I have a rechargeable desk led lamp with USB power input. Although it's supposed to hold charge, it only works when plugged. I opened it and found that the problem is about included lead acid rechargeable battery.

I cut out the broken battery and connected a li-ion 3.7v in its place that I got from an old broken cellphone.

Lamp lights up just fine now but I'm afraid to connect the lamp to a power source since li ion and lead acid are different type of batteries. Am I creating a potential fire hazard or is it fine?

picture of the old battery

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    \$\begingroup\$ It's a severe fire hazard and will almost certainly end badly. A lithium ion battery must ONLY be charged using a circuit that is specifically designed to charge it safely. \$\endgroup\$ – nanofarad Sep 2 '18 at 21:25
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    \$\begingroup\$ Incidentally, if you want to replace the original battery, lead acid would be exceedingly unusual in a desk lamp. Are you 100% sure? \$\endgroup\$ – K H Sep 2 '18 at 21:55
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    \$\begingroup\$ Actually there isn't any label on the battery but it looks like those in solar powered toys for kids. \$\endgroup\$ – uylmz Sep 2 '18 at 22:05
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    \$\begingroup\$ How about posting a picture of the battery? \$\endgroup\$ – Ale..chenski Sep 2 '18 at 22:07
  • \$\begingroup\$ What if I use a Li ion charger circuit between old terminals and the battery? Circuit says it's for single cell li-ion but I don't know if the phone battery is single cell. \$\endgroup\$ – uylmz Sep 2 '18 at 22:09

No, it is not fine. Lithium batteries have different charge profiles, and must be managed more closely than the relatively forgiving lead-acid batteries. Doing this could result in dead battery, or even fire. And, you don't even seem to know the nominal voltage of the lead-acid battery.

All around BAD IDEA

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