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Old 10-10-2021, 12:02   #1
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Float charging Lithium

My Lead Acids were ruined being uncharged over my two year Covid absence (in a yard which did not allow power on board) so I decided to replace with Lithium. All seems fine with two Lithium banks each of three sets of four cells plus a lead acid to protect the alternator, if nothing else.

Now, I must leave the boat in a marina 1,000 miles from my home (who will allow power onto the boat) so to be certain of having power for my bilge pump I plan on float charging my batteries including Lithiums.

I have set the charger at 13.2 volts on the basis that the batteries will not be taking in power unless the pump drains the voltage down. This way I believe my Lithiums will not be damaged by overcharging.

Does this make sense?
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Old 10-10-2021, 13:39   #2
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Re: Float charging Lithium

No. Lifepo4 do not like float charging.

Charge them to about 70% & disconnect them from any charging or discharging equipment. Disconnect also from BMS, solar, wind, shore charger and active balancer and let them sit.

If you must, keep the pump connected to the lead acid and keep that on float charge via a solar panel.
Do not connect lead acid and Lifepo4 in this case.
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Old 10-10-2021, 22:41   #3
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Re: Float charging Lithium

What brand/type lithium batteries do you have? You mention 3 sets of 4 cells and two battery packs with a lead acid battery to protect the alternator. This sounds like you have 4 cells in series and 3 sets of these in parallel x 2 separate battery banks ..... do you have a cell balancer across all 12 cells in each battery bank so all the cells in that bank bank are at the same voltage?

If you have all 12 cells balancing together for each lithium battery, then a trickle charge at 13.2v will keep all the batteries from discharging to the point of failure or damage, the lead acid battery at a high enough voltage to avoid sulphation and all the lithium cells at 3.3v, roughly half way between 100% SOC and 0% SOC and they will be happy at that for ages ..... but if you do not have all the cells balancing together so they each hold the same voltage, then follow Franziska's advice and disconnect everything from the lithium batteries, including being linked to each other across the whole system. One cell going low in any of the 4 cell series strings will kill all the other parallel strings connected to it. All BMS systems require some electrical power to run, if you have no charging connected to the batteries, the BMS will drain them flat and they will gradually destroy each other.
If you leve them om a trickle charge, no matter how low the voltage, and do not have all the cells balancing together, at least one cell from each group of 4 cells in series will gradually climb higher the the voltage of the other 3 cells .... resulting in at least one cell in each group with its voltage gradually dropping, yet the voltage across the battery remains the same ... until one cell dies, then it will kill the other batteries in parallel because they will constantly try to send current to the battery with the weak cell .....

This is he major problem with linking 12v lithium batteries in parallel, all the cells in that battery group must be balanced to each other all the time to stop cell voltage run away ....

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Old 15-10-2021, 03:31   #4
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Re: Float charging Lithium

Thanks Franziska, and T1 for your thoughtful and informed response.
T1 your interpretation of my set-up is correct. The cells do not display a brand, doubtless Chinese, but they seem well made. my BMSs are Daly 100Amp, one for each set of four cells. So I do not have one balancer for all 12 cells in each bank.
The cells in each battery were matched to two decimal points of voltage before being built and I have relied on the Dalys to keep each battery balanced. However, I think you are saying that one battery could leak into another in the same bank without a 12 cell balancer. if so, should I have a 12 cell balancer as well as the 4 cell Daly BMSs? And, if so, could you please suggest a 12 cell balancer brand?

I have now disconnected the Lithium banks from charge being the safest option, but they remain connected as banks. Would 12 cell balancers solve the problem of leaving them for six months or should I disconnect each battery or each cell. The latter cannot be wrong, I suppose!

If I am allowed a further question, I note people talk about charging up to 70% or 80% but not 100%, but I cannot find the guage which shows the state of charge (!) and I understand a voltage reading is not much use because of the flat curve. I do not want to overcharge; more damaging than undercharging, I suppose. So I have only charged at 13.2 volts. Am I misguided?
Your advice is much appreciated.
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Old 15-10-2021, 22:53   #5
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Re: Float charging Lithium

As you point out. Voltage on bank level is a hard way to measure SOC on lifepo4 but if they are reasonable we’ll balanced you should be ok. What you can do is to put some load on the battery for a while to at least be sure it is not to fully loaded. I leave my lifepo4 connected all winter with a small solar panel to charge. But I have a bms that connect/disconnect the bank automatically from both charge an load sources at given SOC, voltages and temps. This have worked very good. Don’t you have a bms or any other means to control your bank?
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Old 16-10-2021, 00:03   #6
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Re: Float charging Lithium

Floating your LiFePO4 batteries at 13.2V will work just fine with your Daly BMS ensuring that cell level balance is within acceptable limits.

Your only risk is a BMS failure which may result in the failure of a single 4 cell pack, much more acceptable risk than the boat sinking because of a minor leak with no bilge pump to sort it out.

I have floated my 4s8p bank with cell balancing at 13.2V for several months during peak Covid with no problems at all.
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Old 18-10-2021, 09:07   #7
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Re: Float charging Lithium

Thanks Flod and Markcouz
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Old 28-10-2021, 16:50   #8
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Re: Float charging Lithium

Floating is fine as long as the voltage is low enough so there isn't a constant charge current. 3.35vpc works well and holds batteries in the 60-70% SOC range, and lets the charger carry any loads.


What you DON"T want to do is set the float voltage above the battery resting voltage such that there is a continuous charge current, as you would for a lead battery.
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Old 30-10-2021, 06:01   #9
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Re: Float charging Lithium

Thanks for your advice, Tanglewood. I think there is a real benefit on a boat having a low voltage (13.4 ish) charge continuously connected. While at sea, and using power, it allows the batteries to be re-charged even if slowly and in dock it is reassuring to know that the bilge pump will not run out of power (heaven forbid that I am ever in need of that).
Yes, lithiums have the ability to recharge remarkably quickly but it would be a very rare occasion that I would need a higher voltage to obtain a fast recharge.
Thanks again.
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