Flight packs get all the attention, but the two small batteries that keep you in control are the ones in your radio and, on gas, glow and big electric planes, the one powering your receiver and servos. When a motor pack runs low, you land a little early. When a receiver pack quits, the plane stops listening.
This guide covers what goes in the transmitter (AA cells, NiMH, Li-ion, LiPo and LiFe packs), how to pick a receiver battery your servos can handle, and how to test an old pack before you trust it with a model. We use Spektrum and Futaba gear as examples, because that's what most of our customers fly, but the rules are the same for any brand. Your radio's manual has the final word on what it accepts.
The Short Answer
Use the battery chemistry your transmitter's manual lists, and set its low-battery alarm to match. For receiver power, pick a pack every servo is rated for: 2S LiFe works with most standard gear, 2S LiPo only with high-voltage servos.
- Never charge alkaline AAs. Charge only rechargeable cells, and only the way the manual says.
- Switching a radio to LiFe or Li-ion usually means changing the low-battery alarm and using a matching charger.
- Older servos are typically rated for 5-cell NiMH. A 2S LiPo peaks at 8.4 V, which is too much for them.
- A voltage reading isn't a health check. Discharge an old pack on your charger and see what capacity it actually delivers.
1. What Goes in Your Transmitter
Transmitters are built around one battery type, and the manual names it. Most problems come from mixing chemistries without changing the alarm, or from charging the wrong thing.
AA cells: alkaline or rechargeable
Entry radios like the Spektrum DXe run on four AA cells. The DXe manual says that if you use rechargeable batteries, charge only rechargeable batteries, and warns that charging non-rechargeable batteries may cause them to burst. Rechargeable NiMH AAs are a fine choice in a radio that accepts them. Keep in mind a NiMH cell is about 1.2 V nominal against an alkaline's 1.5 V, so a set of NiMH cells starts lower on the radio's battery meter. That's normal. Pick a quality low self-discharge cell, and replace or recharge all four together.
NiMH packs
Many radios ship with a NiMH pack. Futaba's T14SG manual lists a 6.0 V, 1,800 mAh NiMH pack (HT5F1800B) as standard. The instructions for that pack note that NiMH self-discharges fairly quickly, so charge a new pack or one that's sat for a while before you fly. They also say the included charger doesn't stop on its own: end the charge at the specified time (15 hours for that charger). Older radios built around a 9.6 V, 8-cell AA NiMH pack still turn up at the field, and replacement packs are easy to find. Match the voltage and plug exactly.
Li-ion and LiPo packs
Newer Spektrum radios use lithium packs. The NX6, NX7e, NX8 and NX8+ take a 1S 3.7 V Li-ion pack, and Spektrum also lists a 6,600 mAh 1S pack for the NX6, NX8, NX10 and iX12 if you want longer run time. Older DX8 and DX9 radios take a 2S 7.4 V LiPo; Horizon says its 4,000 mAh DX8/DX9 pack has a built-in circuit that shuts the battery off to avoid overcharging, plus built-in balancing. Use the charge port and procedure in your radio's manual. The DXe manual, for example, says never to connect an external battery charger to the transmitter.
LiFe packs and the low-battery alarm
LiFe (lithium iron phosphate) packs are a popular upgrade for radios that started on NiMH: lighter, and they hold a charge in storage. But they sit at a different voltage, so the alarm has to change. Futaba's T14SG manual says to raise the low battery warning from 5.6 V to 6.0 V for LiFe, that the LiFe pack needs a spacer in the battery bay, and that LiFe can't be charged with the radio's included charger. Remove it and charge it on a LiFe charger. Skip the alarm change and the radio may let the pack run far lower than it should before it warns you.
2. Choosing a Receiver Battery
Small electric planes power the receiver from the ESC's BEC and don't need a separate pack. Gas and glow models, and many large electrics, run the receiver and servos from their own battery. The right pack is the one every part of that system is rated for, and the servos are usually the limit.
The receiver is rarely the limit
Modern receivers accept a wide voltage range. The Spektrum AR637T manual lists an input voltage of 3.5 to 9 V, so the receiver itself is happy on 4-cell NiMH, 2S LiFe or 2S LiPo. The question is your servos (and, on a gas plane, the ignition). Look up the voltage rating of every servo in the model before you pick a pack.
Pack choices side by side
| Pack | Nominal | Full charge | Best fit |
|---|---|---|---|
| 4-cell NiMH | 4.8 V | about 5.8 V | Older standard servos, light models |
| 5-cell NiMH | 6.0 V | about 7.5 V | Standard servos rated for 6 V |
| 2S LiFe | 6.6 V | 7.3 V | Most standard servos and ignitions |
| 2S LiPo | 7.4 V | 8.4 V | HV servos only (or with a regulator) |
Horizon's article on old RC batteries explains the cutoff: older servos are typically rated for a 5-cell pack at about 1.5 V per cell peak, a 2S LiPo at 4.2 V per cell is too high for those systems, and a 2S LiFe at 3.65 V per cell peak is usually acceptable for older radios and ignitions.
