BMS and mppt victron not showing same voltage

[marcello]

Hi,
My victron 75/15 shows 14.5v while my jbd bms shows 14v. I guess the bms shows the correct voltage.
It's a problem because I'd like to charge my lifepo4 battery with my solar panels until it reaches 14.5v but since the victron already shows 14.5 it goes on float mode and doesn't charge anymore (I set it up to charge to 14.5v max). In reality the battery is still at 14v according to the bms.

I increased the victron max voltage so it can continue charging, I put 15v and it reached them almost right away under the hot sun, But the bms showed 14.4 or something like this.

So do I have to increase the victron max voltage to something like 15.5-16v to get my bms to effectively show 14.5v?

That's strange they don't show the same value. I don't want to put too high values in the victron, 15v is already very high (but I don't know if it is accurate since the bms shows less).

My cells are well balanced.

Thanks for your help

PS: I know 13.6 is enough for a lifepo4, But I'd like 14.5v, I can live with the fact I'll get 2000 cycles instead of 5000, it's still several years of use.

 

[rhino]

Have you used a separate digital multimeter to see what it says? What is the size and length of cable from the Victron 75/15 to the battery? The BMS and charge controller should more or less match so you should not be artificially raising the values on the Victron to compensate since that will potentially cause confusion/problems in future.

 

[GSXR1000]

get a volt meter 1st.. how long is the wire between battery and victron

 

[740GLE]

If you pick up a Victron smart shunt you can create a “virtual network” the smart shunt will link via Bluetooth and send over the battery voltage to the SCC and it’ll adjust as needed.

That’s what I did for my 100/15 Smart SCC and my 460ah 12v system works pretty slick.

Also I have my SCC set for 14v with absorption for 2hr even the weak JBD balancer brings cells to .002v delta. I do have the BMS balance when not charging just when >3.4, as my battery has zero usage/demand for a week or so, it balances for 2hr a day.

 

[marcello]

Thanks for your help guys.
Distance between the battery and the mppt is about 30cm. Cable gauge is 10awg. I added an ANL fuse on the positive wire.
I will measure the voltage with a multimeter when I get back home and update, thanks

 

[#40Fan]

Just saw a video about this today. Turn on the BMS's discharge setting and see if that fixes the difference.

 

[mikefitz]

Your charge voltage is too high and the battery BMS is shutting down the charge path, that's why you see a voltage difference.
Don't increase the charge volts.
Use the Victron default setting, absorbtion 14.2, float at 13.5.
If your are have badly out of balance battery cells the charge volts may need to be even lower, down to 13.8.
Once the battery has been in use for some time at the lower voltage that does not cause shutdown, the charge voltage may be increased.
You say you would like a charge voltage of 14.5, why? It makes virtually no difference in practical terms.
When the BMS goes into protection mode, whilst under charge, there is a possibility of a voltage surge on the system. This can cause issues.

Mike

 

[Solar Guppy]

Thanks for your help guys.
Distance between the battery and the mppt is about 30cm. Cable gauge is 10awg. I added an ANL fuse on the positive wire.
I will measure the voltage with a multimeter when I get back home and update, thanks

10 AWG is much to small and that is your issue ( you are seeing voltage drop in the wiring ). Typically wiring would be more like 2 awg or larger, but to provide a more correct size would require amps being charged / discharged.

From top google hits is some more explanation on amps vs wire size

 

[mikefitz]

10 AWG is much to small

No it's not too small, it's the cable between the solar controller and battery, controller output is 15 amps maximum.
Maximum cable accepted by the Victron 75/15 is awg 10.

Mike

 

[Solar Guppy]

No it's not too small, it's the cable between the solar controller and battery, controller output is 15 amps maximum.
Maximum cable accepted by the Victron 75/15 is awg 10.

Mike

Yes, missed from a quick read that it was mppt charger to battery ( was thinking AIO to battery high current charging ), though only the connection between the two can cause voltage drop, so that need to be understood, or as was suggested use a DVM to verify the reported values.

 

[marcello]

Ok guys here are thé measures:
Multimeter 14.14v
Bms 14.2v
Victron 14.6v

The victron is faulty or ??

Mike I prefer to charge to 14.5 because at 13.6 it's like 95% and I'd like to use my battery to the max.
Thanks

Edit

Just say a video about this today. Turn on the BMS's discharge setting and see if that fixes the difference.

