Lithium Battery Pack and Cold Temperatures

[swalker]

I have been considering putting together a LiFePo4 battery pack for my Magic Mobility X4 wheelchair. I bought this wheelchair as a derelict off of craigslist several years ago. My friend and I spent a few months putting it back into operational condition and it has been a wonderful addition to my wheelchair fleet for the last 3 years.

I use it mainly for outdoor activities, such as getting me to the ski lift for skiing and for hikes on backcountry trails.

A primary limitation of the wheelchair is that it has a very limited range. It will always go 4 miles and on the right terrain will go as far as 6 miles on a charge.

There is room in the battery boxes for a 180AH lithium pack composed of 16 90AH cells. I have ready many posts here about building a battery pack and have done a lot of research elsewhere as well. I think I have the design figured out, but have a few questions about temperature.

Question 1:
I often use this wheelchair in the winter when the outside temperatures reach 0F. While temperatures can get much colder than that here, I could get by without going outside when the temperature is below 0F. During the day, temperatures would typically be above 15F (about -10C). That means I need to be able to operate it in temperatures as low as -18C, but usually above -10C. In this scenario, the battery pack would be charged at normal house temperatures of at least 60F (about 16C) and I would take the wheelchair from that environment directly into the cold. While -10C and even -18C are within the operating temperature range of the prismatic cells I have been looking at (http://www.evlithium.com/LiFePO4-Battery/472.html), does anyone have an opinion about how practical it would be to actually do so? I am aware that there will be a reduction in available energy, but the trips in the cold are always less than a few miles.

Question 2:
We often go camping at a location where the only practical way to charge the wheelchair is to leave it and the charger in the van overnight with an extension cord to a convenient power outlet. We do this in the spring and fall and camp where the temperatures often reach below 15F (about -10C) and rarely as cold as 0F (about -18C), though temperatures would typically be above freezing (0F, 32C). The spec sheet for the cells says the allowable temperature range for charging is 0C to 45C. My thought was to provide some sort of heat for the battery pack, perhaps via something like a low power heating pad suitable for medical use. Hopefully, that would be safe enough and would provide sufficient warmth to keep the battery pack well above 0C and probably closer to 20C. What are your thoughts on doing this?

Question 3:
To fit the 16 prismatic cells (and insulation material) into the battery boxes on the X4 wheelchair, the batteries would be packed pretty tightly, leaving no room for air circulation. Does anyone see any significant potential for thermal issues caused by the lack of airflow around the batteries?

Thanks for your insights.

Steve

[Burgerman]

1. Should be no problem. Been doing that with hobby packs for years. Flying helis etc.

2. Should be OK as long as its thermostatic, and doesent allow anything past 20C. 12 to 15C ideal. But dont charge at less than 0C or preferably 10C. As it damages them. Charge at a slightly lower voltage. Like 3.550Vpc.

3. At the AVERAGE discharge rate we use, its not a problem. And in fact may help.

[swalker]

Thanks for the input!

Steve

[Burgerman]

The reason for charging to a lower voltage like 3.55 or 3.50V is because...

A lithium battery, capacity reduces as temp drops. And so does the fully charged safe voltage. Charging a lipo for hobby use at 0C on a cold winter day on the flying site to the usual 4.200V means that when it warms up its voltage is now at an unsafe 4.3 ish volts. So your helicopter bursts into flames as you drive home with the heater on. Dont laugh, a guy at our hobby club lost his work van that way!

If you look at the PL8 it gives the option to reduce charge voltage by temperature. Starting at 55C.

[shirley_hkg]

You may want Lithium Titanium Oxide battery (LTO ) from now on .

Discharge platform : 2.4V

20,000 cycle lifespan . 50C rated charge and discharge .

viewtopic.php?f=2&t=6861

We have too many hidden treasure in this site ,
sadly buried down under .

[Burgerman]

I looked at those a few times.

They have about 66% of the energy density of LiFePO4. So we cant get as much range!
And no decent charger that will charge the 24V 10S setup we would need.

[Furio]

Hi,

Does it affect adversily lifepo4 battery using it before complete balancing?

[Burgerman]

It depends HOW out of balance and how low you run it. If its a 100A pack, and its 1Ah out, no problem as long as you dont run the low cell out of power...

If its a lot out of balance then its ok to go from the house to the garden... And then check lowest cell.

Why not balance it properly?

[Furio]

A sudden need use the chair prompted to terminate charging in the final stage.The two voltage extremes were 3.430 and 3.591.

[shirley_hkg]

The two voltage extremes were 3.430 and 3.591.

They ALL are FULL , basically.

Better not drain it below 25V .

[Burgerman]

If it was balanced on the previous cycle, thats no problem. It cant be far out.

I sometimes wacK 40A straight into mine from a simple power supply for a while until volts begin to rise significantly above 3.4V per cell. As long as I knew it was perfect the charge before.

[MichaelB]

My off road chair with LiFeP04 batteries lives in my garage which is not heated. I'm in the Lake District and temperature in winter can get to -10ºc sometimes lower. I do occasionally use the chair and it is left with the batteries partially charged and I fully charge on the day. PL8 charger using preset 1.

I can't warm the garage for the whole winter so is it best to bring batteries into the house when charging, can't keep them inside as the missus will go crazy. If I left them outside in the garage what would need changing on the PL8?

[Burgerman]

Not a stock PL8 preset I hope!

