evanjs avatar image
evanjs asked

How do I know when my battery is fully charged? How does the MPPT know?

I have an Victron 150/45TR MPPT charge controller being fed by 600 watts of Renogy panels wired in series.

This is hooked up to two, wires in parallel Renogy 170ah lithium ion batteries.

According to their spec sheet they are "recommended" to be charged to 14.6 volts. I don't want to do that and would prefer charging to 14.2

Looking at the spec sheet, Renogy says the following.

Standard Charge shall consist of charging at 0.2C constant current rate until the battery reaches 14.6V. The battery shall then be charged at a constant voltage of 14.6V while tapering the charge current. Charging will terminate when the charging current has tapered to a 0.02CA. Charge Time is approximately b7 hours. Safe Charging consists of temperatures between 32 ºF and 113 ºF.

Does the battery stop accepting charge from the MPPT once the current has tapered? Does the MPPT know when to stop charging the battery? How do I know when these are full? How does the system know when these are full?

Lastly, what should my absorption voltage and time be? How about the float voltage? Battery University says these batteries should really just be charged in bulk phase to ~90 or 95% capacity and then turn off charging. What does the MPPT aim to charge these at during bulk phase? That doesn't seem to be a setting I can adjust.

Sorry for so many questions, this I am new to all this.

Thanks in advance!

MPPT Controllersbattery chargingLithium Battery
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4 Answers
wkirby avatar image
wkirby answered ·

The battery manufacturer is giving charge parameters assuming that there are no loads connected to it, only the charger. Therefore some of those parameters can't be adhered to. For instance it my never be possible to terminate the charge on tail current if a load is constantly drawing more than that. The MPPT sees the battery and the loads as one.

You set the absorb Voltage to 14.6V (or 14.2V if you like). You can limit the maximum bulk current to 0 2C. In solar applications the bulk current is rarely constant because clouds and stuff. Remember, that includes the loads, so if you have a big load on then the charger is still only going to output 0 2C to the battery and loads. The battery may actually be discharging at this point.

Set Float Voltage to be just above the cell's nominal Voltage - 12.8V or 12.9V should be fine.

In summary, just worry about maximum charge power (0.2C). Maximum Absorb Voltage and Float Voltage. Tail current termination is not currently available as a setting but it will be rare for that scenario to present it's self even if it is enabled.

If you have any more questions, please ask.

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evanjs avatar image evanjs commented ·


Thank you for taking the time to answer my questions.

My end goal is to get the batteries charged to 90-95% and ensure my settings are correct.

In order to do this I will disconnect all loads and let the solar panels charge the batteries to full.

Just to confirm my settings are correct, here is a screenshot of what I am doing.

This morning I used my Victron BMV 712 to look at my battery voltage and it read 13.32. I had disconnected all loads and the solar charger from the batteries the night before so the batteries have been resting for about 10 hours. Looking at the Renogy Manual there is a graph that shows how voltage translates into SOC.

This graph makes me think that the batteries are only around 20% full.

Using the settings above, and without any loads drawing from the battery, Over a few days the MPPT should be able to get the batteries to 14.2 volts and I will have fully charged the batteries.

Summary of questions:

Are my MPPT settings correct?

Do you agree the batteries are about 20% charged?

Do you agree with my current (no pun intended) charging plan to get the batteries to 90-95% SOC?

Is there any risk of overcharging them?

Thank you again for the help!

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mppt-settings.jpg (424.7 KiB)
boekel avatar image boekel ♦ evanjs commented ·

As these are LFP batteries, you don't have to stop charging at 80% just don't stay above 13,8 V after they are fully charged (so set float below 13,8V)

And: Do you agree the batteries are about 20% charged?

No: 13,3 V /4 cells = 3.32 V per cell, when lightly loaded this is about 100% charge you have to look at discharge curves, not charge curves.

And: with LFP cells, SOC is difficult to determine by voltage.

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evanjs avatar image evanjs boekel ♦ commented ·

@Boekel Sorry, but I am very confused. How can the battery be fully charged at 13.3V when the manufacturer calls for 14.6 not that I would charge it that high.

