Charging in device | Need more extra-leaky Schottky? - Page 1
Forum section: electronics / beginners

author: igor78
author_rank: Contributor
timestamp: August 08, 2022, 04:53:16 pm
content: I've replaced a NiMh battery in my handheld radio with a lipo cell. I discovered that the original designer had put a diode D1 between the outside charger and the inside battery. The diode prevents charging IC TP4056 to measure voltage. Is there a clever way of resolving it, except removing the diode? Maybe this is a made-up problem, and the diode should not even be there? How do people do charging in-device?It used to be like thisI think D1 is here to prevent the battery from discharging back into whatever happens to be accidentally connected to the external terminals in the field. And D2 is a overvoltage protection? The original charger floats a NiMh battery with a small current at 3.7V.The terminals look like thisAfter the modification, the circuit looks like thisTP4056 uses the same pin #5 to supply charging current and measure the battery voltage. Here is a block diagram I found online.TP4056 block diagramObviously, with a super low leak 1N4004, the charging IC does not see the battery voltage, and the charging does not work. I've tried replacing D1 with several Schottky diodes I have on hand - 1N5819 (ss14), 1N5822 (ss34), SS510, MBRA340T3G, hoping that the leakage current would be enough for TP4056, but it does not work. Heating the Schottky with a heat gun does help. I presume that temperature increases the leakage current? A super low resistor across the diode does help, like 5Ohm, but it defeats the protective purpose of the D1.The charging IC measures the battery voltage at ~100ms intervals. Here is what it looks like with no battery attached to the charging circuits.What do I do? One obvious decision is to move the charging circuit inside the device, I can't do that - there is not enough room. I can also remove the D1, but I think it just makes charging terminals "hot" all the time. The lipo cell has short-circuit protection, but it would not prevent a slow drain.

author: macboy
author_rank: Super Contributor
timestamp: August 08, 2022, 05:41:03 pm
content: You could add a resistor across the diode. It will provide some means to sense battery voltage and will limit current that can be drawn from the batter through the terminals. Or replace the diode with an appropriate fuse or polyfuse, so that an external short across the terminals will trip the fuse. Remember there is voltage drop across the diode, fuse, or polyfuse, and this might interfere with proper charging; I haven't looked into the TP4056 to know for sure.

author: wraper
author_rank: Unknown
timestamp: August 08, 2022, 06:04:27 pm
content: You could use MOSFET based reverse polarity protection circuit instead of the diode but it will take almost as much space as charger circuit. Is there really no space? As you put a different type of battery, and this is not a super compact device like a smartphone, I guess there should be a gap somewhere to put a tiny PCB.

author: iMo
author_rank: Super Contributor
timestamp: August 08, 2022, 06:05:38 pm
content: Replace the diode with a small reed relay. While charging put a magnet onto the pack.