Best batteries to use for chieftain

[John Clarke]

Gerhard
I've come across used battery packs before on the bay, details see below.
They are a sealed unit reasonably priced. If as you say these things have a good life expectancy would this package be suitable for the tanks.
I cannot make my mind up on the size of the batteries you have pictured on your bench.
They look like they've been swagged from a U boat/Boot

lifepo4 battery packs 24v 1kw 40ah with 20 amp 8s bms lithium iron phosphate
they are used but all are above 95% capacity

size is 21cm x 9cm x 30cm

[Adrian Harris]

The 20A BMS might be a snag if that's all the current drain it will allow.

The one I use in the T-34 is rated at 30A continuous discharge and that's the smallest tank I own.

I would also say the packs I have also use the Headway individual cells - think huge AA batteries - so individual cells can be monitored and replaced as and when they become tired.

Adrian.

[Stephen White]

Gentlemen, when I posted my advice, I was thinking that not everyone who reads it may be as experienced as Gerhard and John. So in order make a simple and clear recommendation, I suggested that for most people, there are just two choices of battery chemistry, lead acid AGM and Lithium Iron Phosphate with a BMS.

I don't subscribe to the argument that real time telemetry will allow you to monitor battery parameters to the extent that you can avoid exceeding some limits. I had real time telemetry working on my Cent when my LiFePO4 BMS tripped in. As it was a momentary condition, I'm sure I couldn't have guaranteed spotting it. It happened at a public show and no harm was done, either to the model or the reputation of our hobby.

Of course, anything is possible as long as you understand the risks but if there is any consideration of running an Armortek model in public, LiFePO4 without BMS is not a risk worth taking.

It may also not be helpful to the less experienced to abbreviate LiFe as in

LiFes

.

Lithium Phosphate (LiPo) batteries are not recommended for Armortek models. Full stop.

John, 40ah seems serious overkill. As I said, my 22ah LiFePO4 lasted four day at TankFest and still had plenty of useable charge left.

[John Clarke]

Capacity is not the issue then, but a 20amp BMS may not have enough poke.

Unlike a leap of faith, it's a bloody mine field!

[Stephen White]

John

I reckon as a rule of thumb that you can get away with a LiFePO4 capacity of about 60% of that required if you use lead acid. For example, the Cent would probably have a 22-24ah lead acid array. I've used a 22ah LiFePO4 and it's got more than enough capacity. Next time around, I'd probably go for a 14ah battery or maybe even less.

LiFePO4 batteries are lighter and smaller than lead acid, so they have even more of an advantage over footprint.

In the early days of LiFePO4, you could only buy cells from which to assemble an array. The newer ones now come as an integrated module with integrated BMS. That's by far the safest and simplest solution.

[Gerhard Michel]

Hi all,

40 Ah cells with a 20 amps BMS are not suitable for a tank in my eyes. I don't own a Chieftain, but my NH Königstiger with steel tracks, about 160 kgs and a very strong traction consumes up to 55 amps when operating under rough conditions like logging a tree or towing a full sized car. This is the reason why I don't like a BMS because a perhaps 100 amps BMS is rather expensive and fully oversized in 99 % of driving conditions. Without a BMS there is no problem to consume very high currents for a vew seconds when using an ESC with 2 x 100 amps constant current like mine.

Driving without a BMS is not a big risk in my eyes when monitoring is done not per operator's view but automatically per threshold monitoring of every single cell, combined with an acoustic warning in the transmitter.

Lead acid batteries have a DoD rate of 50%; therefore you need a 40 Ah battery to use 20 Ah. With LiFes you can use up to 100% DoD, using a BMS or a fine tuned monitoring. Under safety aspects one will use 'only' 90% (consumption controlled by the telemetry), but this although means that a 20 Ah LiFe may be used up to 18 Ah. Such a LiFe battery weighs about 30 % of a comparable lead acid battery and needs remarkable less space.

[John Clarke]

Having no experience of the LifePO4 battery setups and was just about ask some stupid questions, but you beat me to the enter key Gerhard

so

I won't be ordering a blank firer

[Stephen White]

The advice is very simple. Don't operate a LiFePO4 without a BMS. Period. Full Stop. The BMS is there not just to protect the battery when it is operating but also when charging.

This thread was begun by Daniel and there may be readers here who are less experienced. So the simple guidance is that the options for Armortek tank batteries are lead acid AGM gel or lithium iron phosphate with integrated BMS. Anything else is at your own risk and you need to know what you are doing. Can we move on please?

