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Old Mar 16, 2011, 10:34 PM
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deegola's Avatar
Las Vegas, Nevada
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Help!
Charging a 6v flight pack with a standard 4.8v wall-wart

I recently bought a used plane. I didn't realize that the installed battery was a 5-cell 6v pack. I plugged in my standard 4.8v wall-wart charger.

Mt question is, do you think it's still safe to use?
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Old Mar 16, 2011, 11:06 PM
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yes, you can use a standard wall wart designed to charge a 4-cell pack to charge a 5-cell pack. It will take slightly longer but it will charge up fine. The wall wart charger is a fairly unsophisticated device that puts out more or less 50 mA of current at more or less 6 - 10 volts. It is current-limited, so that no matter what you hook to it, it is only capable of putting out about 50 mA. On my tests with a 4-cell pack I was getting a charge of 71 mA and with a 5-cell pack I was getting a charge of 48 mA. A 7-cell pack only charged at a rate of 18 mA. This was with a JR wall wart rated at 50 mA and 4,8 volts. But all the wall wart chargers are of similar design.
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Old Mar 16, 2011, 11:41 PM
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Daedalus66's Avatar
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Quote:
Originally Posted by dmcquinn View Post
yes, you can use a standard wall wart designed to charge a 4-cell pack to charge a 5-cell pack. It will take slightly longer but it will charge up fine. The wall wart charger is a fairly unsophisticated device that puts out more or less 50 mA of current at more or less 6 - 10 volts. It is current-limited, so that no matter what you hook to it, it is only capable of putting out about 50 mA. On my tests with a 4-cell pack I was getting a charge of 71 mA and with a 5-cell pack I was getting a charge of 48 mA. A 7-cell pack only charged at a rate of 18 mA. This was with a JR wall wart rated at 50 mA and 4,8 volts. But all the wall wart chargers are of similar design.
This is good advice, but why don't you tell us the specifics: What is the rated output current of the charger? What is the capacity of the battery. These numbers are readily available. Then we can offer more specific advice as to how long you should charge.
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Old Mar 17, 2011, 01:08 AM
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Miami Mike's Avatar
Miami Lakes, Florida, USA
Joined Mar 2003
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I can confirm what dmcquinn wrote.

To measure current and/or voltage in receiver battery circuits while charging or discharging, you might want to build yourself a testing adapter like this one:

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Old Mar 17, 2011, 10:19 PM
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deegola's Avatar
Las Vegas, Nevada
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First off, thank you all for your replies.

Here's better specifics:
The wal-wart is a futaba - output 4.8vdc 100ma

The battery pack is 6v - 2500 mah (Previous owner not sure if nicad or nimh)

Thanks again!
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Old Mar 17, 2011, 10:32 PM
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Lakewood, Colorado
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Most wall warts don't have any sort of brains at all. They are constant voltage
and simply have limited current capacity so as not to cook a depleted
pack when you first hook it up. It's not regulated, it's just limited. The voltage
they put out is somewhere not far above the peak charge voltage of the pack, so the
current drops off as the pack gets closer to fully charged. My guess is, given
your particular wall wart, it will charge the 5 cell pack only a little bit,
and then then the current will drop off to practically nothing when
the pack voltage matches the charge voltage, long before the pack is charged.
You'd be better off just getting a real charger.

ian
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Old Mar 18, 2011, 10:34 AM
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Daedalus66's Avatar
Canada, ON, Ottawa
Joined Feb 2006
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Just to second that. Except that they are not constant voltage.

Even if it does deliver 100 mA, which is doubtful, a full charge will take days. You want at least 150 - 200 mA charger for that pack, rated for 6v, or a high rate one with peak detect.

The risk is not that you will damage the pack with your current charger but that it will be inadequately charged.
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Last edited by Daedalus66; Mar 18, 2011 at 12:21 PM. Reason: Correction on CV
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Old Mar 18, 2011, 10:46 AM
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I use this type charger all the time for both 4 and 5 cell packs. No problem, just takes a bit longer on the 5 cell packs and, yes, you will get a full charge--just takes a bit longer for a 5 cell than a 4 cell. If you have an ammeter just hook it up in series with the charge lead to see what the current is. As mentioned above, it will taper off a bit as the battery approaches full charge. The nice thing about using these for charging is that, if you forget and leave them on longer than necessary, no damage is done to the battery being charged. In fact, if this is the only type charger you use, you will get more lifetime (number of charge/discharge cycles) than you will get using a peak detecting charger.
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Old Mar 18, 2011, 11:32 AM
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Quote:
Originally Posted by Daemon View Post
Most wall warts don't have any sort of brains at all. They are constant voltage...
No, no, no no, no! That's absolutely wrong. The one thing that wall-wart battery chargers are definitely not is "constant voltage"!

A power supply has constant voltage, to one degree or another, depending upon the degree of voltage regulation, but chargers meant for NiCd and NiMh batteries absolutely do not incorporate voltage regulation. Instead, they use current regulation. In fact, that's the basic difference between a power supply and a battery charger. A wall-wart battery charger has a simple circuit inside with essentially no solid-state electronics, but it's nevertheless designed to achieve a limited degree of current regulation while varying the voltage over a fairly wide range to accommodate the battery's varying state of charge.

When the label on a wall-wart battery charger denotes a specific voltage, that doesn't mean the the charger delivers that particular voltage, it only means that the charger was meant primarily to recharge battery packs that are nominally rated at that voltage. A charger labeled "4.8 volts" is primary designed to recharge 4.8 volts battery packs, but it will deliver a wide range of voltage during the course of recharging a battery as it maintains the current flow within certain limits.
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