Experiences or opinions on the Chargery BMS?

[MondeoMan]

there is any orientation about what could be considered "low charging current"?, maybe /1C?, .01C?

From my experience, watching the voltage rise while charging, I would say below 0.2 C

Not sure if i get this... your guideline about "real" voltage its with the battery specifically underload and its relation with resting voltage at the same SOC?

Sorry, I don't understand the question.

Problem mine doesn't have any algorithm to detect charge termination and reset itself

Many detect charge termination and "synchronize" to 100% SOC when cell voltage exceeds a set value on all cells.

if you set the voltage termination to 12 volts using a relay to disconnect, the unit will disconnect at 12 volts no matter what the capacity reading is. Same with the other settings.

I assume that capacity reading is the SOC (State Of Charge)
So if you want to take good care of your batteries, you should do some test runs, watch the SOC and set the charge voltage to minimize risk of overcharge.
In some cases, the voltage is below the upper knee. In these cases, automated frequent Top-Balancing is a risky proposal.

 

[Gazoo]

I assume that capacity reading is the SOC (State Of Charge)
So if you want to take good care of your batteries, you should do some test runs, watch the SOC and set the charge voltage to minimize risk of overcharge.
In some cases, the voltage is below the upper knee. In these cases, automated frequent Top-Balancing is a risky proposal.

I have two Valence RT batteries and they have a built in BMS. I use a 10 amp lifepo4 charger and it cuts off at 14.62 volts every time. The BMS in my batteries top balance the cells during the final charging stage. I don't use the protection features of the meter we have been discussing. I keep an eye on the voltage reading of the meter while charging and discharging. The voltage of the meter closely matches my multimeter.

Many detect charge termination and "synchronize" to 100% SOC when cell voltage exceeds a set value on all cells.

That's what I thought the JunTek meters do as well. Some more testing is in order when I have the time. Thanks for your replies. Good info.

 

[MondeoMan]

What's the AH rating of the Valence RT batteries?

 

[Gazoo]

What's the AH rating of the Valence RT batteries?

40ah each. Hooked in parallel for a total of 80ah. They are used so don't have the full capacity but it's close.

 

[MondeoMan]

10 A is a slow charge current for 80 Ah.
They're probably already full at 13.6V...the additional charge up to 14.62V is going to be overcharge.

Are you able to monitor individual cell voltages? Do they really need re-balancing at each charge cycle? How big is the imbalance at around 14V?

 

[Gazoo]

I have had these batteries going on a year and have never had any problems. I don't use them often. I charge them FULLY monthly due to their high discharge rate so they will be ready in case of a power outage. I cycle them every couple of months. I have a 300 watt sine wave inverter in case the power goes out. When I discharge them I do so while happily watching my 55" OLED TV. They will run it for at least 9 hours. I run the battery down to 11.5 volts or so. I wait until the next day to charge them. They are very robust. The battery chemistry is LiFeMgPO4. The batteries are cylindrical, not prismatic. Every time I have cycled them over the past year the capacity has remained around the same. I cycled both of them separately a bunch of times before I hooked them in parallel when I was experimenting with them.

LiFeMgPO4 Lithium-iron Magnesium Phosphate Cathode Material Developed By Valence (VLNC) Shows A Strong Intellectual Property Position For The Firm; Industry Analyst Sees A Bright Future

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www.twst.com

http://mesa-tec.com/wp-content/uploads/2014/05/Mesa_Valence_U_Charge_U1-12RT.pdf

I have done a lot of research on these batteries both before and after purchasing them. My intent was to only get one and I had already ordered the charger when I decided to get the second one. Bought them both on ebay and the deal was too good to pass up.

My meter could be 2 one hundredths off....no big deal even if it isn't. They will accept a higher charge before the BMS cuts off but I don't recall what that is. I am guessing the batteries are around 9 years old. They were used in portable medical carts in hospitals. Their big brother XP series were used in EV vehicles. Same chemistry but larger amperage and different type of BMS.

