14.2, 14.4, 14.6 - seems like there is no agreement on charging voltage

[sunshine_eggo]

Personally, I think SOK should eat every battery they ship out that's imbalanced out of the gates. This implies to me that they don't make any effort to match or balance as a builder, and this may be an indication of future service problems.

Cycling typically doesn't do much except during the charge period. If you could float this battery at 13.9V, you should see the V Differ slowly decrease to where you could incrementally increase the "float" voltage.

 

[BradCagle]

On testing these in the Lithium profile, I see that the Minn Kota has a charge voltage for the Lithium profile of 14.2volts

14.2v Is perfectly fine.

14.6v you'll rarely ever reach before the BMS cuts out.

 

[Pierre]

I built a 16s battery with prismatic cell from China. I already had to swop out 4 cells and still have one that peaks before the rest , tripping the BMS. I have run out of cells so I just set charging to stop when the cells delta is around 80mV , caused by this one cell. This cell is then at 3580mV and all the others at 3500mV ( within 3mV of each other ) . This was my work around and I still get almost full capacity out of the pack.

 

[archjeb]

So what balance voltage/delta should I say its 'good enough at'? 50mV? 20mV? Or am I expecting too much here?

 

[sunshine_eggo]

So what balance voltage/delta should I say its 'good enough at'? 50mV? 20mV? Or am I expecting too much here?

If the battery can't be charged to the 14.4-14.6V spec without triggering BMS cut-off, send it back. It doesn't meet spec.

 

[archjeb]

If the battery can't be charged to the 14.4-14.6V spec without triggering BMS cut-off, send it back. It doesn't meet spec.

I guess I'll see what I can work out with SOK.

 

[Jennifer]

2 great pages that discuss this topic:

Page discussing drop in's like your SOK, Battleborns etc

Drop-In LiFePo4- Be an Educated Consumer

Lead is Dead (almost) Warning: This article is long and detailed. It may require two reads so the nuances and detail are not missed. You may be wondering why I am saying lead is dead ?The answer to that is simple; lead acid battery makers dug their own graves

marinehowto.com

This one's a LONG one but down the page goes into this topic in great detail with stats and evidence re the only need to go above 14 is for the internal cell balancing... a great page:

DIY LiFePO4 Batteries On Boats - Marine How To

DIY LiFePo4 On Boats Last edit;3/26/23:The base of this article was written a number of years ago (2010) but this does not mean the information here is outdated. We have been keeping it updated and have added to it when ever we had the time. This article deals

marinehowto.com

 

[BradCagle]

If the battery can't be charged to the 14.4-14.6V spec without triggering BMS cut-off, send it back. It doesn't meet spec.

Most of these Chinese batteries spec it as 14.4 +-0.2
So 14.2v is probably the best they guarantee

 

[sunshine_eggo]

Most of these Chinese batteries spec it as 14.4 +-0.2
So 14.2v is probably the best they guarantee

Then they need to change their datasheet:

SK12V100 Spec Sheet.pdf

drive.google.com

 

[time2roll]

The whole charge to max voltage is a hangover from the lead-acid days. LFP cannot be undercharged and cause harm like lead acid. No need to run max to get virtually full capacity. Time to lay off the lead-acid mentality and enjoy the new world of LFP. Charge at 13.8-14.2 is fine.

 

[hwse]

The simple truth is that you cannot charge to any voltage that is higher than what you have when the highest cell goes into OVP. I recently received a 460Ah sealed battery pack and out of the box, it had a 300mv imbalance and would hit 3.65v on cell #3 and cell #1 was only 3.35v. This happened when the pack voltage was 13.9v. Any attempt to charge above 13.9v would result in a OVP shutdown. Given the very large pack capacity of 460Ah and the tiny 100mA passive balancing, this imbalance will take a long to correct. I am now able to charge at 14.26v with the lowest cell now getting up to 3.465 so I can safely charge to 14.2v. Hopefully later this week I will get it completely top balances to that I can set by absorption voltage to 14.4v and get full capacity out of the pack.

