How to know when your Lifepo house battery is charged

[user 37446]

I've been wondering how peeps know when their Li-on house battery is "full" as you cannot use voltage as the sole determination like with lead acid. If you have a coulomb meter maybe but what if you don't?

Example: You have a 100 amp cheapo $300 "drop in" house battery. You spring for a 20 amp DC to DC charger and an mppt charge controller with programmable settings.

So what tells the dc to dc charger to not keep sending a charge?

Since charge controllers pretty much only work on "time" for their stages, how are you going to figure that?

Or am I overthinking things, as usual. Reading various online articles and watching YT videos hasn't helped me, but then I may be "beyond help" anyway.

 

[jacqueg]

My LiFePo battery has a bluetooth function that lets me know. Also, I believe my MPPV controller would tell me - if I could figure out how to read it!

 

[user 37446]

My LiFePo battery has a bluetooth function that lets me know. Also, I believe my MPPV controller would tell me - if I could figure out how to read it!

How would the charge controller tell you? Cannot just use the voltage, and if you never let the battery stay "at rest" for x many hours without a load or charge, then what?
Fortunately Lithium doesn't care if it stays in a discharged state yet I'd like to know if there's some way to tell without fancy tracking devices.

(I have a Lifepo battery box which gives me watts in/out and remaining life % but when testing it, the "hours remaining" and battery percentages are not reliable once it hits 25% remaining.)

 

[jonyjoe303]

Without being able to read the individual cell voltage you won't be able to tell. Even the coulombmeter will only give you a estimate. But without a coulombmeter, eventually the battery will go to zero, it happened to me several times when I was just relying on voltage to keep track of the battery status.
In summer I have my mppt bulk voltage set to 13.9 volts and even at that low voltage it will fully charge the lifepo4 to 14.6 volts (3.65 volts per cell). At 13.9 volts it charges it slowly. When I set the voltage to 14 volts or higher it will trip the battery bms too quickly. 13.9 volts is the sweetspot for my solar system.
I see about 14.1 volts throughout the day as it charges, that might be a voltage to look at that might give you an idea that the battery is almost fully charged.
But for you to get an accurate reading everything has to be properly calibrated to the battery terminals. I encounter voltage drop on every mppt/pwm controller I have used, I don't rely on them for accurate battery voltage.

This is a picture that shows how I keep track of my 220ah lifepo4. I mainly use the coulombmeter, which is good enough. But the bms which show the individual cell voltage is the most accurate. With the coulombmeter I know that if I used 20 amps the previous day, the following day I have to at least put 20 amps back in the battery to be back where I started.

 

[user 37446]

Thank you for that explanation.

So what I gather is that it's better to purchase a battery that has the app with it that (may) reveal the cell balancing levels... This is getting a bit complex but sounds necessary.

Can you comment on the dc to dc charger knowing when to continue charging and/or can you comment on what to set your solar bulk charge time to?

My (very) limited understanding of programmable mppt controllers is that you can set the bulk voltage for "x" minutes then revert to a "float" of say 13.6.

The only upside to this whole thing that I can determine is that Li-ion doesn't degrade if left in a less than fully charged state (unlike lead acid) so it's ok to "muddle" along with a semi-partially charged battery... kinda-sorta. Hmmm.

 

[jacqueg]

How would the charge controller tell you? Cannot just use the voltage, and if you never let the battery stay "at rest" for x many hours without a load or charge, then what?
Fortunately Lithium doesn't care if it stays in a discharged state yet I'd like to know if there's some way to tell without fancy tracking devices.

(I have a Lifepo battery box which gives me watts in/out and remaining life % but when testing it, the "hours remaining" and battery percentages are not reliable once it hits 25% remaining.)

I have a 200 ah Renogy battery with bluetooth. The bluetooth function tells me that right now my battery level is at 91.8%. It also tells me that my present capacity is 183.6 ah, present voltage is 13.2v, present current is 8.8a (my fan is running, the refer is currently not), and the cell voltages are at 3.3v for all 4 cells. And that's pretty much all I know!

 

[Spaceman Spiff]

I've been wondering how peeps know when their Li-on house battery is "full" as you cannot use voltage as the sole determination like with lead acid. If you have a coulomb meter maybe but what if you don't? .....

A primary function of your batteries internal Battery Management System (BMS) is to protect the cells from overcharging. It will shut off charging when it determines the battery is at 100%. Your cheapo battery has a BMS, I hope. The decent BMS has bluetooth reporting, the cheap ones have LED lights.

..... So what tells the dc to dc charger to not keep sending a charge? .....

