Re-Charging Ni-mh

Comments

• Tracy Allen Posts: 6,667

  2008-09-14 18:02 edited 2008-09-14 18:02

  Are you talking about NiMH/NiCd? I was confusted about the 2.22 volts you mentioned (two batteries in series?) And how it was charging up to 2.975, or was it 12.6; that must be the stack of 6. Either figure would be very high if they are NiMH. Were the batteries hot?
  
  There is a lot of good info on the Energizer site. Here is their app note on NiMH
  data.energizer.com/PDFs/nickelmetalhydride_appman.pdf
  It talks about the different charging strategies. Other articles in their technical database cover other battery chemistries.
  
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  Tracy Allen
  www.emesystems.com
• bambino Posts: 789

  2008-09-14 18:33 edited 2008-09-14 18:33

  Sorry Tracy,
  
  The store bought charger, charges two ni-mh(in this case) AAA batteries. Normally rated at 1.2 volts each, but it charges them in series.
  
  I placed a meter on the neg of one battery and the pos of the other. The discharged batteries started a charge that rated 2.2 volts, and charged up to around 2.95 before leveling out. the charge terminated at 2.975 then the charge light went out and the meter read 12 volts.
  
  The six battery pack is what I intend to build,...... the store bought charger just handles 2 at a time!
  
  Thanks for the link!
• Mike Green Posts: 23,101

  2008-09-14 18:42 edited 2008-09-14 18:42

  There are several ways to charge NiMH batteries. First of all, they are not like lead-acid batteries and you can't charge them with the same circuits. One very simple way to charge them is to use constant current at 1/10 of their capacity. They can be charged indefinitely at that rate without damage, but will take over 10 hours to charge. For example, if you had AA cells with 2300 mAh capacity printed on them, you could charge them at 230 mA. You can easily take an LM7805 regulator and wire it as a 230 mA constant current source as show on the LM7805 datasheet. This will need an unregulated supply of around 5V (2+V for the regulator and 2.5+V for the batteries) at about 250 mA. Read the app note for other suggestions, particularly for more rapid charging like the store bought charger can do.
  
  Edited ...
  
  The 1/10 capacity charge rate cannot be used indefinitely. It can be used to charge a NiMH battery with unknown initial charge for a fixed time (like overnight). You really need to use a more complex fast-charge algorithm that uses voltage sensing as described in the application note if you want to maximize battery life and capacity.
  
  Post Edited (Mike Green) : 9/14/2008 7:16:21 PM GMT
• bambino Posts: 789

  2008-09-14 18:55 edited 2008-09-14 18:55

  Thanks Mike,
  I was planning on using a stamp with the chip array and a temp sensor to charge the six batteries at once. typically a 7.2 cell!
  
  Ps, Tracy the batteries were cool during the charge. I have two more I plan on testing later!
• LoopyByteloose Posts: 12,537

  2008-09-14 19:22 edited 2008-09-14 19:22

  Ni-mh cells are NOT the same as lead acid for charging. They usually have a charge rate between 1/10th and 1/30th of their amp-hour rating. But new technologies may be moving toward 1/5th amp-hour. Of course the supplier will sell you a charger that is as fast as possible because it will also shorten the battery's useful life and no one likes to wait. The store bought chargers really should have a disconnect, but they don't. The manufacturers are more than happy to let their chargers wear out the batteries faster. You should use a timer that disconnects the battery after about 10 hours at most. And with fast chargers that can be merely hours.
  
  But the main thing is that all the Ni batteries are recharged by current regulation. Lead acid batteries seems to be more rough and ready about abuse. I have seen auto batteries take a charge while boiling. I guess the internal resistance is adequate for the job. So you may just hook up a set voltage and don't concern yourself as much with the charge current.
  
  Lithium batteries are very touchy. They will pre-charge at a trickle to first get them ready, then to a high charge rate, and then switch to maintain charge rate at a safe voltage. This makes a lot of sense as the store more energy in the mass and if you upset the chemistry you get a ball of fire.
  
  Temperature is always a good indicator of a safe charge rate. But Ni-mh batteries can actually be damaged by extremely slow charge rates, like 1/50th their amp-hour rating. Look at 'The Art of Electronics' for more info. The third wire on Lithium batteries is supposedly a thermistor that tells the charger to shut down if the batteries is getting too hot.
  