If I can't tell SOC by voltage, it seems all I should really worry about is if the charging parameters are correct. Is my screenshot above accurate?

Is there any risk of overcharging them?


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boekel avatar image boekel ♦ evanjs commented ·

Hi @Evanjs, this 14,6V is during charging (and high current charging also), when you stop charging an LFP battery goes back to it's resting voltage (on a new battery maybe only when lightly loaded).

If you charge an LFP battery to 3,45V per cell (13,8V for a 12V battery) and stay there until current drops, you have charged the battery to 99% and ensure a long life.

Now the confusing part: most LFP batteries are only balanced when above a certain voltage, this can be different between manufacturers (and should be mentioned in the documentation of those batteries). So this is why you might have to charge up to 14,2V or maybe even 14,6V (but if you do this once a week it should be enough)

look at the blue line for charge voltage at 0,5C (50% of capacity, or charge in 2 hours)

And the same for discharge, you see the difference in voltage between charging and discharging:

(but do notice this is under quite a load)

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evanjs avatar image evanjs boekel ♦ commented ·

@Boekel wow, that is confusing.

Let me make sure I understand.

Ideally, this Renogy battery should be charged with constant current at an ever increasing voltage (Victron calls this charging phase Bulk) until it hits peak voltage of 14.6 volts.

Once at peak voltage, the current decreases quickly, referred to as tail current (Victron calls this charging phase Absorption).

Renogy says the charging will terminate once tail current reaches 0.002CA. Does the battery or the MPPT know to stop charging once the charging current has tapered to 0.02CA?

Once the absorption phase is over, the charge controller switches over to float which should be set to this batteries Nominal Voltage of 12.8V.

Now that the battery is fully charged and assuming the charger is disconnected and no loads are placed on the battery, the voltage should drop over a ~24 hour period to it's charged resting voltage of 13.8 according to Renogy's discharge graph for this battery.

Below are two screen shots of charging parameters.

The first one is for top balancing the cells in both batteries. How frequently do I need to top balance?

The second one if for normal charging to ~90-95% SOC.



Last question: what is an appropriate absorption time?

Thank you!

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boekel avatar image boekel ♦ evanjs commented ·

What is confusing?

The mppt doesn't know when to stop charging, that's where a fixed absorbtion time comes in. (this is not yet possible, next firmware version will make this function available)

Where do you get the 12,8V number? that's much too low to keep a full charge. Look at the blue line on the graph, 12,8V is 40% SOC (lightly loaded)

I can't tell you how frequently you have to top balance, Renogy should be able to tell you.

14,2V I think is a bit high, I'd say 13,8V for normal (99%) charge

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evanjs avatar image evanjs boekel ♦ commented ·


I got the 12.8 float voltage from @WKirby's suggestion from above. This is the batteries nominal voltage according to Renogy. Float should only kick in once bulk and absorption are done and the battery is almost completely full. My understanding is lithium batteries should NOT be floated. Seeing as there is no way to disable this, people put the float voltage around nominal voltage. Maybe @WKirby can help explain the reason.

Regarding the 14.2V absorption voltage, I think you're correct, 13.8 charging volts seems like a good way to reach between 90 and 95% SOC given the charging graph provided by Renogy show in in my post above.

Here is the updated normal charging parameters I will use.

Finally - is my absorption time correct?

Thank you for all your help so far, I feel like I understand a lot more about what I'm doing :-)

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boekel avatar image boekel ♦ evanjs commented ·

absorbtion time: see if the batteries are still taking current after 1 hour, if not: fully charged, if still taking current: not fully charged yet.

You are correct, lithium shouldn't be trickle charged, so you lower to the desired 'charged' voltage, that will be about 13,4V, I don't agree with the 12,8V, that's too low.

The thing is: if you charge a battery and don't use it after, you stop charging totally and preferably disconnect charger.
But you're using the battery, so after being charged there will be loads discharging the battery. by setting float at 13,4V the MPPT will try to keep the battery fully charged when loads draw power. if you use 12,8V the battery will discharge to 30-40% SOC before MPPT kicks in again.

