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Old Jun 17, 2012, 11:03 PM
K1w1ana is offline
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Help!

8.4v NiMh conversion to Lipo


Hey Guys and Gals,

I was given a Jabo 2 electric Heli, however the NiMh battery packs are dead and won't charge.

I want to switch to LiPo which would be easier than making new packs.

However I can not find one that has the same voltage, the packs for the Heli are 8.4v 800mah or7x AAA 1.2v batteries.

What could I use to replace the NiMh, 7.4v Lipo?

Any help would be great.

Thanks
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Old Jun 18, 2012, 01:12 AM
Ribble is offline
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NiMh 8.4v 800mah or7x AAA 1.2v batteries has a voltage range of roughly 7.0v to 9.8v.

LiPo's
Two 3.7v cells = 7.4v -- voltage range roughly 7.0v to 8.4v
Three 3.7v cells = 11.1v -- voltage range roughly 10.5v to 12.6v

So, a two cell 7.4v LiPo would work. You would have to be careful not to fly it below 6.0v and buy a LiPo battery charger. Using the NiMh charger would be dangerous (fire hazard) and likely ruin the LiPo battery.

If it were me, I would build a NiMh 7 cell battery pack from AAA cells which would be the correct physical size with perhaps higher mah ratings for longer flight time.


Understanding RC LiPo Batteries
http://www.rchelicopterfun.com/rc-lipo-batteries.html
Last edited by Ribble; Jun 18, 2012 at 01:29 AM.
Old Jun 18, 2012, 03:31 AM
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A 2S (7.4v nominal) 800mAh LiPo pack would be much lighter than seven AAA cells, which I think would compensate for the slightly reduced voltage.

But I think the hazards outlined by Ribble outweigh the advantage.
Old Jun 18, 2012, 11:02 AM
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I think they stopped making this heli in 2005. It's a copy of the original Hummingbird FP heli. Parts will be very difficult to find. So you best make sure you have all the parts and the crystals for the transmitter and receiver before getting too involved. If you are sure you have everything, a 7.4v 800mah battery is your best choice. These helis had very marginal motors for the mass they had to lift and this size lipo weighs a lot less than a comparable nimh battery.

It was standard procedure to replace the the 8.4v nimh battery with a 7.4 lipo on all of the old Hiller head 300 size FP helis that used this same design (Hummingbird, Jabo, Walkera 4#, HBFP, etc.). The practice was so common the manufacturers even started doing it as an "upgrade". So, you know you got the real old one, if you got the nimh version.
Old Jun 18, 2012, 01:16 PM
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How about this LiFepo4?

http://www.batteryspace.com/lifepo41...olyswitch.aspx
Old Jun 18, 2012, 01:25 PM
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I would be very careful about increasing voltage. The boards on many of those helis were marginal and it didn't take much extra load to burn out FETs.
Old Jun 18, 2012, 05:33 PM
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my friend has donated to me a HBFP1 variation, but there are so many different parts on it, I don't know what the helicopter actually is, it is partly HBFP1, partly the V2, but it also has the old IMH battery tray style. I am actually running a 3 cell but I it is known that the Esky 4in1 can take this, I am not sure about other variations. All I know is my 4in1 is an Esky 35mhz, so I imagine you can risk a 2 cell surely.

Yeah - I don't know how long mine will last on a 3 cell, I wouldn't recommend it on yours it might not be able to take the voltage. Lol.

(Good to see people still try and attempt to fly these retro-birds, other than just me.
Old Jun 18, 2012, 06:28 PM
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The HBFP V2 used 3S batteries as standard. It realy made a difference in performance.
Old Jun 19, 2012, 07:59 AM
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Yeah, I know, I don't know what this 4in1 is from but it is taking 3s nicely, and is flying much better on the advanced swashplate settings.
Old Mar 09, 2016, 04:02 PM
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Are you still there Microbaba?
Old Mar 09, 2016, 04:27 PM
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Hi Balr14,
Are you still there? Cranky old fart myself. I've got a deal for you you can't refuse if you're interested in an old nostalgic copter that needs a little TLC.
Come back.
Vince
Last edited by vpmccormack; Mar 09, 2016 at 04:29 PM. Reason: missspelled word
Old Mar 10, 2016, 11:16 PM
Balr14 is offline
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OK, my curiosity is peaked?
Old Mar 10, 2016, 11:34 PM
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As stated above, a LiPo cell is nominally 3.7V and fully charged is 4.2V. A NiMH is nominally 1.2V and fully charged is 1.4V. Packs are specified by their nominal voltages, not maximum/charged voltages.

