Maximizing Run Time?

[AJ Ross]

Hello all! First time owner/builder here, American, and on standby for delivery of an Armortek King Tiger from the 2019/20 batch.

I've several questions, but what worries me most is all to do with batteries. Either one big 24V or a pair of 12Vs rigged together, right? In whichever case, I'd like mine to be able to run for about 5-6 hours straight, if that's even feasible.

It's because I intend to have my Tiger rolling around all sorts of geek conventions for an afternoon at a time. That done, I'd be anxious to have it rumble on back to my car under it's own power. I dread having it die on me in the middle of a convention center, so any tips to help keep a KT charged up for a long day would be very welcome!

All the best,
Ross

[simon_manning]

you will get all sorts of answers, take a spare set of charged batteries for this duration, regards Simon manning.

[Stephen White]

All I can add is that I've run my Centurion over three days at TankFest without recharging and the voltage never dropped below 23.5. That is using a single 24V 22AH LiFePO4 battery.

The problem with lead acid is that the discharge profile is such that the effective voltage drops below a usable level much earlier in the cycle compared with lithium iron phosphate. The latter will continue to produce good power until about 90% discharged. At that point it falls off the edge of the cliff.

I have had one issue with lithium iron phosphate, on a very hot day, after long running on sandy grass. I was monitoring voltage via telemetry and was happy that the battery was producing about 23V when running in a straight line but as I attempted to do a tight turn, the peak discharge tripped the battery management system cut-out and stopped the tank dead. It's easy to reset it by momentarily connecting a 24V charger. If that had been lead acid, I suspect I'd already have run out of usable power long before. LiFePO4 batteries are about half the weight and size of lead acid, so there is an additional reduction in the power needed to run. There is a Knowledge Base topic on battery selection.

[Adrian Harris]

If the running surface is a polished convention centre floor then, as long as you don't spend all day doing donuts, battery drain will be minimal.
If the running surface is rough tarmac/concrete, running time will be less.
If running on grass, then battery drain is much higher, and the longer the grass, the higher the drain.

Running indoors usually precludes the use of the smokers. I think the KT comes with two smokers, so that's a continuous 4A drain on the battery, so that alone will flatten a pair of 22Ah batteries in 5.5 hours. I would recommend you can isolate the smoker(s) either electronically or with a switch.

One of the most active King Tigers around used to have two sets of AGM batteries inside the hull, swapped over by the use of a pair of 100A SPDT toggle switches. This allowed it to be used for weekend shows without needing to be recharged. Not for continuous usage, but usually on grass. The downside is that more batteries inside means more overall weight which means more battery drain.

As Stephen says, LiFePO4 batteries run and run, until they don't. And the battery protection usually won't allow them to run until recharged.

Once you get a feel for running your tank on lead acid batteries, you get to know when they are starting to struggle. They can 'recover' if given a breather, so it's not uncommon to see people driving their tank back to the car in spurts of 10 to 20 feet

A second set of batteries, with an easy method to swap over the power connections (XT60 or XT90 plugs & sockets used on the new modules are ideal), gets you out of all sorts of problems, even if you're running with them sitting on the engine deck.

If you go with two sets of batteries inside, take a third set as a backup, and rotate them to even out the usage. No point having a pristine set, with the longest running time, if it spends all its life in the car.

Adrian.

[Christoffer Ahlfors]

It is feasible, especially on a hard floor at an indoor convention. Grass drains the batteries considerably more. Smoke adds to the drain. My smoker draws about 3,5 A max and produces decent volumes. Sound can be a killer if you go all in. A friend of mine has some hundred W and it draws 11 A when turned up to compete with the real tanks and helicopters around us at shows. (I know, it sounds like something is wrong, but 24V times 10A is 240W with no losses. It is correct!)

I took Stephen's advice and ditched my lead batteries for lithium iron phosphate (NOT Lipo!). Although it came at a cost, it has been one of my best moves ever (Stephen: Thank you, thank you, thank you! ). It is not just a matter of running time. As Stephen is eluding to, the lithium batteries will give the motors access to full voltage the entire duration, regardless of how hard maneuvers you are making. With lead batteries, the voltage will drop (due to the comparatively high internal resistance of the lead/acid chemistry) when maneuvering, reducing the joy and realism. The king tiger has chevrons on its tracks, which grips the ground better and makes it more difficult to turn on surfaces like grass than his Cent or my Tiger 1, which both have smooth tracks.

Capacity wise, I went all in with 60Ah. It has proved enough for two days of intense running in terrain (I only have one model), but my sound system is close to zero. I am a bit surprised that Stephen can run a Centurion (comparable to a king tiger?) for three days on just 22Ah. It could be the newer high efficiency motors that I don't have that make a difference, but I guess you have yourself a range there...

Good luck with your model - it will be an impressive one!

[AJ Ross]

Aye, thanks! Linoleum, lobby carpet, and concrete are the expected operating environment for my KT. A smoker does seem like it might be a bit of a nuisance indoors. Even if most people don't mind it, some convention organizers can be very finicky. Liability and all that. Which is just as well, since smokers use a lot of power as you said.

I'm liking the idea of having a spare battery handy. Might carry it around in a backpack. Although certainly not a lead one!

LiFePO4 looks encouraging, Not Lithium polymer, of course. Been reading a lot of nasty stories about them. I've been browsing a few online shops for a 24V LiFePO4 with a high Amp Hour rating. Now, from what I can tell, 100 Ah would be gross overkill, and way too expensive. But maybe something in the range of 50-60 or so, yeah.

