newbe question - weird futaba charger - RC Groups
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Feb 20, 2011, 02:42 PM
Registered User

newbe question - weird futaba charger

Hello, I'm hoping someone here can help explain these chargers to me as I have been searching around and have still not found an answer.

I recently bought a second hand futaba 9c which came with an FBC-2J charger. On the charger it states output as 45ma AC not DC (see attached picture).

I don't understand how AC would be charging DC batteries and am worried that its actually the wrong charger. I don't want to use it on the radio or receiver until I figure out if this is right.
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Feb 20, 2011, 02:49 PM
Registered User
Digging around in my old box of spares I also found another Futaba FBC-3 oplader which also shows the AC symbol. I think this charger came with a 4ch AM attack transmitter around 25 years ago but i'm not sure.

Is this just a typo? My Tx battery is standard 9.6V 700mah NT8S700B which I think is the original.
Feb 20, 2011, 03:55 PM
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LeszekJ's Avatar
I've got the FBC-30B.
Exactly the same voltages.
Output is DC.
Feb 21, 2011, 01:10 AM
Registered User
Thanks, I'll assume its just a typo then. If I can get hold of a multi-meter I'll try and test it.
Feb 21, 2011, 03:26 AM
Registered User
I guess they're refering to the transformer in the charger, for "Primaer" and "Sek" must be for primary and secondary windings -- the input and output of a basic transformer.

There must also be a rectifier inside the box, in which case I would expect dc multimeter readings to be higher than 5.8 - 11.6 volts shown on the casing. You might see something like 8v and 16v with no load.
Feb 23, 2011, 09:14 AM
Registered User
Thanks Abenn, looks like your right. I got hold of a multimeter and had the following readings:

Tx charge point DC output = 9.23V, AC output 7.63V.
Rx charge point DC output = 7.31, AC output 8.99V.

So there is clearly still AC there but I guess it doesn't really matter as it is being rectified. I looked into this a bit further as I always assumed that to charge a 9.6v battery you needed 9.6v from the charger (and to charge a 4.8v battery you needed 4.8v from the charger). Apparently this isn't the case and voltage isn't that important, it can be a fair bit higher or lower as long as the current is correct.

This made me think that if I just swapped the resistor in the charger for a variable one I could manually adjust the current which should allow me to up this from a 45ma charger to something like 150ma which I can use to charge my 1500mAh NiCd's over 16 hours?

My curiosity got the best of me and so I opened up the charger to have a look. The attached picture shows the board. Current is rectified by just one diode and a resistor, one for rx charge point and one for tx charge point. I guess this is why it still reads an AC voltage.

Anyone got any ideas if this will work if I use a variable resistor?
Feb 23, 2011, 09:35 AM
Low AltiDude
PGR's Avatar
Originally Posted by scotti
I looked into this a bit further as I always assumed that to charge a 9.6v battery you needed 9.6v from the charger (and to charge a 4.8v battery you needed 4.8v from the charger). Apparently this isn't the case and voltage isn't that important, it can be a fair bit higher or lower as long as the current is correct.
That's correct, at least as far as NiMh and NiCd batteries go. Charging them involves current regulation and voltage regulation is typically just a byproduct of the current regulation.

The same is not the case for the lithium-ion family of batteries, though. They require both current control and very precise voltage control to charge them without incident.

Feb 23, 2011, 09:49 AM
Southern Pride
everydayflyer's Avatar
I always assumed that to charge a 9.6v battery you needed 9.6v from the charger (and to charge a 4.8v battery you needed 4.8v from the charger).
1.2V per cell is the nonimal voltage however in fact a fully charged Ni cell ios closer to 1.45 thus a fully charged 8 cell is approx. 11.6 volts.



The "wall wart" chargers while designed to charge a specific number of cells at a specific current. This is usually the rating seen on them. The consist of nothing more than a transformer, and a diode to rectify the AC input. The windings of the transformer are designed to have the necessary resistance (or some resistance may be added) to set the current at the right level for the specified number of cells. In actuality, because a true constant current source would be quite expensive, the "wall wart" is a compromise in design. If you were to measure the voltage without being connected you would find it is somewhat higher that the voltage of the battery to be charged. A Futaba charger for instance, measures 6.7 volts* at the receiver plug, while it says the voltage is 4.8 volts and current is 50 mA. Even his really not true as the current is around 60 mA at the beginning of charge and then settles down to 50 mA as the battery reaches full charge and the voltage approaches 6.0 volts.This is how you can use a "wall wart" charger marked for a specific voltage/current on a battery with more or less cells (voltage). You just have to make sure that the resulting current at the higher or lower voltage is proper for that battery. A simple current measurement while on charge will take care of this.
*keep in mind this is an unfiltered half wave voltage measured on a digital volt meter, so not actually a true DC.
Feb 23, 2011, 10:58 AM
Registered User
Great quote everydayflyer. I've just been reading some of the articles on that site which are very informative.

However, surely there must be a limit to the maximum variation in the charger voltage to the cell voltage? i.e. If I try to charge an empty 8 cell 9.6v with a 5v 100ma charger would the increasing backwards charge from the battery not eventually stop the new charge going in? This would lead to the battery never fully charging?

The reason I say that is because volts are the 'pushing' force behind electricity. Therefore a battery with higher voltage than the charger will have more 'push' in reverse back to the charger. Also is nicds top up at 1.45v per cell the surely you would need greater than 11.6v to fully charge an 8 cell?? This would suggest charge voltage is important - it always has to be higher than the battery voltage?
Last edited by scotti; Feb 23, 2011 at 11:03 AM.
Feb 24, 2011, 10:13 AM
Low AltiDude
PGR's Avatar
Scotti, there are two types of charging schemes which are the basis for nearly every charger on the market: Constant Current (CC) and Constant Voltage (CV). CC chargers control the charge rate by limiting current and CV chargers control the charge rate by limiting voltage and there are also chargers (like LiPo chargers) which use both CC and CV stages in their charging algorithm), but almost all NiMH/NiCd chargers use the CC scheme.

But the thing you need to understand is that by controlling one (C or V) you are, by default, controlling the other. The current which flows from one component to another is always directly proportional to the voltage difference between the components.

Ohm's law dictates it, you need to believe it, and that settles it. For a better understanding of what I just wrote, Google "ohm's law" and do some studying. It's a theory that can explain just about everything a model aviator will do with electricity so it's information which you should try to understand.


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