Why LiFe is the common upgrade
LiFe gives you more capacity for the weight than NiMH, without the LiPo voltage problem. Spektrum's 6.6 V 2,200 mAh LiFe pack is listed for both aircraft receivers and ignitions, with a standard charge rate of 0.2C and a maximum of 1C. If your servos are all HV-rated, a 2S LiPo receiver pack is the lighter choice. Spektrum's Smart 2S LiPo receiver pack adds charge protection and an auto-storage feature that drains it to storage voltage after a rest period you set. Horizon's article adds one rule for LiPo receiver packs: take them out of the model to charge.
3. Test Old Packs Before You Trust Them
A receiver pack that's been sitting in a hangar since last season is the most common way a healthy-looking plane goes down. The fix is a simple bench test.
Voltage isn't capacity
In the Horizon article, a new-but-old 5-cell NiMH receiver pack read about 2.5 V and the plane lost control. The article also warns about a false peak: a tired NiMH pack can reach normal voltage and fool the charger into stopping early. A pack that takes much less than its rated capacity on charge is telling you something.
Run a discharge test
- Fully charge the pack on a charger that has a discharge function.
- Discharge it and read the capacity delivered. For NiMH, the article says to stop at 0.9 V per cell.
- If it delivers close to its rating, it's good. A NiMH pack stored for months or years may need 6 to 10 charge and discharge cycles to come back; if capacity climbs each cycle, keep going.
- If capacity stays well below the rating, retire it and drop it at a battery recycling location.
Watch it in the air, too
Telemetry receivers such as the AR637T report receiver battery voltage to your transmitter. Set a voltage alarm for it, and check the pack with a battery checker before every flying session. Two minutes on the bench beats a walk into the woods.
Conclusion
Power your transmitter with the chemistry its manual lists, and when you switch to LiFe or Li-ion, change the low-battery alarm and the charger too. For receiver power, pick the pack your servos are rated for: 2S LiFe for most standard gear, 2S LiPo only with HV servos. Then test old packs with a real discharge, not just a voltmeter.
Not sure what your servos can take, or whether last year's receiver pack is still good? Bring it to our Cumming showroom and we'll check it with you, or contact the shop.
Gear Mentioned in This Guide
Spektrum 2 Cell 2000mAh 7.4V Smart LiPo Rx Pack with IC3 & Servo Connector (SPMX20002SRX)
A 2S receiver pack for HV setups, with charge protection and auto storage built in. Check every servo is rated for 2S LiPo first.
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Spektrum 2 Cell 4000mAh 7.4v LiPo Tx Battery: DX8, DX9 (SPMB4000LPTX)
Higher-capacity 2S pack for the DX8 and DX9, with built-in balancing and overcharge shutoff.
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Onyx 8 Cell 2000mAh 9.6V AA NiMH Tx Battery: Universal Rx (ONXM2213)
An 8-cell AA NiMH pack for older radios built around a 9.6 V square pack. Match your radio's plug and voltage before you buy.
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Spektrum Smart Battery Checker & Servo Tester XBC200 (SPMXBC200)
Check per-cell voltage on receiver and transmitter packs at the field, and exercise servos on the bench.
Shop nowFrequently Asked Questions
Can I use rechargeable AA batteries in my transmitter?
Yes, if your radio takes AA cells. Charge only rechargeable cells, never alkalines, and expect the battery meter to read lower because NiMH cells run about 1.2 V each versus 1.5 V for alkaline.
What is a LiFe receiver pack?
A LiFe (lithium iron phosphate) pack is a lithium battery at 3.3 V nominal per cell. A 2S LiFe pack is 6.6 V nominal and peaks around 7.3 V, which most standard servos and ignitions can handle, and it's lighter than NiMH for the same capacity.
Can I run my receiver on a 2S LiPo?
Most modern receivers accept it (the Spektrum AR637T is rated 3.5 to 9 V), but a 2S LiPo peaks at 8.4 V. Only do it if every servo is rated for high voltage, or use a regulator.
Do I need to change settings when I switch to a LiFe transmitter pack?
Usually, yes. Futaba's T14SG manual, for example, says to raise the low-battery warning from 5.6 V to 6.0 V for LiFe and to charge the LiFe pack outside the radio on a LiFe charger.
How do I know if an old receiver battery is still good?
Charge it, then discharge it on your charger and read the capacity it delivers. A pack well below its rating, even after several cycles, should be retired.
Sources and Further Reading
- Spektrum DXe transmitter manual (PDF)
- Futaba T14SG instruction manual (PDF)
- Futaba HT5F1800B battery instructions (PDF)
- Horizon Hobby: The Threat of Old RC Batteries to a Model RC Plane
- Spektrum 6.6V 2200mAh 2S LiFe Receiver Battery (SPMB2200LFRX)
- Spektrum 7.4V 2000mAh 2S Smart LiPo Receiver Battery (SPMX20002SRX)
- Spektrum AR637T receiver manual (PDF)
- Horizon Hobby: Spektrum 3.7V 2200mAh 1S Li-ion Transmitter Battery
- Spektrum 3.7V 6600mAh 1S Li-ion Transmitter Battery
- Horizon Hobby: Spektrum 7.4V 4000mAh 2S LiPo Transmitter Battery
