Does anyone know at what voltage level the Tamiya DMD trips out, the tank stops moving, stops and starts its engine and starts the gun elevator going as a low battery warning?

Is there a central repository of knowledge for all things DMD and MF unit related on this site.

I did do a search before asking these questions but found nothing that hit the nail on the head.

Lettuce no!

Cheers

Alan.

Yes alan,,, it is the low battery/battery dead sign... I am not sure the voltage.but it is accurate.. gas out... engine starts and stops...

-nemesis4u

It was the voltage level I wanted to know to see if it coincided with the cutout level for LiPo batteries.

The search for answers continues.........

Cheers

Alan.

Alan,
Running my Tamiya Tiger I Saturday afternoon to the point of "out of gas" stops movin, engine shuts down and starts back up. The battery pack that was on the drive set read 5.84 volts when I hooked it to charge. If about 6 volts is the cut off, it may be to low to protect lipo. I do run lithium in my KT and Panther but they last so long that I dont worry about discharging them below a safe point and I alway check the voltage befor and after running. Hop this helps, Paul

Justa note, a friend of mine left a lipo plugged into his Tam tiger I and left the master switch on for a day or 2, the lipo bloated and when I got it from him to see if there was anyhope of reusing it, there was 1plus volts coming form it, this is a 2 cell pack and that means its dead.

So even if there is a low voltage cut off, try not to run it to that point and donot leave teh lipos plugged in with the master switch on.

The Leo has a particularly sensitive voltage level indicator. The LED on the power will change when low.

I have found the DMD/MF can be driven at slightly higher voltage to avoid premature shutdowns.

I run several of my tanks at 8.4V now to avoid 'brown-outs'. The extra voltage helps with signal retention as well when battery current supply is low.

Yes but what voltage are the batteries down too when the DMD shuts down on normal run?

I guess I will just have to buy a voltage meter and measure a battery right after a shutdown. I had thought that this work would have already been done and was common knowledge in the hobby.

I just checked a NiCd that I had just fully discharged on the work bench in my charger, like Marshalls suggestion and it reported the voltage as 5-6v which it cannot be as it was fully discharged. So this makes me think that Marshalls experience could also be something to do with the charger rather than a real reading of what the starting voltage of his battery is when plugged in to the charger.

So the search for answers goes on...but in so doing has uncovered other interesting stuff like Planedevs battery bloating and heavyaslead being able to run his DMD at a higher voltage without probs. The LiPos of course have a stated charge of 7.4v and this also does not seem to harm the DMD.

Interestingly even a battery rated at 7.2v will come out of the charger with 9v that quickly drops down to an average of around 7ish. I would bet that the Tamiya DMDs have a generous voltage tolerance range.

My transmitter starts off with a charge of 12v and then drops down from there and needs a recharge after it drops below 8v. So the Tamiya DMDs must work along similar lines.

Cheers

Alan

Hello Alan

You may be the first to really reveal this information. The DMD/MF after all the experimentation and hacking still has mysteries to reveal. There is not even a schematic of the device yet, that I am aware of.

Try connecting to a variable voltage DC power supply and watch as you dail in various low level voltage ranges. Indeed the connected voltage will be different than the battery unconnected measurement.

Note that a fully charged 2S lipo pack is 8.4V. Also note that i have had 2 packs bloated on me while running to LVC on the tam pershing a few times. Although the packs still work, they have bigger voltage sags on load. I suspect the DMD/MFU to have a LVC under 5V for it to be able to bloat my packs.

Hello,

I bought a multimeter and have been testing my batteries while running my Tamiya Panther. And I have answers!

A fully charged battery 7.2v rated battery comes out of the charger with about 8.5v in it, this is regardless of battery type be it NiCd, NiMh or LiPo.

After driving about in the tank the voltage drops to the stated 7.2v average fairly quickly and then flatlines at 7.2v for a long time (they are of course designed to do this and this is why they are rated at 7.2v)

When the tank DMD goes into save mode and stops, the battery on coming out of the tank still has a charge of between 6 and 7 volts. I believe the DMD shut down threshold is about 6 to 6.5 volts as even a spent battery with 7 volts still in it dropped to about 6 volts on the test bench under slight load. So one can imagine that in the tank when one runs over steep or difficult terrain the motor draws more power, the voltage drops below 6v - 6.5v under the strain, and the DMD shuts down.

I have repeated this experiment many times as I did not believe it at first. It seems wrong that the battery should still have so much charge in it on DMD failure. But the result was repeated over and over again with each sort of battery.