 

[BradCagle]

Then they need to change their datasheet:

SK12V100 Spec Sheet.pdf

drive.google.com

View attachment 130738

Yeah, that's pretty bold.

 

[Pierre]

So what balance voltage/delta should I say its 'good enough at'? 50mV? 20mV? Or am I expecting too much here?

One always tries for the lowest but I would suggest that at the knees not more than 150mV but on the flat part of the curve about 20 - 50mV , lower being a bonus. I have an active 5A balancer connected permanently , charge or discharge. Works for me , others may differ.

 

[MrM1]

What do you like for Float, 3.38v?

 

[Pierre]

Spot on. I float at 3,40v .

 

[MarcoWasRight]

Max charging voltage is 4.2V per cell.


I’m sure someone will get all bent out of shape about that one sentence. It is the true max safe charging voltage just like most lithium ion chemistries but common sense will tell you that past 3.35V you start to see the voltage climb rapidly and if your charging current is a fixed value then 2+2= there’s not much capacity up there. You decide for yourself how high you want to go. Most of the research and real world experience has shown us that past 3.50V you’re seeing maybe the last one or two percent of capacity. This also depends on how your charger cuts the current off. Most people follow the 0.05C rule.

Also, keep in mind I don’t really use BMS’s unless it’s a pack I know I’m not going to monitor so I’m free to charge up to 3.65 on a balance charger if I want to so long as my charge current is low enough for the balance current to stop any runners. I couldn’t fathom having an off the shelf battery with no access to the balance leads with how slow most of these BMS’s balance and worst of all the non Bluetooth BMS’s that have balance start set to 3.60V

Do whatever makes you happy. Maybe take notice that these BMS’s aren’t making you very happy.

 

[Browneye]

Rookie question here, but should I have stuck with the 2Amp charger? Or is 4Amps low enough to see if the BMS could actually balance that cell?

Or should I take the battery cover off and try to drain that single cell to try to force it some into balance?
On full discharge, its actually lower then the first 2 cells...so it makes me think that maybe there is something wrong with that cell. But since this is my first LiFePo4 rodeo, I have nothing to compare this to other than the 2 other SOK batteries that are all part of the same shipment.

IMHO a 2A or 4A charge won’t make much difference- whatever charge voltage you set, they will taper current as the battery voltage reaches that setting. Some chargers have a duration, some sense tail current - a level that is decidedly full charge and terminate.

Bleeding one cell at high knee is a quick way to bring it down to the level of the other cells. I have done this several times on my diy battery, and is very well balanced up to about 3.55Vpc.

You may need to repeat the process again at an interval. A severely degraded cell that doesn’t match the others is a warranty claim on a bought battery.

I use a 12V auto headlight bulb to bleed individual cells - done near full charge while charging to assist bms balancing. Only takes a minute or two to drop a cell down and let others catch up before the whole pack reaches the full voltage charge setting.
I don’t bounce off the bms over volt settings.

 

[BradCagle]

Also, keep in mind I don’t really use BMS’s unless it’s a pack I know I’m not going to monitor so I’m free to charge up to 3.65 on a balance charger if I want to so long as my charge current is low enough for the balance current to stop any runners. I couldn’t fathom having an off the shelf battery with no access to the balance leads with how slow most of these BMS’s balance and worst of all the non Bluetooth BMS’s that have balance start set to 3.60V

Do whatever makes you happy. Maybe take notice that these BMS’s aren’t making you very happy.

I do the same with a few of my e-bike packs.

But I 100% agree on the balance lead issue. These battery manufactures should provide port where you can access the balance leads. They can call it diag port, and sell a little device which is basically just an active balancer.

 

[John Frum]

IMHO a 2A or 4A charge won’t make much difference- whatever charge voltage you set, they will taper current as the battery voltage reaches that setting.

Some BMSs don't balance unless the BMS is registering >= ~3 amps charge current.

 

[Browneye]

I couldn’t say for SOK.
My JBD balances at milliamperes. And not very darn much, half an amp or something. Balancing isn’t very impressive, or I’m just impatient. Lol