Thanks again for helping a noob get his bearings. I'll be sure to share what I come up with when the time comes!

[Gerhard Michel]

I'm using LiFePO4 batteries in four of my five models, with and without battery management systems. If you don't use a BMS, another kind of monitoring each single cell is mandatory for charging and discharging.

Lead acid batteries should be discharged only to 50 % DoD (depth of discharge) for a good life; this is the biggest disadvantage of lead acid. LiFePO4 may be discharged to 90 % DoD without a significant loss of duration.

E.g. my King Tiger uses eight 40 Ah single cells, which last up to 5 - 6 hours of driving in grass and sand, strongly depending on the amount of curving, which is extremely power eating! My telemetry shows a power consumption of 3 -4 amps for driving straightforward (also in grass and sand), but up to 60 amps for a table turn in rough grounds!

[AJ Ross]

Single cells? Eight of them? Well, I'm not sure if I'd rig something similar (or even know how) but that's some great run time on rough terrain!

And there's another mention of telemetry data. Is that something you can see on the digital readout of an RC transmitter?
(Something else I need to shop for.)

I've also seen a bluetooth thing can connect to an app on your phone to monitor a battery's charge level. That could be handy too.

[Stephen White]

Telemetry is not only easy to fit but very, very useful. Most modern RC systems now come with integrated telemetry. Soektrum for example offer a basic installation, to which you would simply add a voltage sensor, which is easy to install into your power leads. Ive also fitted a current sensor but so far have only been able to read the data retrospectively. Some of the more advanced software defined radios such as FrSky Taranis can give you a very powerful tool to set up alarms and voice prompts which can be very helpful.

Here are example of what you might do with Spektrum

https://www.spektrumrc.com/Technology/Telemetry.aspx

And FrSky



It’s that easy.

Stephen

[Gerhard Michel]

This is a possible display of my transmitter. In the right side you can see the voltages of the single LiFe cells. I programmed a spoken warning for each cell deceeding a minimum voltage when not using a BMS. Another very useful screen shows the consumed capacity in amp-hours, the total voltage and the actually consumed current.



Here you see four of the eight cells with their wiring for measuring and balancing. Two of these packs build my main battery, which is also the only battery in the whole tank. All other voltages (receiver, sound) are generated by voltage converters:

[AJ Ross]

I see! Between your advice and some of the things I've read online, it seems that wiring several small batteries together may be a little better (or at least cheaper) than getting one big one for the sake of run time. I'll have to study further about balancing batteries, though. That's a new one for me. And what's this about voltage converters for the receiver and sound system?

And thank you both for the introduction to telemetry controllers as well. I'll see what I can do to get one, along with the requisite sensors.

Cripes, you'd think being a Fire Controlman would have taught me a bit more about electronics. That or my memory is just crap, right?

But thanks for bearing with me!

[Gerhard Michel]

Single cells are better AND cheaper, When you can spare a BMS! But single cells without BMS MUST be balanced when loading, and you need a reliable system to recognize the end of discharge of every cell (to avoid deep discharge).

Complete batteries use a BMS which adds some cost and prevents the cells from being really fully loaded! It also limits the current of the battery. If no more current is needed this is a benefit; if the full possiblecurrent of the cells is needed than it is a disadvantage.

Single cells need a charger combined with a balancer, which also adds some cost. You can use a charger which can supply the full current allowed for the cells without a limiting BMS.

The more single cells you use in your models, the cheaper is the solution without a BMS on each battery.

I use the iCharger which can charge suitable LiFe cells with more than 40 amps:

[Stephen White]

Not sure what point Gerhard is trying to make here. The “complete battery” is made up from single cells. Both types of array should be protected by a BMS in any event. In the case of my Centurion incident, had I not had a complete battery with integrated BMS, the battery would have been discharged to a damaging level. Telemetry would not have offered any protection because the current draw was instantaneous as soon as I started the manoeuvre. The apparent cost differential also needs to be tested in the market where you buy. Single cells really came in when the technology was first introduced and now the integrated solution of battery, BMS and charger is common. I’ve not done a lot of research but did find a 24v 20AH battery with charger for around $250 in the UK. It was for an e-bike I believe. That’s half what a similar solution cost in UK two years ago. You might also do some weight and size comparisons. The battery I use is 4.6kg and 215 x 150 x 100mm which is half the size/weight of comparable lead acid solutions.

[AJ Ross]

Aye, I'd say a BMS is certainly desirable, if not essential.

It does look like many LiFePO4 batteries have a BMS built in, like these Relions I've been browsing: https://relionbattery.com/products/lithium

As well as these AAPP folks who offer batteries built to customer specifications: https://www.batteryspace.com/lifepo4-pr ... d-dgr.aspx

I wouldn't mind either option. Although, rigging enough 24V cells together with a BMS (like the iCharger) sounds like fun. A bit more involved, with more effort, but some greater pride of ownership as well.

[Christoffer Ahlfors]

I am also not sure which route is best. I did it the Gerhard way and happy with that. Either way, I think you will find that the height of the batteries/cells is a challenge.
My charger is capable of pushing 54A into the battery. So if you opt for a smaller capacity, a ten minute pit stop goes a long way (about 10Ah). All you need is a power outlet, which can be a challenge in an airfield, but hardly in a convention center.