So it seems that the DMD is designed to shutdown at the end of the 7.2v flatline on the performance curve leaving your batteries still with lot of charge left in them.

One should not deplete LiPos below about 3v. So because the DMD shuts down at about 6v there is no danger of damaging your LiPos. (in theory but what happened to the batteries in Planedevs Pershing? Older DMD? different programming?)

We have also learnt that having run your LiPos to DMD failure your should not leave them connected in your tank for days afterwards as they continue to deplete and then bloat and damage themselves. Best practice is to remove them from the tank and charge them up again as soon as possible.

I would imagine that once a battery has reached the end of the 7.2v flatline on its performance curve the voltage drops away very quickly.

Another point that applies to all battery types, except LiPos and LiOns, is that because the shutdown threshold of the DMD is relatively high one should discharge the batteries fully every now and then before recharging them otherwise they may develop a memory (NiMh and NiCd) and thus shorter running times.

So now I am going to use my LiPos more and feel relatively safe doing so, though after the first shutdown I no longer try to limp home on restarts rather than picking the tank up and carrying it back for a battery change out.

If your LiPos do leak, bloat or explode, despite the above, please write in with a bit of background and we can work out what went wrong. Maybe not all DMDs are the same.

I can say for my part that the newer DMDs that came with the Panther perform as listed above. I do have 2 other tanks with Pershing DMDs in that were not subject to my testing, I will carry out some testing on them and report back.

Cheers

Alan.

Thanks, Alan great testing!

That explains why I overvolt the DMD with a larger battery, since its NOT the capacity of battery but its ability to MAINTAIN voltage that extends runtime.

Have a look at this [link=http://www.yuasa-battery.co.uk/industrial/popup/nicd_dischargelow.html]graph[/link].

Yours Simon M.

That graph shows the performance flatline nicely.

It also makes a lot of sense to use higher voltage batteries so that one stays above the magic 6v shut down level for longer.

Most of my batteries are only 2000mA and so with a shutdown level of only 6v it does not take long for them to be used up.

Cheers

Alan.

Hi Guys,

Just to add a few pence in.....with electric flight, we have always known that a nimh/nicad cell should not be discharged below 1v/cell or 6v for a 6 cell packm, or they become damaged.....perhaps the DMD's are based on this assumption also.....

CaptB

Captain Bertie,

I thought that with NiCd and NiMh it was good to completely discharge them so that they did not develop the dreaded memory effect, is this not true.

I know that LiPos should not be completely discharged but that is another can of worms altogether.

I think your idea about the DMD is most probably right too.

Lettuce no!

Cheers

Alan.

Not so with NiMh, these are said to be memory free.

Yours Simon M.

We've debated memory effect on the board before.

IMHO, *true* "memory effect" where frequently discharging to the same partial charge in modern Nicd/Nimh doesn't exist.
But, cells can suffer from crystallization on the electrodes, which results in a lower charge capacity, and can be resolved by a deep discharge. Hence, many people call it "memory effect". The difference is that it's not caused by repeated discharges to the same level, and doesn't require the pack to be drained every single time you use it, as true "memory effect" does.

So, discharge cutoffs at 1v (where the pack is pretty flat anyway) shouldn't be a problem if the pack gets the occasional deep cycle.

D.

Good job Alan, with those results we all can learn more about the MFU.

hi guys,

in my experience with electric flight, what would happen is if you draw too much current from the battery pack and the voltage collapses below 1v/cell, then the pack would in quick time lose it's 'punch' be it nicad or nimh so this was the rule that all flight enthusiasts followed....may not follow for the tanks because we are not drawing heavy loads but I wonder if the 1v/cell rule has been more widely applied.........hmmmmm

CaptB

Hi CaptB
I beg to differ. With tanks you an regularly stall a motor, climb a few hills in the long grass / sand etc.

Yours Simon M.

Hi Samarh,

Well, I wonder what a typical current draw would be......I know in electric flight 25 - 30Amps
is pretty typical. but I take your point, stall current must be very high..........

Cheers

CaptB

I suppose it ought to be like for like for a fair comparison. I'm just looking at it from a technical point of view, i.e. stalled motor = max current, and the motors in HL tanks take 9A in this condition, way less than the flight motors you talk of.

Yours Simon M.

Tank motor demand is variable, plane motor demand is continuous.

Continous load will be hardest on a battery.

Stalls while greater are more dangerous to the electronics burning up if you don't turn it off!