Battery Charger

[Martyn Johnston]

Hi all,
I have an old 7.6V, 1000mAh, Ni-Cad battery that came with its own little plug-in charger.
I've just bought a new 8.4V, 3000mAh NiMh battery.
It didn't occur to me that it might need a different charger (only a Noobie, sorry).

So, three questions.
1. Can I use my existing charger? I guess that'll only charge it to 7.6V? But will it do any harm?
2. Am I supposed to get any specific type of charger (if I buy none) because it's a NiMh battery?
3.Any recommendations on which charger to get (ie 240V or 12 V?)?

Martyn

[Martyn Johnston]

I'm getting confused on batteries.
I now have a battery in my Tx (unknown V(7.2V?)), a 4.8V for my Rx & servos, an old 7.6V NiCad, and a new 8.4V NiMh.
Will one charger cope with all this?

[Alistair Taylor]

Martyn

It's quite hard to say whether your old charger will do any harm to the NiMH battery without finding out more about the charging rate (i.e. the amps the charger tries to put into the battery).

It is Amps what will cause damage.

The basic rule is that you CAN charge NiCads at more than 1C (up to 4C or more is possible although it will reduce their useful life) but charging NiMH batteries at more than 1C will damage them.

1C basically means the Amp-hour rating of the battery x 1.

i.e. your NiCad is a 1000mAh battery. 1000 x 1 = 1000 mAh (or 1 Amp). Charging this battery at 1 amp will mean it is charged in 1 hour. Charging it at 2 amps (or 2C) should be OK and it will be charged in 30 minutes. 4C (or 4 amps) should charge it in 15 minutes.

The NiMH batter is rated at 3000mAh - so 1C is 3000 mAh, or 3 amps. Likewise, charging at 3 amps will take one hour to charge it - but 3 amps is the maximum.

BUT (and it's a big but), not all batteries are the same, and some will only just tolerate 1C (and will therefore have a much shorter useful life). AA batteries don't like big amps for charging, SUB-C batteries are quite happy with them (it's all to do with internal resistance, but I'm not going into this cos I've really got to do some work).

So with this in mind - if your battery charger charges your 7.6v NiCAD overnight, it most likely will not be capable of harming your NiMH.

BUT - if it is designed to fast charge your NiCAD in 1hour or less, it might not be ideal for your NiMH.

Also, if it is designed to shut off once your NiCAD is charged, it will most likely NOT switch off when your NiMH is charged, so you will need to time the charge, or you might overcharge (and damage) the NiMH battery.

Another rule of thumb - It takes 12 - 15 hours to charge a battery at 1/10C (this is the instruction you will find on most battery packs).

For the 1000 mAh NiCAD this means 12 - 15 hours at 100mA.

or 6 - 7.5 hours at 200mA.

For the 3000mAh NiMH this means 12 - 15 hours at 300mA

or 6 - 7.5 hours at 60mA.

In both cases the rate-time relationship is linear - double charge rate, halve charge time.

So - you don't need a "special" charger for you NiMH, but you do need to know how many amps/milliamps it supplies in order to time the charge correctly.

A final note - it is recommended that all battery packs are slow charged (at 1/10C) for the first time, so that they can "form". Ignoring this can reduce battery life.

Alistair

[Martyn Johnston]

Alistair,

Wow, a whole lesson in batteries; I've just learned a load more about batteries than any magazine articles have told me.
Thank you.

If I were to buy a charger to deal with my existing Tx battery, Rx battery, old NiCad battery and new NiMh battery, will a single charger cope?, and would you have a recommendation?

Martyn

[Martyn Johnston]

Alistair,
Me again!
If I use my existing (7.2 V) charger on my new NiMh (8.4 V) battery will it only charge it up to 7.2V? Will that matter? presumably it would only run the motor I've got at the same power the current (7.2 V) NiCad battery does. Maybe that's a good thing. Maybe 8.4 V would be too much for the motor?
Martyn

[Alistair Taylor]

I'd recommend the pro-peak prodigy, just cos it will charge all of those batteries and it's easy to use.

The only minor down side is that it only runs off 12 volts - so you would need to get either a gel-cell 12v battery, or aquire a spare car battery, or get a 12volt power source.

NB - I bought a cheap 12volt power source that would only work with a switching diode added to drop the voltage. As supplied the ouput voltage was too high for the prodigy charger.

Your 7.2v charger would probably be OK charging your 8.4v NiCAD battery, but it's hard to tell without more info on the charger - who makes it? what model is it?

Similarly motorwise - what model is this for? Is the motor a speed 400, speed 600 or brushless?

Alistair

[Martyn Johnston]

So the 12 V charger runs off the 12 V 7Ah gell cell I've just bought for my tool-box; is that right? Or does it need connecting to my (bigger) car battery?
I'd have thought a 12V/240v one made more sense, to charge up the night before flying!

[Alistair Taylor]

Depends on how you're set up.

I reckon the cheapest solution to your situation is to buy one (or two) of the mains chargers sold by Hillcott Electronics ( http://www.hillcott-electronics.net/store/ ).

These will enable you to charge all of your batteries at a range of rates, without costing too much.

NB - these chargers will keep charging if you leave them on, so you either need a mains timer (can be temperamental) or keep watch over them closely (charge overnight at a rate that means they're done when your alarm goes off, or soon after). On the plus side an overcharged NiMH or NiCAD will not turn into a fireball like a Lipo can...if provoked.

Alistair

[Thomas Oliver]

A charger normally has an output voltage about 50% higher than the battery voltage. A fully charged battery will always have a voltage higher than the nominal voltage eg a fully charged good 12v battery will measure about 14v. Measure the free output voltage of your 7.2v charger and if it is about say 20% more than 8.4v it should charge an 8.4v battery. My Futaba outfit AA battery packs are 20 years old and still good. Reason- the original dual charger has an output of only 50ma for both TX and Rc. They can be left on charge for a week and suffer no ill effects at all. Fast charging of AA Nicads and NImh will possibly lead to damage. SubC fast charge batteries are very robust and can stand fast charging well. My Irvine peak detect charger works on both types and charges all types of battery.

[Adrian Jones]

how about one of these?:

http://cgi.ebay.co.uk/Intelligent-Multi-Fast-Charger-for-NiCd-NiMH-batteries_W0QQitemZ300103142989QQihZ020QQcategoryZ34063QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

[Aslan]

I use a multiplex LN5014.12v gel cell.Maplins XM20W regulated dc power supply.Similar set up as Alistairs.Does everything.