Lifepo4 batteries charging confusion

[timelinex]

This isnt exactly solar, but i figured you guys have the most knowledge in this.

I bought 2 Lossigy 100ah batteries and replaced the converter on my 2021 Grand Design 381m with a 60a powermax pm4. So far so good. The Powermax charges at around 40a, which is 20a a piece and the advertised amount to maximize longevity. I'm having a hard time working through what is the right setup for me
To clarify MY goals...I have done alot of reading and know the conservative options are "better", but dont know what that actually looks like in real life. I dont go super often. I obviously want to get a good life span out of my batteries, but if I get 5 years instead of 10, by being more aggressive towards convenience...then that's a good option. But 2 years would not be acceptable. So keep that in mind when answering my questions!

1. The PM4 has 3 stage option or flat voltage. I set it at a flat voltage of 14.4 as I read somewhere to do that for lifepo4. But my concern is that the voltage remains at that even when it's fully charged. I thought I read that's not good. Is the 3 stage better, as it has a float? I do imagine it's a slower charge though, which is a bad outcome...

2. If doing a straight voltage, is 14.4 the right voltage? I asked the company and they just said anything under 14.6 is fine. I dont think it's their engineers answer questions....

3. I know the 40a it is currently charging at it optimal....but I would really prefer faster so I have to run my generator for shorter periods to recharge when boondocking. It's not just a fuel cost issue, but rather noise and disturbance one. I read the 75a one does about 55amps for people. How much worse is it for the batteries to charge at 30a vs 20a? 100a one (40a each?) would be even better, but im guessing that will start pushing things...

It would also be a good thing to have a stronger charger if I decide to add more batteries. Then I won't need to waste money on upgrading the charger.

Thanks guys!

[howson]

I do imagine it's a slower charge though, which is a bad outcome...

"Slower" depends on the specific charge profile as programmed into the charger. I assume the 3 stage is programmed for LifePO4? If so it will have a charge profile that stays in bulk for the majority of the charge cycle followed by a short absorption and then will stay in float.

I think Garret Towne explained this best in his video Four Stages of Battery Charging for Your RV, Skoolie or Van at https://www.youtube.com/watch?v=ljXNptHgDXk

If it's any help, the Victron Multiplus goes through three charge stages (Bulk, Absorption and Float) for the Battle Borns I use. The charger does not stay at 14.4 in float.

As you can see in the real time shot below, the state is "Float" and the voltage is 13.6.

[TucsonJim]

I went through the same questions when i switched to LiFePO4 batteries in 2017. I discussed it with both Battle Born batteries and Progressive Dynamics.

I had purchased two BB 100aH LiFePO4 batteries and a PD 9160AL converter/charger. The PD9160AL only has one charging voltage, and that is 14.4VDC. I was concerned (like you are) that this may be detrimental to the batteries. Both Battle Born and Progressive Dynamics assured me that this set up is fine. As the batteries start to achieve a full charge, the voltage remains at 14.4 volts, but the current drops to near zero. With the proper meters, you should be able to confirm the same situation with your charger.

Some companies such as Victron lower the voltage as part of their charging profile.

Jim

[acoleman43]

timelinex I have a 24v system so my voltages will be about double a 12v system. My batteries are charged by my solar panels about 90% of the time> I have the solar charge controller set at 3 stage (Bult, Absorbtion and float). The cell manufacturer recommends Bulk/Absorbtion at 28.1V (+/- 0.2V) and Float at 27V (+/- .02V). Recommended charging is 20A, and max is 100A per battery.

Because my panels charge my batteries, the charging current varies significantly throughout the day. The most I will typically see to the batteries is about 35A (about 12A each battery) but it is usually about 6-8A per battery.

Generator/shore power will charge using the most current allowed by battery BMS, which is set to 110A (probably should change that to 100A) - however I max I can get from generator per battery (If my math is correct) is about 40A, which is a little more than I can get from shore power. Remember I have a 30A trailer. With a 12V system and 2 batteries, you will be able to hit the 100A charging if desired.

Also, I typically keep my batteries close to 100% charged while in storage - which isn't optimal.

In 2 years I've lost about .02% to .04% battery capacity or about .01% to .02% per year. In other words a battery that was 100AH is now 99.98AH. Batteries have about 45 charging cycles on them in 2 years.

If "abused" batteries get 3000 charging cycles, and I average about 23 cycles per year, that's about 130 of years I will get out of them. If I baby the batteries and get 6000+cycles, that's 260 years.

Don't sweat the details too much. Get out there and camp. If you need to push 100A through them to quickly charge them, do it. You won't be doing this every day of the year....year after year. Or if you do and get "only" 3000 cycles, that's still many, many years of good use out of your batteries. The stress worrying about this isn't worth it in my opinion. Just get out there and enjoy the trailer

[ncitro]

timelinex I think a lot of your information came from me (sounds like what I normally recommend). Unfortunately I do not have your batteries, and you really need to trust the information you have on your batteries if you want to go past the conservative choices in favor of more performance. This is where more name brand batteries have an advantage, that information will be easier to get and with a larger install base will be more trustworthy.

That said you need to realize you're relying on the batteries BMS to allow current to flow or not. What happens in fixed voltage is the BMS sees the cell voltage is low and allows current to flow. Once the cell voltage rises to a set point it stops letting current flow. That's why you still see voltage out of the charger but no current flowing. The cells aren't sitting at the charger voltage because the BMS has turned off flow. That said the BMS is seeing charger voltage at it's input, so you need to trust you have a quality BMS that can handle that. The charge current limitation is usually a function of the BMS (can it handle the current) and the cell quality (will they degrade over time at a higher charge current). I don't know this for sure but I'd assume if your recommended charge current is lower than the BMS (usually 50A) then they'd using lower rated cells. I don't have any proof of that but it's all I can think of that makes sense.

[Jkwilson]

Constant 14.4V is fine for lithium-ion batteries. No need for any other modes. The stages or modes that charger manufacturers claim are just descriptions of how a lead-acid battery charges combined with the power limitations of the charger. A charger doesn’t change anything between bulk and absorption mode. It keeps doing the same thing at all times. Only when going to float mode does the charger change its output. Lithium-ions don’t behave like lead-acids, so those modes don’t exist.

RV converters are more than chargers, as they are also power supplies, so they change output voltage based on demand.

The BMS in a lithium-ion handles the end of charge decision, so a 14.4V DC power supply is a perfect charger for them.