John Eucist Posted June 27, 2016 Share Posted June 27, 2016 This is my "oral irrigator" and I noticed that the charger is DC5.4V while the device says DC2.4V. Is this normal? Link to comment Share on other sites More sharing options...
US69 Posted June 27, 2016 Share Posted June 27, 2016 this can be as long as the device is using not the normal li Ion cell with 3.6 volt, and therefore lets say 2 normal nimh batteriee with each 1.2 volt.... not loadable batteries in this size have 1,5 volt...loadable about 1.2 or 1.25.....so 2 can have 2.4 volt.... the output is bigger to have a faster load? Link to comment Share on other sites More sharing options...
esaj Posted June 27, 2016 Share Posted June 27, 2016 It might be that the wall-transformer gives out 5.4V, which is then regulated down to 2.4V at the actual charger? Really just guessing... I tested some "wallwarts" (transformers that plug into mains) marked as 12V output that were for Philips wireless headphones (the transmitter-part), and they gave out 18V when measured with the multimeter, so go figure I trust their engineers still know what they're doing better than me Link to comment Share on other sites More sharing options...
John Eucist Posted June 27, 2016 Author Share Posted June 27, 2016 21 minutes ago, esaj said: It might be that the wall-transformer gives out 5.4V, which is then regulated down to 2.4V at the actual charger? There is no "actual charger" (besides the 5.4V one for the wall socket) unless it's embedded in the irrigator itself. Anyways, it charges fine but I just thought it was unusual.https://www.amazon.co.uk/Panasonic-EW-DJ40-DentaCare-Rechargeable-Irrigator/dp/B002NKLWEI Link to comment Share on other sites More sharing options...
Keith Posted June 27, 2016 Share Posted June 27, 2016 They have to be a pair of NiMH cells because of the voltage. They are nominally 1.2V each but reach 1.44V when fully charged. NIMH cells are constant current charged - not constant voltage, so the voltage has to be high enough to give that current even when they are at full voltage. I.e. Voltage isn't critical beyond being high enough to supply the required current. Fast chargers use something called "Delta peak" to terminate the charge. At about the point the battery is almost fully charged its voltage drops slightly and that is used to terminate the charge. However at a 1/10C charge (120mA for a 1.2Ah cell, etc.) NiMH and NiCad will not overcharge even if left on for days, so most chargers don't switch off but drop back to a 1/10C trickle charge which will also do the last bit of topping up and hold the battery fully charged. Link to comment Share on other sites More sharing options...
John Eucist Posted June 27, 2016 Author Share Posted June 27, 2016 @Keith so are you saying that the disparity in the printed voltages between the charger and the device (with the battery) is is not unusual for NiMH cells but would be unusual if it were Li-ion? Link to comment Share on other sites More sharing options...
Keith Posted June 27, 2016 Share Posted June 27, 2016 1 hour ago, John Eucist said: @Keith so are you saying that the disparity in the printed voltages between the charger and the device (with the battery) is is not unusual for NiMH cells but would be unusual if it were Li-ion? Yes that is correct, all Lithium battery types are very voltage sensitive and can be damaged if it is too high or too low. NiMH and NiCad cells are more robust. They are current sensitive to a degree though so they should be charged at constant current and you need some headroom in the supply voltage to be able to maintain that current which is typically no more than 1C although some will take more. The main issue with them is that if they are charged at too high a current, or for too long at a high current they start to gas, if they are vented (as any designed for fast charging will be) then nothing drastic happens other than they a reduction in capacity, unvented cells could explode (really more pop like a balloon) they will not catch fire like Lithiums may, but they could get quite hot. Funny story (with hindsight) years ago I was charging an old NiCad at too high a voltage as they pack was old, completely drained and didn't want to take a charge (4 cell 4.8V pack being charged from a 12V source but still taking a very low current - I was trying to see I could bring it back to life.) I got called to lunch. When I came back the pack had disappeared and I couldn't find it anywhere. Several days later I was going to develop a film and got my developing tank out of a cupboard, inside the tank was the NiCad. It had exploded, flown across the room into the open cupboard, landed on top of the tank and was so hot it melted its way through the lid. Replacing that tank cost more than buying a new NiCad in the first place! Link to comment Share on other sites More sharing options...
John Eucist Posted June 28, 2016 Author Share Posted June 28, 2016 5 hours ago, Keith said: . It had exploded, flown across the room into the open cupboard, landed on top of the tank and was so hot it melted its way through the lid. Replacing that tank cost more than buying a new NiCad in the first place! Woah that's scary! Are NiCd and NiMH less of a fire hazard than Li-Ion and LiPo? Link to comment Share on other sites More sharing options...
steve454 Posted January 2, 2017 Share Posted January 2, 2017 On 6/27/2016 at 4:37 PM, Keith said: inside the tank was the NiCad. It had exploded, flown across the room into the open cupboard, landed on top of the tank and was so hot it melted its way through the lid. Replacing that tank cost more than buying a new NiCad in the first place! Was that before or after you crashed your R/C plane through someone's window? Your living a charmed life Link to comment Share on other sites More sharing options...
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