Parallel nimh cells connection

[GTR450]

Hello I have question about connect to 40 nimh cells with 8.4 - 8.7 Volts parallel.
I need balance whole battery pack for hybrid car.
Is it safe to connect 40 cells parallel and wait?
Every cell has more than 5000mAh capacity, but I need the same voltage.
One more question is if I can discharge whole pack in parallel connection by RC charger to 8.2 VOLTS. Because car computer shows error if they re more than 8.2V, and how can I meassure Internal ressistance?
I need help

 

[newfutile]

You need to check all the voltages are the same with a meter before parallel connection,any that ate not will need to be charged /discharges to meet the rest,if you discharge the pack to a lower than the full charge voltage you wont have much capacity left.if the car computer is not taking all the power you can add diodes in series (about 0.6V drop for silicon).
for internal resistance you can buy a meter that may work , or you could calculate using a constant load/resistance and ohms law.

 

[GTR450]

You need to check all the voltages are the same with a meter before parallel connection,any that ate not will need to be charged /discharges to meet the rest,if you discharge the pack to a lower than the full charge voltage you wont have much capacity left.if the car computer is not taking all the power you can add diodes in series (about 0.6V drop for silicon).
for internal resistance you can buy a meter that may work , or you could calculate using a constant load/resistance and ohms law.

But I think the voltages never be the same, and when I connected old cells together they still had same voltage after 24hours (before it was like one cell = 7.8 another 8.3 another 8.6...) after its 8.4 every one so It was not good idea? So it is a bad idea to discharge whole parallel conected cells by rc charger? Its better by some diode?

 

[Lucien Nunes]

NiMH cells are 1.2V per cell, so it seems that you are talking about 7-cell (7S) battery packs. What you are proposing is to make up a battery of 7S40P. If you are concerned about the peak current when interconnecting 7S packs to each other because they are in different states of charge, simply connect them together via a resistor to let them balance. A resistor of 0.22 ohm 10W (or even a long length of wire of that kind of resistance) would allow batteries in quite different states of charge to equalise at the rate of a few amps. Don't forget the fuses for individual packs - if one has a shorted cell the others will soon cook the good ones and you will get thermal runaway and a meltdown. There is a possible ongoing issue of keeping the 7S packs themselves balanced on cyclic charge, if they are in different states of health. How well they share the work will depend on the charging regime.

 

[GTR450]

NiMH cells are 1.2V per cell, so it seems that you are talking about 7-cell (7S) battery packs. What you are proposing is to make up a battery of 7S40P. If you are concerned about the peak current when interconnecting 7S packs to each other because they are in different states of charge, simply connect them together via a resistor to let them balance. A resistor of 0.22 ohm 10W (or even a long length of wire of that kind of resistance) would allow batteries in quite different states of charge to equalise at the rate of a few amps. Don't forget the fuses for individual packs - if one has a shorted cell the others will soon cook the good ones and you will get thermal runaway and a meltdown. There is a possible ongoing issue of keeping the 7S packs themselves balanced on cyclic charge, if they are in different states of health. How well they share the work will depend on the charging regime.

Thank u very much, every cell or module i dont know how to say it consist of 6cells he looks like that one on the picture but I have 40 of them connect like on picture 2, they are connected + - + - + -, but i read that when I want to balance them, I must connect all the negatives and all positives together, Please can u tell me how to connect resistor to them ?

 

[GTR450]

NiMH cells are 1.2V per cell, so it seems that you are talking about 7-cell (7S) battery packs. What you are proposing is to make up a battery of 7S40P. If you are concerned about the peak current when interconnecting 7S packs to each other because they are in different states of charge, simply connect them together via a resistor to let them balance. A resistor of 0.22 ohm 10W (or even a long length of wire of that kind of resistance) would allow batteries in quite different states of charge to equalise at the rate of a few amps. Don't forget the fuses for individual packs - if one has a shorted cell the others will soon cook the good ones and you will get thermal runaway and a meltdown. There is a possible ongoing issue of keeping the 7S packs themselves balanced on cyclic charge, if they are in different states of health. How well they share the work will depend on the charging regime.

Please just answer for my last question, u mean to pút resistor between All negative and All positive?

 

[newfutile]

I assume what Lucian mean is to temporarily connect all negative together ,and between each positive connect the 0.22 ohm 10 watt resistors.this will allow the more charged cells to charge the les charged cells without too much current flowing .
i would not this this unattended especially at the start when the highest currents will flow and things may get hot ,if anything resistors or cells get to hot to touch it may be an idea to disconnect .

 

[GTR450]

I assume what Lucian mean is to temporarily connect all negative together ,and between each positive connect the 0.22 ohm 10 watt resistors.this will allow the more charged cells to charge the les charged cells without too much current flowing .
i would not this this unattended especially at the start when the highest currents will flow and things may get hot ,if anything resistors or cells get to hot to touch it may be an idea to disconnect .

Okay thank u ill try IT, and why 0.22ohm and 10w? I think i dont have 7S but 6S it could be the same right ?

 

[newfutile]

I just copied Lucian‘s resistor values , i expect he has worked it out , possibly with your declared 0.5 volt differential between cells ( V=I x R) so I = V/R : 0.5/0.22= 2.27 amps flowing though the resistors (hopefully the worst case) and for a limited duration.

higher value resistors would limit the current more (your cells will have a maximum charge level ) this would take longer to equalise the voltage .the 10 watts are to dissipate the heat (these are usually ceramic or metal finned)

 

[GTR450]

I just copied Lucian‘s resistor values , i expect he has worked it out , possibly with your declared 0.5 volt differential between cells ( V=I x R) so I = V/R : 0.5/0.22= 2.27 amps flowing though the resistors (hopefully the worst case) and for a limited duration.

higher value resistors would limit the current more (your cells will have a maximum charge level ) this would take longer to equalise the voltage .the 10 watts are to dissipate the heat (these are usually ceramic or metal finned)

Okay I understand, but lowest cell has 8.66 and the best has 8.77, Its dangerous to connect ? Today I want do this and let it like this for two days, and then wanna put battery pack to the car..

 

[newfutile]

It will most likely be ok ,you just need to keep up with any temperature rise on the cells and resistors, or better still try to charge the lower ones to the higher value

 

[Lucien Nunes]

If the voltages are within a few tenths of a volt you will get a spark if you connect up directly but there won't be any damage. Only need the resistor to bring them closer if you have some that are a volt or two different. I would just hook the resistor, which as I say could just be a length of wire, the resistance value isn't very important, onto any odd packs before connecting them to the already connected group.

 

[GTR450]

I really appreciate your help. I try to connect them with a wire on both sides, and my last question is when they ll connected parallel
LIKE ++++ | ---- they match and every of them will be like 8.7 V, can I connect some bulb or something between + - when they re connected to discharge them to like less then 8.4, because car computer shows error if they are higher than 8.4

 

[GTR450]

If the voltages are within a few tenths of a volt you will get a spark if you connect up directly but there won't be any damage. Only need the resistor to bring them closer if you have some that are a volt or two different. I would just hook the resistor, which as I say could just be a length of wire, the resistance value isn't very important, onto any odd packs before connecting them to the already connected group.

I did it like this so can I connect some Bulb on them ? To dischsrge to 8.4?