LiFePO4 best practices

[Gary H]

Hi all,

I have a LiFePO4 bank that I charge primarily with solar. Since float charging is not recommended once the bank is full, I have programmed my Victron battery monitor to cut off charging sources when the bank reaches 99% full.

My question is at what state of charge should I start charging again. In other words, is it better to do a greater number of charge cycles with just a couple of percent depth of discharge or would it be better for the bank to go to a greater DOD and do fewer, but longer charge cycles?

I'm living on the hook, so my instinct has been to not waste sunlight and keep the bank as full as possible, but I don't want to do so at the expense of shortening the bank's life span.

Thanks!

[exMaggieDrum]

A good read on best practices on LiFePO4 batteries can be found here:

Batteries - Thoughts & Musings" target="_blank">LiFePO4-IRJDSUNE9932123321222xxeww-Batteries-IRJDSUNE9932123321222xxeww---IRJDSUNE9932123321222xxeww-Thoughts-IRJDSUNE9932123321222xxeww-&-IRJDSUNE9932123321222xxeww-Musings

under the marinehowto.com website produced by CF contributor MaineSail.

I believe he addresses your question directly and from what I have seen it is not the best practice to keep the batteries full, contrary to what your might think. Very counter-intuitive compared to lead acid technology.

There is this article: Proper Care Extends Li-Ion Battery Life | Mobile content from Power Electronics found referenced in this article: https://marazuladventures.files.word...batteries8.pdf

Some good reading here. I am just starting my own research so can't comment on the accuracy or quality of the statements but they seem very creditable to me.

[Gary H]

Thanks for the links, but I still haven't found a direct answer. Maine Sail discharges to about 80% DOD and then recharges. I'm using mostly solar so that kind of scheme is not ideal for me. Unfortunately, he no longer consults on LiFePO4 installs unless they are local.

Maybe there just isn't any data on this yet. I might have to be the guinea pig.

[Pyxis156]

Gary,

This spring I installed a Genasun LiFePO battery system and faced the same issue, albeit with charging while plugged into shore power instead of with solar. I didn't want to keep the batteries floated at full charge for the reasons you state above, but I wasn't keen on making the batteries constantly work between full charge and, say, 50% and shorten their lives for no reason. My solution has been to set my battery charger to float at 26.4V (I have a 24V battery system) which allows the batteries to draw down to about 60% and then the float takes over and provides the power the boat needs to maintain. The net result is I keep my batteries at 60% whenever plugged into shore power.

I am not sure if you can set the output voltage of your solar controller easily but if you can, then perhaps setting it to 14.2V (or whatever voltage you see at the % discharge you desire to hold the batteries at) will do what you want. The caveat is being able to change that easily since when you are actually on the boat cruising you will want the solar to charge the maximum amount possible.

On a related note, you will likely have to experiment to determine the optimal float voltage because the charge curves of these batteries is so flat. I had to do a fair bit of trial an error to figure out a value that kicks the charger in at the right time, and it is dependent on the amount of load your boat experiences when at anchor awaiting your return. The voltage value that works is dependent on the size of your battery bank and the magnitude of your "maintenance load".

Hope this helps...

[exMaggieDrum]

Gary, this is from the Maine Sail article that I gave the link for. For some reason I thought it addressed your question, although it is just one person's opinion:

"When we come back from a weekend on the water, and our battery is at 50% SOC, I DON'T CARE!!!! I shut down the boat, and the solar and go home. LFP batteries actually prefer to sit at 50-60% SOC rather than at 90-100%.. As I said earlier this is a mental paradigm shift we need to overcome in our human behavior/thinking around our batteries..."

Your question is very important though. I would like to know the definitive answer. I have a fairly large solar array and I would like to know if it is best to leave the batteries on the juice or not.

[SV THIRD DAY]

I just set my solar controller voltage to 13.8v. This way I stay away from the risks of over charging. Been running my 400AH LiFePO4 bank that way not for a little over a year and it's been working great. I can't over charge unless the solar controller fails....easy pleasy...

[Gary H]

Thanks for the replies. I am already using conservative charging voltages so there is no danger of overcharging in such a way that a single incident will damage the bank.

What I am trying to get an answer to is sort of the extra credit question on the exam. It is my understanding from reading the giant thread on LiFePO4 that charging should be stopped COMPLETELY once the bank is full (no floating). Obviously, if the bank reaches the voltage of a charging source no current is flowing anymore, but you will be keeping the bank at this higher voltage for longer than is necessary. This is what I am avoiding by having the battery monitor cut off charging via relays when the bank reaches 99% full.

