Parallel Charging for RC LiPo Battery Packs

With the high performance applications that we are all after today, power consumption is about as great as it’s ever been in RC. Whether you are piloting an EDF jet, driving an RC Car or racing an RC fast electric boat a 100 amp power draw is not uncommon. With this much power being drawn, a single charge will be spent quite quickly. The easiest way to combat this is more batteries and faster methods to get them all fully charged. This is exactly where many use the concept of parallel charging for RC LiPo batteries.

The Benefits of Parallel Charging

The concept of parallel charging is not anything new, people have been charging this way for decades. However, using this charging method for LiPo’s has changed the way we have been charging in the past. We are now able to get many battery packs safely up to a fully charged state relatively quickly. The big impact that this charging method has is the ability to charge multiple packs within the same time frame as you may be able to charge one LiPo battery pack.

The Charger you will need for Parallel Charging

Depending on the type of charger you have will ultimately depend on your parallel charging capability. Not all chargers will be able to parallel charge multiple packs within the same time as charging a single pack. In addition to the charger you also must have a parallel charging adapter that will allow multiple battery connections.  Let’s look at an example to get a better understanding of the charging power requirements.

Parallel Charging Board consisting of balance taps and main charge lead

Parallel Charging Board consisting of balance taps and main charge lead

Parallel Charging Example

Let us assume that we have four 6s 5000 mAh battery packs and we are happy with charging this battery pack at a rate of 1C. Charging this battery pack at 1C will translate in to a charge rate of 5 amps. Check out the C rating page for more information on C ratings and how they are used.

A fully charged 6s battery will have a voltage of 25.2 volts. While charging this battery pack at 5 amps, the maximum charging load will equate to 126 watts. In other words, to charge only one 6s battery pack, your charger must be capable of charging at a rate of 126 watts. What happens when we wish to charge four battery packs in parallel? Well, we must multiply our 126 watts by 4, giving us a rand total of 504 watts. The multiplication of 4 is actually applied to the charging current of 5 amps. The result is 20 amps of current will be fed to the 4 batteries being charged in parallel. 20 Amps x 25.2 volts results in 504 watts of power.

Charging Specification Requirements

Your charger must be capable of 3 specifications to charge as above. Firstly, you must be able to charge a 6s LiPo battery. Secondly, you must be able to charge at a rate of 20 amps. Lastly, the charger will have to have the capability of charging at 504 watts total power. These specification are quite demanding and many chargers will not be able to handle the load. If you plan to parallel charge, grab a charger that has the specification you require.

For other charging specifications use the RC Charger Wattage Calculator.

The do NOT do’s of Parallel Charging

Charging batteries can be dangerous when there is an insufficient understanding and rules are not followed. Make sure that you avoid doing the following:

Do not charge LiPo’s in Parallel with varying charge levels

It is highly recommended to charge only battery packs in parallel that have reached the same level of discharge as a percentage. Do not parallel charge battery packs that are at a high degree of varying charge levels. For example, battery packs at a remaining capacity of 70% and 20% must not be charged together. The general rule that I follow and would recommend is charging battery packs that are within 30% or 0.1 volt. Anything outside of this is too much.

Do not charge LiPo’s with varying cell counts

When charging LiPo battery packs in parallel, you can not have LiPo’s of a different cell count. One rule you must follow with parallel charging is being certain all LiPo’s batteries to be charged are equal in cell count.

Do not force the Balance Adapter to charge the batteries

Balancing LiPo batteries happens through a balance tap that is a separate harness on all LiPo batteries. All LiPo batteries that are being charged in parallel must have the main power plug plugged in to the parallel charging adapter. Forgetting to plug the main power leads in to the charging adapter will cause the charging to occur in the balance leads. The balancing harness is not designed to handle much more than one amp of current. Forcing more current through the balance leads will cause them to over heat, leading to a dangerous situation. Be certain to always connect the main power leads to your parallel charging board first. Do not connect the balancing plug first.

Do not use resistance data when parallel charging

Battery resistance information will only display when you have a charger that is capable of determining its value. Charging LiPo’s in parallel will throw off the calculation that the charger performs for resistance. For example, two equal batteries have a resistance of 10 milliohms. Placing these two batteries in parallel will reduce the resistance that the charger sees to 5 milliohms. Additional packs added in parallel will further reduce the total resistance that the charger is capable of determining. Do not use the resistance value provided by your charger when parallel charging. Otherwise, you will be lead down the wrong path when evaluating your LiPo’s overall health.

Do not immediately begin charging your LiPo batteries

When charging your LiPo batteries in parallel, it is critical to not begin charging right away. The point of this is to allow your packs to balance out among all the battery packs that are connected in parallel. The time that you must wait until starting the charging process is relative to the difference between the least discharged pack to the most discharged battery pack. There are many devices that will allow you to plug the battery in to a voltage / capacity meter in order to determine the state of charge.

The chart below identifies the amount of time required to wait prior to charging the battery pack.

Time to Wait before Starting the Charger for Parallel Charging

Time to Wait before Starting the Charger for Parallel Charging

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