  The new AAA Ni-mh have almost as much capacity as the AA batteries of five years ago. They are a very good choice for micro-controller projects. The AAs still seem to be the best $$$ value in rechargables and the best for optimal capacity. The 9v rechargables have very small capacity.
  
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  It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
  
  Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
  
  Post Edited (Kramer) : 9/14/2008 7:32:54 PM GMT
• bambino Posts: 789

  2008-09-14 19:53 edited 2008-09-14 19:53

  @Tracy,
  That link recommended 1C up to 90% charge,terminated by Temp sensor.
  Then slow charged up to 100% at .1C terminated by timer.
  Then Trickle charged at .025C to eliminate self discharging.
  If I am using 900mAh cells in my battery Pack(6 AAA batts in series), do 1C, .1C, and .025C corelate to 9, .9, and .225 mA?
  
  @Kramer
  I have steered away from Lithium for the reasons you mention. I would that I could stick with Lead Acid, but they would not handle the vibrations.
  I'm just hopeing ni-mh is docile enough to accept my engineering skills!
• Tracy Allen Posts: 6,667

  2008-09-15 01:52 edited 2008-09-15 01:52

  For 900mAh cells, according the the Energizer documents:
  -- 1C is 900mA, until the cell temperature rises about 5 degrees above ambient or until the cell voltage passes the "hump". The temperature has to be detected inside the battery pack or at least in very good contact with it.
  -- 0.1C is 90 mA, for an additional 1 to 2 hours
  -- 0.025C is 22 mA, trickle charge thereafter.
  Provided by a regulated current source. I'm not sure how you planned to do it with the Darlington array chip, and where the primary charging power will come from, but it does have to have means to regulate the current. (In contrast, Pb acid chargers usually work in terms of regulated voltages).
  
  Why do you state that sealed Pb-acid wouldn't handle vibrations? I hadn't heard that.
  
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  Tracy Allen
  www.emesystems.com
• bambino Posts: 789

  2008-09-15 13:52 edited 2008-09-15 13:52

  Hi Tracy,
  The acid in the battery sloshing around would be a dynamic influence on my accelorometer sensors. Everything in my device has to be static weight to get accurate readings.
  
  I was planning on using a R2R ladder on the chip array, opening one or more darlingtons on the chip would change the current flow. And by swithing the ground eliminate the need for fet's and fet drivers. Just a high current regulator on the pos side and hopefully keep componant count down. The link said nothing about the trickle charge having to be of higher pressure, I think the one voltage source should work!
• Tracy Allen Posts: 6,667

  2008-09-15 17:58 edited 2008-09-15 17:58

  Ahh. Most sealed Pb-acid batteries use some kind of electrolyte suspension system, such as a fiberglass mat (AGM absorbed glass mat, or gel, silica gel). That allows them to be used in any position, for discharge. I wonder if there would be much residual slosh?
  
  For current regulation, an op-amp and one transistor run with PWM might not take up more space than the R2R and the darlingtons. It will be important to have accurate current regulation.
  
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  Tracy Allen
  www.emesystems.com
• bambino Posts: 789

  2008-09-15 23:43 edited 2008-09-15 23:43

  Tran+Op+PWM may be the way I go now, as the machinist is complaining about the room I am going to need for the charger circuit.
  If I go with an external charger circuit(which I have to admit is a better idea), then I will have more room to manipulate the positive input side.
  I have really poor luck with transistors, I would probably use a mosfet. "less burned fingers and exploding dies that way" Everytime I screw up a fet circuit, it just doesn't work no more.
  
  You may be right about the slosh in those batteries, but the crew has already black balled the idea of using them.
  
  May be a good topic for another thread "Other peaple's use of my creation's : and how I lothe them"
  Anyway, It's good to know you guys are here! I hope to start on the charger next month! "sooner if I can clean my plate"
• LoopyByteloose Posts: 12,537

  2008-09-16 17:32 edited 2008-09-16 17:32

  You can easily use a LM317 voltage regulator as a current regulator according the schematics in their PDF. And there are one transistor current regulator schemes too if you want to save money. Since you are using AAA cells, the charging current is not going to be more than one amp.
  
  ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
  It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
  
  Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
• bambino Posts: 789

  2008-09-17 13:53 edited 2008-09-17 13:53

  I have a lot more room now, and time.
  I imagine I'll be looking at a lot of circuits to do this with now before I start!
  Thanks!