Question on Effect of Cold on Battery Pack

[coosa]

My uncle wanted me to go look at a tract of land he's considering investing in on Saturday morning, and wanted me to bring my cart because the roads on the place are in bad shape. So I loaded up the cart and met him there; it was 20 degrees when we unloaded the cart.

JohnnieB said in another thread that you lose 15% of you battery capacity for every 20 degrees below 80. (At least, I think that's the figure he gave; I can't find the thread now). So I thought I would lose something like 40% of the distance it would go. This is a 36 volt cart. I've never tested to see how far it will run on a charge. The hardest I work it is on a steep hunting club where I may run it 6 or 7 miles on a trip. It has one of those cheap EZGO meters on the dash to show charge remaining, and its never dropped below the halfway mark. After one of these trips, pack voltage is 37.3, so going by the charts, I've never used more than about 25% of the capacity. I think it would do 12-15 miles before dropping below the 50% charge at around 36.2, but I have never pushed it that hard to find out. My batteries are T-105 and about 2 years old.

We rode around for several miles and I noticed the meter was dropping much faster than it ever has before, but I've never used it before at 20 degrees. Once the meter hit the halfway mark, I told him we needed to turn around. It quickly dropped to about the 40% mark and the cart slowed to a crawl. We wound up having to walk a half mile back to the truck. Fortunately, the road was good enough that I could drive to the cart and load it. I should have tested the pack voltage then, but I just wanted to load it and leave.

When I got home that afternoon, temp was up to 50 degrees and the cart felt strong when I took off the trailer. I tested the pack voltage and it was 37.6, which would indicate I had more than an 80% charge left. I did not expect that; I thought I'd be well below 36 volts. I charged it and it runs normally now with pack voltage at 38.2 this morning.

My question is, what does the cold weather really limit on the batteries? The distance I actually ran the cart was about 20% of what I thought the cart would normally run. Once the temp went back up, I still had an 80% charge, even though the cart would barely move at one point earlier. So does really cold weather essentially make it impossible to run the batteries down more than 50%? Or does all of this indicate I have a battery problem? I'd just like to better understand what happened so I don't run my cart down again. Thanks!

[JohnnieB]

Lowering the temperature does two things with batteries, reduces the storage
capacity and reduces the amp flow, so you get a double whammy. The max amps the battery will produce is less and the size of the "gas tank" is smaller, so your cart won't be as peppy as it is in warm weather, or go as far on a single charge.

The electricity is stored chemically and the chemicals are less reactive when cold and more reactive when hot.

I find at least three different rates of change per degree of temperature from Trojan alone.

1. Trojan website linked to below suggest a good rule of thumb is a 10% difference in storage capacity (AH) for each 15°F below (or above) 77°F. That equals 6.67% per 10°F.
http://www.trojanbattery.com/Tech-Su...mperature.aspx

2. The text in the attached cut&paste from a Trojan publication states that the battery looses 50% of its storage capacity at 0°F vs 80°F. That is 6.25% per 10°F.

3. However the line on the graph in the same cut&paste indicates the storage capacity only drops to just under 60% at 0°F. That is 7.5% per 10°F. (Which translates to the 15% per 20°F that I have posted in other places)

At 20°F, your battery pack had somewhere between 55% and 62.5% the storage capacity it has at 80°F. Being two years old, they have lost storage capacity due to aging, so they were probably only about 200AH instead of the 225AH they started with. So you only had about 110AH to 125AH of stored energy and they would have been sluggish pumping out amps.

Also, if it had been colder than 20°F the preceding night, your batteries were colder than the ambient air temperature. Batteries have a huge thermal mass and it takes a long time for them to warm up to the daytime air temperature after being cold soaked overnight.

Hope this helps.

[coosa]

Thanks JohnnieB, that does help. It died sooner than I expected, but what really surprised me is the way the pack voltage made it back to a figure indicating 80% capacity without any charging.

[JohnnieB]

Battery voltage drops when amps are drawn, batteries alone in a 36V pack is about 1.0V per 100A of current being drawn. The voltage drops almost instantly, but recovers slowly. It takes over an hour to fully recover, but in about half an hour, most of the voltage has been recovered.

Attached is a chart that I made when I was running a 36V pack of T-105 batteries. The first voltage reading was after a 10 mile run (I was trying to estimate my carts range) and was taken just as soon as I could get the meter leads on the main terminals after stopping cart. The last one was an hour later and the voltage had climbed from 37.05V to 37.57V.

That is only an increase of 0.52V, but that's 13.7% SoC, which makes a huge difference when guesstimating max range for a 10 mile trip. When I do range estimates these days, I let the batteries rest at least half an hour before looking up the SoC. That underestimates the range a little, but the error is on the safe side.

I hate to mention this, but the time it took for the half mile walk plus the time it took to get the trailer to the cart was probably long enough for the battery pack to recover enough to drive cart on the trailer. (Or at least almost hated mentioning it )

One way to increase range on cold days is putting the batteries on charge for an hour or more before using it. That will warm up the electrolyte some.

[coosa]

It's ok, JohnnieB, I don't mind the mention of the walk. :)

In fact, I had a pretty good idea that I was gonna be lucky to avoid walking when I drove the cart off the trailer that morning. I've worked with electric carts enough to know they don't like 20 degree weather. I didn't let it run to a dead stop. Once it was apparent I couldn't make it back to the truck, I parked it on a level place where it would be easy to load, and it did have a lot more power when I got back with the truck and it had plenty of power to back onto the trailer.

I guess the lesson is that when it dies in cold weather, just wait until the temp warms up and it will be ready to go some more.