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  #1  
Old 03-28-2011, 03:52 AM
bgoner bgoner is offline
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Default Do I have to balance these Nicads?

SAFT SBM112 Nicads http://www.saftbatteries.com/doc/Doc...ce9b7c26dc.pdf

Would it be a problem to charge these in series?

Do I have to balance these Nicads before I start charging them?

Can I find a scheme for a device which connects both terminals of the battery and monitors voltage if it gets too high or too low the device would act as a shunt so that I won't overcharge or overdischarge one of the batteries? Thanks!

Would this charger work for me? http://www.rusbaevents.com/cgi-bin/V...W/VMcharger.pl

Last edited by bgoner; 03-28-2011 at 04:13 AM.
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  #2  
Old 03-28-2011, 05:52 AM
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Default Re: Do I have to balance these Nicads?

Quote:
Originally Posted by bgoner View Post
SAFT SBM112 Nicads http://www.saftbatteries.com/doc/Doc...ce9b7c26dc.pdf

Would it be a problem to charge these in series?
I think NiCads are often used in series.
Quote:
Do I have to balance these Nicads before I start charging them?
If they need equalization, I believe the way to do it is to discharge them completely, to zero volts per cell.

Quote:
Can I find a scheme for a device which connects both terminals of the battery and monitors voltage if it gets too high or too low the device would act as a shunt so that I won't overcharge or overdischarge one of the batteries? Thanks!
I don't think any type of BMS is required. Very occasional checking and then a discharge equalization if needed.
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Old 03-28-2011, 01:11 PM
MN Driver MN Driver is offline
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Default Re: Do I have to balance these Nicads?

No, don't discharge to zero volts.

Cylindrical nickel chemistries must be charged in series but it seems its different with flooded where it can be done both ways. I personally wouldn't want to disconnect the cells to try to charge them in parallel as that would be a nightmare. I would have a series string with none in parallel. To balance them I would charge each one on its own untill full and then connect them in a string.

The specsheet is different than the typical charging methods I've used which are dT delta Temperature (measure temperature rise at a specific current and when temperature rise exceeds a certain amount over a given period of time its charged), or -dV or negative delta voltage (where voltage drop occurs with internal cell heating at the end of charge) to determine the end of charge. With flooded cells it seems that -dV goes away or becomes incredibly difficult to detect. I'm a bit confused over the specification sheet provided by SAFT as it seems to have float application information but not cyclical application charging. Are these designed for an uninterruptable power supply application like a server backup or telecommuications backup?

Its tough to find people who use these because they are heavier than lithium and a bit more picky with charging and generally were hard to get when they were the superior option that was available. I wouldn't discharge these lower than .9v under load(lowest cell), or 1 volt per cell average throughout the string. Reversing these cells destroys them, sometimes they are resiliant but you want to avoid it, I wouldn't discharge a pack like this under load below 20%. My 2000 Honda Insight using NiMh doesn't discharge itself below 1v under load or more than 1.15v open circuit or low load(140v out of 120 cells), when at the very bottom of the state of charge gauge it will drop current very low and wait for a 12-cell string(10 of these strings are compared to find a mismatch in balance) to have a cell drop in voltage and then it cuts the assist and will charge. The discharge limits are the same for both chemistries except NiCads are more intolerant to being held at one SOC range and like to be discharged through a decent portion of their pack often.

Do you have these cells already? It may be a challenge to figure them out but they do last a long time.
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Old 03-28-2011, 02:07 PM
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Default Re: Do I have to balance these Nicads?

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Originally Posted by MN Driver View Post
No, don't discharge to zero volts.
I just did this last week on a 10 cell NiCad battery to bring it up after 6 years on the shelf. Worked pretty good. These were 3/2D cells, 7Ah. I thought I had done the same thing on an old Nife flooded 10 cell battery a few years ago. It was very large, about 100 pounds. Never did seem to be able to get many Ah out of it. It might have been 30 years old to start with.

So what you're saying is that you should not discharge flooded NiCad to zero?

major
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Old 03-28-2011, 02:14 PM
bgoner bgoner is offline
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Default Re: Do I have to balance these Nicads?

Quote:
float application information
didn't get that

Yes they were used for telecommuications backup. Maintain mix of charge and discharge from 5h to 30mins.

Quote:
heavier than lithium
But I guess they are a much better choice than Lead acids...?

Quote:
I wouldn't discharge a pack like this under load below 20%
didn't get that... are you refering to debt of discharge?

Quote:
when at the very bottom of the state of charge gauge it will drop current very low and wait for a 12-cell string?(10 of these strings are compared to find a mismatch in balance) to have a cell drop in voltage and then it cuts the assist and will charge
do you mean that when you fully discharge them to ~1V-1.15V you check them if some have more or less voltage than the average so that you could manually balance them? or am I getting it wrong?

Well they are reserved for me, no one will be touching them
I heard they are badass batts which maintain short circuit and could live even fully discharged. Also could be refreshed when you change the liquid and gain more than 100% of their sheet charge...
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Old 03-28-2011, 02:29 PM
bgoner bgoner is offline
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Default Re: Do I have to balance these Nicads?

Quote:
So what you're saying is that you should not discharge flooded NiCad to zero?
I've heard that they could cope with that, even stay completely discharged, but in the SAFT specs the deep discharge is set to 0.6V.

Don't go under 1.0V for everyday use, but for complete recycling go to 0.6V from time to time.
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Old 03-28-2011, 10:31 PM
MN Driver MN Driver is offline
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Default Re: Do I have to balance these Nicads?

.6v is the number I've seen before. In a series string though, if you short circuit the pack or drive it too low you'll get reversal but as a single cell, yes they probably will handle zero volts but I wouldn't do it. There are plenty of resources out there that describe the limit at .9v under load and .6v under very low loads. Just avoid driving the pack down to the point where you will get reversal from the lowest capacity cell and you should be fine. It depends on how different the capacity is amongst cells, 20% is probably more conservative than you need to be but these are used cells so you probably want to find out the capacity of the cells before ragging on them in an automotive application. I've read some instances that describe long-term storage of NiCad as being under short circuit and then tossed into a refrigerated space. Information is a bit conflicting but the information you get from SAFT should be the information to go by.
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Old 03-29-2011, 12:45 AM
gottdi
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Default Re: Do I have to balance these Nicads?

Charge in series and use a proper charger that can terminate the charger properly. If they are low in fluid then charge them full first then put in the fluid. Not before or you will have an overflow most likely.

http://www.evdl.org/lib/index.html

Read all the stuff under nickel based batteries. ALL of it.
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