I am in the process of ordering prismatic cells from China. I am planning on building a 24 volt pack and using with an inverter/charger or MPP Solar. I can't exactly do a solar layout as I live in an apartment but I do like the all in one for its value and simplicity to hook it up. This will be another experiment and I am looking forward to it.

Almost forgot to add Will Prowse has videos about these batteries. I ordered mine before he posted his videos on youtube. Since then the batteries have gone up in price.

 

[MondeoMan]

Just as a side-note, if you look at the datasheet, they give 14.6 V as "fully charged voltage", but also a 20 A charge current for a 40 Ah battery.
That's a 0.5 C rate, which causes a significant rise in cell voltage during charge, due to "Ohm's law", as BiduleOhm called it.

In reality, it's not just Ohm's law. The battery also behaves as a capacitor.
The chemical charging process is not exactly instantaneous, so when a high charge current is used, the battery accumulates a "superficial" charge. This superficial charge contributes to the rise in cell voltage, until it is absorbed by the slower chemical charging process and the battery voltage drops to its resting voltage.

https://res.mdpi.com/d_attachment/applsci/applsci-07-01002/article_deploy/applsci-07-01002.pdf

 

[Gazoo]

I understand what the voltage is for a fully charged lifepo4 battery. I have known this for years. I also understand I should be charging the batteries at 40 amps. However since I have not had any evidence of the batteries characteristics changing, and since I don't use them that much, I am not concerned about it. I am not going to go through the expense of purchasing a 40 amp charger. I would rather put that money towards my new build.

This build was more of an experiment than anything else. I love the batteries. But as I said nothing has changed since I have had them. The charge always cuts off at 14.62 volts and the current is 500ma's. Remember my meter could be off a tiny bit? The batteries behave the same way as when I first received them and tested them a year ago. Maybe due to the battery chemistry? Who knows?

I am too tired to read through the article you linked to...lol. But doesn't it refer solely to Li-Ion batteries? Does it apply to lifepo4 batteries. There is surely much out there to debate. If anyone on this forum has ended up with reduced battery capacity from charging at lower C charge rates then the manufacturers' recommendation's I am all ears. I would like to know.

 

[Gazoo]

Just as a side-note, if you look at the datasheet, they give 14.6 V as "fully charged voltage", but also a 20 A charge current for a 40 Ah battery.
That's a 0.5 C rate, which causes a significant rise in cell voltage during charge, due to "Ohm's law", as BiduleOhm called it.

In reality, it's not just Ohm's law. The battery also behaves as a capacitor.
The chemical charging process is not exactly instantaneous, so when a high charge current is used, the battery accumulates a "superficial" charge. This superficial charge contributes to the rise in cell voltage, until it is absorbed by the slower chemical charging process and the battery voltage drops to its resting voltage.

https://res.mdpi.com/d_attachment/applsci/applsci-07-01002/article_deploy/applsci-07-01002.pdf

I got to thinking about this and am wondering if you thought my batteries were developing a capacitance due to my mention of high self discharge. The batteries have a high self discharge partly due to the status lights on them constantly blinking, and the BMS staying in Active Mode. There are three modes for this battery, Park Mode, Shelf Mode and Active Mode. Mine stay in active mode which has a self discharge rate of less that 4 amps per week. I don't have the interface to change the settings of the battery.

4 amps self discharge for two batteries connected in parallel equals 8 amps. That is equal to 32 amps per month and is close to what I have noticed when testing them.

Back to the charge rate. Per the Valence manual they specifically state to use a charge rate of 20 amps or less. I could keep them on a float charge. Valence states leaving them on a float charge 13.8-14.6 volts will not damage the batteries in any way. I choose the charge them fully monthly.

Charging The Batteries; Select A Proper Battery Charger; Charging The Battery - Valence RT User Manual [Page 12]

Valence RT Manual Online: charging the batteries, Select A Proper Battery Charger, Charging The Battery. For Normal Charging, Use A Battery Charger That Has An Output Of 20A Or Less Until The Voltage Reaches Recommended Charge Voltage And Then Automatically Tapers The Current Down To...

www.manualslib.com

 

[MondeoMan]

am wondering if you thought my batteries were developing a capacitance due to my mention of high self discharge.

