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Battery charging -Ni cad, Nickel Metal Hydride

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Larry

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I have a scanner that I am changing over the battery pack ( 4 cells in series) from a Nicad to a Nickel Metal Hydride. This will change the mah from about 2400 to 7200. Im going to use the Nicad wall charger. My question is will the current demand for the new battery pack exceed the demand of the wall charger? The new battery pack should charge at about .7 amps (7.2 ah / 10). The old Nicad should have been about .24 (2.4 ah / 10). The other question is How is the charging current demand determined? Would it be calculated from internal battery resistance?
Thanks
 
Charging current depends on how fast you want to charge it. NiCad chargers such as you're described aren't even really chargers they're just simple transformers/rectifiers rated to only output a certain current, it will take 2-3 time longer to charge the NiMH battery and once it's full the constant overcharging current will damage the NiMH pack over time. If there is in fact charge circuitry in your charger then the chances of damage to the NiMh pack increase as the NiCad charger will likley not detect when the pack is fully charged.
 
The charge rate is determined by the charger, not the batteries. If you remove the NiMH batteries once they are fully charged (about 45 hours at 0.24A for a fully discharged battery), they should not be damaged since the battery is being charged at less than the C/10 rate. You might want to measure the actual charging rate with a multimeter to make sure you are not overcharging the battery.
 
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Where are you getting 7200mah from? High capacity AA NiMH cells are only around 2000-3000mah or so, or are you using larger form factor? If they're AA batteries there's no way the capacity is 7200mah.
 
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There are 4 batteries @ 1800 mah each.
The mah rating does not change when you put batteries in series. It's still 1800 mah.
 
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Old AA Ni-MH cells were 1800mAh and a charge lasted for one month. New ones are 2450mAh and a charge lasts for months.
 
Larry voltage adds in series not capacity.
Capacity adds only in parallel. It sounds like you may have downgraded your battery pack, unless the 2400ma capacity before was based on 4 600mah cells in series as well.
 
The old Chinese Ni-Cad AA cells in my solar garden lights are only 600mAh.
 
corrected info

Thanks for bringing the battery capacity to my attn. I stand corrected. I need to correct some info I gave, My old Nicad battery pack is actually 600 mah at 4.8v ( 4 cells in series ). The new Nickel metal hydride battery pack is 1800 mah at 4.8 volts. The wall charger is 12 volts @200 MA. I usually change over my equipment to NiMH after my NiCads give out and its worked out very well but Im very careful about overcharging. I think thats the key. Im back to my original question. What will actually determine how much current the battery pack will draw during charging? Is it entirely up to the wall charger-my 12v/200ma will provide 200 but a 12v/400ma will charge twice as fast? Ive heard about everything on this. Will the batteries internal resistance be considered?
Thanks

Larry
 
It's primarily up to the power supply. Cell resistance will show up but it's not the primary limiting factor. This is the problem because a constant 200mah charge when an 1800ma pack is already full is NOT safe for the pack, it will dergrade it's chemistry much much faster than somethign that knows what it's doing. You need an actual intelligent charger, which knows when the pack it's charging is full and stops charging until enough current has passed out of the battery to allow it to again detect when the pack is full when the charge state starts again. The voltage levels and vectors a charger needs to detect a full capacity state only occur at very specific stages near full charge. Approximations can be made, but ALL battery packs need good chargers. A stupid wall wart will shorten the life of the battery pack by 100% or more.
 
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If the charging circuit does not have a current-limiting circuit then the 200mA adapter will be seriously overloaded by a dead battery that will try to use 500mA or more. 12V is too high for only 4 cells in series. Each cell is 1.4V to 1.5V when fully charged so the current-limited charger should be 5.6V to 6.0V.
 
If the charging circuit does not have a current-limiting circuit then the 200mA adapter will be seriously overloaded by a dead battery that will try to use 500mA or more. 12V is too high for only 4 cells in series. Each cell is 1.4V to 1.5V when fully charged so the current-limited charger should be 5.6V to 6.0V.

Audioguru I agree with you on the 12v. In my case Uniden supplied it along with the scanner probably 15 years ago. Goes to show how much thought they put into it. A smart charger is probably the way to go.
 
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