you plan to connect . The terminals will provide the connection point to each battery and should be tight and clean , as should the battery terminals themselves . These terminals should also be capable of appropriate / reliable termination with the gauge of cable you are using . Finally , the wire gauge itself should be the same for every jumper cable and connection cable being used , this guarantees that all cabling can handle the same current level and there is no heating of cables under high load due to size variation . ( Thinner cables will present a higher resistance under load , so will heat up more at the same length than a thicker cable might )
3 - Batteries of equal age , size , resistance / state of health and capacity must be used in this application . It is extremely important that the batteries used are identical both electrically and physically . This will mean the batteries operate successfully in unison as one larger unit . The resistance of each battery is most important as it will impact the rate at which an individual battery charges and discharges , so when identical units are wired in parallel , the two batteries will truly act as one . Battery current size is also paramount . Connecting a 100-amp battery to a 120 amp battery will not yield best results as the smaller battery will dictate the overall performance of the package . Equally , if one of the two batteries is down on performance and a fresh new battery of the same current rating is added in parallel , the lesser battery will instantly draw down the new battery and the overall package will operate as two of the lesser batteries in terms of performance . If you are considering a parallel battery package it would be advantageous to start with two batteries of the same size , current rating , age and even batch number if possible , that way you are assured of the desired outcome .
4 - Sufficient charger capacity to satisfy the larger battery pack during the recharge process should be considered Charging your parallel battery pack should be considered as well , because a charger suitable for a single battery will charge a parallel pack at approximately half the rate or twice the time depending on your point of view . Using a 100-amp AGM battery as an example , these batteries generally charge between a low level of 10 amps to a higher level of charge at 20 amps per battery . Once a parallel setup is completed , a charger capable of 20 amps is now charging each battery at approximately 10 amps which is generally considered a little on the low side . Replacing your charger to achieve the most appropriate supply level is not mandatory but it will bring back the charging timeline and parameters to a level you are accustomed .
5 - Recognition of the types of batteries this wiring format will suit Finally , it should be noted that not all battery types perform the same way in the parallel format . The parallel format suits AGM and Gel batteries and will also work with wet cell batteries as found in motor vehicles , however it will not work the same way with Lithium batteries which have a BMS system on board , these batteries will be limited by that onboard BMS system meaning that a parallel setup of 2 x 100 amp BMS equipped Lithium batteries , will still only supply 100 amps output , as the BMS units will control the current output to match the BMS parameters . Ultimately the Lithium supply will last longer than AGM or Gel batteries due to the larger depth of discharge that a lithium battery can provide , but the current delivery itself will not exceed the capacity of the Lithium ’ s onboard BMS .
So , if a parallel battery setup has been in the back of your mind , there is nothing to stop you from having the benefits available to you . Simply use the correct products to set it up and enjoy the peace of mind offered by the larger battery capacity you will have available to you !