Charging problems hot batteries etc

I hope someone can put my mind at ease with my current delemma.
I'll try to keep it short.

I did buy a 4-8cell nimh battery charger today with auto cut-off, but my batteries are getting so hot that I rather stopped the charging process as I feel the charger will not cut off automatically. I am tempted to leave it and see what happens but don't want to push my luck.

This charger costed me 5 times that what I paid for a car charger made for 7.2-8.4V battery packs for my Twin Detonator offroad RC truck. But the cheap charger shuts off perfectly. The batteries are just getting hot then it's only trickle charging. That is with a 5-cell 3300mah pack.

The batteries that are getting hot with the new, more expensive charger is 2700mah AA's. My plan is to charge them while leaving them in the battery cradle of my TK40 as I don't want to charge them 4 at a time with its original charger. The charger can charge at 1, 2 or 4 amps. I used the 1 amp setting.

Just some questions.
1. What might be the reason for the charger not shutting off?
2. What is the highest safe temp for the AA's when charging them?
3. How does the charger know what voltage is the battery pack as it does not have an input setting?
4. How does the charger know what amps is the battery pack? This battery pack should be 5400mah if I'm correct as its 2 sets of 4 batteries in parallel, standerd setup of the TK40.

Any suggestions would be appreciated as I don't want acid all over my carpet

Hello Lightseeker2009,

It looks like you are charging 4 NiMh cells in series. Is that correct?

When you hook several cells together, you form a battery pack. If the cells in the pack are not well balanced or matched, the charger will not register the -dV end of charge signal, and just keep on charging. This will cause the fully charged cells in the battery to heat up.

A cell is too hot when it is uncomfortable to have it placed on the tender skin of your wrist or forearm. Warm is OK, but hot is not. Many people experience discomfort when the cell temperature approaches around 140 F. This is considered an upset condition and it is a lot better if the cells stay below 120 F.

The charger monitors the voltage of the battery pack through the connection leads. With a -dV termination, it is looking for a drop in voltage from the whole pack.

The charger doesn't have any idea of what the capacity of your cells are. It simply keeps charging until it sees a -dV signal, or until it times out if it has a safety timer. It is not a good idea to charge NiMh or NiCd cells in parallel.

Tom

Additionally the TK40 battery pack is 4s2p as noted, in other words two sets of four cells in parallel. This should not be charged as a single pack. NiMH cells must not, must must must not be charged in parallel. This can produce a pack melt-down from over heating.

However, if you charge each set of four cells separately as two individual 4s packs things should work better, as Silverfox explained it.

Thanks guys for the inputs.
I did not know its not good to charge nimh's in seriesBut to be fair, all battery packs made for RC cars and boats etc the cells are in series, and are charged accordingly.
Like I've explained, the 7.2V 3300mah pach for my truck is going strong for 2 years now without problems and its charged at a current of 5A. That cheap charger just does its work perfectly. If its wrong to charge in series, why does it work so good with my 7.2V battery pack?

But to be honest, I wanted to buy a 8-cell charger with LCD screen that monitors and charges each battery on its own. But there is not even one available anywhere in South Africa I could import one but I needed one as in now and could not wait 30 days like I did with the TK40.

lightseeker2009 said:

I did not know its not good to charge nimh's in seriesBut to be fair, all battery packs made for RC cars and boats etc the cells are in series, and are charged accordingly.

Click to expand...

It's OK to charge them in series. What you must not do is charge them in parallel.

The TK40 battery pack has two sets of four batteries connected in parallel, so you must remove at least one set of four before charging. You could either charge all eight batteries in series after removing them from the TK40, or you could charge each set of four separately.

lightseeker2009 said:

But to be fair, all battery packs made for RC cars and boats etc the cells are in series, and are charged accordingly.

Click to expand...

Remember that high quality R/C battery packs are made from matched batteries so they peak at the same time. That's why, in general, you can charge them as a pack.

R/C is another one of my hobbies.

But then of course it depends what batts your using as well....im also in rc's....onroad racing is what i love to do on the weekends. I would charge each cell individually......or try to match the batts by mah or voltage if you can.