LiFePo4 charging Trials and Tribulations

Grumpy_G

(yes I stole the idea for the title...)

TL;DR Charging your LiFePo battery with constant 14.4V keeps it happy. 

A few weeks back I posted about a charging issue a friend had with his LiFePo battery. This is a closer look at charging LiFePo batteries which might help with troubleshooting. Over the past few months I had access to Batteries from SOK, Renogy, LiTime, PowerUrus and Vatrer, solar charge controllers from Victron (MPPT), Renogy and Zamp (PWM), 120V powered chargers from Victron, PowerUrus and an adjustable lab power supply off Amazon (same one Will Prowse uses in some of his videos). 

The problem

LiFePo4 batteries want to be charged with 14.2V - 14.6V and can ingest large amounts of current until close to full charge state. The battery management system (BMS) simply cuts off charging once it deems the cells fully charged. LiFePo batteries with integrated Bluetooth monitoring will often show the status of the “Charge” and “Discharge” cutoff switches in the monitoring app. Note the “switch” is not a physical contact but a metal–oxide–semiconductor (MOS) transistor. 

In contrast lead-acid batteries need the charger to employ charging curves to balance charging speed vs. battery health. The phases are bulk, absorption, equalization (for some battery types) and float charging, representing different charge voltages and currents. A typical logic for charging lead acid batteries is as follows: Charge with maximum power until absorption voltage is reached at the charge controller output, then stay at absorption voltage for a time, then lower the voltage to float (ignoring equalization because it does not apply to all lead acid batteries).

LiFePo batteries essentially only charge in bulk mode which is as much current as possible within the limits of the charger. The charging variations of most solar charge controllers can be detrimental to achieving a full charge for a LiFePo battery. 

Batteries

The batteries are all Chinese made with similar construction using 4 prismatic cells. The BMS is essentially a cell-level charge controller. Unsurprisingly they all have the same charging behavior: With a charging voltage of 14.2-14.6V they pull the maximum current the charger can deliver until the BMS turns charging off. Charging with lower voltages does work to some extent, but the current drops off as the charge level increases and charging takes longer or never finishes. Think of charging voltage as “pressure” that pushes charge into the battery against its internal “pressure”. If the voltage is too low (typically 13.2V) the battery does not charge regardless of charge level. 

Solar charge controllers

The following are empirical results with the lab power supply as a stand-in for the solar panels. 

- The Victron 75/15 has the least aggressive settings for “Smart Lithium” with 14.2V absorption voltage and 2 hrs of absorption time. This is somewhat of a double whammy as it kicks into absorption early and only continues for two more hours before dropping too low to continue charging. 

- The Renogy Wanderer 30 in LiFePo mode uses an absorption voltage of 14.4V and no timeout. It basically behaves like a simple 120V powered dedicated LiFePo charger. Works well. 

- My puny pre-LiFePo Zamp ZS-8A was a surprise. Although sold as 8A it delivered a constant 10A current, I’m guessing marketing didn’t want to encroach on the more expensive 10A models with display. In AGM setting (14.4V absorption/4hrs) it did fully charge the battery. My hunch is because the controller is at its limit it never sees 14.4V until the battery turns off charging. 

120V powered chargers

- The Victron Blue Smart IP65 5A again uses absorption/float stages in the “Li-Ion” setting which is not needed. To be fair the charger has many settings and features for lead-acid battery charging, it is somewhat “overqualified” if you will. It does work well and has the good Victron bluetooth interface. 

- The PowerUrus charger is a dedicated LiFePo charger and is simply a 14.4V/10A DC power supply with some short-circuit protection. The charging LED turns green once the battery turns off its charging MOS switch. Simple and effective. 

- The lab power supply set to 14.4V works just like the dedicated LiFePo charger with added benefit of voltage and current display. It was handy for experimenting with different charging voltages and different solar side power levels.

