electronics help needed nicad battery charging ?

[deerslayer660]

  I find that i need to be able to charge 16 aa nicad batteries from a motorhomes

13.8 vdc. batteries are rated at 700 mah.  i plan to charge these with 2 seperate

charging curcuits 8 bateries each in series  this makes a batery pack of 9.6 vdc

at 5600mah correct?. i plan to use curcuits made up useing 2n3055 transistors to regulate current and ues the motorhome voltage as is. but at this time i am haveing troulble finding a curcuit that works for what i am tring to do so maybe somebody has a curcuit i can use? problem 1 is time these batteries run numorous handheld

devices gps-  handheld radio- digetal camera- exc going out sometimes 20 miles into lake eire and the outboard motor dos not charge batteries so i do not want anything else on boat battery I have to be able to charge these in 9 hrs or so. So my plan is to charge these batteries at a c3 rate for about 3 hrs and than switch manualy to a

c1 rate here is ware my brain starts to hurt  c3 rate schould be at 1680 ma?

but i plan 1500 ma   and c1 schould be at 560 ma correct? again this is for one bank of 8 batteries. is all my thinking correct so far?   the curcuit i have been trying

to make work so far is a 220 uf electro across + and- for a filter 1 wat ris 220ohm

between + and base of 2n3055. than  between emitter and ground i tried a 5k audio volume pot with a 15 ohm 1/2 wat parelled <all i had at time> this gave adj of some ware around 400ma to 780ma . not enough for what i need so lowerd base ris to 47 ohm

and pot went up in smoke. anyone know how to properly calculate for these restances

including wat size ? or anybody have a better idea  george

[wdyasq]

Look at Radio COntrolled charging devices.  There are for R/C airplanes and cars and these folks regularly charge off 12V sources.  Loosing a receiver battery in a flying R/C airplane is not an inexpesive option.  These folks have it down to a science of charge times and battery condition.

Ron

[deerslayer660]

   schould have probly included tat batery pack is + to + and - conected to colector

of 2n3055 thanks george  ps also will use a led from base to ground for ckt on indicater

[ghurd]

You also want a float voltage. I go about 1.45v per cell, with a float current of 3-4%.

Controling only by input current and time will ruin the batteries.  So will very many 1-hour charges.  I design for 10 hour charges, but have a 6 hour charger just in case.  A self resetting overtemp shut off is a good idea.

Nimh batteries will get over 3X the capacity.

I could explain a circuit I use if you have interest.  Nothing fancy or complex, but they have been running for about 4 years with NO ruined batteries.  Only 3 parts.  Meets the recommendations of a major battery manufacturer for proper charging (their own chargers do not).  Let me know if you are going to upgrade batteries and I'll figure the parts.

G-

[GeoJohn]

Hey Deerslayer660,

You would actually get two 9.6V battery packs at 700mAh. Current only adds in parallel. If I recall, on a simple current controlled charger, 1/10 of the mAh rating of the battery is what you need. In this case a 70mA current source. You can do this with a LM317 and a couple of resistors. (Available everywhere) I made a couple of chargers like this for use on a trip to Africa. I can draw up a circuit tonight and post it to this forum. I may even be able to post a picture of the chargers I built.

[alcul8r]

My understanding is that these batteries are used in different devices, then all charged in series with each other.  I would discourage this, as you will have some close to depletion and some with a lot of charge left.

If they are not discharged together I don't think they should be charged together.

[ghurd]

Yes. Good catch!

G-

[deerslayer660]

   yes i agree that its not good on batteries charging this way and NImh would have a higher mah cappasity but these batteries cost me only .63$ a cell the NIMH  i have

cost me 2.00$ cell and only last about as many charges the way i abuse them the thing is the fishing trip is what this is all about if i have to buy batteries durcell for exampell will cost around 12.00$ a trip  and i pay 10.00$ for 16 700mah

cells if i only get three weekend from the cells i save 22.00$ but in reality i got 10 trips on last years cells  though most times they were charged with a single cell charger at home this requires changing 4 batterys every day all week long forget to change one day and your short for the weekend . so all being said I still have a problem I understand that 8  700 mah  batteries in searies gives 9.6 vdc at 700 ma