So when should charging resume? When the bank is down to 98% full? If I was relying on a shore charger or an often used alternator, I would wait until the bank was down to 20% to recharge, but I am primarily living off of solar. (I don't have a shore charger and engine use is infrequent).

[Singleprop]

Since you are living on the hook there will be a continuous amps draw from the batteries to keep freezers, fridge, radio's and sensors working, so would your batteries ever be fully charged? Would they ever go into the float stage?

My thinking is that the continuous amps draw keeps the system out of the float stage even if you have the solar system delivering more than the amps usage...so theoretically the battery bank is full, but being discharged all the time and charged all the time as well.....confusing...

Can anyone confirm this?

[colemj]

I don't think I understand the problem clearly. Are you wanting to:
1. leave the boat with all loads turned off
2. leave the boat with loads on, like reefer, etc?

If #1, why not just draw the bank down to 50%, shut everything off and leave?

If #2, how is this any different than living aboard? I assume you have set your solar charging to stop charging at some higher voltage and begin charging at some lower voltage. If so, then it will just be longer between charging regimes if your power usage is lower when you are gone. The batteries, however, will still cycle through (say) 85-50% SOC.

Mark

[CarstenWL]

I don't think I understand the problem clearly.

If I understand correctly the valid question is:

- Charge the batteries to say 90% SOC
- Turn off ALL charging sources
- Turn charging sources back on when Battery is at say 30% SOC

OR

- Charge the batteries to say 90% SOC
- LEAVE charger on to keep battery at around 90% SOC

I think the first choice will be better in the long run but problem is if solar is your only charging source and you are at 30% SOC and have a week of cloudy weather in front of you ....

I am too interested to hear what 'the experts say'.

Regards,

Carsten

[NahanniV]

Originally Posted by SV THIRD DAY

I just set my solar controller voltage to 13.8v. This way I stay away from the risks of over charging. Been running my 400AH LiFePO4 bank that way not for a little over a year and it's been working great. I can't over charge unless the solar controller fails....easy pleasy...

What kind of controller do you have ?

Is it just charging to 13.8V and holding there ?

How long are your bats. usually held at 13.8V. during a day?
Do you manage with 100% solar, or do you need to top up regularly with another source ?

Cheers,
JM.

[Gary H]

Carsten, you have it right. That is a much better way of asking the question.

Mark, I am dealing with number two on your list. The issue is not voltage, though. I already charge at a constant voltage, and there is really no voltage difference in a LiFePO4 cell unless it is VERY full or VERY empty.

What I'm trying to get at is whether it is better for the cells' long term health to allow the bank to discharge more fully before beginning another charge cycle.

[offpist]

I found that setting the Float to 13,25V lets the LIFEPO4 settle at about 70% SOC when connected to shore power for long time off season.

[donradcliffe]

I looked at the RC forums, and most suggest that the LiFe can be stored at full charge. Your situation is different than storage, as you have a daily load on the battery.

While looking, I found this manual, which I think may be a bit conservative, but has interesting comments on min and max temperatures and getting thebattery wet.

http://manuals.hobbico.com/hca/lifesource-manual-v2.pdf

[SV THIRD DAY]

Originally Posted by NahanniV

What kind of controller do you have ?

Is it just charging to 13.8V and holding there ?

How long are your bats. usually held at 13.8V. during a day?
Do you manage with 100% solar, or do you need to top up regularly with another source ?

Cheers,
JM.

Hi JM

I I have a few Blue Sky 2000E controllers.
Yes, once the charger sees 13.8V on the battery bank. So once my batteries get to 13.8v, no more amps flow into the battery from the solar controller.
Living on the mooring with NO shore power cord and no real need to run the generator for battery charging, solar is all I need to keep my batteries charged. But based on my Amp meter (which lies more than a politician) my typical SOC during the day is 50% to 80ish %. When I want to impress myself with these batteries I will run my 1500W hot water heater for 30 minutes just to drag the SOC down a bit for fun. Typically my batteries are at 13.8v for...maybe 4-5hrs per day, load and sun dependent.

After the last year on LiFePO4 batteries....I would NEVER go back.
It's easier than the experts want to make it and even better than they hype it!