Converter in the power center

The non-LiFePo converters switch to absorption mode at 13.6V and stay there for almost two days. 13.6V is too low for maxing out the possible charge current and charging gets painfully slow towards the end. Depending on other loads on the converter the battery may reach 85-95% after a long time. See picture below where the current is well below the 10A maximum of the power supply. 

Troubleshooting tips

Compare voltages at the charge controller and the battery. A large difference can cause the controller to kick into absorption early. Increase absorption time if the charge controller goes into float before the battery is fully charged. 

Kitchen lab setup  

Sharon_is_SAM

“The charging variations of most solar charge controllers can be detrimental to achieving a full charge for a LiFePo battery.”  Am I missing something.  Everyone on the forum reports that they are able to fully charge their lithium batteries via solar.  

Also, how do you adjust the charge controller to increase absorption time?

SLJ

I just turn off the LiFePO4 batteries when on shore power and let a Victron Smart Charger charge them back to 100% if needed. I don't bother letting the older WFCO do any charging. Wasn't interested in replacing the older WFCO when I switched to LiFePO4 batteries as there was a lot of problems with the WFCO auto-sensing version. We are seldom on shore power anyway unless at home parked in the garage. When boondocking the Victron roof controller and my Renogy controller for the portable panels seem to do well together most of the time.

AnOldUR

Grumpy_G said:

Kitchen lab setup

Had to laugh at this. We keep our T@B in a one car garage, so there's only room for a small work bench. Much to my wife's dismay, the kitchen counter often ends up being used for projects.

Grumpy_G said:

- The Victron 75/15 has the least aggressive settings for “Smart Lithium” with 14.2V absorption voltage and 2 hrs of absorption time. This is somewhat of a double whammy as it kicks into absorption early and only continues for two more hours before dropping too low to continue charging. 

The nice thing about the Victron controller is that you can adjust the settings. I've changed the absorption voltage to 14.4 as to the battery manufactures recommendation. After reading your post, I checked out the absorption time and see that it can also be modified. What would be a good setting for this? Can it be set too high or will the BMS stop absorption charging before any damage could occur?

Grumpy_G

Sharon_is_SAM said:

“The charging variations of most solar charge controllers can be detrimental to achieving a full charge for a LiFePo battery.”  Am I missing something.  Everyone on the forum reports that they are able to fully charge their lithium batteries via solar.  

Also, how do you adjust the charge controller to increase absorption time?

Occasionally members post about their solar setup not performing to their expectations or ask about what settings to use for the XYZ battery. My post is meant to give some background to help with those questions. The newer T@bs with the Victron controller won't have problems but if the T@b is older and/or had changes to solar or battery installation things might not work that well. In my friends case the T@b just had the connector at the tub and the issue was a combination of wiring and charger settings. 
It depends on the solar charge controller if and how the absorption time can be configured. For the Victron MPPT controller the setting is available in custom expert mode IIRC. 

The nice thing about the Victron controller is that you can adjust the settings. I've changed the absorption voltage to 14.4 as to the battery manufactures recommendation. After reading your post, I checked out the absorption time and see that it can also be modified. What would be a good setting for this? Can it be set too high or will the BMS stop absorption charging before any damage could occur?

As mentioned above the BMS turns off charging and protects the battery. Both the Renogy solar controller and the PowerUrus 120V charger stay at 14.4V indefinitely without ill effects. If I had to change absorption time for any reason (e.g. as a workaround in my friend's case) I'd set it to 8 or even 10 hours which effectively is the whole day. 

elbolillo

Good synopsis @Grumpy_G

It illustrates that there are a ton of many variables to consider depending on the specific combination of equipment being used. One of the biggest variables would be the BMS. There are a lot of unknowns around the details of each manufacturer's technology in their BMS and consulting with them should at least get a baseline from which to start.

Grumpy_G

I clearly failed in my objective of making things less complex The BMS is a "black box" that just requires constant charging voltage of 14.4v +- 0.2V, the internal workings are irrelevant.