Proper Nicad parallel pack construction charging procedure

[johnws6]

Hi all, I'm looking to get your thoughts and feed back on a project I'm currently constructing. I'm making a rotary self propelled sweeper and am using an old nicad pack I had from a previous unfinished project. It's a pack made of 160 Sanyo N3000CR Nicad cells. I made the pack from garolite tubes epoxied together and I'll be constructing a HDPE box for the enclosure for them.

My question is to charging these. I am going to have them as a 24 volt, 24 Ah pack. So we're looking at 20S8P. I know I cannot charge them as one pack, and have to break apart each of the 8 series strings and charge individually. My plan, which I'm not sure will work, is to have ( 8 ) 24 volt cordless tool chargers that are hooked up to each of the 8 series strings permanently. If I was to design these chargers to be on board the sweeper and hooked up directly to each pack would this create a problem? The idea is that I could connect all 8 chargers into a single strip to have one main plug to connect to an outlet. The pack would still be connected in parallel permanently and I don't know if this will pose a problem? Any help is appreciated. I uploaded two photos to show what the pack itself looks like and my current progress on the machine itself.

Shot at 2012-08-28

Shot at 2012-08-28

 

[dnmun]

soldering them is bad. but if you hook the charger up permanently then the resistor across the output caps of the charger will drain the pack deader than doo wah diddy.

 

[johnws6]

I thought leaving them connected would be a discharge issue, just seemed easier than having a huge lego block of anderson power pole connectors. Would it work if I were to leave the packs in parallel permanently connected but run the chargers to each individual series string?

 

[dnmun]

you can have the chargers connected to each string but you have to isolate the charger with a diode or remove the resistor across the output capacitors.

 

[johnws6]

I think I see how that would work. I did a little research and found these might be suited for this application?

http://www.mouser.com/ProductDetail/ON-Semiconductor/MBRD630CTT4G/?qs=sGAEpiMZZMtvcUztdGSumFE0r4pYHY%2fvOS0lZOAECu4%3d

Having the pack connected in parallel full time however won't affect anything? Thanks for all the help.

 

[dnmun]

not sure what surface mount schottky diodes will do for you. i thought you said you were gonna not connect them in parallel.

 

[johnws6]

Ah yes that's true. Would this be better, since I could wire it after the positive lead on the charger?

http://www.mouser.com/ProductDetail/Vishay-Semiconductors/SB540-E3-73/?qs=sGAEpiMZZMtvcUztdGSumL%2fJtICIUTwc5WTgrK3twkE%3d

 

[dnmun]

if your charger has very low current that diode would be adequate. the other part was a surface mount diode and this is an axial diode so it is easier to work with. but it seems like it would be simpler to remove the resistor from the output of the charger or disconnect the charger.

 

[Jeremy Harris]

Make absolutely certain that you don't parallel any cells, even accidentally, without proper reverse current isolation. As I found out a few years ago with a nasty fire, nickel chemistry cells have to be very carefully isolated with strategically placed Shottky diodes if you're planning a parallel arrangement.

The problem is that when being charged the cells exhibit a slight increase in voltage just as they approach peak charge, followed by a slight decrease in voltage as they become fully charged. This can result in catastrophic current flows into the least capacity cell (even if it's only a fraction of a percent less than its fellows) from the cells that are just peaking, leading to rapid overheating and, in my case, cell explosions.

If you have 8 independent strings with 20 cells in each, then fit a Schottky diode in series with the output of each, before the parallel connection. If you want to still be able to charge the cells as a group then you can fit the diodes in the negative leg of each pack. I found it handy to use dual Schottky diode packages, in TO220 cases, to do this. The diagram below shows a way of doing it that works for charging. Ideally include temperature sensing in the pack, because the diodes on their own don't guarantee that one string won't reach full charge and start to heat up before the others. Using temperature sensing to reduce the charge current if this happens would make the pack safer.

 

[johnws6]

I remember reading about the 3700 mah nimh cells you had erupt a number of years back, and that has been a reminder as to make sure this battery pack would work.

It looks to me then that I should have a diode between each battery pack and charger. Also I need to have one on the output of each pack before the parallel connection is made? Is that correct so far?

 

[Jeremy Harris]

I remember reading about the 3700 mah nimh cells you had erupt a number of years back, and that has been a reminder as to make sure this battery pack would work.

It looks to me then that I should have a diode between each battery pack and charger. Also I need to have one on the output of each pack before the parallel connection is made? Is that correct so far?

Sounds good to me.

I'd hate to see anyone else get caught with this. It wasn't obvious to me that you could get this condition where a fully charged set of cells would draw lots of current from less fully charged cells in a parallel stack until I worked through it after my fire, and, AFAIK, it's only NiCd and NiMH cells that have this oddball voltage drop after the charging peak that causes the major problem.

I have heard of people paralleling cells directly for discharge only, and I guess it probably works as long as you can be certain that all the cells are at exactly the same state of charge. Personally I'd stick to using isolating diodes between the strings.