Oh man it worked, as soon as I turned the discharge on the victron showed 14.2! Cool !

Well that means I have to let the discharge on to get the right voltage so the victron can continue charging correctly. That's not cool, sometimes I'd like to turn off discharge. Well I'll live with it. Thanks again !

 

[#40Fan]

Oh man it worked, as soon as I turned the discharge on the victron showed 14.2! Cool !

Well that means I have to let the discharge on to get the right voltage so the victron can continue charging correctly. That's not cool, sometimes I'd like to turn off discharge. Well I'll live with it. Thanks again !

Good to hear. It is odd that you need to have the discharge on, but it worked for the JK BMS the guy in the video was using.

Too bad I didn't catch my spelling mistake before you quoted me. :D

 

[mikefitz]

I don't think I suggested a 13.6 charge voltage, I implied the Victron setting of 14.2 was more than adequate, however they are your cells so treat as you wish. Turning off the charge or discharge FETs will drop volts across the BMS. It's not wise to use the battery In this state as the temperature rise in the BMS fets may reduce service life.

 

[marcello]

I don't think I suggested a 13.6 charge voltage, I implied the Victron setting of 14.2 was more than adequate, however they are your cells so treat as you wish. Turning off the charge or discharge FETs will drop volts across the BMS. It's not wise to use the battery In this state as the temperature rise in the BMS fets may reduce service life.

Thanks for your reply.
Do you know if charging the battery at 14.5 let it stay longer at 100% than charging it at 14.2?
This 0.3v gap may w represents 1 hour at 100%? Don't know...
If it means nothing then probably 14.2 is enough.

Also when you say

Turning off the charge or discharge FETs will drop volts across the BMS. It's not wise to use the battery In this state as the temperature rise in the BMS fets may reduce service life.

I don't understand, do you mean I have to leave the discharge mode on all the time and don't touch it? Thanks

 

[lexio]

 

[marcello]

They recommend charging to 3.4v... ie 13.6v

From a comment under the video : Based on what I've read, LiFePo4 can withstand up to 4.2 volts before it starts to degrade. It's the charging to HIGH SOC that is harmful, not the voltage

....

Anyway 14.5v is fine by me, many people using this voltage to cut the charge without any problem. Honesty if this voltage allows to get a 10 years use instead of 15 is very fine. After 10 years there will probably be other technologies on the market, lifepo4 will be considered obsolete. I prefer having 100% capacity all the time in my van instead of 90%.

Plus we don't really know for sure, as the guy says :
@OffGridGarageAustraliail :
That's a long term study, I cannot really simulate. I would be interested in this too. How much cell life do you sacrifice by charging to a higher voltage?

.....

For lithium I think it's more important not to dischatge them too low, compared to charging them too high.

But I respect all choices off course.

 

[lexio]

From a comment under the video : Based on what I've read, LiFePo4 can withstand up to 4.2 volts before it starts to degrade. It's the charging to HIGH SOC that is harmful, not the voltage

Yes charging above 3.65v will degrade your battery never charge above that voltage. what you have read is old info.
as you can see in the video if you charge to 3.5v a cell your battery is full even without float charge.

 

[marcello]

Yes charging above 3.65v will degrade your battery never charge above that voltage. what you have read is old info.
as you can see in the video if you charge to 3.5v a cell your battery is full even without float charge.

Thanks, yes i will lower the voltage to 14 instead of 14.5 which seems useless stress

 

[740GLE]

All depends at how long it stays at say 14v or 14.4v

If there is constant load on the battery, 14.4 won’t hurt anything as it’ll quickly bleed off soon as the sun goes down, if the battery is in “storage” charging, no real load, 14v is more than adequate to bring the battery up to 100%.

 

[marcello]

All depends at how long it stays at say 14v or 14.4v

If there is constant load on the battery, 14.4 won’t hurt anything as it’ll quickly bleed off soon as the sun goes down, if the battery is in “storage” charging, no real load, 14v is more than adequate to bring the battery up to 100%.

Well I have a little fridge running continuously in the van but it doesn't need too much energy, maybe something like 2 or 3Ah (vevor 25L). So i don't know if this fits the criteria of "constant load on the battery".

