Maximizing Run Time?

[AJ Ross]

Too right! After more online browsing and a discussion with the local battery monger, I finally understood:

Batteries connected in SERIES combine their voltage, but not their amp hours.

Batteries connected in PARALLEL combine their amp hours, but not their voltage.

So maybe I was slow on the uptake, but it's clear to me now. Forgive me.

So! A few low voltage, high amp hour batteries wired in series could be just the thing; along with a BMS and the adjoining sensors, etc. As demonstrated by Mr. Michel, I think. Could be just the thing.

That, or spend a little extra to have it all compacted into one or two of the big ones. I'm still weighing the options. It might help to hold off until I have my KT kit in hand and have assembled the hull. Then I'll be able to come to grips with the space I'll have to work with.

Continuing to gather tools and supplies in the meantime. A big-ass lift table is in the mail from California, so that's one less thing.

[Gerhard Michel]

A BMS is a simple and comfortable thing, which e.g. seems to 'transform' a LiFePO4 battery containing four cells to a simple lead acid battery containig six cells. Then you can use a simple lead acid charger with 12 volts fitting a car battery. But most of BMS limit the peak charging voltage beneath the possible value for each cell, and also limit the peak output current of the battery. The first reduces the possible capacity of the whole battery, the last MAY limit the output current below a value necessary for driving the tank in extreme situations (e.g. pulling a car or so), depending on technical data of the BMS.

Therefore a BMS is a good thing for carefree using a LiFe battery, but it limits the capacity and the peak current of charging and discharging. Originally a BMS was constructed for starter batteries in vehicles, where a long durability and a low internal resistance are much more important than the highest possible capacity.

Using LiFe cells without a BMS needs a charger with balancing function for charging and a reliable LiFe monitoring to prevent deep discharge, but you can get the full capacity and the peak current of the battery if necessary.

[Christoffer Ahlfors]

Good strategy Ross!
1 Get the lifting table (you will need it from basic hull assembly and forward) and save up to the batteries and charger for later.
2 Measure the available space, plan the layout and order cells or batteries accordingly.

It came on to me to do just a quick space check before clicking the buy button on the cells I had in mind. It turned out that they would not have fit in any orientation. We have to realize that our models are so small...

[AJ Ross]

Aye, thank you, Mr. Ahlfors. Internal space does seem to be at a bit of a premium in some instances. Although, I suppose I'm helped by leaving out a smoker and a model Maybache engine. Nothing against them, just had to make the right choice for me.

And thank you, Mr. Michel. I think a comparison might be made between doing without a BMS and overclocking PCs. Both can increase performance, but are liable to do more harm than good if you don't take care. And I know little about the specifics of either, honestly. So I think I'll be playing it safe.

There do seem to be a variety of BMSs available with different specs, or ones that can be adjusted to certain settings. I'd hate to mistakenly install one that'd choke off most of the current my tank needs to run.

I've also been looking into the possibility of buying individual 18650 lithium cells and configuring them into a 24V pack with with 50-60 amp hours. It'd be a fun project, albeit also one I've never attempted before. Worth considering, with proper caution.

Then again, the safest and simplest option might be to just rig a pair of 12V LiFePO4 batteries with BMSs already built in and call it done.

I might be overthinking the hell out of this, but at least I'm having a good time of it. Thanks to all involved!

[Stephen White]

AJ, the other options may be a fun challenge but for safety and reliable performance, a single 24v LiFePO4 battery with integrated BMS is undoubtedly the soundest option (and the smallest/lightest).

[AJ Ross]

You're right, Mr. White.

After all this this pacing around, I finally remembered what the navy calls KISS.

"Keep It Simple, Stupid"

And it's not as though I have a spot welder in the closet, is it? No, sir. Maybe someday, but not in this instance.

[Adrian Harris]

I've also been looking into the possibility of buying individual 18650 lithium cells and configuring them into a 24V pack with with 50-60 amp hours. It'd be a fun project, albeit also one I've never attempted before. Worth considering, with proper caution.

That would be quite a project.

