LiFePo4 longevity in a dual battery setup

[Saabpilot]

Thanks Will for the excellent video on battery longevity. I realised that I have a problem.
I am about to Install 125ah LiFePo4 into my new Ford Ranger using a Projecta Lithium DC-DC charger.
Part of the set up is a solar blanket but for the most part, the alternator (connected to the Projecta) will hold the battery at full change.
So the battery will set there for months on end at 100%...sometimes in a hot car.
Yes, the charger is “5-stage” designed for Lithium and the Battery has a BMS on-board, but its still going to hold the charge at 100% most of the time because it will get very little use.
What I would like to do is limit the battery upper charge level (Say 75%) with the ability to restore to 100% when required.. (ie a camping trip running a fridge).
Does this make sense? Following up with the supplier the reaction was pretty dismissive... basically, you don’t need to do that.. the DC-DC Charger will look after the battery.

I have some thoughts on how this could be done but I would sure appreciate some advice!

Love your book... Highly recommended.

 

[gnubie]

It floats at 13.5V which is generally accepted as safe for a lifepo4 battery that is in at least weekly use (ie being discharged). I wouldn't leave it on float if the discharge interval is more than a week personally. If you intend to leave it sit for longer the 75% SOC and bring to fully charged when needed plan looks good to me. lifepo4 has a very low self discharge so 75% would let you sit it unattended for months with no load before it needed attention.

The absorption charge is 14.5V. That's pretty close to the limit for several BMSes so check your battery to make sure it will not trip off with that voltage. The manual says it drops to float once absorption charge current falls to 2.5A. 2.5A tail current is fine for your 125AH battery but if you have any loads on the battery you may find the charger never drops back to float mode, and that will not be good for the battery. There doesn't appear to be any time limit on absorption charging so that's definitely something to be aware of.

100% for extended periods is not a good idea. It's even worse when you mix in high car temperatures in summer. In-car temps will get well over 50c. Will it be in the cab or in a box in the rear or? You will need to work out a way to keep the battery cooler either way, no matter what.

 

[Sojourner1]

Saabpilot what you describe in your op is a recipe for disappointment. Can't you just pull the lfp battery out and store it at 50-60% in a cool place when not in use, then reinstall a day prior to use to fully charge it?

 

[Saabpilot]

It floats at 13.5V which is generally accepted as safe for a lifepo4 battery that is in at least weekly use (ie being discharged). I wouldn't leave it on float if the discharge interval is more than a week personally. If you intend to leave it sit for longer the 75% SOC and bring to fully charged when needed plan looks good to me. lifepo4 has a very low self discharge so 75% would let you sit it unattended for months with no load before it needed attention.

The absorption charge is 14.5V. That's pretty close to the limit for several BMSes so check your battery to make sure it will not trip off with that voltage. The manual says it drops to float once absorption charge current falls to 2.5A. 2.5A tail current is fine for your 125AH battery but if you have any loads on the battery you may find the charger never drops back to float mode, and that will not be good for the battery. There doesn't appear to be any time limit on absorption charging so that's definitely something to be aware of.

100% for extended periods is not a good idea. It's even worse when you mix in high car temperatures in summer. In-car temps will get well over 50c. Will it be in the cab or in a box in the rear or? You will need to work out a way to keep the battery cooler either way, no matter what.

It will be mounted in the space behind the rear seat. It’s a slim profile device in a aluminium case....good heat sink. I may just plan on removing it. Not like I don’t get a warning that we’re going camping!

 

[Saabpilot]

Saabpilot what you describe in your op is a recipe for disappointment. Can't you just pull the lfp battery out and store it at 50-60% in a cool place when not in use, then reinstall a day prior to use to fully charge it?

Sojourner1. That’s exactly what I will do!
elegant simplicity... thank you.

btw. Here’s the reply I got from the manufacturer of the battery.


Thanks for your enquiry, the Projecta Dc Charger will be more than enough to look after the Battery . The 5 stages of the charger monitor the battery status and will regulate the amount of charge into the battery if required.
When the battery is full it will monitor this as there is no problem having a fully charged battery it doesn't affect the amount of cycles of the battery. On top of this the battery has a BMS built in that also protects it from over and under voltage.
I would not recommend putting in a VSR.

end of quote.

All this said. It would seem like a good idea to control the SOC as an “idle mode” or “green mode” or something sexy. Maybe as a feature of the BMS or charger... just a thought.

In the future a lot of people will install Lithium batteries in this way and not give a second thought about the SOC/temperature problem.

I appreciate your excellent advice.

thanks guys!