"Smart Chargers"

[frogs3]

As a newbie to hotwires, I decided to power my homebrew M@g85 with a set of Eneloops in a FM 9AA-3D holder.

When I first brought home the batteries, sure enough, they had enough stored power to light the 1185 for about 30 minutes.

I recharged them with the Tenergy Universal Smart Charger [for the 7.2-12V Battery Pack range], with the wires soldered properly to the FM connector, using the lower ma setting of 0.9A. It took about one hour to get a green light for the pack.

All went well the second round.

Then I recharged a third time, and the light ran for about 5 minutes.

I put four of the cells from the pack into a Mercury charger (500-800ma), and after 5 hours, the red light stayed on. Tried another 4 cells this morning, and had the same result. I have used this small charger to "top off" the original set of Eneloops, which took about 1 hour, and they keep my year old Mercury NiMH's running just fine. Does it just take a long time with this slow charger?

Unfortunately my DMM is not working so I can't check anything.

Any ideas about what is going on here? Is the Tenergy not so smart to pick up the signal from this pack? Are the Eneloops defective?

Guidance from the experienced members would be appreciated.

-HAK

 

[SilverFox]

Hello HAK,

Let's check the math...

You are charging 2000 mAh cells at 0.9 amps. At this charging rate, it should take around 2.5 hour to come to a full charge with fully discharged batteries.

Your charge finished within 1 hour, so either your batteries were only half discharged, or your charger ended the charge too soon. You ran for around 30 minutes, so I think your batteries were mostly discharged, so that leaves the charger.

Charging at 500 mA, the 2000 mAh cells will take around 6 hours to charge.

When charging a battery pack, you need to monitor the heat of the pack. If the charger ends the charge and you are charging in the recommended 0.5C - 1.0C range and the cells have not warmed up, you have had a false peak in the voltage and need to start the charger again. This can also indicate that some of the cells are out of balance. To re balance the pack you need to charge at 0.1C for 16 hours starting from a discharged state, or you can pull the cells and charge them individually.

Tom

 

[frogs3]

Using your principle, I let'em charge for about 6 hours, 4 batteries at a time on the small Mercury charger, and they did come to full charge. One set needed a bit more time, but they all did fill up.

I don't understand how several could have gotten so far out of balance so fast, and how a "smart" charger could be not so smart.

Tonight I'm going to give the pack a try and see what runtime I get now.

Any suggestions, while I'm at it for a DMM to check the batteries? I had a RS cheapie that went back for failure in less than a week, so I am ready to spend more to get a meter that works reliably.

Thanks,

HAK

 

[SilverFox]

Hello HAK,

For casual use, I usually recommend a voltmeter in the $20 - $30 range. You should be able to find them in any well stocked hardware store.

Fluke meters are good for serious use.

Tom

 

[LuxLuthor]

Harvey, you can find a better quality ($30-60) DMM at Sears if you don't have a local electronics shop. Based on my experience and dicussions with their own tech support department, I would stay away from Triplett brand.

 

[barkingmad]

If all you are doing is measuring the voltage does it really matter - i.e. no point spending a lot on a meter - certainly a Fluke etc. would be overkill.