Theres a few issues with those. The presets for LiFePO4 and lead we use are modified in around 10 ways in parameters that are hidden that you cant see. For a number of reasons.

You should not ever charge below 0C for any reason. But if its a few degrees warmer OK. If its 10C then 3.500V is OK. Set balance deadband to 4mV. Set charge current to around 1/4C. But use one of my presets to modify. Set balance start to 3.500V.

You can also set the charge voltage to automatically drop as temperature falls.
See here, set to reduce voltage by 100mV per cell at 10C: So giving you 3.600v at room temperature, dropping with temp to 3.500v at 10C as mentioned above automatically.

[Burgerman]

Since we already charge at a lower voltage of 3.600v that some do, 100mV may be excessive. I can set 50mV lower at 10C instead. Also charge aceptance slows in cold so you need to charge slowly and it may take a while.

[MichaelB]

Thanks, you just did a preset for me, I guess what I need to do is duplicate it, make the changes above and save it as a "cold weather" preset. Does the PL8 know the air temperature and alter automatically?

[Burgerman]

On the preset you have no.

Set it to 50F or 10C to reduce by 50mV. If its set to charge at 3.600V per cell. Then it can.

(I had thought you were using a stock 3.650V charge preset)

Then it will work at 3.600 indoors, and lower the voltage in your garage/freezer At 3.550, at 10C and to 3.500v at OC in theory... The problem is that if you charge to 3.6V when cold, that may be 3.7 to 4V when warmed up as you drive! Not so good.

So you still only need that same preset. If its below say 2 or 3C dont charge unless essential. It can cause whats called lithium plating, which ruins the cells. Even though it all appears to charge fine. Cant you just hide them indoors under the bed or something?

[MichaelB]

Thanks again. If I get this when enabling the cold weather 10ºc setting it will read the temperature and adjust automatically. What about charging current? It was reduced in the screen shot should I change this?

I can't get anything in the house without her noticing, I'm sure that she has a hidden cctv camera

[Burgerman]

The charge current is whatever you set. What Ah is the pack? Set amps to 1/4 of this. Set termination current to 1/250 to 350th of pack capacity. So if 80Ah, set termination current to 270mA.

[MichaelB]

60Ah pack so 1/4 = 15A, would that be the same for warmer temperatures I'm sure preset that you did for me was higher. Termination current 200mA is OK?

[Burgerman]

All of that is OK, just enable the 50mv/50F 10C temp gradient.

Or upload here and I will.

As far as charge amps, then you can charge as fast as you want when its warm. Even 40A is still below 1C. But they dont like that when cold. But even when warm, its better to charge at a lower level if you like your battery!

[MichaelB]

I think that I'll just have an above 15ºC preset with higher current and a cold weather preset with the changes suggested that way I can fast charge in summer and take a bit more care in winter. Or just bring the batteries in the house to charge if my support workers can sneak them in

[Burgerman]

Sounds like you should keep the support workers and get rid of the wife.

Similar to my situation. Only it was a GF and she got rid of me!

[MichaelB]

Sounds like you should keep the support workers and get rid of the wife.

Similar to my situation. Only it was a GF and she got rid of me!

Well, I can't be far away from the same as you. Charging batteries in the house might be the tipping point. Then I would have no-one to go out with me when I want to go off road so no need to charge batteries.

[Burgerman]

Dont be silly. The world is full of young fit, single women.

[ex-Gooserider]

I know that here in New England, we can buy heating tapes intended for wrapping around exposed plumbing. Typically they have an AC cord, with a sealed blob in it that contains a thermostatic switch, and a few feet of flat heating element wire.... They come in different sizes, mostly different lengths depending on how much pipe needs protecting, obviously the longer lengths draw more power, but most are on the order of a few hundred watts.

The theory is that you wrap it around the pipe and insulate it except for the blob. The thermostat is set by the manufacturer to turn on near freezing and off when warmer... The claim is that they put out just enough heat to save the pipes from freezing. The ones I've tested by sticking the blob in the freezer and holding the tape in my hand seemed to get up to no more than 'coffee cup' temperature...

Would wrapping one of these around a pack inside a battery box (the tape is only a 2-3mm thick) be a possible way of solving the cold charge problem for our arctic members? Obviously it would mean needing to keep the chair plugged into AC whenever it's parked, but you need AC to run the PL-8 power supply anyway....

ex-Gooserider

[swalker]

I appreciate all the input and have followed the tangential discussions with great interest.

I feel comfortable moving forward knowing there are several solutions that could keep the battery packs above freezing while charging.

I had considered medical heating pads, for their perceived safety. I will continue to investigate other alternatives, including heating tape (it is commonly used where I live).

I have continued to look for batteries that will fit the most energy into the space available in my X4 wheelchair. In my search, I came across these: http://www.lifepo4-batteries.com/sale-1 ... ation.html

They are about 71mm wide, which is about 1 to 2mm too wide each to fit 8 of them in the space I have. Too bad, because that would give me 240Ah at nominal 24 volts.

The web site for those batteries claims they have a charging temperature range of -10C to 75C. That is really surprising to me. From what I have learned about the electrolyte used in LiFePO4 cells, none should be charged below 0C.

Any thoughts on LiFePO4 batteries that can be charged below freezing? Sounds like the marketing department is working overtime on that claim.

Steve

[Burgerman]

Yes marketing. Esp the 75 degree end! Thats way too hot.

[shirley_hkg]

LTO for -30℃

A 12V 20Ah pack .

Or brand new TOSHIBA cells .