Help me charge this mobile drill battery? - Page 1
Forum section: electronics / projects

author: homeless_peep
author_rank: Contributor
timestamp: November 05, 2017, 08:07:24 pm
content: Hello,I've got a budget drill (lol, it's brand is actually"budget") that I got for 20 euros some time ago. I lost it's charger and it's probably either impossible or uneconomical to get a new one. I'd like a safe(-ish?) way to charge it, so I can keep using it.The battery notes "Ni-Cd 14.4V, 1000mA". I've found a dead simple circuit that uses a diode and a resistor to limit the current. I tried to use it, and here's an interesting thing I observed: Even though the battery is now very low (the drill barely works), it's voltage is still 14.4V?! I'm missing something here and my battery theory is non existent.Thoughts/suggestions?

author: amyk
author_rank: Super Contributor
timestamp: November 06, 2017, 12:30:56 am
content: Quote from: homeless_peep on November 05, 2017, 08:07:24 pmThe details are complex, but to summarise, the voltage is decided by the chemistry; and as long as there is enough reactant to allow the reaction to continue, the battery can still show its nominal voltage. The major difference between empty and full is the lack of ability to supply current --- try to draw a small current from an empty battery, and the voltage instantly drops.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 06, 2017, 05:24:01 am
content: The best ready made solution is to get one of those universal R/C chargers. They're not that expensive nowadays and can come in handy for a lot of things.

author: homeless_peep
author_rank: Contributor
timestamp: November 07, 2017, 09:05:50 am
content: Quote from: amyk on November 06, 2017, 12:30:56 amI suspected as much. What if I use a constant current source?

author: ElektroQuark
author_rank: Unknown
timestamp: November 07, 2017, 09:17:16 am
content: A 14.4V NiCad pack has 14.4/1.2 = 12 cells. (1.2V is the nominal voltage for NiCad cells).Each cell will be completely charged when at about 1.50 volts the in current is bellow 50mA.So, 12*1.5 = 18V is the voltage the pack has to be charged to.After charging the cell voltage will settle to about 1.35V, so the pack will be at about 16.2V.

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2017, 09:53:04 am
content: Quote from: homeless_peep on November 07, 2017, 09:05:50 amYes, constant current is the standard, recommended way to charge NiCds. It helps to equalize the charge of each cell in the chain. The basic charge rate for all NiCd cells is the C/10 rate, any NiCd cell will be happy with this. In your case, charge at a constant current of 100mA for 14 hours or more. (14 hours rather than 10 to account for inefficiencies in the chemical process). A NiCd can be left charging at C/10 rate for a very long time (several days or more) without trouble. It's been establised that they will accept C/30 indefinitely. There's no need to set an upper voltage limit.The cells in a typical drill pack will probably have been designed to tolerate higher rate, eg C/5 (200mA) or faster, but the charge time must then be strictly limited. As long as you don't mind waiting, C/10 for greater than 14 hours willalwayswork without any risk of damage due to overcharging.

author: homeless_peep
author_rank: Contributor
timestamp: November 07, 2017, 12:57:53 pm
content: Oh well, I decided to bite the bullet, because I need to finish this job asap. Got another drill, used it's charger, problem solved.But at least I learned something. I can see my mistake now- I should have gotten an 18V supply instead of a 15V, to have headroom. The stock charger is 18V. I won't take it apart, but I kinda suspect what's inside the charger adapter (to which the 18V supply connects). Maybe I'll attempt to replicate it, just to learn something. Thank you all for your help!