How Long Does It Take To Charge A NiMH RC Battery?


How Long Does It Take To Charge A NiMH RC Battery?

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If you recently purchased an RC car that comes with a NiMH battery or if you are just curious about how long it takes to charge an RC battery then you are in the right place! RC batteries are obviously vital to using RC cars so knowing how long it will take before you can use your car again is pretty important information. 

The time that a battery takes to charge depends on the size (mAh) of your NiMH battery as well as the amps that you are charging it at. If you have a 3,000 mAh battery and your charger charges at 1,000 mA then it will take 3 hours to charge the battery. 

Unfortunately since there are so many variables in batteries and chargers then the exact time for your battery to charge has to be figured by you based on the battery size and charger you own. For example, this charger found on Amazon can charge up to 5,000 mA so if you have a 3,000 mAh battery it will take under an hour to charge! 

It’s easy to figure out approximately how long it will take to charge a battery once you know the proper formula. Battery capacity is measured in mAh, milliamp-hours. Charge currents are measured in A, amps, divide by 1000 and you have milliamps. Milliamps times hour makes milliamp hours. 

You can use any combination of milliamps and hours to charge a 1800mAh battery, as long as the product of milliamps and hours makes 1800, and hours is no smaller than 1. So you can charge at 180 mA for 10 hours or 900 mA for 2 hours, or 450 mA for 4 hours, and so on. These figures can only work if the battery is entirely out of charge, that is, if it is run until it automatically shuts itself off/dies. 

If your battery is not 100% dead you really can’t tell if your battery is 9% full or 80%. If you put your battery on a 450 MA charge for 4 hours, and it is 85% charged to begin with, you will heat the battery. 

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NiMH batteries do not like to be overcharged. You will want to ensure you have a  peak detect charger, as it’s capable of charging NiMH batteries without allowing them to overcharge. 

NiMH charging basics

NiCad and NiMH batteries are the most difficult batteries to charge accurately. For lithium-ion and lead-acid batteries, it is easy to control overcharge because you are required to set a maximum charge voltage. The nickel-based cells lack a float charge voltage so the charging is based on forcing current through the battery. For other batteries, the energy is fixed in stone, but for NiMH, that not the case.

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Parallel charging

Parallel charging makes the cells and batteries hard to charge in parallel. This is because you can’t be sure that every cell is the same impedance, and so some will take more current than others even when they are full. Meaning you need to use a different charging circuit for every string in a parallel pack or balance the flow in some other way.

The coulometric charging efficiency of nickel-metal hydride batteries is typically 66%, which means you must put 150amp hours into the cell for every 100amp hours you get out. The quicker you charge, the worse this gets.

The minus delta V bump that is indicative of the end of charge is much less pronounced in NiMH than NiCad, and it’s very temperature dependent. New NiMH batteries can exhibit bumps in the curve early in the cycle, unusually when cold. Besides, NiMH is sensitive to damage on overcharge when the charge rate is over c/10. Since the delta-V bump is not always easy to see, a slight overcharge is probable. This is why the power stream does not recommend using simple minus delta V as a termination criterion for nickel-metal hydride batteries.

However, modern algorithms have been developed to enable correct charging without using a thermistor. These chargers are the same as the delta-V charger but have a unique measurement strategy to detect a full charge, usually involving some pulse cycle where the voltage is measured during the pulse and between pulses. For multi-cell packs, if the cells are not all at the same state of charge, and if they are not balanced incapacity, the cells may fill up at separate times. 

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Fortunately, NiMH does not mind being overcharged at C/10 or less, which allows the charger to balance the cells during the trickle charge. As the battery reaches the end of command, oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured with a thermistor. This is the safest way to detect the end of charge during a fast charge.

Overnight charging

The easiest way to charge a nickel-metal hydride battery is to charge at C/10 or below. So a 100 Mah battery would charge at 10Ma for 15 hours. This method does not need an end of charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst that prevents damage to the battery on overcharge, but this recycling can’t maintain if the charge rate is above C /10. The minimum voltage you need to get a full charge varies with temperature.

Even though constant charging at C/10 does not cause venting, it does warm the battery slightly. For your battery to last, extended use of a timer to prevent overcharging is the best practice. 

Faster charging

It’s possible to charge at C/3.33 for 5 hours when using a timer. It’s a bit risky since the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of damaging the battery. One way to make sure this doesn’t happen is to have a charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours. The advantage of this method is to eliminate any chances of battery memory.

Trickle charging 

In standby, you might want to keep a nickel-metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and 0.05 C. The voltage required for this is dependent on temperature, so be sure to regulate the flow in the charger.

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Guidelines for charging nickel-based batteries.

  • The charge efficiency of nickel-based batteries is close to 100% up to 70% charge. The pack remains calm, but it begins to warm up with decreased ability when it gets closer to a complete charge.
  • Nickel-based batteries must stay cool/ cool down on trickle charge. If warm, the trickle charge is too high.
  • The consumer charger does not always terminate the charge correctly. Remove the battery when warm to the touch. Stop using a charger that heats cells.
  • Charge only at room temperature. You should avoid charging your batteries when hot or at freezing temperatures.
  • Nickel-based batteries are best fast-charged.
  • Nickel and lithium-based batteries require different charge algorithms. A NiMH charger can also charge NiCd; a NiCd charger would overcharge NiMH.
  • Do not leave a nickel-based battery on the charger for days. If possible, remove the packs and apply a brief charge before use.

Conclusion

The amount of time that it takes to charge your NiMH battery for your RC vehicle will vary based on the size of the battery as well as the charger that you are using. If you are using a faster charger then it will obviously charge the battery faster! 

The more expensive a charger that you get the more options that it will have and often the faster that it will charge your NiMH battery. There are a variety of cheap chargers but if you spring for a better, more expensive one it will cut down on your charging time by A LOT. For example, this Tenergy charger is able to be used for virtually all RC catteries that you might have and it is incredibly fast as well. 

If you are new to RC cars it is normally better to buy a quality charger up front so you never have to worry about your charger ever again even if you change the type of battery in your RC vehicle. 

Matt Robbs

I love to spend time with my wife and 3 kids. There is no better way to get them off the couch and outside than for us to grab the RC cars or boat and enjoy the sunshine!

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