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How to charge LiFePo4 battery to target voltage

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Sal23

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I didn't find any thread here on how to charge a 12V LiFePo4 battery (with BMS) to a target voltage that is not 100% SOC. So, asking here. Nothing special about those batteries - one of those LiTime, PowerQueen etc. batteries (400+ Ah). If this is already discussed in another thread before, a link to that thread will be appreciated. More details below.

I will like to charge some of the batteries to a target voltage (say 13.2v) for storage.

How would one go about doing this? Like measure some kind of delta between charging and post-charge rest voltage (after a fixed amount of current charging) to figure out what charging voltage to apply to get the target rest voltage? And then calculate target absorption voltage by adding that delta to target rest voltage?

I have a Victron 12V 30A charger that I can program with custom absorption voltage, time etc. I am familiar with charging algorithms, but not how to adjust the settings when the voltage target is very specific. An error of +- 0.05v should be fine (given the still flat voltage curve around 13.2v level).
 
The voltage drop on stopping charging will depend on the amps of the charge. So today my peak charge rate on my 48V batteries was 104A and they showed 54.2V, charging has stopped and they have dropped back to 53.5V and SOC has not budged. So the only way to do this is say charge to 13.5V then turn off the charger and then measure in 1hr. If you get 13.1V then charge to 13.6V and repeat.
 
You don't find it because it's not even remotely practical, and it's often more detrimental than it is beneficial. Most pursue this to maximize cycle life, and the easiest way is to simply reduce your charge voltage to 3.45V/cell with 1-2 hr absorption time.

LFP batteries need to see elevated voltage to ensure the BMS can keep them top balanced. Failing to regularly charge to the balancing region means the cells will drift, capacity will be lost.
 
Yeah, it's rather futile to try to charge at a specific voltage in the very flat portion of the charge curve. 13.2V could be 30% capacity or 80% capacity. Counting coulombs is much better way to determine the specific voltage for your system to target. I presume your goal is to 'store' them at 50% or so for relatively long periods of time? Be sure to give them the opportunity to top balance with some regularity to avoid them drifting apart in capacity. This should not be a significant issue with properly/well balanced cells from the start.

Andy has some videos on this very topic. Informative and I find entertaining as well. Gotta love him.

 
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I am curious how would battery vendors do this specific targeting? Can they do it with one of those LiFePo4 chargers w/o coulomb counting? Vendors like PowerQueen usually send batteries at almost identical voltage (max 0.01v difference), around 13.2v (~70%-90% SOC in 13.2-13.3v range for LiFePo4).

So, I was looking to do the same thing for long-term storage of unused batteries (and full charge once in a while to keep them top balanced - they already seem well-balanced to begin with). Like you folks pointed out, should avoid the charge voltage that is too flat (which is 13.1-13.2v which corresponds to 40-70% SOC range).

I have found Andy's videos very useful, especially simple battery analyzer like YR1035 which I will get. Nothing about mechanism of charging to specific storage voltage by battery vendors though.
 
The manufacturer simply charges by amps and time for a specified period, aka they put in 40Ah into a 100Ah battery/cell. In a built battery, the BMS has a small parasitic drain, in addition to the tiny self discharge of the cells, so depending on how long it's been since it was put together and charged, the SoC upon receipt will vary.

Voltage in the flat part of the curve masks the vast majority if not all capacity inbalance. That is the 'problem'. This is why you find lots of cell/batteries at the same voltage. The capacity imbalance is only realized in the knee of the charge curve.

Typically, cell makers recommend charging to ~30-40% for long-term storage, just as they ship them and disconnect all loads. The cell specs should give you the self-discharge % by month. From this you can get fairly close to figuring how frequently you should allow them to top balance before discharging for storage again.
 
Sal23 said:
I am curious how would battery vendors do this specific targeting? Can they do it with one of those LiFePo4 chargers w/o coulomb counting? Vendors like PowerQueen usually send batteries at almost identical voltage (max 0.01v difference), around 13.2v (~70%-90% SOC in 13.2-13.3v range for LiFePo4).
Click to expand...

