Batteries - Lithium Iron Phosphate or Lead Acid Gel?

[Stephen White]

I know this has been debated before but I thought I'd share some research. I need to order some batteries for the Cent and thought that since it's a heavy beast, I'd take another look at the alternative to Lead Acid. I'm a complete electronics biff, so I need simple explanations. I found this site useful for a good review of the competing merits:

http://www.wheelchairdriver.com/BM-MK3- ... rchair.htm

This site has an article on constructing a LiFePO battery pack using Headway cells:

http://zenid10.wordpress.com/category/4 ... hium-pack/

And here a UK supplier:

http://eclipsebikes.com/lifepo4-battery ... 6e49b8a107

I did some rough comparisons:



Although Lithium got a bad press, with horror stories of batteries exploding, with an appropriate battery condition monitor and careful charging, the risks should be mitigated. On the plus side, the cost over comparable life cycles are pretty much the same as lead acid (eg 15ah?).

In reality, lead acid output declines as the charge is consumed so in reality about half the capacity is usable. For lithium, the battery delivers the reference voltage until it falls off the cliff (ie the BCM cuts the load). So in theory, Lithium could deliver the same endurance as lead acid with a smaller capacity pack.

Because a lithium pack is made up from individual cells, the size and shape of the pack could be tailored to the space available in the hull. It could be very long and use the full width of the hull for example but be very low, avoiding fouls with the turret.

But for me, the overriding potential advantage, which is considerable, is the weight saving which benefits performance in many ways.

Comments, advice please?

Stephen

[Adrian Harris]

I've ordered a DIY kit from "Battery Matrix", which appears to be very similar to what Eclipse are selling, but direct from the Orient, so a few pennies cheaper.

Hopefully, it will be with me by the time of Shepton Mallet, so I will be able to demonstrate it there.

I'm interested to see that the article which shows how to build a battery pack only uses one link per pair of cells back to the BMS. I wonder what happens to the charge/discharge distribution between the two cells in each pair ?

Adrian.

[Adrian Harris]

I've just been reading an article on Günter Gäbelein's T34, which states - if I can translate German correctly - that his model runs from a 22.2V 5Ah LiPO battery.

I wonder if Günter could comment on what sort of running time this provides ?

Adrian.

[Stephen White]

Adrian

I agree it would be good to hear from Guenther. From what research I've been able to do, it suggests you can get equivalent endurance from about 25% less capacity eg a 15maH pack. That's with the bonus that you get very little drop in voltage until if falls off the cliff.

Interested to know how long Battery Matrix take - they look pretty good but I've seen some questions on one forum.

all the best

Stephen

[Armortek]

Hi all

My knowledge of LiPo batteries is as limited as everyone else. My concern would be how these batteries react to the regeneration charging that the Armortek system delivers. Our system is capable of producing very large short term currents that re-charge the batteries during braking and turning. I have measured recharge currents of 80 amps, albeit for short periods. Can LiPo batteries cope with this safely? I don't know, but I would be cautious until someone who really does understand the issues can advise.

Mark

[Adrian Harris]

Thanks for your input Mark. I must admit I had forgotten that the newer modules provide regen current.

I'm not sure which BMS will be supplied with the packs I have ordered, but the most likely one is rated for over-current protection in the range of 50A-500A.

Whether the BMS can handle the model providing charging current on the "output" lines remains to be seen.

Adrian.

[Stephen White]

Good call Mark. Just one significant observation though, we're talking about Lithium Iron Phosphate (LiFePO4) rather than Lithium Polymer (LiPO). I gather the latter have a track record of spectacular self-disassembly, whereas the former, if protected with an appropriate BMS, are far more stable. I also understand that the internal construction of modern LiFePO4s is such that they have anti-explosion valves - sounds dramatic but apparently mitigates the risk.

Nevertheless, it would be good to have someone in the know pronounce on whether the regenerative capability is OK with LiFePO4 cells.

Stephen

[phil fitzpatrick]

Hi People
I have been using big Lipo batteries safely in my Helicopters for a long time.
Like Stephen said there is a big difference between Life and Lipo.
Here is a good site but it compares the life with Nicd and Nimh
www.lifesourcebatteries.com/reasons,html
A significant thing is they only advertise for receiver and transmitter applications.
But that didn't stop old Barns Wallis experimenting and doing the business
Lipo and Life batteries need charging with a dedicated charger for individual cells
and monitored individually.
Life just like Lipo packs have a charging connection ie a white plug so a 6 cell 22.2v
5000 ah would have 7 connecters in the plug.
A 5000ah battery should be charged at 1C max so 5amps is max and recommended is 0.5C
so 2.5 amps.
Any battery that requires individual cells to be charged and balanced separately
would object to 80 amps up their rear end me thinks
Lipo batteries are very sensitive of the 3.0 volt discharge limit,it can kill the battery dead,I cant see the Life being that much different so BMS or as in the
helicopter the speed controller and BEC,I think these settings are critical.
Lipo batteries run very hot and after cooling charge with no appreciable warming.
I don't know about the Life running carracteristics.
But I wander if the Lipo would not like the dungeons of the Tank Hull.
if making a pack up of individual cells I think you would have to give a lot of thought
to matching the proper charger and its requierements.
Also if connecting several ready made packs they would have to be disconnected and
charged separate.
Lipo should deffinately be removed from the model so you can whatch and feel for
swelling and heating,the signs of impending doom.
I like my Tanks Heavy so I choose the biggest amp hour and weight of batteries I can
get in.
When my King Tiger goes out to play I want the slugs to grow legs and run or explode
when I go over them
Teach them for eating my plants
Very interesting subject I like it.
Cheers
Phil

