Lead Acid Cell Balancing

[Mosaic]

Hi all:
I am thinking about creating a gadget that mounts lead probes through wet cell cell caps to achieve:
1) Cell level balancing/ equalization (perhaps switched capacitor)
2) Early warning of a weak cell (not taking EQ due to shedding or corrosion etc.)
3) Is suitable for general battery apps, such as Solar, golf cart, automotive or so on.
4) Does not come with the battery but with the application requiring the power, eg. Golf Cart, Gen Set, Marine etc.
5) Sleeps when cells are balanced to not waste power.
6) Can offer an RS232 link for a data log retrieval, tacking such things as cell voltages for SOC.
7) Can analyze single cell internal resistance and total for battery internal resistance.
8) Can be left on the battery during normal battery use.

I wanted to listen to your thoughts on this idea and its possible implementation.

 

[4pyros]

How do think a simple probe in the cap of a lead acid battery can be used to control the charge state of individual cells?

 

[NorthGuy]

Quite dangereous. If someone shorts positive and negative plates while instaling the tool, this may not end up well.

 

[Mosaic]

The bottom of the probes would have to be plastic capped and of the proper length.. The probes operate thru electrolytic contact only.

Since you can measure cell voltage via dipping into the electolyte, I don't see why charge currents can't also be delivered. After all that's what the electrolyte does...move ions.

 

[NorthGuy]

Lead Sulphate on the plates is part of the charging reaction. Won't work without it.

 

[4pyros]

I don't see why charge currents can't also be delivered.

With small probes in a big cell? I dont think so.

 

[ronv]

https://www.electro-tech-online.com/threads/hazardous-thing-happened-at-work.142910/unread

 

[Mosaic]

Lead Sulphate on the plates is part of the charging reaction. Won't work without it.

You mean the probes themselves?

 

[NorthGuy]

You mean the probes themselves?

I mean that to do any sort of charging (discharging too) you have to connect to the plates.

 

[Mosaic]

Yes, I suppose that is perhaps a requirement...I'll have to do a test.
Since it's possible to measure cell voltage using a pair of probes dipped into adjacent cell electrolyte only, I was wondering if appropriate probes (lead or carbon) could actually deliver charge to the plates via the electrolyte.


EDIT:
Ok, the test revealed that you CAN draw power from the electrolyte as well as directly from the terminals, with an efficiency loss due to reduced surface area of the probe (copper wire 18AWG, 1 " bare). I lit a 5A bulb and it suffered about a 10% voltage drop compared to the actual battery terminal voltage.
This means that moving on to use thick carbon brushes which are reasonably porous should improve the connection. That's the next test.

 

[NorthGuy]

If that was possible, the plates would self-discharge through the acid because both negative and positive plates are submerged.

I guess you have drawn the power from the regular cells connected in between the cells where you placed test electrodes.

 

[Mosaic]

That would be reasonable. But it does indicate that power can be moved around cell 2 cell for equalization via capacitive switching and electrolyte transportation/conduction

 

[4pyros]

That would be reasonable. But it does indicate that power can be moved around cell 2 cell for equalization via capacitive switching and electrolyte transportation/conduction

I still don't think this is possible, you need two electrical points per cell.
How would you connect to the two cells on the ends?

 

[jpanhalt]

That would be reasonable. But it does indicate that power can be moved around cell 2 cell for equalization via capacitive switching and electrolyte transportation/conduction

1) It's DC. What do you mean by "capacitive switching?"
2) What do you mean by "electrolyte conduction [and/or] conduction?" NB: The electrolyte in a battery may be conductive, but you need a complete circuit for it to conduct electrons. Can you draw that circuit for a battery that is disconnected? What is your understanding of the mechanism for self-discharge?

John

 

[Mosaic]

jpanhalt: The method for intercell charge balancing involves low ESR capacitive charging (from cells with higher voltage) and discharging (into cells with lower voltage) via a semiconductor switching mechanism until the cells are 'equalized'
We are not discussing self discharge here. It is an attempt to create a device/method that can achieve inter-cell equalization where there are no preexisting accessible cell terminals.

 

[Mosaic]

How would you connect to the two cells on the ends?

To access & isolate cell #1, use the +ve Terminal on the battery and a probe into the electrolyte of Cell #2.

Cell # 6 would use an electrolyte probe from Cell #5 and the -ve terminal.

 

[4pyros]

To access & isolate cell #1, use the +ve Terminal on the battery and a probe into the electrolyte of Cell #2.

Cell # 6 would use an electrolyte probe from Cell #5 and the -ve terminal.

OK then you will need more than just "a gadget that mounts lead probes through wet cell cell caps".

 

[Mosaic]

OK then you will need more than just "a gadget that mounts lead probes through wet cell cell caps".

That would be correct...but such is the nature of idea developement.

 

[4pyros]

Ok so how would you take some charge off cell #2 and put it in cell #3 if when they are in series?

 

[Mosaic]

Well the switching arrangement would have to permit trying the higher voltage Cell#2 to a capacitor of some decent size with low ESR. When charged based on say a 4x time constant or evaluated by by a comparator, it is switched to 'discharge' into the lower voltage cell #3..
The purpose of the capacitor is to provide the isolated charge source, as we cannot short adjacent cells to balance charges. It may be possible to short non adjacent cells though.
Cell # 2 will be accessed/isolated via carbon probes into cells #1 & 3, cell #3 is accessed/isolated via the probes in cells #2 & #4.

I suspect there may be optimizations where the capacitor is charged by the whole battery and then discharged into a single undercharged cell to bring it up as a faster means of shifting energy around. That might simplify the switching arrangement.