Lifepo4 Battery

[Allan Shannon]

Colleagues

Would a 36AH 24v LiFepo4 battery be okay to power a tiger 1 and if so how long a run time would I be able to get?

Regards Allan

[Brian Ostlind]

It will probably vary based on your driving style.

[Stephen White]

Allan

A qualified yes. You should just check the max peak current. A value of around 60A momentary would be good. It should also have a built in BMS (battery management system).

36Ah is overkill. With Lithium Iron Phosphate, you can reckon to get perhaps 140% better endurance compared with lead acid (flatter discharge curve) and the benchmark for lead acid is about 20Ah.

How long to run? Impossible to say. It depends on running surface, temperature, driving style etc. You can say that a Lithium Iron Phosphate will give you pretty much the full voltage until about 90% exhausted, whereas with lead acid, voltage reduces significantly beyond 50% discharge. So you will run for longer with the former.

All I can tell you is that my Centurion has run at a three day show, with perhaps six or seven 20 min runs each day and still had more than 60% capacity, ie I didn’t have to recharge at all. That was on hot summer days running over astroturf obstacles. The battery is a 24V 20Ah module.

[Gerhard Michel]

Hi Allan,

I use each 8 LiFe cells (26 V) of 36 Ah in my Jagdtiger, 40 Ah in my King Tiger with Porsche turret, 20 Ah in my King Tiger with Henschel turret and other 20 Ah in my Pz. IV. Please note that these batteries are the only power sources in my tanks, for driving, sound (up to 250 watts), RC and so on.

Therefore I can say that 20 Ah are sufficient for moving the tank for about 2 to 3 hours at a 'normal mission' , but strongly depending on driving conditions! Nearly all my tanks use steel track links with an excellent grip. This results in a huge power consumption when turning. All my tanks use telemetry (to avoid a BMS) and therefore I can also see the actual current at my transmitter. Driving straight forward on a plain ground consumpts overall 4 to 6 amps, whereby the sound may consumpt the same current as the driving motors (!). Tableturn in grippy grounds may consumpt up to 60 amps and more! The driving conditions may cause a 15 times varying power consumption, and no one can predict the real run time. Therefore it is strongly necessary to use either a BMS or a very reliable control method for each single cell! In my eyes the latter is the better method because you can always see the power conditions (e.g. the used capacity!) and can stop the mission BEFORE your tank stops by itself at a place, where the recovery is difficult. Our sandy driving area e.g. is not suitable for a family car to recover a tank model...

[Stephen White]

Allan

Gerhard has provided some useful data but it's worth emphasising the purpose of a BMS. It protects against a momentary excursion beyond the safe parameters of the battery. It will detect a condition where the battery is about to exceed its limits and will shut it down, very effectively, before any damage is caused. The battery can be allowed to rest briefly and then the BMS can be easily reset by supplying a charging voltage briefly. I carry a 24V lipo and the charger for that purpose. It is misleading to think that you can monitor the battery via telemetry and avoid the permanent damage that will come with a momentary spike in demand. It's not about monitoring the discharge of the battery, which, as Gerhard says, is more important with a Lithium Iron Phosphate battery with it's sharp fall off in capacity at the lower end of the curve.

In the one case I've had of a BMS activation, it was a very hot day, I was turning at slow speed on quite thick grass but the battery was still at 65% capacity. I suspect the motion pack attempted to draw more the 60A and the BMS said no. After about ten minutes, the battery was successfully reset and the tank kept running afterwards for some time. I was monitoring discharge via a telemetry voltage readout but would not have been quick enough to stop before the demand on the battery in turning would have caused damage. If you’re operating the tank near the public, such as at a show, you shouldn’t be monitoring the transmitter all the time rather than spotting that child about to run in front of the model.

Or in simple terms, don't use a LiFePO4 battery without a BMS. Some may question that advice but it's meant to give you the best option for protecting your investment.

[John Heath]

I decided on a bit smaller LiFeP04 batteries as I wanted to be able to easily change them in the field. A pair of 12v 16ah Tracers batteries will fit through the engine hatch .
John

[Gerhard Michel]

Hi Stephen,

of course a BMS is a good thing to protect a LiFe battery, but I refuse to say it is the only one. Like you I think that your BMS switched off due to an 'excessive' current peak, but that limitation results exclusively in the electric data of the BMS and not in any damage which may happen to the battery. LiFes can endure much more current for seconds (e.g. my 40 Ah cells more than 400 amps), which no BMS would allow. My motors can consumpt 2 x 119 amps (short circuit current, for less than one second without damage), but I use a limitation to overall 80 amps continuous current by magnetic fuse and my ESC limitates the current digitally to 2 x 100 amps real-time. If I would like to maintain these fringe conditions I would need a 200 amps BMS!

I also agree that a permanent personal monitoring of the transmitter display would be a bad solution, because especially near the public we all have to monitor our models (in Hausen there are about 40 models at the same time in action, only a few meters away from the spectators). Therefore the transmitter gives me an acoustic warning if only one cell exceeds the parameters. In my opinion this is sufficient to save both the model and the battery.

To get more experience in using a BMS I bought one (with 100 amps current limit) for my King Tiger with the 20 Ah cells. Yes, it works well, and till now it didn't switch off. But I'm unsure when using this tank in our battle field, because IF the BMS would stop in the field I would be forced to rescue the tank with rope and man power, because a 'dead' BMS can only be reactivited by short charging the battery

[Christoffer Ahlfors]

Alan, Gerhard is touching on an important aspect regarding driving time: A 300W sound system to compete with 1:1 counterparts in the background (to prove rather than gesture the answers to the usual questions: CAN IT MAKE ENGINE SOUNDS? CAN IT SHOOT?), will consume around 15A @ 24V. This is far more than the drive motors will consume on average. Add to that another, say 3A for smoke, depending on light conditions and your preference. Oh, 15A if it's a T34, of course...

[Gerhard Michel]

Hello Christoffer,

fortunately a 300 W sound system doesn't consume 300 W at any times; this is the peak current (e.g. gunfire). But yes, my 250 W sound system needs up to 3 amps, when the tank stays with motor idle. This is an average value because the volume is variable, controlled by transmitters volume control. With my Beier sound module I can use each sound in its own loudness; e.g. full power fur gun fire, strong power for backfire when retarding and moderate power for all other sounds. Therefore the power consumption of the sound system ist a complex thing.