 

[dnmun]

if you connect them at the base in the way he laid out they are not really connected, they are isolated by the diode. using the TO-220AB package so you have a dual cathode is more cost effective than using the axial diodes as well.

you do not need a high speed diode, do you have an idea of how much current you will expect max? the voltage rating is not important in this situation. maybe a few amps from each series?

discharging in parallel is very risky because you can have one of the cells in the string reverse if the attached parallel string draw it down below zero and reverse it with current flowing through it.

try this if you have very low current from each string, 3.5A max on each leg:

http://www.ebay.com/itm/Vishay-6CWQ03FN-30V-7A-Schottky-Recti-Diode-DPAK-25pcs-/400150451002?pt=LH_DefaultDomain_0&hash=item5d2ad3533a#ht_500wt_949

here is the spec sheet:

http://media.digikey.com/pdf/Data%20Sheets/International%20Rectifier%20PDFs/6CWQ03FN.pdf?cshift_ck=null&client_id=5042

 

[johnws6]

I remember reading about the 3700 mah nimh cells you had erupt a number of years back, and that has been a reminder as to make sure this battery pack would work.

It looks to me then that I should have a diode between each battery pack and charger. Also I need to have one on the output of each pack before the parallel connection is made? Is that correct so far?

Sounds good to me.

I'd hate to see anyone else get caught with this. It wasn't obvious to me that you could get this condition where a fully charged set of cells would draw lots of current from less fully charged cells in a parallel stack until I worked through it after my fire, and, AFAIK, it's only NiCd and NiMH cells that have this oddball voltage drop after the charging peak that causes the major problem.

I have heard of people paralleling cells directly for discharge only, and I guess it probably works as long as you can be certain that all the cells are at exactly the same state of charge. Personally I'd stick to using isolating diodes between the strings.

That's been a concern I've been trying to address as well, as far as making sure the cells are at somewhat of an equal state of charge. With 160 cells though, it's hard to guarantee they ever would be. As far as the charging route goes, the chargers I'm looking to hack and use at this point are 24 volt craftsman dual chargers or hitachi 24 volt chargers. The Hitachis are decent since you can actually adjust the rate on the internal board through a small dial. I could set this rather low, say 200 ma, and equalize the packs this way but still have peak and temperature detection for each of the 8 packs.

As far as discharging the cells I have 3 curtis controllers that I have to use. They all have the programmable 4 prong molex connector, and I could possibly set the discharge cut off voltage at a reasonable rate if I can find somewhere around me that does this. I will have to check out the spec sheets online and see what the stock cut off is on those, if there even is one at all.

I think it would be best to isolate the chargers and the parallel packs, it seems to be the safest bet. This machine will probably, I'm guessing, weight right around 120 pounds? I'll have to get some more pictures in this thread, but I'm using a 24 volt mag motor a40-300 for the brush drive and a 24 Volt clark floor machine for the drive wheels. The one below in the ebay link is the one I purchased, and am awaiting to receive. I'll be cleaning it up and painting it to make it look as good as I can.

http://www.ebay.com/itm/390402663165?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I'd have to guess that I would need about a continuous 60 amps at 24 volts when this machine is actually in use going through light snow and moving about. I won't know the exact amount until it's all operational and have all the amp and volt gauges attached.

 

[johnws6]

As a follow up I snapped a few photos to give a representation of what I was explaining in prior posts. The photo below is the two chargers I currently have, and would need 6 more to charge the 40S8P pack.

By johnws6 at 2012-08-30

Here is a photo of the inside of the charger itself. The resistor was soldered by me for a previous project on the output leads. I'll be using a thermoresistor.

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

Here is a shot of the rotary dial that adjusts the output of the charger. It seems you can adjust this down to almost nothing all the way up to 7 amps or more.

By johnws6 at 2012-08-30

Here is the motor that arrived, it is a 36 volt motor, I purchased a 24 volt motor from the same seller on ebay but they sent this by mistake and don't have the 24 volt one. It's about 35 pounds I would say. I removed the brush cover and am going to be painting the motor and re wiring it.

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

Here is a the mag motor c40 300 i'll be using to drive the brush.

By johnws6 at 2012-08-30

By johnws6 at 2012-08-30

 

[johnws6]

if you connect them at the base in the way he laid out they are not really connected, they are isolated by the diode. using the TO-220AB package so you have a dual cathode is more cost effective than using the axial diodes as well.

you do not need a high speed diode, do you have an idea of how much current you will expect max? the voltage rating is not important in this situation. maybe a few amps from each series?

discharging in parallel is very risky because you can have one of the cells in the string reverse if the attached parallel string draw it down below zero and reverse it with current flowing through it.

try this if you have very low current from each string, 3.5A max on each leg:

http://www.ebay.com/itm/Vishay-6CWQ03FN-30V-7A-Schottky-Recti-Diode-DPAK-25pcs-/400150451002?pt=LH_DefaultDomain_0&hash=item5d2ad3533a#ht_500wt_949

here is the spec sheet:

http://media.digikey.com/pdf/Data%20Sheets/International%20Rectifier%20PDFs/6CWQ03FN.pdf?cshift_ck=null&client_id=5042

From each string I'd say 7-10 amps would be about right, that's about 56 to 80 amps. I'm not so sure how much the total system would draw on the batteries. The curtis controllers have under voltage cutback at 17 volts, so I'm thinking that might help to prevent the cells from reversing at least somewhat.

 

[amberwolf]

I have to say I saw the title and had my heart race a little, thinking "no, not *another* noob to nickel-chemistry that wants to blow himself up!" Then I skimmed the thread and saw you do already know enough not to do that, and have excellent advice so far.

But just in case, I'll drop in here with: I'd second the diodes on each string for discharge, for the reasons already given. It should be interesting to see how this setup works out in actual use. If you make a multipole charging plug that automatically connects them separately to each charger, and then a paralleling plug that includes the diodes for discharge, it should be easy to work with.