Flooded NiCad Battery bank Balance circuit (1.2v 32Ah x 20 cells)

[Theunis Prinsloo]

Hi Guys - your help here is highly appreciated !

I have 20 x NiCad (flooded cells) of 1,2v 32Ah each, wired in series into a 24v bank. Float voltages per cell is 1.40 - 1.42 V/cell.

(I actually have 80 cells connected in 4 x 20 cells each iaw for a total of 128 Ah)

Have looked at off the shelf BMS's but is above my pay grade - (and really only need balancing)

A simple but effective circuit that I can build to balance each cell would be appreciated.

I had a look at the attached circuit found it on YT - something similar should be able to do the job.

I am aware that the cell voltage of 1.4v is very low for most components to function on -  will mean that 24v from the bank will be needed to power the circuit

Any ideas please share - 

Thank you for your interest and help in this project

 TP

[HarryA]

Tenergy claims to have a Nicd charger for 12 - 24v but I could not find it on their web site. They are less expensive on Amazon.com then on their site.  I would think it less expensive than building one from scratch if it is under 100$.  They require an external power source. I use a Tenergy to charge my quadcopter's battery.

As you are fabricating you own battery you could set it to charge say 10 cells at a time but that requires 8 charges; mine will do 15 cells.

see: https://power.tenergy.com/chargers/for-battery-packs/12v-24v-nimh-nicd/

[Theunis Prinsloo]

Hi Harry - thanks for knocking on this door ! 😁

 

Thanks for getting back. I have the system connected to an 3Kw invertor so the charging is not the problem. I need to balance the batteries while they are charging and while on float. I have an idea how it should work but I'm not n electronic wiz so need n bit of assistance on the build. Taking the above example should work I need small change to this circuit to get it to read n 1,4 volt cel instead of the 3.7 v cell. 

any assistance would be appreciated.

Thanks 

TP

[HarryA]

 In the circuit above the 2SD1805 is an NPN  not a PNP as required. The TL431 is a precision programmable reference.  I could try the circuit in the simulator if I can find a spice model for the TL431 or otherwise simulate it.

I see: "The 2SB1205 is a complementary PNP transistor for the 2SD1805."

Characteristics of 2SB1205 Transistor

• Type: PNP
• Collector-Emitter Voltage: -20 V
• Collector-Base Voltage: -25 V
• Emitter-Base Voltage: -5 V
• Collector Current: -5 A
• Collector Dissipation: 10 W
• DC Current Gain (h_fe😞 100 to 400

[Theunis Prinsloo]

On 11/4/2021 at 3:38 PM, HarryA said:

 In the circuit above the 2SD1805 is an NPN  not a PNP as required. The TL431 is a precision programmable reference.  I could try the circuit in the simulator if I can find a spice model for the TL431 or otherwise simulate it.

I see: "The 2SB1205 is a complementary PNP transistor for the 2SD1805."

Characteristics of 2SB1205 Transistor

• Type: PNP
• Collector-Emitter Voltage: -20 V
• Collector-Base Voltage: -25 V
• Emitter-Base Voltage: -5 V
• Collector Current: -5 A
• Collector Dissipation: 10 W
• DC Current Gain (h_fe😞 100 to 400

Hi Harry

my electronics knowledge is quite limited - I can build and test n circuit thanks to YT ect.. But the intricacy's of component values etc. not well versed.

I more or less know what to do but to get there is the dilemma.

1. will need to use the pack voltages of 24 vdc via n ± LM7805 circuit to drive the BMS's on each of the batteries. 

2. Then a censing circuit to read the cell voltage 1.140 v adjustable (preferably multiturn pot) and use this to activate the BMS in 1. to balance each cell.

3. I think both circuits need to be isolated from each other by an OPTO or something.

If you have something in mind would be appreciated

TP

[HarryA]

One could use a pair of Arduino mega 2560 microcomputers each has 16 analog inputs. Using two boards each monitoring 10 cells. As each cell is measured relative to ground from the its first  cell the additional voltage would indicate the voltage on that cell, That is if the voltage at the 5th cell where  say 6.5v to ground and the 6th cell where 7.9v the computer would "know" that cell  is at 1.4v and thus charged. It would switch it out of the circuit and replace it with a shunt resistor.  

Also there are Arduino devices to monitor current up to 5 amperes using a Hall effect device.