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I have a Dell Vostro 3500. I want to replace my old battery, but there's something I don't understand about the ones they are selling on eBay and Amazon.

See this specification for the battery:

Specification:
Condition: Brand New - Grade A cells.
Battery Type: Li-ion
Voltage: 11.1 Volt (10.8 Volt compatible)
Capacity: 9 Cell / 78Whr

Is it okay to charge this kind of battery with a 19.5V charger? Or should I buy a specific charger for this battery?

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It's a big difference so it's somewhat surprising but the standard charger for the Dell Vostro is 90W at 19.5VDC and the battery (either the 6 or 9 cell variants) are 11.1VDC, per this spec sheet

This is the default 6 cell battery

I highly recommend you buy a Dell branded battery; either direct or from a reseller. You'll avoid the risk related to the previous link and be assured compatibility.

Reason for difference in battery vs. charger voltage:

In nearly every case the voltage (and total power rating) provided by a laptop charger will be more than what is needed to charge the battery alone, as you want the battery to be able to charge while actively using it, so the charger needs to provide enough power to do both. Typically the voltage is cut down for the charging circuit using whats called a buck converter (DC to DC step down converter) and the current flow to the battery is controlled with a PWM (pulse width modulator) which allows the charging circuit to slow or stop the charging rate based on the charge level, battery temp, etc.

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    The voltage to almost every appliance in my house is 110 volts. Yet I can run one appliance at a time or many. You don't need more voltage to do more things at once. You seem to be confusing voltage with power.
    – David Schwartz
    Commented Aug 18, 2016 at 16:46
  • I could have worded that better, fair point. The limitation is obviously the wattage of the adapter with respect to charging and running the laptop at the same time.
    – Argonauts
    Commented Aug 18, 2016 at 19:04
  • thanks for clearing thinks for could i ask you another question is it okey to buy a compatible battery with not matched part number ?
    – Kamel Mili
    Commented Aug 19, 2016 at 3:51
  • I wouldn't risk it personally but its your judgement call. If they claim compatibility and have a good rep than its probably low risk. If neither are true then I highly recommend against it
    – Argonauts
    Commented Aug 19, 2016 at 3:54
  • i bought one and it didn't worked :'(
    – Kamel Mili
    Commented Feb 8, 2017 at 21:24
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You don't have a 19.5 volt charger. You have a 19.5 volt power supply that supplies power to the laptop. The charger is part of the laptop. There's no particular reason the power supply input should have any particular relationship to the battery voltage because the laptop contains appropriate conversion circuitry. Typically, the charger voltage is significantly higher than the rated battery voltage.

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