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I have some background with electronics, but that was in the "old days", so my knowledge of lithium battery types and charging processes is very limited.

I have a bunch of gadgets with one of those baggie-type lithium batteries. Some of them are size/weght restrained, but with a couple, size is no issue, so i was wondering if I could replace the provided battery with a (lot) larger ones. Those gadgets also have charging circuits (i've read the charging is voltage-controlled, so in theory, the capacity shouldn't matter for the charger, atleast in theory). All batteries are single-cell.

So, is this possible at all, and what should I be extra careful about, when selecting batteries (eg. when ordering cheap large ones from ebay), and what are the limitations?

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Most likley yes. 99.9 % of all lithium battery chargers are simple CC/CV chargers and they will just spend more time in their CC state until done. Others have built in ICs for management and some even have coulumb counters.

Is it just plus and minus comming out from the battery or are there more cables?

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    \$\begingroup\$ If the battery is 7.4V like in the picture, that means it's a 2S battery, in other words it has two cells in series. Those should really be balance charged. \$\endgroup\$ – Dampmaskin Jun 1 '16 at 12:21
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    \$\begingroup\$ I put the picture just to show how it looks'. The actual battery is 3.7V type \$\endgroup\$ – Zgembo Jun 1 '16 at 12:22
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    \$\begingroup\$ I have experience with those :) The batteries I have, all have ~5cm+ cables, just with different connectors (some directly soldered), so no problem with heat there. \$\endgroup\$ – Zgembo Jun 1 '16 at 12:33
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    \$\begingroup\$ @winny, it is a basic safety feature of any good charger. I have designed around 5 or 6 products with built-in Lithium ion chargers. They all had timeouts. Review specs on lithium ion charger IC's. They all have timeouts. You cannot just float a lithium ion cell in CV mode indefinitely, so there has to be a timeout in case the battery never achieves its normal termination condition (based on current acceptance). \$\endgroup\$ – mkeith Jun 2 '16 at 8:30
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    \$\begingroup\$ @mkeith Either an integrated PWM with some external means to give it a crude current limit (sometimes actual output current sensing but also straight peak current cycle-by-cycle current limitation on the primary), but the PWM IC itself was made for running continuously and no external timers, or a full-fledged dual-OPamp with precise voltage and current regulation but still no timeout. \$\endgroup\$ – winny Jun 2 '16 at 9:03

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