No, sir.

All cells, new and old, show this effect: a temporary rise in cell voltage during charging, that slowly drains off after disconnecting the charger, and then the terminal voltage of the battery drops to the resting voltage.

The rise in voltage is in direct relation to the charge current. Higher charge current => higher cell voltage during charging.

 

[Gazoo]

Oh...I get what you are saying now. I never looked at it that way. Thanks for clearing that up.

Yeah when I receive my prismatic cells I will surely treat them differently than I do my Valence batteries. When I am sure they are balanced and everything, I plan to not charge or discharge them fully.

 

[MondeoMan]

Let me put it another way...

I started with a new 200 Ah cell that I have drained until the resting voltage was stable at 2.75 volts for about two days.

Then I started charging with a constant 4 A for 2 hrs and then constant 40 A.
After 5 more hours, the cell voltage was 3.48 V, if I remember correctly. Sorry I did not write this down.
How much charge did the battery get?
4 A * 2 h + 40 A * 5 h = 208 Ah.
Now let's say charge efficiency is 96% => 200 Ah loaded into a 200 Ah nominal capacity cell, and the voltage is below the mythical 3.65 V

Discharged again to 2.75 V, until voltage was stable.
Then I started charging with a constant 4 A for 2 hrs and then constant 20 A.
After 10 more hours, the cell voltage was 3.40 V.
How much charge did the battery get?
4 A * 2 h + 20 A * 10 h = 208 Ah
Again, considering a typical 96% charge efficiency => 200 Ah loaded into a 200 Ah nominal capacity cell, and the voltage is even lower this time.

Now what do you think would happen if I set my charge voltage (Absorption) to 3.45 V, and the Bulk current to 20 A?

 

[Craig]

Let me put it another way...

I started with a new 200 Ah cell that I have drained until the resting voltage was stable at 2.75 volts for about two days.

Then I started charging with a constant 4 A for 2 hrs and then constant 40 A.
After 5 more hours, the cell voltage was 3.48 V, if I remember correctly. Sorry I did not write this down.
How much charge did the battery get?
4 A * 2 h + 40 A * 5 h = 208 Ah.
Now let's say charge efficiency is 96% => 200 Ah loaded into a 200 Ah nominal capacity cell, and the voltage is below the mythical 3.65 V

Discharged again to 2.75 V, until voltage was stable.
Then I started charging with a constant 4 A for 2 hrs and then constant 20 A.
After 10 more hours, the cell voltage was 3.40 V.
How much charge did the battery get?
4 A * 2 h + 20 A * 10 h = 208 Ah
Again, considering a typical 96% charge efficiency => 200 Ah loaded into a 200 Ah nominal capacity cell, and the voltage is even lower this time.

Now what do you think would happen if I set my charge voltage (Absorption) to 3.45 V, and the Bulk current to 20 A?

SO you are absolutely proof positive it took 40 Amps at the end of the 5 hours? What was your charge voltage? To get it to accept 40 amps.

 

[tim0shel]

Some good info in here. I have just got my Chargery up and going on a temporary setup. Going through the parameters I have gotten little tied up on how I should set the voltages. I am thinking to only charge to 3.4 and discharge to 2.57

 

[Craig]

Some good info in here. I have just got my Chargery up and going on a temporary setup. Going through the parameters I have gotten little tied up on how I should set the voltages. I am thinking to only charge to 3.4 and discharge to 2.57

I set mine cell voltage cutoff at 3.65 but set the SCC charge controller to limit to 27.2V (24v System) This way one or 2 cells can go a bit over 2.45 if necessary.

 

[Gazoo]

Let me put it another way...

I started with a new 200 Ah cell that I have drained until the resting voltage was stable at 2.75 volts for about two days.

