Battery life in the cold

[Nealh]

Now you have tried again, the cold may have a small effect on the battery cutting out but the main one is voltage sag esp when towing up an incline. The sag causing cut out will be more noticeable as the voltage decreases, it is in the higher assist levels or under throttle use that the sag will be more prominent as the overall voltage decreases. A sign that the battery is ageing a little.
How old is the battery and the spec's ?

 

[D C]

I have only had my conversion a few weeks

was the battery brand new a few weeks back? It sounds as Neil suggests, to be a battery fault rather than a cold weather problem. I had similar symptoms a few years backwith a battery that was around three years old and although it looked healthy and charged to 42 volts it would cut out under load after a few miles.
Sorry Neil, crossed post, yours is more informative than mine

 

[artsyhonker]

was the battery brand new a few weeks back? It sounds as Neil suggests, to be a battery fault rather than a cold weather problem. I had similar symptoms a few years backwith a battery that was around three years old and although it looked healthy and charged to 42 volts it would cut out under load after a few miles.
Sorry Neil, crossed post, yours is more informative than mine

Yeah, to my knowledge the battery is new -- I paid a mechanic to source the kit and fit it to my bike. I'll get back to him and let him know the battery isn't behaving and see what he says. Worst case scenario I'm willing to throw money at a new battery, but obviously I'd rather not.

It's a heavy bike (even without the motor and battery) and I do carry a lot of stuff and I'm not light myself, so I don't expect a huge range, but hitting the edge of it when the battery is allegedly over half full is vexing.

 

[Nealh]

I have a pair of 6 y/o batteries that can cut out if I use high assist or throttle once the voltage drops below about 38v, they sag badly but if I only use PAS1 they can still give 40 odd miles locally with no inclines. Ageing factor/ higher cell resistance showing it's head, I can still tow a trailer with them.

 

[WheezyRider]

Yeah, to my knowledge the battery is new -- I paid a mechanic to source the kit and fit it to my bike. I'll get back to him and let him know the battery isn't behaving and see what he says. Worst case scenario I'm willing to throw money at a new battery, but obviously I'd rather not.

It's a heavy bike (even without the motor and battery) and I do carry a lot of stuff and I'm not light myself, so I don't expect a huge range, but hitting the edge of it when the battery is allegedly over half full is vexing.

What is the battery size? I see bikes for sale with 8Ah packs, which in real world situations, is really only going to give 6Ah max, then add a heavy load, a hill and cold weather to it, it is not going to last long. How many Ah is your pack?

The charge indicator lights on battery packs are frequently next to useless. They will often give 3 bars of lights even when the pack is almost empty. The best thing to do is get is a proper handlebar mounted voltmeter. Also, if you don't have one already at home, get yourself a multimeter and measure the battery voltage.

If you have the opportunity, recharge your pack whenever you can, don't let it run down to near empty if you can avoid it. BUT avoid fully charging too often (this is where a handlebar voltmeter is useful).

Never recharge your pack outside if there is the chance of it going below zero degrees overnight.

 

[vfr400]

Now you have tried again, the cold may have a small effect on the battery cutting out but the main one is voltage sag esp when towing up an incline. The sag causing cut out will be more noticeable as the voltage decreases, it is in the higher assist levels or under throttle use that the sag will be more prominent as the overall voltage decreases. A sign that the battery is ageing a little.
How old is the battery and the spec's ?

The more a battery sags, the more it heats up and the less it's affected by outside temperature.

 

[WheezyRider]

The more a battery sags, the more it heats up and the less it's affected by outside temperature.

It depends whether the energy dissipated in the pack is sufficient to overcome the cooling rate of the pack. In cold weather, it can take lot to warm a pack up. Then, if you have already asked a lot of a cold battery, by the time it gets warm, it's already lost a fair bit of its capacity.

 

[vfr400]

It depends whether the energy dissipated in the pack is sufficient to overcome the cooling rate of the pack. In cold weather, it can take lot to warm a pack up. Then, if you have already asked a lot of a cold battery, by the time it gets warm, it's already lost a fair bit of its capacity.

Ohm's law. Voltage drop =current x resistance or current = voltage drop/resistance
Heat = current squared x resistance, so heat = voltage drop squared/resistance

Heat goes up with the square of the voltage drop. Unless you've drilled holes in your battery case, the cell-pack is very well insulated from ambient temperature.

 

[WheezyRider]

Ohm's law. Voltage drop =current x resistance or current = voltage drop/resistance
Heat = current squared x resistance, so heat = voltage drop squared/resistance

Heat goes up with the square of the voltage drop. Unless you've drilled holes in your battery case, the cell-pack is very well insulated from ambient temperature.

All very well, but internal cell resistances are small and battery packs have a significant heat capacity. Battery packs like Silverfish designs even have metal cases...

