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i am building a charging station for my lipo batteries for my hobby. i have 2 lead acid batteries, 12v@18ah and another 12v@22ah want to couple these in parallel to bump up the the amps. i have heard the facts about charging and discharging batteries with different amps. but my question is will 4amps be that big of a difference? if so i will have to go out and find another battery to match one of the two. thanks for the info

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    \$\begingroup\$ Probably OK if they are same sub-type (gel cell vs AGM vs flooded) and same stage of life (new/old). I would DEFINITELY put a fuse on each cell separately, and carefully monitor cell temperature the first few times you charge and discharge them. Also, you need to voltage balance them before you connect them in parallel. \$\endgroup\$ – mkeith Jan 4 '17 at 2:40
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    \$\begingroup\$ 18ah is amp-hours, not amps. \$\endgroup\$ – Pete Becker Jan 4 '17 at 3:06
  • \$\begingroup\$ dont know for sure what sub-type they are i got them from a electric shop that uses the batteries for emergency exit lights. i know they were removed because the shop went to a different system so instead of throwing them out i took them, i was planning to charge them using a battery tender since that does slow charging. \$\endgroup\$ – frank Jan 4 '17 at 4:00
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    \$\begingroup\$ also would there be a difference for the application intended when it comes to "ah" and "amps"? \$\endgroup\$ – frank Jan 4 '17 at 4:03
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You may freely have batteries of the same voltage and chemistry connected in parallel, regardless of their capacities, and charge and discharge them as one.

However, there are several precautions to observe.

Lead acid batteries are not always the same 'chemistry' for charging. The two main types are wet cell, and sealed gel cell. With the former, you fully charge with a constant current, as you are able to top up the water lost. With the latter, you use a constant voltage, and at full charge the current falls low enough that the cell is able to recombine its gases internally to retain its water.

You will probably be lucky in that, as both batteries were from an emergency lighting system, they are overwhelmingly likely to both be gel cells, as emergency systems tend to float charge their batteries to keep them in readiness.

Assuming they are both the same type, then it's good to have them at the same voltage before you connect them.

Lead acid cells can support very high output currents, so it's essential to fuse protect the wiring from them. This really means one fuse per battery, which has the added advantage of protecting either battery from a short failure in the other.

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