Cheap ones rely on your batteries BMS to shut off charging. Expensive ones will also monitor the battery and could shut off charging sooner.

..... Since charge controllers pretty much only work on "time" for their stages, how are you going to figure that? .....

'Stages' are for lead-acid batteries; Li-ion batteries have charge and off. The BMS will shut off charging when the cells are 'full'.

 

[user 37446]

I have a 200 ah Renogy battery with bluetooth. The bluetooth function tells me that right now my battery level is at 91.8%. It also tells me that my present capacity is 183.6 ah, present voltage is 13.2v, present current is 8.8a (my fan is running, the refer is currently not), and the cell voltages are at 3.3v for all 4 cells. And that's pretty much all I know!

Well that seems to be the key then, pay the extra for the app.
Mucho thanks for the report and recommended fix.

 

[user 37446]

A primary function of your batteries internal Battery Management System (BMS) is to protect the cells from overcharging. It will shut off charging when it determines the battery is at 100%. Your cheapo battery has a BMS, I hope. The decent BMS has bluetooth reporting, the cheap ones have LED lights.


Cheap ones rely on your batteries BMS to shut off charging. Expensive ones will also monitor the battery and could shut off charging sooner.


'Stages' are for lead-acid batteries; Li-ion batteries have charge and off. The BMS will shut off charging when the cells are 'full'.

I don't have a Li-Ion battery yet. I currently use an old group 24, 70a marine SLA and a 35a AGM when I travel. I was hoping to upgrade/replace both with a single 100a Lifepo unit but was struggling with the charging.
What you wrote is reassuring and it seems all of the basic $300 batteries have a BMS and a few other safety systems in place. As long as the BMS does its job then I suppose all is well.

For now finances and "do I really need it" keep me using what I have, which for a PT user is ok.

 

[maki2]

It will soon become obvious just from the general volts readout on your controller if the battery is failing.

 

[user 37446]

Kinda yes and no. What seems to happen more is that the SLA battery voltage is ok (12.5-12.7) but it can't handle much of a load for very long. Probably sulfated and I don't have a reliable way to desulphate it.

I tried running a 20 watt panel straight to the battery with no charge controller, to allow the 20 ish volts to help boil off/dissolve the accumulated crud, but that didn't seem to do much. There wasn't any case swelling after an hour and my IR gun didn't indicate any heat buildup. (The mushroom cloud had me worried but it dissolved after a bit.)

It is just an old reconditioned marine battery that has seen its time I guess. It's fine for a 12v fan but not for a large load.

 

[jacqueg]

I don't have a Li-Ion battery yet. I currently use an old group 24, 70a marine SLA and a 35a AGM when I travel. I was hoping to upgrade/replace both with a single 100a Lifepo unit but was struggling with the charging.
What you wrote is reassuring and it seems all of the basic $300 batteries have a BMS and a few other safety systems in place. As long as the BMS does its job then I suppose all is well.

For now finances and "do I really need it" keep me using what I have, which for a PT user is ok.

Yes, I took a hugely deep breath when I bought the Renogy. The deciding factor for me was weight. My van has a V-6 engine...

I have to say I am VERY pleased with it. It just works.

 

[user 37446]

Thats great to hear that it is working for you!

 

[user 37446]

Kinda yes and no. What seems to happen more is that the SLA battery voltage is ok (12.5-12.7) but it can't handle much of a load for very long. Probably sulfated and I don't have a reliable way to desulphate it.

I tried running a 20 watt panel straight to the battery with no charge controller, to allow the 20 ish volts to help boil off/dissolve the accumulated crud, but that didn't seem to do much. There wasn't any case swelling after an hour and my IR gun didn't indicate any heat buildup. (The mushroom cloud had me worried but it dissolved after a bit.)

It is just an old reconditioned marine battery that has seen its time I guess. It's fine for a 12v fan but not for a large load.

Update: I bought a charger that has a “recondition” setting for FLA/SLA chemistry. Says don’t use on AGM.

I tried the recondition setting after I had used the charger to fully charge the battery. Whatever magic the recondition setting is supposed to do didn’t work for me, it “examined” then immediately shut off.

Amazon comments had said that the recondition feature brought back dead batteries so unless I got a bad one, maybe a fully charged battery fools it. Don’t know.

 

[maki2]

Sounds like you need a new battery.

I have a Renogy mppt controller and a very good quality AGM battery. My controller is set to do a monthly desulfating cycle. Still going strong after 3 years of continuous use and it was stored and changed occassionly for just over a year before that. But I have never let that battery go below 52% and only rarely do Ibtake it below 60%.

I want it to last a few more years so I take care not to ever abuse it.