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Why does some manufacturer which I know using NiMH battery in their products recommends to charge the product for like 24 hours the first time you charge it, yet the seller usually tells to charge only 8-10 hours the first time you charge it. Which one should we follow? If we should charge it based on the sellers recommendation, then why did these manufacturers recommended to charge it 24 hours in the first place?

As side note the battery is usually around 1300-1500 mAh 2.4 V.

And There is no indicator whatsoever which tells how much charge is left on the battery. The charger itself is around 100 mA 3V (is this categorized as slow charging? ).

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    \$\begingroup\$ All my NiMH would have exceptionally low capacity on the first charge, so initial one would be done in hours and the battery would last only minutes. Over the first three or four cycles, something "forms" inside the battery (I'm not a chemist) and you will notice that both charging time and of course discharge time will increase to normal values. I doubt you will notice any difference between 8-10 or 24 hour initial charge. \$\endgroup\$ – winny Nov 14 '18 at 11:46
  • \$\begingroup\$ This recommandation seems just stupid, charge duration doen't depend on time, but on charge protocol, current, & time ratio... & even on full-state detection. This recommendations is not logical, it really depends on the charger itself & the battery state before charging it. The only case it can be intelligent to do so ... is if charger is a slow mode charge, to regulate/refresh battery after many many very fast charges in factories ....else inside chemistry is till the same as mentionned by Winny \$\endgroup\$ – francois P Nov 14 '18 at 11:46
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I would ignore the seller's advise and follow the manufacturer's advise to charge for 24 hours when charging for the first time. The seller didn't design the product so how can they know better than the manufacturer?

What can happen is that after a long time in storage the NiMh cells lose their charge completely. NimH cells can handle that. After the full discharge NiMh cells can be "revived" by just charging them slowly. Many products always charge NiMh cells slowly because that is the cheapest, safest and easiest solution.

When the cells are fully depleted it might take some time for them to "recover" especially when charged slowly. That might be the reason to charge for 24 hours. That way the cells have a couple of hours to regain their nominal voltage and when they do there is still enough charging time to fully charge them.

Lithium based cells generally should not be fully discharged so they need a protection circuit to prevent that. Also Lithium based cells do need a proper charge controller, just trickle charging them continuously is not recommended as these cells cannot handle that very well.

So charge control and full-state detection as mentioned in a comment only applies to Lithium based cells, not NiMh cells. For NiMh cells charging can be done safely using a resistor and a diode at C/10. Charging then takes long but completely safe.

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    \$\begingroup\$ Sorry, but this is not really based on factual accuracy. In particular, simple slow charging of NiMH is not a particularly good idea - they're not as tolerant of overcharging at low current as NiCd were. If you are going to do it, you want something slower than C/10. Most decent NiMH chargers do in fact have charge state detection. \$\endgroup\$ – Chris Stratton Nov 14 '18 at 14:24
  • \$\begingroup\$ @ChrisStratton Sure but what are accurate facts regarding batteries? The (what you call) decent chargers with state charge detection are often fast chargers and then charging must stop to prevent overheating. At C/10 a full cell indeed gets warm and this can have a negative impact on lifetime. But also at C/20 this is still a possibility. Charging time and speed are always balanced against lifetime. You get the longest lifetime from a battery when you never use it. \$\endgroup\$ – Bimpelrekkie Nov 14 '18 at 14:38

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