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I am recharging a 14v LiFePo4 4s8p battery from a Phillips wall transformer from scrap. The phillips wall transformer is rated 13V 400ma on it, but it measured 18V on the multimeter.

I put it on anyway. The voltage when it is charging is 0.5 volts higher when connected to the transformer, rising from 11.7V to 12.3V in about an hour.

What damage can incur from charging a 14V battery on a small 18V power supply? until it reaches 14v? I know that some chargers limit the charge current to 0.5V over the battery current, does that also make sense for lithium chemistry?

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  • \$\begingroup\$ Yes, have added the information above. \$\endgroup\$
    – bandybabboon
    Commented Jul 9, 2020 at 7:24
  • \$\begingroup\$ Diconnect the transformer immediately and never again connect it directly to batteries! It is not a battery charger that is smart enough to charge them safely. Based on the measurements you made, it is simply an unregulated 13V supply, as it has 13V × 1.414 = 18V unloaded peak voltage. \$\endgroup\$
    – Justme
    Commented Jul 9, 2020 at 7:40
  • \$\begingroup\$ What makes sense for lithium chemistry is to use a proper lithium charger. If you use a random power supply, you'll at best trash the battery, and at worst start a fie. \$\endgroup\$
    – Neil_UK
    Commented Jul 9, 2020 at 8:28
  • \$\begingroup\$ "I know that some chargers limit the charge current to 0.5V over the battery current," says you aren't clear on the difference between current and voltage. From that starting point, your only answer is to get a proper charger. \$\endgroup\$
    – user16324
    Commented Jul 9, 2020 at 8:45
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    \$\begingroup\$ @Justme I think it's worth pointing out that it is possible to charge batteries with such a transformer but you need extra circuitry between the transformer and the batteries to prevent overcharging, and we call that circuitry "a charger" \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Jul 9, 2020 at 10:20

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There is nothing to limit the supply voltage so it will overcharge batteries. Do not connect randomly found power supplies directly to a battery, you'll end up damaging the batteries and lithium batteries can explode and start a fire if incorrectly charged. For charging, use a charger that is compatible with the battery type and voltage.

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  • \$\begingroup\$ Cheers I'll get it on the solar charger a bit later. The question specifically states "what's the danger of charging them up to 14V on an 18V power supply", so the danger of overcharging is known and covered in the original question. it's good to emphasize it, but it's not electrically the answer to the question, I knew that already, obviously. I found 3000 batteries in the a bin, i am familiar with the the cell specifications, PTC and cross-section photos and other tests. I've put an alarm on them so I check that they stay withing the chemistry's voltage range... it's am experiment! \$\endgroup\$
    – bandybabboon
    Commented Jul 9, 2020 at 12:26

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