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senormechanico · 18-08-2019, 10:30

Some of us have long felt like our comments were a voice in the darkness.
It's good to see confirmation of those comments from knowledgeable, experienced professional sources.

sparrowhawk1 · 18-08-2019, 15:38

Here's a YouTuber with real experience explaining why he prefers bottom balancing. https://youtu.be/iN4ok93hhWY

sparrowhawk1 · 18-08-2019, 16:50

He also explains that if you have good cells and you stay away from the ends they shouldn't go out of balance. Which seems to be what people with experience are saying. Also the cheaper bms's should not be trusted and the expensive ones don't really make much sense with lower cost systems.

**Dockhead** · 19-08-2019, 01:25

Quote:

Originally Posted by sparrowhawk1

He also explains that if you have good cells and you stay away from the ends they shouldn't go out of balance. Which seems to be what people with experience are saying. Also the cheaper bms's should not be trusted and the expensive ones don't really make much sense with lower cost systems.

"Staying away from the ends" is from what to what, in your view? 20% to 90%?

john61ct · 19-08-2019, 03:22

Not to do with any absolute SoC % number, nearly as much as the rate of change in voltage as you get toward the extremes

Where the voltage "shoulders" start to steepen will vary greatly depending on the current C-rate.

So charging at a very high rate allows you to go to a much higher voltage, while still well below 100%.

Charging at a low enough voltage, can harmfully overcharge even below 3.45Vpc if held long enough.

Conversely, discharging at a high C-rate you can safely go to a much lower voltage, because you are still at a higher SoC.

At very low currents, you can go harmfully low and still be well above the mfg low V rating.

That said, say rates are between 0.01C and 0.3C, stopping charging at 3.45Vpc, maybe holding for 10-20min, will get you well above 95% compared to say 3.6V held for the same period.

Not as measured by a coulomb counter, but actual CC load testing for useful energy actually stored.

At the other end, same amps rate range, stopping discharge at 3.0Vpc will not be leaving much over 10% usable energy compared to going lower - which of course hurts cycle lifetime a lot and could be disastrous, so should not even be occasionally tested anyway.

When people use a coulomb counter to estimate these SoC levels, they often conclude longevity cycling "between 20-80%" is required, a 40% sacrifice of capacity, but that is grossly overblown.Attachment 198180 Attachment 198181 Attachment 198182

senormechanico · 19-08-2019, 11:29

We all knew he wouldn't answer...

**newhaul** · 19-08-2019, 11:30

Quote:

Originally Posted by senormechanico

We all knew he wouldn't answer...

can't describe what you don't have .

sparrowhawk1 · 19-08-2019, 12:43

Quote:

Originally Posted by Dockhead

"Staying away from the ends" is from what to what, in your view? 20% to 90%?

I was just summarizing the video. For exact details watch, I don't want to misquote.
I'm just passing on info from someone with experience.

andyalford · 20-08-2019, 20:58

Quote:

Originally Posted by sparrowhawk1

Here's a YouTuber with real experience explaining why he prefers bottom balancing. https://youtu.be/iN4ok93hhWY

I have watched lots of Will Prowse's videos, and he is responsible for me buying Lithium batteries. So I tip my hat to the guy.

But he spent many years of his youth racing RC vehicles and drones, screwing every last Ah out of the battery to get the range. And that is why he is very comfortable with bottom balancing.

He has tested a lot of his batteries to destruction, so has a lot of experience. Personally I plan to only buy one set of LFP in the next 10 years+ so I have decided to go with Top Balancing.

andyalford · 20-08-2019, 21:37

Quote:

Originally Posted by Dockhead

"Staying away from the ends" is from what to what, in your view? 20% to 90%?

A slight segway to my question about emergency bilge pumps...

My plan is to have the ML-RBS 7700 contactor disconnecting my Loads Bus at 2.90V representing 12.5% SOC

(In my initial design, I am considering 100% SOC as 3.60V and 0% SOC as 2.80V - the default values for the REC Active BMS for Victron CAN.)

And I plan to connect my 2x Emergency Bilge Pumps and the BMV912 Battery Monitor to the Battery side of the Contactor. (Diaphragm 5A pump at the bottom, then Centrifugal 13A above that.)

My thinking is that I want to save the boat, even at the cost of the batteries. So if I am not on the boat, I want the Emergency bilge pump to continue until the last possible second.

I haven't read anyone else doing this? Am I making a mistake? (again!?)

CatNewBee · 21-08-2019, 02:59

Quote:

Originally Posted by yachtshah

A slight segway to my question about emergency bilge pumps...

My plan is to have the ML-RBS 7700 contactor disconnecting my Loads Bus at 2.90V representing 12.5% SOC

(In my initial design, I am considering 100% SOC as 3.60V and 0% SOC as 2.80V - the default values for the REC Active BMS for Victron CAN.)

And I plan to connect my 2x Emergency Bilge Pumps and the BMV912 Battery Monitor to the Battery side of the Contactor. (Diaphragm 5A pump at the bottom, then Centrifugal 13A above that.)

My thinking is that I want to save the boat, even at the cost of the batteries. So if I am not on the boat, I want the Emergency bilge pump to continue until the last possible second.

I haven't read anyone else doing this? Am I making a mistake? (again!?)

We are doing it, we leave the busses on, including solar, but set to lower voltages, bilge pumps can run indefinitely until they break.

**newhaul** · 21-08-2019, 15:43

Something interesting is happening with one of my 4s4p 100ah banks that I have not brought online yet. It was placed into storage after top balance 6 months ago at 13.030v
Just checked the voltage after 6 months not used but bms hooked up. Now reading 13.025v
Is that expected or is that to little of a drop for 6 months ?

john61ct · 21-08-2019, 16:32

Can't be too thin nor too rich.

Also no such thing as too little self-discharge 8-)

Same instrument measuring?

They drift a bit too. . .

senormechanico · 21-08-2019, 16:42

Newhaul,

What is the resolution of your meter? Does it change in hundredths of a volt?
The meter may have drift with temperature as well.
I wouldn't worry about it.