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evanjs avatar image evanjs boekel ♦ commented ·


Ok, I will test the absorption time and see if they are still accepting current after 1 hour in absorption mode. If they are then they need more time in absorption mode to get them to 95% charged.

Once I do this I should put the absorption time back down to 30 min or so for future charge/discharge cycles, right?

I see what you mean about setting the float too low and now agree it should be set to 13.4

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boekel avatar image boekel ♦ evanjs commented ·

Just try it and see how it behaves.

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evanjs avatar image evanjs boekel ♦ commented ·

Ok, I tried it earlier today and here is what happened.

Absorption voltage was set to 13.8 and absorption time was set to 1 hour. Float is set to to 13.4

The batteries started at 13.3 volts and after 20 or so minutes of bulk, it went into absorption mode. It stayed in absorption mode for about 25 minutes before switching to float.

In order to test if the 100% SOC was accurate, I decided to disconnect the solar panels and draw down the battery. I turned my fridge on to 0 degrees, turned on the inverter and hooked up my portable sander while I sanded some plywood.

Overall the BMV states I used about 31 amp hours and said that I had 91% SOC left.

This makes sense seeing as the battery bank is 340 amp hours so if it had been drawn down by 8% it would have used 30.6 amp hours.

I will continue to discharge the battery down to 20% before I reconnect the solar system just to test how a full charge/discharge cycle looks.

Anyway, I'd like to thank everyone for their input, especially @Boekel!

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svtti123 avatar image
svtti123 answered ·

Yes the charger knows that when it hits absorption setpoint to begin a constant voltage charge with current gradually decreasing with battery resistance increasing when then tail current hits i believe 2 amps the charge ends or if the absorption timer times out it ends what ever happens first.

You dont need to charge a lithium ion battery to full voltage that does put stress on the battery and therefore shorter cycle life. You can charge the batteries to any usable voltage level you like below the 14.6 volts 14.2 is good.

The chart below is for a nickel Cobalt aluminum or NCA battery i dont know your exact chemistry. Most lithium based if not all batteries follow the same rules. Check on your chemistry to be sure. As yo can see skipping the absorption charge gives you less of a charge. Setting the absorption time to 0 effectively does just that. Controller switches from bulk to float skipping absorption.

So you could start with 0 absorption time slowing increasing it till you get the desired results you want.

The float voltage will keep the battery at float voltage lets just say 14.0 volts if the battery voltage decreases say 13.9 the current will increase to get the voltage back to 14 volts. So any load or battery losses you will the see current increase as the controller trys to maintain 14 volts until the battery voltage drops to approximately 200 mv for a certain amout of time below the float voltage setting i believe . For example a heavy load. This causes a new bulk absorption float cycle. So you need some float settings.

I would go with the battery manufacturer recommended float setting till i was comfortable with my own settings.

You never mentioned battery balance?

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boekel avatar image boekel ♦ commented ·

All 12V Lithium batteries are LiFe(Y)PO4 and have a very different charge curve than you posted, please don't confuse this.

(yes 12V lithium could also be LTO but I haven't seen them in the wild yet)

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wkirby avatar image
wkirby answered ·

@Evanjs I want to back up what @Boekel has said.

What @Svtti123 has written is correct, but the chart is misleading and irrelevant to this particular question.

As for battery balance, are you using the Renogy BMS? Do you realize that (according to the documantation) if the Renogy battery locks out that only the Renogy BMS can re-activate the battery again? I'd use the Renogy BMS with these batteries.

renogy170.jpg (141.9 KiB)
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inhabor avatar image
inhabor answered ·

So did someone finally figured out the charging settings for the Renogy lithium battery? I can use the information. Thanks

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evanjs avatar image evanjs commented ·

Reading through this may help. I think there are 11 comments in it.

Ultimately I set the absorption voltage to 13.8 and the float to 13.6.

The biggest thing I did not understand when doing this was the voltage that the batteries are charged at vs the resting voltage of a full charged battery. The maximum charging voltage Renogy recommends is 14.6. Once the battery is fully charged and if you were to remove the charger and any loads, the battery voltage would settle to around 13.6 volts.

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