So a 8.4V NiMh pack is 7 cells. Fully charged it's 9.8V. Fully discharged it 7.0V.

A 8.4V LiPo is 2 cells, fully charged. Nominally, it's only 7.4V. Fully discharged it's 6.0V (but the copter might stop flying at a voltage higher than this).

The 2 cell LiPo pack is 7.4/8.4 = 12% less voltage. Since motor speed is proportional to voltage, and power is proportional to RPM, the copter will fly at full throttle on a full charge with about 12% less power on the LiPos than it would on the NiMHs.

It looks like a Sanyo Eneloop 800mAh NiMH is about 12.5g, so 87.5g for the entire pack of 7.

It looks like your average 800mAh LiPo cell comes in at about 20g, so 40g for the entire pack of 2.

Depending on the all up weight of the copter, that 47g (about 2 ounces) lighter LiPo could represent a significant weight reduction. Enough that (as suggested above) it might compensate for the reduced voltage.

You need to get a LiPo voltage monitor, which plugs in to the balance tap of the battery pack and beeps really loudly when a cell hits the programming minimum. Set it for something like 3.5V/cell, plug it in before you take off, and land when you hear it beep. You don't need a "LiPo friendly" ESC for this.

A 3 Cell LiPo is 12.6V fully charged. That'd probably blow up the electronics on the copter.
Old Mar 11, 2016, 09:30 AM
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Quote:
Originally Posted by rocketsled666 View Post
As stated above, a LiPo cell is nominally 3.7V and fully charged is 4.2V. A NiMH is nominally 1.2V and fully charged is 1.4V. Packs are specified by their nominal voltages, not maximum/charged voltages.

So a 8.4V NiMh pack is 7 cells. Fully charged it's 9.8V. Fully discharged it 7.0V.

A 8.4V LiPo is 2 cells, fully charged. Nominally, it's only 7.4V. Fully discharged it's 6.0V (but the copter might stop flying at a voltage higher than this).

The 2 cell LiPo pack is 7.4/8.4 = 12% less voltage. Since motor speed is proportional to voltage, and power is proportional to RPM, the copter will fly at full throttle on a full charge with about 12% less power on the LiPos than it would on the NiMHs.

It looks like a Sanyo Eneloop 800mAh NiMH is about 12.5g, so 87.5g for the entire pack of 7.

It looks like your average 800mAh LiPo cell comes in at about 20g, so 40g for the entire pack of 2.

Depending on the all up weight of the copter, that 47g (about 2 ounces) lighter LiPo could represent a significant weight reduction. Enough that (as suggested above) it might compensate for the reduced voltage.

You need to get a LiPo voltage monitor, which plugs in to the balance tap of the battery pack and beeps really loudly when a cell hits the programming minimum. Set it for something like 3.5V/cell, plug it in before you take off, and land when you hear it beep. You don't need a "LiPo friendly" ESC for this.

A 3 Cell LiPo is 12.6V fully charged. That'd probably blow up the electronics on the copter.
It would be nice if it was that simple, but you are forgetting about the difference in internal resistance between NiMh and LiPo cells.

It's been a long night at work, so I will let someone else explain.
Old Mar 11, 2016, 09:51 PM
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The effect of IR works in favor of the LiPo (meaning the NiMh is likely a higher IR than the LiPo). But this is a small heli, it can't be drawing a lot of amps, so the IR drop probably isn't an awful lot. The difference in pack voltage is likely to be the dominant factor.


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