Drop-in Internal Balancing

[Jman]

Have a few questions about these.

I have read a few times on this forum that you have to be careful not to exceed the shunt balances very small current handling usually around 1A max.
So how do manufacturers get away with allowing all kinds of charge currents at those high voltages of 14.4-14.6V? Mine allows up to 0.4C max constant charging rate. And when those voltages are reached the battery looks to be still taking in alot of current, alot more than 1A.
So if you hold the absorb voltage at these higher voltages for a while can it damage the balancers?


Finally I keep reading balancers kick in at 14.3-14.6V range, and that they are prone to killing the battery. If one stays low at just 13.8V then does that mean the balances can't kill the battery? I think not as these things do more than balance, like constantly monitor cell voltage. Agree or not?

thanks

[Mike90250]

The Balancers with simple bleeder shunts are mostly designed to interact with the charger, when the 1st shunt activates, it also commands the charger to reduce amps. Most solar chargers don't have this ability and then batteries blow up or the BMS goes ape. That's why many factors have to be designed/engineered for a system to work well for years. Throwing a mishmash of parts together seldom works right, but so many folks nowadays never got a chance for hands-on learning and are finding out the hard way, it's not all book learning or test taking skills.

The other way balancers kill batteries, is the bleeder shunt fails, and discharges a cell. That's poor BMS design.

The more complex a system becomes, the more failure points are in it.

[Ampster]

Have a few questions about these.

I have read a few times on this forum that you have to be careful not to exceed the shunt balances very small current handling usually around 1A max.
So how do manufacturers get away with allowing all kinds of charge currents at those high voltages of 14.4-14.6V? Mine allows up to 0.4C max constant charging rate. And when those voltages are reached the battery looks to be still taking in alot of current, alot more than 1A.
So if you hold the absorb voltage at these higher voltages for a while can it damage the balancers?

The shunts are not wired in series like the batteries. When they are on they reduce the current for the cell whose voltage is above the set point. Because when the shunt is on, it is actually in parallel with the cell. Therefore it will only bleed off the current that it was designed for. Review Ohms law for a more detailed explanation.

Finally I keep reading balancers kick in at 14.3-14.6V range, and that they are prone to killing the battery. If one stays low at just 13.8V then does that mean the balances can't kill the battery? I think not as these things do more than balance, like constantly monitor cell voltage. Agree or not?

A good Battery Managent System is not cheap. The ones I have used to balance lithium packs cost over $500. All those are programmable and the solid state relay circuits are designed to fail in the open position. That is not the case with less expensive BMSs. The BMSs that I use are programmable and I set the balancing voltage according to the chemistry that I am using.

Some people even suggest you don't need a BMS for a system as simple as 4 cells in series. In either event it is recommended that you initially balance the cells by placing them in parallel for a few days before assembling them into a pack. That way your pack should start out balanced. One of the benefits of using a BMS is the monitoring so I agree that that their usefulness is more than balancing. Remember you can be the BMS and all you need is a good multimeter to measure the voltage of each cell..

[Mike90250]

...... Remember you can be the BMS and all you need is a good multimeter to measure the voltage of each cell..

But you have to be reliable, and watch for subtle trends before it becomes a problem.

[Ampster]

But you have to be reliable, and watch for subtle trends before it becomes a problem.

Not a problem for Sunking who advocates for no BMS.

[PNjunction]

Have a few questions about these.
.
.
So how do manufacturers get away with allowing all kinds of charge currents at those high voltages of 14.4-14.6V? Mine allows up to 0.4C max constant charging rate. And when those voltages are reached the battery looks to be still taking in alot of current, alot more than 1A.
So if you hold the absorb voltage at these higher voltages for a while can it damage the balancers?
.
.
Finally I keep reading balancers kick in at 14.3-14.6V range, and that they are prone to killing the battery. If one stays low at just 13.8V then does that mean the balances can't kill the battery? I think not as these things do more than balance, like constantly monitor cell voltage. Agree or not?

The huge charge current, even while in CV stage (aka absorb) is one of the benefits of LFP as compared to lead-acid. The extremely flat charge discharge curve means that if you set your CV voltage high, the amount of time you spend in CV / absorb is very small / quick.

Conversely, you can fully charge and even OVER charge an LFP at merely 13.8v as your CV (absorb) voltage. It just takes waaaay longer to finish the charge to full. But it *will* get there, usually indicated by a 0.005C current rate being reached, and going no lower.

Unfortunately for most consumers all they understand is voltage as an indicator of battery safety or health. It's not your fault. If you study up on how CC/CV works, be it with voltages for lead or lithium, the underlying principle is the same.

Shunt-balancing resistors CAN burn up when tasked to "balance" or produce waste-heat for long periods of time. Balancing should be a quick process. If not, not only are the low cells taking forever wasting heat in the dump resistors, but ALSO holding already-charged cells at or near full charge for longer periods of time than necessary. This is often overlooked by the casual user.

[PNjunction]

Not a problem for Sunking who advocates for no BMS.

I do too, but only if one has the proper tools for monitoring. So do the advanced LFP wheelchair users who don't want to get stuck out in the boonies. They however only use balancing during the charge process, NOT discharge. Thus a failing bms won't catch them 20 miles away from home. These guys are smart however, not the plug-n-play types.

