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I'm trying to fix my Robbe Futaba transmitter. According to the manual it can handle 9.6 V. The old battery was broken because I didn't disconnect it for a long time (5 years), so now I'm trying to figure out which battery is the best one.

The original battery has "8 x NiMH 1500 mAh 1.2 V" batteries (in series).

  1. Is it fine if I find a NiMH battery of 9.6 V but higher mAh?
  2. Is it a good idea to find a higher voltage battery?
  3. Can I use a 11.1 V 2200 mAh Li-Po battery on it?
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    \$\begingroup\$ Higher capacity only means that there is more energy is stored in the battery, that is a prolonged usage. As for the voltage, you should consult the specification of your transmitter and see what range is acceptable. \$\endgroup\$
    – Eugene Sh.
    Commented Nov 9, 2021 at 14:36
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    \$\begingroup\$ It depends on the regulator inside. Remember a nominally 11.1V 3S LiPo is 12.6V when full. You could also try LiFe 3S which is only 10.8V when full and 9.9V nominally or see if it runs on a 2S LiPo. \$\endgroup\$
    – DKNguyen
    Commented Nov 9, 2021 at 14:50
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    \$\begingroup\$ I recommend you read electronics.stackexchange.com/q/34745/2028 to understand voltage and current of power supplies (including batteries). \$\endgroup\$
    – JYelton
    Commented Nov 9, 2021 at 16:25
  • \$\begingroup\$ Damn i just realised of your answers now. Im new here and i didnt know there are small answers.. damn this forum its so nice, you are all insane!! Yes sure, i will read and investigate more about this, thank so much @JYelton !! Have all a nice day :) \$\endgroup\$
    – Alex Arte
    Commented Nov 9, 2021 at 16:40

1 Answer 1

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Can you use a battery with the same chemistry, same rated voltage, but highter rated capacity?

Yes.

Higher capacity (measured in mAh) means that for the same use, the battery will last longer. It should not have any other effect (assuming the circuit does not rely on the battery's internal resistance).

Can you use a battery with a higher rated voltage?

In general, it is not a good idea. However, circuits might (or might not) have tolerances. Without knowing the circuit, it is probably a bad idea to try.

Also, the batteries themselves do not have a voltage always strictly equal to the rated voltage. The rated voltage is often a mean voltage or an approximate voltage the battery will keep most of the time. Batteries usually have a higher voltage when fully charged and a lower voltage when discharged enough.
For example, for an NiMH cell discharged with a current C/10 (where C is the rated capacity in mAH), the discharge curve is the following and the voltage starts at about 1.5 V:

enter image description here

The bottom line is: the real voltage of your battery pack when fully charged is 8 x 1.5 V = 12 V. So you should be fine applying maximum 12 V to the device. BUT...

Can you use an 11.1 V 2200 mAh Li-Po battery instead of a 9.6 V 1500 mAh NiMH battery?

Maybe, maybe not. It is best not to try.

As discussed above, the rated battery voltage is not the real voltage applied. The typical Li-Po battery has a rated voltage of 3.7 V and the following discharge curve:

enter image description here

The voltage goes from around 4.2 V or 4.3 V down to 3 V or 2.7 V (depending on the protection circuit). It means that your 11.1 V battery (composed of 3 cells in series) has a real voltage of around 3 x 4.3 V = 12.9 V when fully charged, which is higher that the maximum voltage of your NiMH battery.

Without knowing the circuit and the maximum input voltage it can tolerate, it is better not to try it.

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  • \$\begingroup\$ Oh my god, you are the best! Im so sorry for asking this questions and not having much idea.. but damn, thank you so much!! And according to the answers, i will play safe and go for 9.6V NiMH battery with 1500mAh or higher mAh. Lets see what i can find out!! You are the best! \$\endgroup\$
    – Alex Arte
    Commented Nov 9, 2021 at 16:05
  • \$\begingroup\$ @AlexArte Never be sorry for asking questions, that's what this whole site is for. Enjoy! \$\endgroup\$
    – Sacha
    Commented Nov 9, 2021 at 16:11
  • \$\begingroup\$ Same battery chemistry, higher capacity rating is usually the best way to go. If you use it with the old charger it may take longer to fully charge -- but then, it'll take longer to fully discharge, so that's OK, right? \$\endgroup\$
    – TimWescott
    Commented Nov 9, 2021 at 16:14
  • \$\begingroup\$ Yep thats right TimWescott, i dont mind if it will take more time to charge! And the time with the 1500mAh was soo long, if i find out a 2000mAh or more it would be amazing :) \$\endgroup\$
    – Alex Arte
    Commented Nov 9, 2021 at 16:36
  • \$\begingroup\$ Yes, the 1500mah NiMH should be common as dirt in the popular AA form factor. \$\endgroup\$ Commented Nov 10, 2021 at 1:16

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