Then I started charging with a constant 4 A for 2 hrs and then constant 40 A.
After 5 more hours, the cell voltage was 3.48 V, if I remember correctly. Sorry I did not write this down.
How much charge did the battery get?
4 A * 2 h + 40 A * 5 h = 208 Ah.
Now let's say charge efficiency is 96% => 200 Ah loaded into a 200 Ah nominal capacity cell, and the voltage is below the mythical 3.65 V

Discharged again to 2.75 V, until voltage was stable.
Then I started charging with a constant 4 A for 2 hrs and then constant 20 A.
After 10 more hours, the cell voltage was 3.40 V.
How much charge did the battery get?
4 A * 2 h + 20 A * 10 h = 208 Ah
Again, considering a typical 96% charge efficiency => 200 Ah loaded into a 200 Ah nominal capacity cell, and the voltage is even lower this time.

Now what do you think would happen if I set my charge voltage (Absorption) to 3.45 V, and the Bulk current to 20 A?

What was your charge voltage? I understand your examples. I do not understand the end of charge voltages. If your example of (4 A * 2 h + 20 A * 10 h = 208 Ah) revealed a cell voltage of only 3.4 volts then I suspect your charging voltage wasn't high enough. Sorry, that makes no sense to me.

 

[MondeoMan]

SO you are absolutely proof positive it took 40 Amps at the end of the 5 hours? What was your charge voltage? To get it to accept 40 amps.

The accumulated AH reading was confirmed by the Coulomb Counter (Victron BMV-712).

I am sure it took 40 Amps at the end of the 5 hours, it was still in Bulk charging and I was checking on it every hour.

Absorption was set to the mythical 3.65 * 8 = 29.2V, but I did not let it get there. Manually stopped charging a bit before 28V.

 

[Craig]

The accumulated AH reading was confirmed by the Coulomb Counter (Victron BMV-712).

I am sure it took 40 Amps at the end of the 5 hours, it was still in Bulk charging and I was checking on it every hour.

Absorption was set to the mythical 3.65 * 8 = 29.2V, but I did not let it get there. Manually stopped charging a bit before 28V.

So you do not really know then. It is also possible the cell's capacity is greater than 200Ah. I am still going to maintain that if you charge at the parameters set by the manufacturer you will not damage the cell. The catch is the manufacturer also says best use is between 10% and 90%.

 

[MondeoMan]

So you do not really know then.

Sorry I do not understand. I do not really know ... what?
I know the charging current and the duration that this current was maintained.
I also have a second confirmation of the accumulated charge (AH) via the Coulomb Counter.

You take a 200 Ah cell, resting at 2.75V, and you charge it 4A for 2 hrs followed by 20 A for 10 hrs. The cell voltage does not exceed 3.40 V during charging.

I say the cell is fully charged because it accepted a bit over 200 Ah (but we allow for some losses).

Can you give a better explanation?

It is also possible the cell's capacity is greater than 200Ah.

That's the thing about LiFePo4. It does not stop accepting charge when full to the nameplate capacity.
ANY LiFePo4 cell accepts more charge than the nameplate capacity. The problem is, if you frequently exceed the nameplate capacity, the cell will wear down faster.

The nameplate capacity is chosen by the manufacturer as a compromise between manufacturing cost and cycle endurance of the battery.
The exact same cell would work with "200 Ah" nameplate rating for 3000 cycles or "220 Ah" nameplate rating for 2000 cycles.

So if you want to benefit from the number of cycles from the datasheet, assuming the manufacturer is honest, then you better not exceed the nameplate capacity too frequently.

I am still going to maintain that if you charge at the parameters set by the manufacturer you will not damage the cell. The catch is the manufacturer also says best use is between 10% and 90%.

There is another catch: the Fully Charged voltage is typically given for a very high charge current (0.5 C-rating).

Based on what I've measured, if you charge with a lower current and the chemical reactions have enough time to absorb the charge "in real time", as it flows in, then the voltage will not rise as much.

 

[Craig]

@MondeoMan Whether we fully agree or not this is a very interesting concept. I just wish it was in its own thread. Do you think you could sum up your thoughts and hypothesis then put them in its own thread in the battery section.. Since this really has nothing to do with the Chargery and I bet a lot of people are missing out on something that could be very important.