[Jman]

There are dozens of brands, they are used for camping, caravans, in cruiser boats now. There is no communication at all between BMS and charger.
So how do they get away with being charged by any battery charger as long as the voltage & charge rate isn't too high?
My guess is the shunt balancers are higher amperage like 10-20A, why not? Or some other trickery is going on inside?

[Mike90250]

There are dozens of brands, they are used for camping, caravans, in cruiser boats now. There is no communication at all between BMS and charger.
So how do they get away with being charged by any battery charger as long as the voltage & charge rate isn't too high?
My guess is the shunt balancers are higher amperage like 10-20A, why not? Or some other trickery is going on inside?

The trickery is in the promotion. They build them as cheap as possible, sometimes a one-shot fuse, and then the pack is dead. If the $200 BMS only uses 3A shunts, you can bet the cheap ones don't exceed that.

[Jman]

Ignore my previous post, I didn't see all of Ampster's 1st post for some reason. Mystery solved with the dropins.

However PNJunction you say at lower voltages like 13.8V takes way longer to 100% compared to higher voltages. My testing shows my dropin will charge pretty fast to full with just 13.8V. Fast enough I can't complain going from AGM.
At 0.3C charge rate, 13.8V is only about 20-30mins slower to 100% (well at least 99) than with 14.6V. Is that what you mean by way slower?

[PNjunction]

By slower, I mean that if held to a CV voltage of 13.8v, achieving an end-current of 0.005C will take much longer than if you charged to 14.4v, and then waited for the current to drop to 0.005C (the natural taper).

Because one doesn't want to totally charge to 100% full with lfp all the time, this longer amount of time is convenient for many to just pull the plug when they look over at the battery every once in awhile. At the higher voltage, the time to reach 0.005C end-current is VERY fast indeed.

But, this is with a stable power supply. If you put solar into the mix, with little available charge from deteriorating conditions, setting your CV high and getting in as much as you can before the sun goes down, even though you never reach the 0.005C end-current could be a better strategy than being totally conservative at 13.8V CV on a cloudy day!

In other words, when I had my LFP's going solar, I knew there wasn't enough time to finish the job anyway. If I had it set low to 13.8V, it wasn't enough for my time period.

However, on a stable supply from AC - sure, no problem. Had the luxury of time to be at that voltage and just pull the plug. So one always has to take the *application* and *local conditions* into account. The strategy in Alaska with LFP might be well different than someone on the equator.

Here's a hint too about the simple "bleeder" resistors being so small going from ideal to worst case in 3 steps:

*IF* the cells are perfectly balanced to begin with, then all the cells will hit 3.5v (lets say that is your individual cell setting before the bleeders activate), and there will be NO bleeder resistance to deal with since the cells will all be in absorb tracking each other perfectly. In the lab. Perfect world. The bleeders don't even activate.

What if they are just a *little* bit unbalanced? The theory goes that ok, one cell tries to go high, and the dinky bleeder resistor activates for a little while. Why doesn't it burn up? Because the cells are theoretically well into the absorb stage (taper if you will), and could be only dissapating very little current or so for 30 seconds on and off here and there. Very little current = little heat.

The reality is that a lot of guys just attached a bms to badly unbalanced cells, and those bleeders remained active for HOURS or DAYS. Now you've cells being held at near 100% charge longer than they need be, and bleeder resistors running at max capacity for hours or days due to the severe imbalance and really prone to failure. That's why a manual *individual* charge was always recommended before doing this to at least get them in the ballpark with each other.

And of course, if one bleeder activates during discharge - bad news. Different application, but it is why the LFP wheelchair guys ONLY use balancing during charge, and remove all that during discharge around town.

[Jman]

Thanks PNjunction

I had thought about lower voltages being too slow for solar, but in my case I'm using large panel wattage for a small battery & so it shouldn't be that much of an issue. But on a boat or small offgrid it would make sense to me to stay at 14.2-14.3V as a good all round number.

[Jman]

I have some more questions if you guys could answer please.

How long does it take roughly for the bleeder boards to re-balance something like 200-300mv difference?

Reading what Sunking writes that top balance only balances voltages & not capacity (like bottom) does Top balance do anything for fractional C use?

I don't see how the dinky bleeder capacity is a problem, like your charging at 10A, and you have just 1A bleed off, still leaving 9A into the cell. Wouldn't it be for just a very short time and doing this only once in a while?
Even so, why not set charger's rate low & then run a load so only a few amp is flowing into the battery?

[Mike90250]

..................I don't see how the dinky bleeder capacity is a problem, like your charging at 10A, and you have just 1A bleed off, still leaving 9A into the cell. Wouldn't it be for just a very short time and doing this only once in a while?
Even so, why not set charger's rate low & then run a load so only a few amp is flowing into the battery?

Dinky bleeder resistors are only good for very slow charge rates and well matched batteries, and then only for a couple minutes before THAT cell is overcharged and the rest of the pack still low.

Are you going to be home all day and watch for when you should run a load ?

[Sojourner1]

Jman which dropin battery are you using? Does this dropin allow you to see what is happening (SOC, VPC, Temp per cell) by phone app? You realize that the dropins require you to periodically charge to 14.4v +/- (depending on manufacturer) for the cells to automatically balance.