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I will be using a TI BQ24074 to charge a battery and power a load via a solar panel. Because of the way the BQ24074 works with a solar panel, it will stay in the DPPM loop all the time (while there's sun). This means termination will be disabled and the charger will not terminate charging when the current drops to the termination threshold (typically, C/10). In this scenario, the charger will simply leave the battery charging voltage at 4.2V as long as there's sun, while the charge current slowly tapers down. Assuming a full charge on the battery at sunrise, worst case scenario is that the battery is left at 4.2V during the whole day (say, 12 hours of sunlight). I would expect that then the battery would discharge slightly overnight (because it will be operating a small load), and then the cycle would repeat on sunrise.

Are there any clear disadvantages to battery life to operating like this? That is, letting the charge current taper on its own, instead of terminating charge at C/10?

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    \$\begingroup\$ Have you read: batteryuniversity.com/learn/article/… ? \$\endgroup\$ Jun 1 at 18:32
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    \$\begingroup\$ You will kill the battery’s expected lifespan, that’s all. limiting the time duration above3.9V is crucial \$\endgroup\$ Jun 1 at 18:35
  • \$\begingroup\$ Thanks both! The battery university link specifically states "A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery at the peak cut-off as short as possible." Good that I asked. I think I could turn off the charger once the battery reaches 4.2V, and turn it on once again at some lower threshold voltage, maybe 4.1 V? What lower threshold would you suggest? \$\endgroup\$ Jun 2 at 0:53
  • \$\begingroup\$ Depending on the cell you are using and the number of charge discharge cycles you expect your battery to live, I would recommend to already stop charging way lower than 4.2V. Best case 80% SOC depending on the OCV curve. The same is recommended on the discharge voltage level for extended cycle lifetime. \$\endgroup\$
    – A.R.C.
    Jun 9 at 13:33
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Low voltage batteries like lead and nickel chemistries tolerate overcharge to some extent without damage.

The higher voltage of lithium cells mean that the active materials plate out in a non-rechargable form if the flow of current continues past fully charged.

What's the disadvantage of plating out your active materials in a non-rechargable form?

At best, you end up with a non-rechargable ... , well, not battery, but paper-weight or door-stop. At worst, you set something on fire while you're doing that.

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Current wisdom is that Li-Ion cells shouldn’t be kept above 80% charge for extended periods. If the intention is to keep cells on more or less continuous charging then it would be prudent to limit at a lower voltage, around 3.8 or 3.9. From your description it sounds like your cell capacity is ample and so the reduced capacity may not be a problem.

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A million swelled-up Samsung Galaxy tablets and phones!

Don't continuously charge flat-wafer type Li-ion batteries found in phones and tablets.

If left continuously hooked to the charger, these wide, 5mm-thick Li-ion batteries will fill with gas (H2?) and turn into pillows. They bend/shatter the tablet screens. Sometimes they partially short. ( w/heat-discolored LCD displays.)

We saw this happening, then later, eventually, Samsung spread the word: don't leave your device plugged into the charger for weeks at a time!!!

Um... we unwisely employed these tablets as smart wall-mounted displays for barcode-scan guns. We plugged in the charger permanently. Two years later, half of them started to swell up. Five years later, the worst had new batteries ...but some had cracked their mounting bezels. Today I have a hazmat bucket with 95 destroyed batteries.

We replaced all the tablet batteries with a little DIY battery board to emulate the Samsung batt., plus a 2.5-farad 5V supercap, to let them ignore 120V line glitches under 3sec. (No, the Samsung tablet will not run if plugged into the charger wo/any battery. However, it apparently WILL run if first you plug in a Samsung tablet, THEN you remove the battery. (The device seems to run a bad-batt charge-test only at power-up.) Heh, you can run a no-battery tablet for a few days, as long as the AC supply never glitches or fails.

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