Cordless screwdriver battery replacement - Page 1
Forum section: electronics / beginners

author: Peabody
author_rank: Super Contributor
timestamp: October 26, 2024, 03:35:11 am
content: I'm trying to repair my neighbor's cordless screwdriver. It works, but has very little power. The battery says Bideli PULY lithium ion 3.6V 1300mAH 3-16-06. During charging, the charger is at 6.65V, but the battery is stuck at 3.12V. That voltage doesn't increase, and charging never stops. Tomorrow I will make sure the power supply and charger are ok, but of course I expect it needs a new battery. 3-16-06.The only 18650 solder tab cell I can find at either Digikey or Mouser is the Sparkfun PRT-13189. it has twice the capacity of the original battery, but I wonder about the current it can deliver. Sparkfun says:"These 18650 Cells have a standard discharge current of 0.2C to a maximum of 1C and can handle about 300 charge cycles."I just don't want to go to the trouble of doing this fix only to find that the cell's output current isn't enough. What do you think?https://www.sparkfun.com/products/13189

author: Poroit
author_rank: Frequent Contributor
timestamp: October 26, 2024, 07:15:17 am
content: G'day Peabody,Do you own a Benchtop Power supply that you can power the screwdriver from to determine how much current it needs to operate correctly?

author: Peabody
author_rank: Super Contributor
timestamp: October 26, 2024, 04:30:35 pm
content: It's a moot point now. I found that the charger wall wart is fine, and surprisingly the battery still works, but the charging circuit is dead. No charging current is sent to the battery, and no material load is drawn from the wall wart. I tested it with another battery I know is good, but it wouldn't charge that one either. And I tested the screwdriver battery with my own TP4056 charger, and it takes a charge with no problem. So it's the charging circuit that's died.The board is a mass of SMD parts, and I didn't see anything that looked like a TP4056 circuit. Without a schematic I would have no real chance of fixing it, but it would take hours to try. So I'm going to declare it dead.

author: Peabody
author_rank: Super Contributor
timestamp: October 27, 2024, 01:05:05 am
content: If it's the charger circuit that's dead, then I wonder if I can disconnect the 6.8V wall wart input from the existing circuit, and wire it instead to a TP4056 module, the output of which connects to the battery. The existing connection from the battery to the motor would remain. It seems to me this would work. I would have to change the program resistor from the default 1.2K, which provides 1A charging current, to something that produces more like 500mA, but otherwise it should work if I can find room for the module.Then there is the question of whether the charger module should also include battery protection (I have both versions of the module on hand). I can't find any ridge on the 18650, so I assume it's an unprotected cell. That means the protection is on the board somewhere. But the motor still runs, which I think means the protection circuit is still working too even though the charging circuit isn't. I would much prefer to use a charging module without protection. First, because it's smaller, but more important because it wouldn't limit discharge current. The protection would be a DW01 and an FS8205A dual mosfet. The overdischarge current limit would be a function of the Rds(on) of the mosfets, and I don't know at what current the protection kicks in. I don't have a bench power supply, so can't readily test that. But the motor won't be drawing any more current than it did before, so it should still work.The other thing is that the TP4056 is for nominal 3.7V cells (4.2V CV charging). I can't change that. The existing battery is labeled 3.6V. Will that cause a problem? I guess I could do a test charge and see where it ends up on termination.

author: henryuan
author_rank: Newbie
timestamp: October 29, 2024, 05:37:46 am
content: Maybe you misunderstood the difference between a charger and a switching power supply.If you want to charge the 3.6V lithium battery directly, what you need is a lithium battery charger 4.2V:https://www.henryuan.com/e_products/42V-Lithium-Ion-Battery-Charger-6-1.htmlIf there is a DC to DC charging circuit in the product, or there is a DC to DC charging circuit in the charging station, then you only need a constant voltage switching power supply.Perhaps the following content can help you choose a suitable lithium battery charger:https://www.henryuan.com/html_news/?8-How-to-choose-a-suitable-charger-for-lithium-battery-packs-8.html

author: Peabody
author_rank: Super Contributor
timestamp: October 29, 2024, 02:16:32 pm
content: The problem is that the DC charging circuit in the screwdriver, powered by a 6VDC wall wart, has died, and no longer sends any current to the battery. I don't have a schematic, or really any ability to diagnose what's wrong. It's all tiny SMD stuff, and I don't see any familiar charger chip like a TP4056. So I thought it might be possible to wire in another charger in the form of a TP4056 module, which I have on hand, in parallel with the dead charger circuit. I tried that, and it seems to work, but the battery is so old and flaky that I didn't get very far. I've concluded that since a new battery, with tax and shipping, would cost about 40% of the cost of a new screwdriver, it's probably just time to get a new screwdriver.