[Dennis Jones]

Be aware when charging lipos, I accidentally recharged a 2 cell pack in a helicopter with the switch set at 3 cell on the charger. Left it on charge with the battery in the helicopter in the garage only to come out later and find smell of burning and the only thing left of the helicopter was the aluminium tail bar and a big burn mark on the bench top, so make sure as Phil says take the battery out of the model bfore charging. Mine wasn't a particularly large battery either.

Dennis.

[Stephen White]

Good point Dennis. Can I just emphasise again though that my original post was about LiFePO4 batteries not LiPO. I gather that with a BCM, LiFePO4s are much safer. It would be good though to have someone with current experience of them comment. Best regards. Stephen

[Andrea Daviero]

Here in Italy most of armortek users, including me, are using LiFe batteries (8s) and no one of us had a problem with them.
I was the first one with my sdkfz7 wich is powered with 2x 4S 20Ah Life batteries, they were installed on it about 3 years ago and when I charge it I can see it still charge 20Ah...
My batteries does not have a BMS on them, I use a dedicate charger with balancing function.
My friends uses 22Ah Life LTE batteries (12,8V) from lipotech.it at a good price but with no cell balancing. However, as the cells are bolted each other they can be modified either for a BMS or for a balancer plug for the charger. I did that on Iacopo's Panzer 3 wichi is now powered with those batteries and there is still few space for other things...

LiFePo batteries are less Dangerous than LiPo ones, they can be safely recharged inside the tank. The battery life is very very higher, and the voltage is pretty costant untill the battery goes empty. So the tank will run without audio volume loss, without performance loss until the last few Ah of power. That's pretty better istead of pb batteries wich gives you less power and a great voltage loss from charged to discharged.

The tanks powered by LiFePo here in italy are:
1 T34-85, normally runs offroad for more than an half day without needing a recharge, ampl. volume set to max
1 A34 Comet, same as above
1 Tiger I mid, as above
1 pz3, not yet tested
1 sdkfz7 wich runs almost a day offroad with sounds, lights, wipers.

[Stephen White]

Thanks Andrea, sounds like a good set of arguments in favour of moving away from pb. Adrian, it would be good to hear how you get on, perhaps at the SW Model Show, if you're going.

Regards

Stephen

[Adrian Harris]

Thanks for the information Andrea - it's good to know these batteries are already being used

I will be attending SWMEE but the batteries haven't arrived yet

Adrian.

[phil fitzpatrick]

Hi All
I couldn't find out how to copy a link and put it on for you.
But if you google
warning!!! alien motion lifepo4 battery
It should take you to a posting on the Aprilia Forum.
This man from Chester UK took photographs of his Lifepo4 battery after having a ride on
his Aprilia bike and he could smell burning.
The photgraphs show it all,there is no denying that this could have turned into a fire.
Perhaps there are good manufactures and bad ones or maybe a manufacturing fault.
There are other posts on the forum equally with or without problems.
RE chargers
In my searches I came across Optimate,they have just brought out a suffisticated
lifepo4 battery charger.
I think Optimate and accumate might be related by the way it was described?
But the spec was good.
Cheers
Phil

[Stephen White]

Thanks Phil, very interesting. I've been using an Accumate charger for my PB batteries and it's worked perfectly. I'm very interested to see that Techmate are now introducing a smart charger for LiFePO4s.That provides one level of protection with safe charging. The other risk to be mitigated concerns discharge, where a battery condition monitor is required to prevent dangerous discharge conditions.

The example you quoted looks like a case of someone putting too much demand on too small a battery without a BCM. Here's the response from the manufacturer:

"Thank you for the detailed information & pictures. *First off, the AM8 battery is slightly too small of a battery for the RSV4 and all its electronics. *What you experienced makes sense. *When an engine is hot, it takes more cranking energy than when it is cold, due to the increased pressure of hot gasses in the cylinders. *When the bike was hot and failed to turn over, the battery was outmatched by the cranking demand of the motor. *When the engine won't turn over, the energy output from the the battery has nowhere to go, so it effectively short circuits the battery. *The battery heated up until it melted it case and when it gets too hot, the cells vent and fail."

A bit of Darwin Factor perhaps?

I'm still tempted to go with LiFePO4 - lighter, constant voltage, longer life. The only unresolved factor for me is Mark's point about the capacity of the battery to absorb the peaks from the regenerative power system but I need to do some more research to see exactly what BCMs can do.

Thanks for the input, as ever.

Stephen