How long to charge x-thousand mAH RC car batteries

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mik3ca

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I recently bought a bunch of 7.2V RC car batteries from Ebay and upon arrival, they were pretty much fully charged. Now I used up a couple of them (their voltage now is around 4V) but I want to recharge them

Both are Ni-MH and one is 6800mAH and the other is 4000mAH.

The 6800mAH looks like one of these:


And the 4000mAH ones look like this:



For each battery order, I also received the same charger as the one shown above this text.
However, for one order, the charger came in plastic wrapping and the other came in a box with the following texts:


Other than that information I can't understand the best way to go. I read somewhere that such batteries should be charged for 6 hours but I wonder if whoever wrote on the box is trying to tell me the light does something special when the battery is fully charged?

I'm confused. How long should I charge these batteries?
 
Put one to charge. Check if the light goes green or off after 8 hours. If not, leave charging up to 24 hours and check light. Or read/paste here manuals for the charger (about its indicator light) and batteries.

NiMH are usually 1.40 V/cell at the end of charge. For the 6 cells in the battery, look for 8.4V/pack at the end of charging.
 
If the charger limits the current to 220mA then a 6800mAh dead battery will be fully charged in about 6800/220 x 1.4= 43.3 hours. The 4000mAh dead battery will be fully charged in 4000/220 x 1.4= 25.5 hours.
If the charger limits the current to 100mA as shown on its label then the charging time is more than doubled.
"The indicating lamp for the power source, the charge to instruct" is a very poor translation from Chinese. You get what you pay for. The charger is very weak (cheap) for these batteries.

My North American name brand charger charges a North American name brand 2500mAh Ni-MH battery with 1550mA for about 2.2 hours which is 2500/1550 x 1.4.
 
NiMH cells have a very characteristic "hump" at the end of the charging cycle.
The terminal voltage rises and then falls.
When the voltage starts to fall, the charging cycle is complete, if you keep on charging after the hump, the cells will get hot.
During the main part of the charging cycle where the voltage is rising, the cells remain cool.

If you want to ensure that your battery is fully charged, measure the voltage and look for the voltage falling.


Here is a recording which I made of an 11 cell battery pack which I made as I was developing a charger for it.
A PIC detects the falling voltage and switches from full charge rate to trickle charge rate.
The cells in the pack are F size and cost me a fortune!


The gap in the trace where the charger switched to trickle mode, is due to the data logger having a brain fart.

JimB
 
I forgot to say that in Energizer Battery Company's Ni-MH manual it says the charger should detect a full charge then disconnect the charging, OR have a trickle charge of 1/40th of the mAh rating (like from this weak little charger) which can be continuous.
Then the chargers you received are for tickle charging which takes a long time if the battery is dead. These weak chargers probably don't even detect a full charge.

Go to a local hobby store with your batteries and ask for a charger that charges them in a reasonable time (a few hours or overnight), detects a full charge then disconnects the charging.
 
I thought there was a reason that mA was scary. and now audioguru's math told me. Now I'll need to get a higher mA one.
 
The very cheeeeep ebay charger ad does not say it detects a full charge then disconnects the charging. It might cook the battery until you smell it burning. I wonder if its polarity is correct? It says it is a balance charger. No it is not!
Buy a GOOD charger from a local hobby store that has been in business for a while and wants returning customers. They know the products they sell and can explain what they do. ebay sellers do not have a clue about the cheeeeep junk they sell.
 
audioguru said:
Buy a GOOD charger from a local hobby store that has been in business for a while and wants returning customers.
Click to expand...
Who in southern Ontario would sell such a thing? Like company names? I tried Sayal Electronics and they don't sell any item that comes with tamiya connectors like my (what you'd call cheeeeeeep from ebay) long lasting 6800mAH batteries.
 
Sayal sells surplus electric and electronic parts from India. Did you notice that everybody working there are also from India? I bought some wall warts made long ago for dialup modems and clock radios there at very low costs.

I go to Hobby Hobby in Streetsville and there is Great Hobbies and Big Boys With Great Toys in Mississauga. Google lists many more in Mississauga and also hobby shops all over Toronto.

Hobby Hobby has 187 items in their Charging department and if a suitable charger does not have your Tamiya connector then they have the connector for you to solder on. They sell hundreds of RC cars and sell 4 Tamiya cars (expensive!).
 
if a suitable charger does not have your Tamiya connector then they have the connector for you to solder on
Click to expand...
So I could just pick any battery charger from anywhere, rip out the battery terminals and connect a tamiya connector to it with 22AWG wire as long as the charger output voltage is 9 and the current is at least 1000mA?
 
My former company used D cell NiMH 10 cell packs in the early days of NiMH batteries. Many or even most of these battery packs failed on delivery - the technology to make D cells just wasn't there.

I built a system that exercised 4 packs at a time through several chatge/discharge cycles so we could pick out the working packs. I did exactly what JimB said - stop charging when you reach the voltage inflection point. Temperature does start to climb at that point, so it would also be a reliable indicator.
 
I wonder if I should just scrap the idea of buying a charger but instead build one using a DC wall wart as a power supply, but then again its risky because half the resources on the internet (including parts listed on ebay that someone calls cheeeep) are not giving the information necessary.
 
mik3ca said:
So I could just pick any battery charger from anywhere, rip out the battery terminals and connect a tamiya connector to it with 22AWG wire as long as the charger output voltage is 9 and the current is at least 1000mA?
Click to expand...
You need to learn about why a Ni-MH battery must have its charger circuit properly detect a full charge then disconnect the charging or switch to a 1/40th trickle charge. Go to Energizer.com and download their Ni-MH battery manual.

parts listed on ebay that someone calls cheeeep) are not giving the information necessary
Click to expand...
ebay sells junk and do not speeky zee Engrish so they and you don't know anything about the (wrong, used or defective) junk they sell.
 
audioguru said:
ebay sells junk and do not speeky zee Engrish so they and you don't know anything about the (wrong, used or defective) junk they sell.
Click to expand...

You have painted ebay with this broad brush (and racist too) any number of times.... Quite frankly, you are wrong. Yes, there is crap and there are dishonest people on ebay. There are also many vendors selling good products at good prices. Give it a break.
 
audioguru said:
You need to learn about why a Ni-MH battery must have its charger circuit properly detect a full charge then disconnect the charging or switch to a 1/40th trickle charge. Go to Energizer.com and download their Ni-MH battery manual.
Click to expand...
Looks like I'll be on a long journey for a charger.


ebay sells junk and do not speeky zee Engrish so they and you don't know anything about the (wrong, used or defective) junk they sell.
Click to expand...

I have lots of experience with that site. I bought good things and things I know as junk I got a refund for it. I will admit I did save alot of money as a result from shopping on ebay and other overseas vendors. Heck, they sell good LCD character displays (16x4 character) for about $5 USD which is cheaper than what electronic stores would charge for them.
 
mik3ca said:
Looks like I'll be on a long journey for a charger.
Click to expand...
Local hobby stores have hundreds of chargers used for the batteries in RC cars, airplanes, drones and boats. Their knowledgeable staff know which ones will be suitable for your batteries.
 
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