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I've got an RC car with a 4xAA battery pack as one unit. I looked for a replacement battery and typically these are 700-1000mAh rarely exceeding this capacity. The 700mAh battery costs as much as 4 separate AA 2800mAh batteries.

I was wondering why are the typical capacity for these types of batteries on the market are so low as opposed to individual AA accu batteries? Can I just assemble 4 x 2800mAh individual batteries and get a 2800mAh battery pack that will run 4 times as long? Will the original charger work OK for this?

Or are there any issues with this approach? Maybe there's a reason they don't do 2800mAh batteries for RC cars (or they're not widely spread)?

Thanks for your advice

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  • \$\begingroup\$ Weight and charge balancing. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 15 '17 at 17:02
  • \$\begingroup\$ Some batteries can support higher current than others. The capacity is often inversely proportional to the max output current. If you want a good answer, you should post the specs of your 4xAA batteries. Capacity, max output current, brand, model, chemistry, anything you can find. \$\endgroup\$ – Dampmaskin Mar 15 '17 at 17:04
  • \$\begingroup\$ I would think the difference in weight to be negligible? What is charge balancing? \$\endgroup\$ – Just a user Mar 15 '17 at 17:04
  • \$\begingroup\$ @Dampmaskin these specs are not in the details on the website I am looking at. There are numerous (subpair) brands in this price range. But they are all NiMh as opposed to the original NiCd. Does this affect anything? \$\endgroup\$ – Just a user Mar 15 '17 at 17:13
  • \$\begingroup\$ @IgnacioVazquez-Abrams the original battery doesn't look like it has any battery balancer. Just 4 batteries stuck together with 2 wires coming out. \$\endgroup\$ – Just a user Mar 15 '17 at 17:16
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Rechargeable AA come in either Nickel-Metal Hydride or Nickel-cadmium

If the battery pack is 4.8 volts the capacity will not increase from a single 1.2 v battery.

If you connect four 1.2v 2000 mAh in series you will have 4.8 volts @ 2000mAh.

You CANNOT use a NiCd charger to charge NiMH. NiCd can be charged in 1-2 hours where NiMH require 3-4 hours. NiMH have a low tolerance for over charge.


You can use a NiMH charger to charge a NiCd. NiCd are very forgiving and safe.

If the voltage matches the original batteries, they will very likely work in your car. A drone or a precision racing model may have some problem due to weight differences.



Nickel-cadmium (NiCd)

Capacity: 600 - 1000 mAh 1.2v

Pros

  • Rugged, high cycle count with proper maintenance
  • Only battery that can be ultra-fast charged with little stress
  • Good load performance; forgiving if abused
  • Long shelf life; can be stored in a discharged state, needs priming before use
  • Simple storage and transportation; not subject to regulatory control
  • Good low-temperature performance
  • Economically priced; NiCd is the lowest in terms of cost per cycle
  • Available in a wide range of sizes and performance options

Cons

  • Relatively low specific energy compared with newer systems
  • Memory effect; needs periodic full discharges and can be rejuvenated
  • Cadmium is a toxic metal. Cannot be disposed of in landfills
  • High self-discharge; needs recharging after storage
  • Low cell voltage of 1.20V requires many cells to achieve high voltage



Nickel-metal-hydride (NiMH)

Capacity: 800 - 2300 mAh 1.2v

Pros

  • 30–40 percent higher capacity than a standard NiCd
  • Less prone to memory than NiCd, can be rejuvenated
  • Simple storage and transportation; not subject to regulatory control
  • Environmentally friendly; contains only mild toxins
  • Nickel content makes recycling profitable
  • Wide temperature range

Cons

  • Limited service life; deep discharge reduces service life
  • Requires complex charge algorithm. Sensitive to overcharge
  • Does not absorb overcharge well; trickle charge must be kept low
  • Generates heat during fast charge and high-load discharge
  • Hig self-discharge
  • Coulombic efficiency only about 65% (99% with Li-ion)
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  • \$\begingroup\$ +1 because the answer has some useful material - but also some which to some extent misses the point. You can also (probably) still get NiCd AA - and it is possible that the discharge currents are a major factor. On short circuit a good retail NimH gives 10A + but "doesn't like it". I do not know the spec for his NiCd As. \$\endgroup\$ – Russell McMahon Mar 16 '17 at 4:05
  • \$\begingroup\$ @RussellMcMahon Thanks, I mistakenly put the wrong title over the NiCad. \$\endgroup\$ – Misunderstood Mar 16 '17 at 4:18
  • \$\begingroup\$ @russell so you think 4x NiMH = 6V? And that lithium cells are 1.2V? \$\endgroup\$ – Passerby Mar 16 '17 at 4:19
  • \$\begingroup\$ ... energizer e2 lithium is normal 3.7V chemistry with a built in buck converter to 1.5V. Energizer does not market a 1.2V lithium battery in AA form factor, and it would be stupid of them to do so. \$\endgroup\$ – Passerby Mar 16 '17 at 4:43
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    \$\begingroup\$ I am not aware of a rechargeable 1.2V Li-Ion or Li-Poly battery / cell. Please post a link where this can be found. \$\endgroup\$ – Dwayne Reid Mar 16 '17 at 4:51
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Batteries made for use in RC racing cars are designed to deliver high current with low voltage drop. To do this they have relatively thick plates. High capacity batteries use thinner plates to leave more room for active material, which increases their resistance and reduces the maximum discharge current. Depending on how much current the motor in your car draws you may get away with using high capacity NiMH cells, but you probably won't get the run time you would expect based on their stated capacity.

Cheap high capacity consumer cells often have a low maximum discharge current and are not tolerant of abuse. Many of them do not achieve their stated capacity at any current because they are 'optimistically' rated. However they may still be better than the original cells in your car (which wouldn't be using AA cells if it was designed for high performance!).

Before deciding which batteries to buy you should examine independent test data to determine whether they will be suitable. Here is a website which has test data for several popular AA rechargeable cells, whose performance can be compared at various discharge currents. Choose cells which hold their voltage up best at high current (5-10A) rather than selecting for highest capacity.

Building your own pack is possible, but unless you have access to a spot welder you will have to solder the cells together. This must be done quickly to avoid cooking the cells (particularly the positive end which has a plastic separator that must not be allowed to melt). First clean the battery contacts with fine sandpaper, then immediately 'tin' them using a high wattage iron and rosin-cored solder (should take no longer than 1 second). For the connections between cells I use thin desoldering braid saturated with solder.

Your existing charger will probably work fine, but will take 2-3 times longer to charge the higher capacity battery. Chances are that it is a simple 'trickle' charger that just delivers a fixed current with no shutoff, which is safe provided the expected charge time is 10 hours or more. If it is a more sophisticated 'peak detect' charger (designed to charge the original battery in about 1 hour) then it may not be suitable because NiMH batteries have a different charging profile.

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  • \$\begingroup\$ Eneloops and similar are high capacity (2000 mAh) and tolerate easily 4-8C discharge. I would say, those are the suitable replacement for old NiCd packs (privided a correct charger is used). \$\endgroup\$ – FarO Nov 1 '17 at 10:22
  • \$\begingroup\$ lygte-info.dk/review/batteries2012/… \$\endgroup\$ – Bruce Abbott Nov 1 '17 at 19:31

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