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The referenced article over on http://www.instructables.com/id/4V-Lead-Acid-Battery-Charger/ indicates a Li Ion charger for mobile phones may serve to charge a low AH SLA.

I held the impression a Li Ion charger actively interacts with firmware aboard the mobile battery to control the charging.

Is the mobile charger capable of charging an SLA? In the absence of necessary firmware, is there risk of damage to the charger, or the SLA, or the target mobile Li battery after the mobile charger is used to charge an SLA?

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    \$\begingroup\$ Phone chargers are (mostly) just a dumb 5V SMPS - all the conversion to the actual battery voltage and current limiting is done in the phone. But lead acids are pretty robust, and just dumping 5V in is probably not going to kill it. \$\endgroup\$ – Someone Somewhere May 20 '18 at 12:16
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    \$\begingroup\$ @SomeoneSomewhere: I disagree. Most SLAs can be float-charged @ 2.3V/cell, or fast-charged @ 2.4V/cell. I wouldn't connect a 4V pack to 5V without monitoring it carefully. You can't just hook it up and walk away. \$\endgroup\$ – Dave Tweed May 20 '18 at 12:22
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    \$\begingroup\$ As I said, probably. You are certainly not going to get good longevity, especially when adding a 5% tolerance onto the output voltage. But I wouldn't expect immediate flames. \$\endgroup\$ – Someone Somewhere May 20 '18 at 12:25
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    \$\begingroup\$ It looks like the linked post is based on a fundamental misconception, though: You can use any Li-ion charger which is used to charge phone's 3.7v Battery as they both are charged with a 4.2V DC. - phone chargers don't output 4.2V, and that would be slightly insufficient anyway. \$\endgroup\$ – Someone Somewhere May 20 '18 at 12:25
  • \$\begingroup\$ Ergo, a 4V SLA may not be charged by a mobile charger. Therefore the subsequent queries are not relevant. @someonesomewhere: You want to make that an answer? \$\endgroup\$ – Everyone May 21 '18 at 2:32

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