How do I know NiMH done charging (wall charger)?
 

Probably a stupid question that I think I know the answer to: I have a new RC car that came with a stock battery and wall charger. The battery is a 7.2 v 3800 maH battery. Unfortunately, there were no instructions in regards to the battery/charger. My boat came with a stock battery/charger and with that setup, you charge until the battery gets hot. Is this the way all wall chargers/NiMH batteries work? This battery has been charging for like 6 hours and it still isn't hot. I haven't run my boat in a while, but don't remember the battery taking that long to charge, though the battery is smaller 7.2v 1100 mAH.

RE: How do I know NiMH done charging (wall charger)?
 

I am certainly not an expert on batteries or charging, but I will share what I think I understand about the subject. First off, I have never heard of charging a battery until it gets hot. I always thought that if you are forcing electricity into a battery to the point that it gets hot, you are doing something wrong. Now, about your wall charger. Most of them will have the charge rate printed on them somewhere. Most of them are somewhere between 50 and 100ma. So to make it easy, if it is charging at 100ma, you divide the capacity of the battery (3800) by the charge rate to get charging time. So it would take 38 hours in this case to charge the battery. If your wall wart puts out 50ma, it would take 76 hours to charge fully. In the case of these higher capacity batteries, it might be a good idea to get a charger that will charge at a higher rate.

RE: How do I know NiMH done charging (wall charger)?
 

NI cells will warm up when full. The voltage will also drop slightly. One thing is for sure, your 3800 pack should take 4 times as long as the 1100. ;)

RE: How do I know NiMH done charging (wall charger)?
 

Look at the rating of the charger. If it is 160mA output and your battery is rated3,800mAh it will take 23.75 hours to recharge.

(Of course it's never that simple - but it's a start).

RE: How do I know NiMH done charging (wall charger)?
 

Your on the right track there Charlie. Here we have a 3.8 Amp battery, with a wall wart charger putting out .16 amps. And with charging with that low of an amperage, the battery will never get HOT ! Maybe SLIGHTLY warm after 24 hours.

Frank

RE: How do I know NiMH done charging (wall charger)?
 

At low currents the pack wont get hot so you have to do some math:
Time=1.5* Pack capacity/charging current (for fully discharged pack)
( charge efficiency of nimh or nicd at low currents is around 70%)

RE: How do I know NiMH done charging (wall charger)?
 

The charger shows an output of 9VDC, 500mA.  By the math given, the battery should take 7.6 hours to charge.  The battery started getting warm, probably al little after 7 hours, and I charged for around 8 hours.  The battery never got hot, just stayed warm.  My other battery/charger instructions actually tell you that the battery will get pretty hot and even mentions that when the battery is just warm, it isn't done charging.  That battery does get pretty hot.  .

RE: How do I know NiMH done charging (wall charger)?
 

when you checked the temps, did it have time too cool down again.

RE: How do I know NiMH done charging (wall charger)?
 

For getting pretty hot it needs enough power to dissipate.
when a cell is charging with 0.5A of current, the maximum disippated power is 1.5Vx0.5A=0.75W and its not enough to make the cell pretty hot.
but when the charge current is 2A the power loss is around 3watt and now it can get hot.
getting hot is just for fast charging and not overnight charging...

RE: How do I know NiMH done charging (wall charger)?
 

All the information you need to know about batteries www.hangtimes.com NiMh batteries should get warm when they are fully charged not hot. You will damage the cells if they get hot and you need to have a charger powerful enough to get the battery fully charged, most wall chargers don't put enough amps to fully charge a 2500mAH battery. A charge rate of 2.5 amps would take an hour to charge.

RE: How do I know NiMH done charging (wall charger)?
 

One slight error in your comments. Anytime the charger is putting current into the battery that is greater than the self discharge rate, the battery will become fully charged. I've never seen a wall charger that does not output more than the self discharge rate of the battery so, ipsofacto, the battery will become fully charged if left on the charger long enough.

RE: How do I know NiMH done charging (wall charger)?
 

Just do like my neighbor does.. Brings his batteries over to me for charging..hehe

RE: How do I know NiMH done charging (wall charger)?
 

If the pack is warm its charged, don't let your batteries get Hot.If the pack gets hot you are damaging it.

All good stuff, but how do I know when NiMH batteries are charged and when they are not?

Is there a battery tester around which will show the charge level?

Can I read the Voltage with a multimeter?

Cheers
Max

A DVM will be able to tell you the voltage in your battery pack but better still IMHO is to get yourself a good quality battery charger that will remove all the guess work out of knowing if your battery is fully charged or not.

Thanks, Karol.

I bought a EV-Peak C4 charger, but it never shows when NiMH batteries are fully charged, nor the state of the battery during the charging process.

Do you have a suggestion for a suitable charger?

Cheers
Max

Though I'm not very familiar with your EV- Peak charger it seems to be a fairly good quality smart unit which should fulfill most all of your charging needs. Once the correct information is input prior to any battery charging /discharging process the charger will sense when that particular process is complete by stopping the process. Maybe you need to familiarize yourself a bit more with how your unit operates.

Two things happen with every Nixx type battery when they reach full charge. The terminal voltage will decrease very slightly for a bit of time and the cells will start to warm up ....get hot if left on charge long enough. Both of these indicators can be used to tell you when to stop charging. Quality chargers sense the voltage change and stop the charge cycle. Some chargers still have a thermistor input for temperature sensing. Either of these changes can be used to determine full charge and quality chargers use either or both though most hobby chargers don't use temperature sensing any more. Industrial Nixx battery chargers almost always use both of these conditions to determine full charge cut off points.

Thanks, Karol and TR.
That seems to be the consensus. I've been back to the store, but they are focused on LiPo's - so not much help.
I might take a C pack with me next time I go in and see if I can get a better explanation from them.
All the best
Max