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I have this UPS https://www.alibaba.com/product-detail/Solar-Manufacture-Multi-function-Battery-Mini_60710518453.html

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This UPS contains 12V 12AH built-in battery, with the ability to add an external battery to extend the operating time (no external battery capacity specified in the manual), I opened this UPS and found that the external battery input terminal is connected in parallel with the built-in battery! I plugged in a 60AH battery and found (without AC input) that the external (60Ah) charged by the built-in battery (about 0.25A) So is UPS design secure? is connecting two batteries with different capacities in parallel a good idea?

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closed as off-topic by Nick Alexeev Feb 10 at 23:52

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    \$\begingroup\$ This seems like a really poor design. \$\endgroup\$ – Hearth Feb 10 at 20:29
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It's not a "bad design" per se. The risk however is that if the internal battery or the external battery are at different states of charge when you connect them there could be a substantial exchange of power. This could be as low as a small spark to as high as a welded connector, if the wire is to small for the unintended load it's going to heat up. A larger capacity UPS usually has batteries in series to provide greater capacity. But they are designed to do that. So on the device you have you can add capacity in this way you just need to be aware of the potentials.

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All lead-acid batteries have plates in electrolyte, and the left half of each plate is 'in parallel' with the right half. Putting multiple lead/acid batteries in parallel is just the same as expanding the plate area.

If the battery chemistry and impedance do not match, both batteries do not equally serve the load when discharging. That implies that a battery can reach end-of-life and require replacement, but the (presumably smart) controller will not be able to determine this from its built-in test on the pair of batteries.

If you use a similar battery type to the internal battery, and always replace the pair at end-of-life, outboard battery is a normal way to extend the time that output power will be available from the UPS, without altering the load limitation. Unless there's a battery fault (internal short circuit) it is unlikely that a charged battery would deliver excess current to a less-charged battery of the lead-acid type and same nominal voltage.

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