Quantitative test results: TM MP7 charger significantly overcharges batteries

[MadMax]

Preamble:

TM AEPs and MP7s come with brick chargers which include a timer shutoff function to charge the small packs which power AEPs and MP7s. Unfortunately the chargers are designed for the Japanese market which uses 100v 50Hz electrical supply. North American household mains is 120v 60Hz.

I tried to order a bunch of brick chargers meant for higher input voltage than the standard 100v input charger and was dismayed to find that the standard 100v input charger is being described as a 110v input charger when the stickers clearly indicate that they are for 100v input. I note that there are no MP7 chargers on WGC or UNC marketed as a 100v charger so I think the 100v chargers are being described as a 110v charger.

So I'm stuck with 20 brick chargers which are designed for 100v input.

I decided to take some measurements to determine how the Japanese chargers perform with our household mains voltage/freq. I also looked into ways to reduce the charge delivered to a battery pack to see if Japanese MP7/AEP brick chargers could be effectively modified for use here.

Summary:

The stock TM charger overcharges batteries by 145mAh. This does not mean that you end up getting an extra 145mAh capacity in your 500mAh rated battery. The 145mAh overcharge is burned off in heat which can damage battery chemistry if you overcharge too far. 145mAh represents 29% overcharge.

The final temperature of the front name plate on the brick was 75C at an ambient temp of 30C. The final battery pack temp was 42C

A modified TM charger overcharged batteries by 68mAh which is about right. Typically a timer charger should deliver about 10% extra charge to assure a full charge on a pack.

The final temp of the front name plate on the brick was 70C at an ambient temp of 29C. The final batt pack temp was 40C.

The TM charger was not hugely modified. Two diodes were spliced in series with the battery pack connector so they incurr a 1.4v drop which reduces voltage applied to the pack and consequently results in a lower current pushed through the cells.

Tables:

Current at particular times (measured every 5min)

Cumulative charge at 5min intervals

General Conclusions:

It looks like it's quite feasible to modify the TM charger to appropriately charge the MP7 packs. However I'm not sure if an external operating temp of 70C is acceptable according to CSA standards. I note that the standby temp of a brick is only 40C so the charger may actually be okay in terms of safety.


Test Discription:

Two TM chargers were slightly modified to facilitate current observation. One of the two wires was cut to make connections to a current meter. Current was observed every 5min and recorded.

At the end of the 2hr charge cycle, the temperature of the battery pack and front nameplate of the charger was measured with a thermocouple (K-type I think). Thermal junction grease was applied to the test surfaces at the beginning of the charge cycle to facilitate temperature measurement.

One charger was left unmodified (except for the current meter splice). Two diodes were added in series to the charging circuit of another charger to reduce voltage applied to the battery.

A further third charger was forcefully disassembled to determine the operating nature of the device. It was found:

-High voltage reduction performed by a fixed ratio transformer (not a switch mode power supply)
-Simple bridge rectification with 330uF capacitor
-CD451BE Texas Instrument cycle counter used to provide timing via RC circuit (not dependant on 50Hz)
-No voltage regulator downstream of the transformer (i.e. voltage supplied to the charging circuit is dependant on input voltage)


Description of calculations to determine cumulative charge:

Each 5min current measurement represents an instantaneous current at a given time. Notionally the current varies continuously with time so sampling every 5min only delivers an approximation of the actuall current vs. time relationship. Luckily there doesn't appear to be a huge amount of fluctuation so a 5min sampling period appears to work pretty ok. I crunched Excel to interpolate an average current between periods and multiplied it by 1/12 (5min = 1/2hr) to determine a mAh delivered over individual periods. Crank out a running total to determine the cumulative mAh delivered at each given sample. The last result indicates the total current accumulation delivered over the charge period.

[^Hyperion^]

That is definately instructive. I beg for sticky

[Maelstrom]

guess it was a good thing that the first step i took was cutting the wires then using my smart charger on it. then again i only charged it once before i butchered the stock battery to make an external battery mod

[MrEvolution]

Originally Posted by MadMax

Two diodes were spliced in series with the battery pack connector so they incurr a 1.4v drop which reduces voltage applied to the pack and consequently results in a lower current pushed through the cells.

-High voltage reduction performed by a fixed ratio transformer (not a switch mode power supply)
-Simple bridge rectification with 330uF capacitor
-CD451BE Texas Instrument cycle counter used to provide timing via RC circuit (not dependant on 50Hz)
-No voltage regulator downstream of the transformer (i.e. voltage supplied to the charging circuit is dependant on input voltage)

good thing im taking electronics @ Niagara college, or else I wouldnt know what all that stuff meant.

[Spas-Tick22]

So pretty much go out and buy a Volt/Hertz converter before hooking up any Japanese electronics to any wall socket?

[Scarecrow]

Just from observation I can say without a doubt that the battery gets waaaaaaaaaay too hot. I actually cut my connector off my original adapter and use a 120v to 7.6vDC adapter that charges at 60mah and while it takes a lot longer for the battery to charge, but, it does not get hot at all now and the battery charges fine.

Never really occurred to me to post about it, but, good on you Carl, thanks for the heads up.

[ScottMcLeod]

Thank god I've only charged my batts once! I JUST came across this.

I'm going to cut into the wires and use it with my smart charger from now on...

[ThunderCactus]

Originally Posted by Maelstrom

guess it was a good thing that the first step i took was cutting the wires then using my smart charger on it. then again i only charged it once before i butchered the stock battery to make an external battery mod

lol I do that with ALL my rechargeable batteries!

[DarkAlman]

including your AA's...

I got a PM from someone recently mentioning that he tried my MP7 charger fix (The one I posted months back) and the battery still over charges (My fix being cut the wire and plug the cradle into a smart charger)

But he decided to do so with a BB bastard charger.

So cutting off the wires and plugging them into a charger will work...but make sure you use a quality charger.

[redberets44]

Sorry, I don't know much about batteries/chargers but instead of charging it for 2 hours couldn't you just charge it for under that?

[ThunderCactus]

You could charge ot for less time, but how are you going to know if its full or not?
Better to just use a good charger that wont accidentally set your battery on fire.

[redberets44]

True, but I usually keep a very close eye on my batteries (even the ones on hooked up to smart chargers), once I get find a spare smart charger I'll probably rewire it.

[Ricochet]

Buy the IMAX B6 from hobbyking, best bang for your buck. Also the Turnigy-Nanotech lipos are like $10.00 or less on there. Order from the US warehouse listing, not the Asia ones.