Nokia 100 AMPS phone - Battery replacement capacitor? - Page 1
Forum section: electronics / repair

author: Halcyon
author_rank: Unknown
timestamp: October 15, 2025, 11:50:17 pm
content: Apologies if this is a re-post, but I couldn't find it anywhere; I might just be remembering a conversation I had with someone about this topic.I have an old Nokia 100 analog phone in my collection. Unfortunately it's one of those phones where even if you're plugged into the charger, it won't power on without the battery being present (and somewhat working - if the pack voltage is too low, the phone won't stay on).The original battery was an old 800 mAh Ni-Cd, which is long dead (and started to leak), so I ripped the cells out to prevent any further corrosion.My question is, how feasible is it to install capacitors that would be enough to allow the phone to think there is a battery present and power up (or even operate for short periods of time without being plugged in)? Being an old AMPS phone, it won't be transmitting anything, so the current draw would be fairly low. Being a brick phone, there is plenty of room in the battery casing.The charger itself outputs 780 mA at 12v DC, however the original battery pack is rated at 7.2v. From memory (and I would have to pull everything out to test), but the 12v was exposed directly on the corresponding positive and negative pins on the back of the phone itself.I've attached photos of the battery shell, and the circuit board inside. The two outer pins are positive and negative, not sure what the two centre pins are doing, it's hard to follow any traces due to the plastic bracket. I've had a bit of a poke around with a multimeter and these are my findings:Pin 1 - Battery positive terminalPin 2 - UnknownPin 3 - UnknownPin 4 - Battery negative terminalPin 1 to battery contact: Continuity (0 Ohms)Pin 4 to battery contact: Continuity (0 Ohms)Pin 2 to Pin 3: Open circuit

author: erkko
author_rank: Contributor
timestamp: October 16, 2025, 12:34:59 am
content: I would just try, had I some large caps around. Then again, 7.2v sounds a lot like 2S 3.6v lion cells would. And I dont have much large caps to speak of...

author: Whales
author_rank: Super Contributor
timestamp: October 18, 2025 02:43:00 am
content: A lot of cheap NiCd devices would charge a cell through a simple resistor. The NiCd and NiMH chemstriescantolerate "trickle" (small current) overcharging, it comes with ugly compromises but proper NiCd/MH charging is reasonably complicated to do safely and properly.I would recommend hooking up a zener in parallel to the capacitor to simulate the voltage clamping effect of the cell. The device might damage itself if powered on without this clamp, or it might detect the lack of "battery" and refuse to operate.QuoteTo get more than a few seconds of power you will need to use supercaps. To get 7.2 volts you will need several of them in series with some sort of balancing (starts getting complicated).

author: amyk
author_rank: Super Contributor
timestamp: October 18, 2025 06:26:57 am
content: Replacing NiCd with NiMH is likely to work well.

author: ftg
author_rank: Frequent Contributor
timestamp: October 18, 2025 09:03:59 am
content: I'd replace the NiCd with NiMH as well.But I have also done that capacitor replacement for a few devices with success.You'll only loose some time by trying.