Stop charging lifepo4 when battery is full

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johan-granqvist asked Feb 16, '21

Stop charging lifepo4 when battery is full

Hi, I am new to the lifepo4 market but has read a lot about it. In almost all articles about charging it states that you should charge your lifepo4 until its full and then stop charging. But the chargers i have found almost always implement the three stages, bulk, absortion and float (allmost all articles say that you should not float a lifepo4 battery). I am planning to by a battery (not victron) with an external bms with hvc and lvc to cut off loads and charges at certain volts, but i would rather have the charger to sense the voltage and can be programmed to stop charge at a selected level, and use the hvc more like an emergency brake. Is this possible with any not-so-expensive victron charger? Will i encounter any problems if i use the hvc to disconnect the battery from a blue smart charger that is plugged in to AC?

battery charging Lithium Battery

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mvader (Victron Energy) answered · Mar 19, 2021 at 06:18 PM

Set float value in the charger low enough, and then it wont “float” the battery.

example: take a lithium battery, fully charge it. Leave it alone for an hour (nothing connected to its terminals)

Now measure the open circuit voltage. Gives a good indication. And will be around 3.37V per cell for lifepo4 = 13.5 V = the recommended “float” voltage.

connecting a charger that is set to that voltage wont charge the battery. What it will do is make for the loads to be powered by the charger; which is what anyone expects: a battery + charger, and then connecting a load, shouldnt result in the battery being discharged.

Above is a bit cryptic english perhaps, but what I’m trying to explain is that what really matters here is looking at the voltage, and understanding what happens when applying a certain voltage onto a battery(cell).

stopping charge can be done by disconnecting or switching off a charger, but you can also do it by having the charger lower its output voltage (to around open circuit voltage).

Explained differently: imagine a system with a battery + constant load of 2 amps. Some sort of lamp or pump or something.

if you’d have a system that would disconnect / disable the charger everytime the battery is full, then you’d be continuously first charging the battery, then charger goes off, then battery is discharged with that load, then at some point charger will be enabled again, chargers the battery full. Disabled again, etc forever.

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mvader (Victron Energy) ♦♦ commented · Mar 19, 2021 at 06:20 PM

Conclusion: battery will be charging during bulk. Then during absorption (voltage contstant at approx 14.4V) you give the balancers chance to balance. And then after a fixed time of 1 or 2 hrs reduce output voltage = go to the “float” stage.

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Answer 1 · 2021-02-18T12:11:02Z

schnema answered · Feb 18, 2021 at 12:11 PM

Hi,

I do have 16 lifepo cells in series. They are connected to a MultiPlus 2 GX. As BMS I use the GWL smart BMS. It will trigger a over- or undervoltage protection. You can programm the MultiPlus Aux 1 and Aux 2 contacts in order to react on the signal.

I have set 56V (16x 3.5V) as chage limit. As my setup is not yet complete, I did not yet discharge the battery. But I can confirm, the Multi stopped the charging process at 56V. The Voltage dropped to 54V and still no recharge. If I am not wrong, this can also be programmed. But for my use case not importat.

Maybe someone else can confirm this or add more informations.

best regards,

Marc

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Answer 2 · 2021-02-18T21:50:52Z

alan answered · Feb 18, 2021 at 09:50 PM

Simply set Absorption time to 1h maximum. This is the lowest setting in all victron charge equipment and the setting victron themselves recommends for their own LiFePo4 batteries and others.

you should never depend on the Bms HVD and LVD as a control method, this is very rough on equipment.
Instead having the LVD and charging limits of the equipment on the inside of the window.

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Answer 3 · 2021-03-19T13:14:54Z

johan-granqvist answered · Mar 19, 2021 at 01:14 PM

I think i can use the Phoenix charger and control it by the BMS. The Phoenix has a remote on off input. But exactly how is a subject for future investigations

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shaneyake commented · Mar 20, 2021 at 12:51 AM

You don't need to turn the charger on and off. Just leave it on with the correct float voltage as mvader has said and the battery will never trip HVD. Lifepo4 batteries also don't mind setting at 100% SOC for long periods. If you have a BMS with CAN like FreedomWon or Pylontech you won't need to worry about this as the BMS will tell the charger what charge current it wants.

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jrg shaneyake commented · Mar 22, 2021 at 01:31 PM

Lifepo4 batteries DO mind setting at 100% for long periods. It's one of the things that lowers their life. All battery specs that I've seen state to not leave them at 100% and that somewhere between 40-60% SOC is best for optimum life.

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seb71 jrg commented · Mar 22, 2021 at 01:46 PM

In a solar energy system you normally discharge the battery (at least partially) each day (mostly after sunset).

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