voltage gradually climbs until it reaches my absorbtion setting of 14.25v then the charge drops to 20 to 30 amps for a couple of hours until the last few percent is charged then it drops to a float of 13.5v which is low enough so the battery doesn't take any charge unless there is a load on it.

 

[AdrianChen]

Victron 12/2000/80-30 inverter charger with new
2 x 280 amp Lithium pro under-seat batteries.
They ran flat yesterday as in 0% and shut off been on charge since 00:30 voltage started at 12.7 now up to 13.52 77% charged. Thought Lithium charged at a higher voltage? First picture just after turning charger on, second 09:00

View attachment 1024825

View attachment 1024824

If you're using a 3-phase charger, the process starts with the "Bulk" phase. During this phase, the charger increases its output voltage until it reaches the maximum current it can deliver. For example, my charger can deliver up to 120 amps, so when charging an empty lithium battery, it will charge at 120 amps. The voltage is only important to the extent that the charger needs to overcome the battery’s internal resistance. It does this by raising the voltage enough to achieve the maximum charge current — or the maximum charge current the battery can accept.


In the charger's settings, there is a voltage threshold at which the charger switches to the "Absorption" phase. In my charger, this setting is adjustable, but in some chargers, it’s preset. During the absorption phase, the charger sets its output voltage to a specific level (in my case, 14.25V). The battery will continue to accept a decreasing current until the set absorption time expires. In some chargers, this time is adjustable, while in others, it may depend on the depth of discharge (especially with lead-acid batteries). Lithium batteries typically only need enough time for cell balancing because they’re essentially fully charged by the end of the bulk phase. In contrast, lead-acid batteries would be around 80% full at the end of the bulk phase.


After the absorption phase, most chargers will drop the output to a "Float" voltage (in my case, 13.5V). At this point, the battery will no longer accept a significant charge, but the charger will still provide a small amount of current to keep the battery fully charged. Some chargers may then reduce the voltage to a "storage" level, which is meant to compensate for the battery's self-discharge over time.

So I think to answer your query I would say the advantage with Lithium is that is will accept a much higher charge current than Lead acid, so during bulk charging it is the charge current that is the chargers focus....