Nope. They tend to be closer to 50%. Hazmat requirements dictate < 30%, so they all poo poo that. The batteries were likely last charged 90+ days ago, so their voltages are fully at rest. Bare cells are typically received at 3.28-3.29V (13.12-13.16V), and they tend to be in the 40-50% range. The voltage to SoC charts are approximate.

Easy mode is it to have a properly configured quality battery monitor that actually measures % SoC that's regularly sync, and you can charge/discharge to a target knowing is relatively accurate.

Be absolutely prepared to have a poor performing battery after long term storage. The individual cell SoC will drift, and you will almost certainly be unable to charge to full voltage due to cell imbalance. You will likely need to make provisions for holding the battery at elevated voltage (full charge) for days and maybe even weeks to allow the BMS to re-top balance the cells.
 
sunshine_eggo said:
Be absolutely prepared to have a poor performing battery after long term storage. The individual cell SoC will drift, and you will almost certainly be unable to charge to full voltage due to cell imbalance. You will likely need to make provisions for holding the battery at elevated voltage (full charge) for days and maybe even weeks to allow the BMS to re-top balance the cells.
Click to expand...
Yep, makes sense. I figured rebalancing might take 1-2 days but yep, it can be more than that.

For the battery bank, I will be using a victron shunt to sync to 100% SOC to keep track of the SOC w/ Cerbo based ESS (that communicates it to Quattro). As for individual batteries that are in storage, will do full charging once in a while to fix the SOC drift, and avoid extended rebalancing effort due to too much drift.
 
Sal23 said:
Yep, makes sense. I figured rebalancing might take 1-2 days but yep, it can be more than that.

For the battery bank, I will be using a victron shunt to sync to 100% SOC to keep track of the SOC w/ Cerbo based ESS (that communicates it to Quattro). As for individual batteries that are in storage, will do full charging once in a while to fix the SOC drift, and avoid extended rebalancing effort due to too much drift.
Click to expand...

I'm a hardcore smurf, so I may be over-reacting to your statement, but if you're specifically referring to Victron ESS Assistant, there are ZERO reasons to use it, and a few dozen reason not to. If you're simply saying you're sharing the shunt SoC with the Cerbo to provide battery voltage, current and possibly temperature data to all Cerbo connected devices, youbetcha! Make sure you enable DVCC, SVS, SCS and STS as applicable.
 
sunshine_eggo said:
I'm a hardcore smurf, so I may be over-reacting to your statement, but if you're specifically referring to Victron ESS Assistant, there are ZERO reasons to use it, and a few dozen reason not to. If you're simply saying you're sharing the shunt SoC with the Cerbo to provide battery voltage, current and possibly temperature data to all Cerbo connected devices, youbetcha! Make sure you enable DVCC, SVS, SCS and STS as applicable.
Click to expand...
Yep, just sharing shunt SOC (including temperature). I now know who to reach out to if I have questions about doing this correctly :) I think I have it figured out but just in case...
 
sunshine_eggo said:
I'm a hardcore smurf, so I may be over-reacting to your statement, but if you're specifically referring to Victron ESS Assistant, there are ZERO reasons to use it, and a few dozen reason not to. If you're simply saying you're sharing the shunt SoC with the Cerbo to provide battery voltage, current and possibly temperature data to all Cerbo connected devices, youbetcha! Make sure you enable DVCC, SVS, SCS and STS as applicable.
Click to expand...
Dynamic ESS is I think something for the Europeans and other upper echelon regions :)
 
Very much useful information here, but I am still bothered by one issue. How long can I store the cells charged to 100% SOC. Let's say that in two weeks I expect the power plant to fail. Is it too long to store the cells like this?
 
It would depend on the mfg...

To know the SOC it takes a digital meter and just measure the voltage. There is a very specific chart that you can use to find 50% SOC.
 
Last edited:
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I know of a company in China that makes adjustable voltage charge/discharge testers. I have one of their devices.
You can find them by doing a quick search for YPSDZ.
However, I don't think it's cost-effective to use them in small quantities.
 
beo said:
Very much useful information here, but I am still bothered by one issue. How long can I store the cells charged to 100% SOC. Let's say that in two weeks I expect the power plant to fail. Is it too long to store the cells like this?
Click to expand...
A couple of weeks will be fine
 
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