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I am using a 750mAh 25C 3.7v LiPo battery for a custom electronics project which currently draws ~1.5 amps, far below the rated operating current of (0.75 * 25 = ) 18.75 amps. However, at the battery terminals (before any PCB traces) I am noticing a drop from 4.03v to ~2v, below the 3.3v I require for my MCU. Why is this drop so significant? Is the battery bad?

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  • \$\begingroup\$ In a word, yes. Unless (possible facepalm moment here) you are not fully charging the battery. Even when nearly discharged, in general a battery will put out a high voltage with no load applied, so you can't use that as a check on state of charge. \$\endgroup\$ – WhatRoughBeast Jul 14 '18 at 11:35
  • \$\begingroup\$ @WhatRoughBeast that's not really true of this type of lithium cell. While the voltage drops under load, the unloaded voltage is a rather useful indicator of the state of charge - not a linear one, but the slope is useful. Much of the usable life, especially in placing something into use is when it reads in the 4.2 down to 3.7 range. Once it's reading 3.7 volts unloaded, there is still some capacity that can be drawn but in a high load application it would be better to swap it out and recharge. \$\endgroup\$ – Chris Stratton Jul 14 '18 at 14:47
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Assuming it was fully charged before, either your battery is bad or its rating is hugely overstated.

LiPo batteries should never be discharged below 3V, but for longevity it is best to have cut-off at 3.2-3.5V

Note that 1.5A is relatively high load for only 750 mAh battery of any C rating. I am not saying they shouldn't be used this way (RC toys and electronic cigarettes use same batteries at currents many times more than that) but cell lifetime will be affected.

The usual definition of C rating is "how fast the battery can be discharged safely". The safety bit should be understood as not blowing up or exhibiting immediate drop in performance. This does not mean, however, that discharging at full C rating has no negative effects. In fact, by pushing the cell to its limit you severely reduce its lifelong capacity. I've seen many 400 and 750 mAh cells used in RC helicopters completely giving up after only 50-70 charge-discharge cycles.

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  • \$\begingroup\$ the poor service life in such toys tends to result more from poor charging, and also from failing to cut off the discharge before too low a voltage, than from the actual rate of discharge. \$\endgroup\$ – Chris Stratton Jul 14 '18 at 14:43
  • \$\begingroup\$ I did purchase the battery off of eBay. I also decreased the load to 350mA (for testing purposes) and still experienced a voltage drop of over 1v. \$\endgroup\$ – Multrix Jul 14 '18 at 22:37
  • \$\begingroup\$ Dispose of it (take it to RadioShack or whatever you have in your area) and ask for refund if seller offers it. \$\endgroup\$ – Maple Jul 15 '18 at 1:43

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