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If I have two 12V 18A and I whant to charge them in series making a 24V 18A battery. With how many Voltage and Amps do I Need to Charge this 24V without damaging the batteries.

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    \$\begingroup\$ What kind of batteries are they? Please provide a link to the manufacturer's datasheet. \$\endgroup\$ – Elliot Alderson Aug 30 '18 at 1:04
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    \$\begingroup\$ Do you mean "18A", or "18Ah"? \$\endgroup\$ – WhatRoughBeast Aug 30 '18 at 1:13
  • \$\begingroup\$ Most likely lead acid. Standard max voltage 14.4VDC. \$\endgroup\$ – Jason Han Aug 30 '18 at 1:14
  • \$\begingroup\$ @JasonHan, yeah. Most likely. But it is better to have confirmation from the OP. \$\endgroup\$ – mkeith Aug 30 '18 at 2:41
  • \$\begingroup\$ Thees are the batteries I bought on eBay. rover.ebay.com/rover/0/0/… \$\endgroup\$ – Maicol Rivera Aug 30 '18 at 22:15
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You need to get the voltage and current charge information for the battery. It may be written on the side of the battery, otherwise you'll have to look up the data from the manufacturer.

The numbers you're looking for are "float voltage" (more conservative) or "cycle voltage" (less conservative). The charge current may not be specified, but if it is then use it. If it is not, then "1C" is generally a safe charge current. To calculate 1C, get the capacity of the battery in amp-hours (often written "Ah"). It may be that the "18A" figure you state is actually "18Ah". Then replace "Ah" with "A" to get the "1C" charge rate. For example, if your battery has a capacitor of 18Ah, then the 1C charge rate is 18A.

Finally, and critically, when you put the two batteries in series you must double the charge voltage, but keep the charge current the same.

For example, suppose your float voltage is 13.8V and your 1C rate is 18A. Then to charge two batteries in series, apply a maximum voltage of 27.6V and a maximum current of 18A.

Remember, you can always make this safer by lowering the max current. I advise starting at 1/4C or lower and only increase to 1C if you really want fast charge and are well prepared to handle the potential heat and gas production. See Peter Bennett's 0.2C recommendation in the comments below.

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    \$\begingroup\$ If the batteries are in parallel, you would have a 12 volt bank, with twice the current/amp-hour capacity - so with batteries in parallel, you use the single-battery voltage, but can double the charging current. The recommended charge rate for flooded lead-acid batteries is about 0.2C. \$\endgroup\$ – Peter Bennett Aug 30 '18 at 2:13
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    \$\begingroup\$ I think it should say 'series' rather then 'parallel'. Third paragraph. \$\endgroup\$ – TestDeviant Aug 30 '18 at 6:38

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