If you select an 18650 battery with a voltage of 3.6V and a nominal capacity of 3000mAh (3Ah) then you would need:

8 x batteries in series to give you a 28.8V pack with 3Ah capacity.

20 x packs in parallel, to give you 60Ah.

Which is a total of 160 batteries, each of which would need to be monitored, given they are LiPo, not LiFePO4

Not impossible, as there is at least one person on here doing exactly that, but for home power storage, but quite a lot of effort over getting a commercial battery pack.

Adrian.

[AJ Ross]

Aye, Mr. Harris, that's true.

I suppose I was keen on the idea for the sake of novelty, more than anything. This is all still pretty new to me.

For now, my determination is to get something in the neighborhood of 50+ amp hours and see if that's enough to keep a King Tiger going along a convention floor for an afternoon. I'm favoring either one 24V, or a pair of 12V in series. Both would need a BMS, of course. Either built-in or externally mounted.

There's also the question of how much weight these power pack will add. I get the impression that Armortek kits aren't overly delicate, but I'm erring on the side of keeping it light.

[Adrian Harris]

> but I'm erring on the side of keeping it light.

Once you've picked up one of the hull plates on an Armortek King Tiger, you'll wonder why you ever worried about the weight of the battery!

Adrian.

[AJ Ross]

Well, yes.

I just felt I aught to keep the added battery weight under, say, 60 lbs or so. (27.2 kg)

I know an Armortek KT can drag a guy in a crate across a dirt yard:

But still, I do tend to over-think things sometimes. Thanks for bearing with me.

[Gerhard Michel]

Oh, IMHO it is not a bad thing to observe the weight of a battery, because in my Jagdtiger the difference between lead acid and LiFe was more than 18 kgs! The lower the weight, the longer the run time; especially in rough grounds! Please remember the headline of this thread......

Some towing results of our models are really remarkable. In another forum an 1:6 scale model is towing a 2.8 tons car over a slight ascent in tall grass!

[Stephen White]

AJ, "we're done here" I thought but maybe not. Sounds as though you've worked through to a conclusion you're happy with. 60lbs of battery weight would be excessive. You also have to remember that the torsion bar suspension would be challenged by that much weight. I would aim for about 10-15 lbs max. The Tracer 24v 20ah LiFePO4 array is 4.6kg ie about 10lbs. You could put in two but I suspect one would give you all the running you're after. The other consideration is size. Typically, the integrated LiFePO4 batteries occupy half the space of a comparable AGM array. I've been able to slide my battery into the forward hull of the Centurion and still fit a driver figure, leaving the space under the turret for electrics and the engine space where you could fit a one sixth Maybach. Over-thinking is an occupational hazard with these models. I like to follow the principle that a wise man learns by his mistakes but a wiser one learns by others'.

All the best.

Stephen

[Steen Vøler]

Haha, I remember doing that video that was a fun weekend we had in Denmark

[AJ Ross]

Aye, Mr. White, you're spot-on. I didn't seriously expect I'd come close to 60lbs, but it certainly sounds daft beside your recommendation for no more than 10-15. These Tracer brand batteries do look promising, though. "Bespoke applications" indeed. I hadn't been aware of these. Thank you!

And that was you, Mr. Vøler? Your Tiger pull video is one of my go-to exhibits when I try to explain all this Armortek business to my friends.

They all think I'm nuts, but also can't wait to see the finished product. Cheers!

[Christoffer Ahlfors]

Overdoing things is an interesting topic. I replaced 80 kg of lead batteries (150Ah) with 11 kg of lithium (60Ah). The model came out way too light after that, so I put some of the lead batteries back (as many as I could fit...) as ballast. After all, prototypical weight of a tiger 1 is 250 kg. (I am well below that.) The running time went from ten minutes (usable time of full maneuverability) to two days. I was using standard lead batteries - not AGM, so a bit of an unfair comparison to poor old lead.

My opinion is that massive weight adds to realism when a model moves through terrain, crushing its way forward, rather than bouncing over obstacles. If we wanted light weight, there are other brands out there...