How to Charge Parallel Connected Batteries

Written by david robinson
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How to Charge Parallel Connected Batteries
Charging parallel batteries requires knowledge of their electrcial characteristics. (battery image by Edsweb from

Rechargeable batteries are everywhere. From wrist watches to car engines, we rely on batteries to deliver portable power. However, as we use up their power, the time comes when they need recharging. With individual small batteries this often involves popping them into a charger for a few hours. However, when several batteries connect in parallel to form a battery array, the charging method must be different to cope with the electrical properties of parallel circuits. Charging parallel batteries requires a basic understanding of electricity and access to one or more battery chargers.

Skill level:
Moderately Easy

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Things you need

  • Battery charger (suited to the voltage and current of the batteries to be charged).

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  1. 1

    Identify the voltage of the batteries. Batteries linked in parallel should have identical electrical characteristics so they will all have the same voltage. Parallel circuits do not increase voltages, so parallel battery packs have the same voltage as the individual batteries within them.

  2. 2

    Calculate the current rating of the battery. In a parallel circuit, current is cumulative. Add the amp/hour ratings of the individual batteries to find the total amp/hour capacity of the parallel battery.

    For example, nine-volt rechargeable batteries may have an amp/hour capacity of around 175 mAh, so three in parallel would have a combined rating of 3 x 175 mAh, or 525 mAh. Likewise, five car batteries, rated at 200 Ah and linked in parallel, have a rating of 1,000 Ah.

  3. 3

    Use a battery charger capable of delivering approximately the same voltage as your parallel battery. So, for a parallel battery delivering 12 volts, use a 12- to 14-volt battery charger such as a typical car battery charging unit.

  4. 4

    Deliver the correct current to the charging batteries. Use a charging current equivalent to 10 to 30 per cent of the battery's am/hour rating to charge lead/acid batteries. Most commercial battery chargers designed for lead/acid batteries will do this for you.

  5. 5

    Provide a current between 10 per cent and the entire amp/hour rating to Nickel-cadmium (NiCd) batteries depending upon the length of time spent charging. Each different type of battery chemistry requires different charging times, so consult the data that came with your batteries to establish the optimum length of charge.

  6. 6

    Connect the battery charger across the positive terminal at one end of the parallel battery and a negative terminal at the other end. Allow the battery to charge at the voltage and current applicable to the type of battery, for the length of time recommended in the battery technical data sheet.

Tips and warnings

  • Lead/acid batteries can be "trickle charged"; fed a constant low current charge as a way of charging them. One car battery charger can charge two or three lead acid batteries linked in parallel.
  • Some types of batteries, in particular Lithium Polymer (LiPo) batteries, require charging only in charger units designed specifically for those batteries. Overheating LiPo batteries is easy to do and may result in fire, injury or explosion.
  • Always monitor batteries while they are charging. Smoke, excessive heat or bubbling sounds indicate overcharging. If these symptoms are evident, cease charging immediately.

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