Also, a question I've never seen replied to is, since both 14v and 14.4v mean 100% charge, will this 100% state stay longer if the battery is at 14.4v instead of 14v, when it's under load?

 

[Tomthumb62]

Also, a question I've never seen replied to is, since both 14v and 14.4v mean 100% charge, will this 100% state stay longer if the battery is at 14.4v instead of 14v, when it's under load?

Not really. I forget the details, but at 14.0v, the battery is something like 98% full. It can take a long time to get that last 2% in, which isn’t really a great use of your solar production. If you had even a slightly bigger battery, you’d get more storage out of that solar production than the last 2%.

Also, once it hits whatever high point you’ve set it at, say 14.4v, and you remove the charging source, it’ll drop back down to about 13.4v fairly quickly. It’s STILL 100% full at 13.4v, it’s just now at its resting voltage. This assumes no or little loads.

 

[marcello]

Not really. I forget the details, but at 14.0v, the battery is something like 98% full. It can take a long time to get that last 2% in, which isn’t really a great use of your solar production. If you had even a slightly bigger battery, you’d get more storage out of that solar production than the last 2%.

Also, once it hits whatever high point you’ve set it at, say 14.4v, and you remove the charging source, it’ll drop back down to about 13.4v fairly quickly. It’s STILL 100% full at 13.4v, it’s just now at its resting voltage. This assumes no or little loads.

Thanks, I thought 14v was already 100%. In fact even 13.8 is 100% (I thought)
What voltage should I charge it to, to get 100%?

 

[740GLE]

I don’t care about getting every whr out of my battery, so I value low stress on the cells vs max storage.

I have it set for 14v and absobtion time of 2hr, this allows the cells to balance. My smart shunt says it gets to 100% because that’s who I programmed it to recognize a full battery. If I’m leaving a few AH on the table I won’t lose any sleep.

 

[marcello]

I have it set for 14v and absobtion time of 2hr, this allows the cells to balance.

Does it mean you have to not use the battery during 2 hours to let it balance ?

 

[Tomthumb62]

Thanks, I thought 14v was already 100%. In fact even 13.8 is 100% (I thought)
What voltage should I charge it to, to get 100%?

I know, it’s confusing.

Resting voltage is different than charging voltage.

A lifepo4 battery, resting for at least a few hours at 13.1V, with no loads or charging, is at approximately 40% SOC. In order for the battery to reach 100.000% complete charge, it needs to reach 14.6v and remain there for a certain amount of time. Once it goes into float charge or charging is removed, and all loads are off, it will settle out at approximately 13.3-13.4v. IT’S STILL FULLY CHARGED AT THIS POINT.

And yes, 13.8v is basically considered fully charged, but I think it’s something like 98-99%. If you look at two glasses of water and one is 98% full and the other is 100%, can you tell the difference without quality scientific measuring tools? I mean, can you tell by just looking at them? No. And in the real world, it just doesn’t matter. If a few watt hours are going to be life or death for you, then just make bigger battery bank. Add on another 100Ah and call it a day.

Also, the absorption time needed after the battery has reached its set full point (bulk or boost set to 13.8-14.6v) can sometimes be difficult to achieve on solar. To my limited knowledge, absorption needs to be about 15-90 minutes and the voltage is held at the set point as the amperage slowly and consistently reduces. If you have any passing clouds or what not affecting solar production, that will upset your absorption phase. How much this matters I don’t know.

What I do know is while lifepo4 C 12v CAN be charged to 14.6v, many on this forum don’t bother for a number of reasons. One being is the theory that charging to the top most voltage is stressful to the cells. Think about it. You have a suitcase that can hold 14.6 liters of clothing. You pack 14.0 liters and dang the thing is pretty full, it’s getting hard to pack any more in there. You then spend another hour packing in the last 0.6 liter and then when you try to zip up the suitcase (this is now the absorption phase), the thing is bulging and you might be a bit concerned about the stress being put on to the zipper.

I have my solar charge controller set for bulk (also known as boost on some devices) to 14.0v. I don’t need the extra few bits of juice and it’s theoretically a bit less stressful on the cells.

My shore charger I only use a couple times a week or less and I set it a bit stronger to 14.25v. An occasionally fuller charge is less stressful than daily. In theory.