[John Clarke]

I absolutely agree with you Stephen, Daniel asked what are the best batteries to use for Chieftain.
The safe options are clear. But if you were to use LifoPo4 batteries, what would the BMS be rated at for the use in Chieftain.?
If the speed controllers can allow a continuous output of up to 32amps x2, would it be practical to look at BMS ratings around 40 to 60 amps.?
I have looked at buying LifoPo4 batteries with a viable capacity, but if the BMS is not rated high enough the tank will stop and theres no point in buying them.
I know this is a frustrating simple question to ask, but it is a substantial outlay as compared with the cheaper almost no limit to discharge (constructional limit) from a lead acid AGM gel battery.

[Adrian Harris]

One of the reasons this is such a hot potato is that it's almost impossible to compare overall costs.

Basic up front cost looks easy to compare:

A Tracer 24V 20Ah LifePO4 pack will costs £499 whereas the a pair of Yuasa 12V 22Ah batteries will cost you £74 plus a pair of chargers.

But with the flat discharge curve, the Tracer pack is probably equivalent to a pair of Yuasa 36Ah batteries, which is still only £120.

My Headway pack is 15Ah and costs £290 but I would say gives the same running time of the Yuasa 22Ah batteries above.

And you have to factor in the number of charge/discharge cycles you get from LiFePO4 as well as the different self discharge rates.

(Sorry Stephen)

Adrian.

[Stephen White]

John, my Tracer 22ah is rated at 60A max momentary. That should be fine and has proved to be in practice. Individual batteries vary so you'll need to check the specs.

Adrian, you're quite right, the cost issue is critical and contentious. I'm at pains to point out that my assessments have always been about "whole life costs" ie capital cost, running cost, number of cycles before replacement. If you wanted a very simple headline, Lithium Iron Phosphate costs twice as much to buy but lasts twice as long. So in whole life cost terms, it's cost is equivalent to lead acid AGM. For some, the up-front cost will be critical, while for those who intend to run the model a lot and for many years, Lithium Iron Phosphate begins to look like a good deal.

Two further points. It's clear that Lithium Iron Phosphate offers a number of very real operational advantages, flatter discharge curve, quicker charge time, much reduced voltage sag during storage, smaller, lighter etc etc.

I also pointed out a further cost advantage. Given the flatter discharge curve, you can select a lower capacity with Lithium Iron (I reckon about 40-50% lower), which further reduces the capital cost. So rather than the Tracer 22ah array I've got in my Cent, I'd now buy two 7ah which reduces the capital cost to £238. This includes BMS and chargers.

I'm not trying to sell batteries by the way, just giving new technology a fair and objective assessment, with a healthy regard for the risks and downsides.

[Richard Goodwin]

Just to put something else into the mix, I believe that you will only get around 1 hours runtime with the Yuasa 22Ah batteries which would suggest the current drain is higher than previous tanks. So if you want more runtime, its a case of either upgrading the Yuasa batteries to 36Ah or more, or switching to LiFePO4 but one would have to ensure that the BMS would allow a continuous discharge current of at least 60A wouldn't you? In addition, I'm not sure if a BMS can handle the regen current since presumably, it is only applied over a short period and may not give the BMS time to react unless someone knows otherwise?

[Stephen White]

This thread is like the multi-headed hydra.....

Richard, good point about the Yuasa lead acid batteries. The BMS on a Lithium Iron Phosphate battery will protect in real time against exceeding all the critical limits. It won't allow the battery to continue delivering power if a critical limit is in danger of being exceeded.

We do lack definitive data on the current draw of our models. I don't believe anyone, including Armortek, has taken any consistent and comprehensive measurements.Having said that, I believe the regenerative current issue is a red herring. The anecdotal figures I have seen suggest our models are not drawing anywhere near 60A, continuous or momentary. So the Tracer battery for example is perfectly safe.

Whatever the real figures about current draw and re-charging, and they'll vary between models, a lead acid battery has no protection other than to monitor telemetry with aural and visual warnings, which doesn't cover the momentary case. A lithium iron phosphate with BMS has continuous protection.

[Richard Goodwin]

Does anyone have any measurement data wrt current draw of the motors on any model?

[Adrian Harris]

I've got a FrSky current monitor fitted into a Sherman, so could take some readings next week.

I know driving the T-34 with depleted batteries will blow a 30A fuse in the power module on the original Armortek branded electronics if driven on grass.

I think driving the Cent bridge layer whilst carrying the bridge it was pulling up to 60A whilst turning on grass but that was when I was playing around with telemetry last summer and I'm not positive it was that high. The Taranis will log telemetry data to the SD card, so I should be able to take the Shermie for a spin round the garden and then plot the data out. I'll have to find out if it will plot the stick positions at the same time, as that would show what user input was causing the current drain.

In 2007 I did a current test with a Sherman by putting it against a fixed object, standing on it and then driving it in reverse with the tracks turning. This caused a peak current load of 70A. But that was with different brand motors, different gearboxes and 4QD electronics, so not sure how applicable that is to the current models and electronics.

Adrian.