9V Alkaline battery charger charging really fast at 5mA ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: January 11, 2018, 08:04:42 pm
content: I used a TL431 adjustable voltage reference, and made a BC558 current source of 5mA and hooked up with 10.22V on the emitter w/ 12.22VccSo in about 30min the 9V bat went from 7.5V ? to 9.483V right now. Its slowed down a lot now. I think that battery was dying in my Brymen 869, so thats pretty dead I guess.So if the battery voltage went up that quick, will it drain almost as quick ? so only another 2-3hrs of life from the rechARGE??? Whats the norm for this ?Fun from scratch circuit with a LM358 will light up at 9.9V. Now to try and add an auto shutoff from what I know/remember of 555 timer or opamp, but not using a relay.So after about 1hour with the 9V batt between the PNP collector and ground, its at 9.855V so I unhooked it and it reads 8.2 just with the DMM loading it with 10MO or so, and the battery's internal resistance.And its dropping by 10mV/s or so......so is that it ? Thats the best it can be recharged ? I'll run it down with 5mA at the start with an LED I guess to see.Yeah running it at 450uA +DMM loading, it drops about 1mV/s at 7.81V

author: Paul Price
author_rank: Super Contributor
timestamp: January 11, 2018, 08:30:36 pm
content: First of all, mostalkaline batteries are not re-chargableand will leak a corrosive fluid if you attempt to recharge them.While charging NIMH, even at lower than expected current, the 9V battery voltage will rise fast at first. But this doesn't mean the battery is anywhere close to being charged. As charging progresses, voltage rise becomes much slower.A fully charged 9V NIMH battery will charge to about 10.5V to 11V and even then without connecting it to a circuit for use, the battery voltage will drop down to about 9.5V within 48-hrs. The normal charging current for a 9V NIMH battery is usually 35 to 70mA, depending on the capacity of the battery, and you should be able to see the recommended charging current v. time written on the battery case, with charging times from 8 to 24 hours, but this depends on the charging current. Slow charging is better for battery life.A battery is charged to about 140% to 150% of its mAH capacity, since you need to put much more energy into a NIMH battery than you can expect to be able to get out of it in use. Charging any battery is not 100% efficient.Overcharging can damage the battery and trickle-charging is not recommended. You can construct a simple timer using a 555 chip to generate 10 to 100-sec pulses that clock a CMOS binary counter chip (CD4060 for instance) to make an accurate timer. You can also cutoff charging when the charging voltage reaches about about 11 to 11.5V at the battery. If you like the counter idea, use some n-th output of the counter chip to reset a CMOS D-FlipFlop that controls a MOSFET that switches on/off the charging current returning to the charger. This charger-timer circuit can be a good electronics learning project.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 11, 2018, 09:13:03 pm
content: Charging an Alkaline battery?Have you any photo of the battery to see what is written on it?

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2018, 09:40:04 pm
content: Any battery can be recharged, sort of. Whether it's really usable depends. Years ago I experimented with various methods of recharging alkaline batteries with mixed success. I found that if I was careful I could recharge one that had been only partially discharged and a few cycles was usually possible. Unfortunately they usually started to leak within a few cycles, sometimes right away and that's why I gave trying.These days well performing NiMH rechargeable batteries are widely available so there's no sense in screwing around with alkalines unless you just want a science experiment. Certainly don't put a recharged alkaline into anything valuable as they almost always leak sooner or later.