for 1 hour at 9.6 vdc draw   so if i charge my 9.6vdc at say 12vdc and 70 ma for 1 hour each cell will be up 70 mah give or take a  little   and a c/3 charge rate would be about 210 mah?  and a c/1 would be 70 mah?  yesterday i had 8 batteries that i discharged and had these on the charger at 750ma for a 1 hour peroid and batteries never got hot today i put 4 in my gps and battery charge indicater shows

only a 1/4 charge  other 4 show the same  if each cell can hold 700mah how can putting only 700mah  70ma x 10hrs = 700mahs charge all the batteries ? im lost  

[ghurd]

Well, sounds like your charger or batteries have a problem.  750ma charge current should have had them quite warm.  Cells 'bounce back', or read higher after they haven't been used.  Maybe they were about 25% when they began 'charging', or 'bounced back' to 25%, meaning possibly they did not charge at all (loose wire or something?).

And maybe some of the math or symbols are 'different'.  C/1 is 700ma. C/2 is 350ma. C/3 is 233ma. C/10 is 70ma.  Capacity/X=B.  700mah/2h=350ma.

Forget losses and such for right now.  Amp-hours is amps per hours. Put in in, get it out. Put in 1 amp for 1 hour, get out 1 amp for 1 hour, or get 1/2 amp for 2 hours, or 2 amps for 1/2 hour.  So put in 70ma for 10 hours, get 70ma for 10 hours, or 700ma for 1 hour.

With 750ma for 1 hour, the buggers should have been about full (and more!) and tested that way.

Could one of the batteries be bad (open or shorted)?  My experience says it happens after a long time of not being used.

Test them for the open V.

A spark charge or "Burning the Wiskers" can fix them sometimes.  One AA to a 12v battery. Wire Neg to Neg. Wire from the 12v pos. Spark it a microsecond to the pos of the AA once or twice.  Keep the spark away from the 12v battery (explosions, etc.)

I CAN NOT RECOMMEND IT!   But I do know it does work somewhere between usually and sometimes.

My chargers work efficiently without battery damage.  Some say they are to this or too that, but they work.

Lake Erie?  Where about's?  Deerslayer660 sounds like Eastern basin, 12ga Mossberg Auto.

G-

[deerslayer660]

  hi ghurd inland a littel soulth of mansfield oh. i dont pay for campgrounds motor home is a converterd invalid hi top 3/4 ton streach van i put a inverter and micrwave in also has a tv- stove top- small sink- propane furnace and fridge. and thats why i ihave to charge

off motorhome battery to far to drive 2 hr each way  I just finished charging with

my curent regulator off of my 25 amp bench supply and thats what i found when checking batteries befor starting a bad battery

thats what i was going to post when i saw your last post and yes i zaped it it now works for now  how long your guess as good as mine  ha ha   i charged at 350 ma

for 3 hrs funny when first starting had 450 ma going into battery bank after about 40 min started droping resting at end of charge at 300ma i did nothing to curcuit

 the batteries were old 12oomah nimh never got hot untill just befor i disconected

even than just barely warm drive voltage was about 12 vdc  soi think i will leave it  the way it is and make another to do the secound eight  i will charge for 3 hrs at 200ma for 2.5 hrs or so and switch to a c/10 of 70ma the rest of the nite for 700ma  ware you from to know eastern basin   i fish from toledo to far east clevland

starting in western basin hopely next month. internet and other lake freinds make finding fishing a lot easer allways looking for a partner.   george ps i used to charge with just a radio shack 1 amp power converter set at 6vdc and 2 packs of 4

in parallel  nise handwarmers in the morning lol  george

[ghurd]

That could work.  I carry extra Alkaline AAs just in case.

Many battery charging circuits are dynamic, and will therefore adjust to compensate for design and voltage parameters.

I fish Sandusky (sometimes) to NY.  Not lately though (last time there with the boat, the shore did not reach the water! 2003-ish, no time to fish big since).

G-