Have you actually measured pack temperature with time, starting from <5 Deg C? You might be surprised how long it takes to warm up.

 

[vfr400]

Have you actually measured pack temperature with time, starting from <5 Deg C? You might be surprised how long it takes to warm up.

Yes I have - whenever I do a capicity test on a battery. At 10 amps most get pretty warm after a while, that's without much sag.

Take a healthy 18650 with a typical internal resistance of .075 ohms. At 10 amps, a 36v 40 cell battery will have 2.5 amps going through each cell. The heating is then 2.5 x 2.5 x .075 watts per cell = 0.47 watts. For the whole 40 pack, that's 18.75 watts.. The ones in the middle are very well insulated by the other warm cells around them, so it's difficult for them to lose any heat.

If your battery were saggy, the internal resistance would be higher and the heating would be higher in proportion.

 

[D C]

I paid a mechanic to source the kit and fit it to my bike. I'll get back to him and let him know the battery isn't behaving and see what he says

That seems like the best plan, it sounds like the battery maybe isn't up to the job

 

[WheezyRider]

Yes I have - whenever I do a capicity test on a battery. At 10 amps most get pretty warm after a while, that's without much sag.

Take a healthy 18650 with a typical internal resistance of .075 ohms. At 10 amps, a 36v 40 cell battery will have 2.5 amps going through each cell. The heating is then 2.5 x 2.5 x .075 watts per cell = 0.47 watts. For the whole 40 pack, that's 18.75 watts.. The ones in the middle are very well insulated by the other warm cells around them, so it's difficult for them to lose any heat.

If your battery were saggy, the internal resistance would be higher and the heating would be higher in proportion.

So how long is "a while"? Have you plotted a graph of temperature vs. time?

My packs are 60 cells, so the 6p configuration means the current per cell is 10/6 A and the overall mass is even larger.

And you'll end up with differential heating as those cells in the centre will warm up faster than those on the exterior, leading to further problems. Also, it will depend on how the cells are laid out in the pack, which will change the surface to volume ratio and hence the effective heat build up/loss ratio.

Anyway, let's do a back of the envelope calculation to estimate how long it should take to warm up a pack, without any thermal losses:

Ok, so let's take 20W of power into a pack. Let's say the pack weighs 4kg. It is of mixed composition of lithium, polymers, steel and hydrocarbons. The specific heat capacity of lithium is 3.58, for steel 0.47 and for most polymers/hydrocarbons its approx 2 J/g/K

Let's say it all comes out at an average of 2 J/g/K

Say we want to heat the pack from 5 Deg C to 20 Deg C

So we end up with 2x15x4000 = 120,000 Joules to get the pack up to 20 Deg C from 5 Deg C

If we are inputting 20W, ie 20 J/sec, then it will take 120000/20 = 6000 seconds to reach 20 Deg C, assuming no thermal loss from the system, so 100 minutes, or 1 hr and 40 min. In which time, the capacity "damage" will have been done.

So you can see, although these numbers are approximations, I have doubts that the heating rate will be as rapid as you think and I'd like to see some real world data.

 

[artsyhonker]

What is the battery size? I see bikes for sale with 8Ah packs, which in real world situations, is really only going to give 6Ah max, then add a heavy load, a hill and cold weather to it, it is not going to last long. How many Ah is your pack?

The charge indicator lights on battery packs are frequently next to useless. They will often give 3 bars of lights even when the pack is almost empty. The best thing to do is get is a proper handlebar mounted voltmeter. Also, if you don't have one already at home, get yourself a multimeter and measure the battery voltage.

If you have the opportunity, recharge your pack whenever you can, don't let it run down to near empty if you can avoid it. BUT avoid fully charging too often (this is where a handlebar voltmeter is useful).

Never recharge your pack outside if there is the chance of it going below zero degrees overnight.

Thanks -- this is good to know. I suspect it's 8Ah... hmm.

 

[artsyhonker]

Thanks -- this is good to know. I suspect it's 8Ah... hmm.

(...and no, I don't ever charge it outside! That would terrify me, the adapter does *not* look rated for outdoor use.)

 

[WheezyRider]

(...and no, I don't ever charge it outside! That would terrify me, the adapter does *not* look rated for outdoor use.)

Sorry, I should have been clearer, I meant outdoors as in inside a shed, garage or outhouse If it is 0 degrees or below outside.

 

[artsyhonker]

Sorry, I should have been clearer, I meant outdoors as in inside a shed, garage or outhouse If it is 0 degrees or below outside.

Fair enough!

The front hallway gets nippy, but doesn't really go below about 11°C at night, which should be fine.

Meanwhile: I've ordered at 36V 10Ah bottle-style battery. If it really does have better capacity than what I've been using, I'll be pretty happy, as the casing etc is smaller. If not, well, I just bought a very expensive phone power pack...

 

[OleP]

Hi, I'm new here but have read this thread with interest, as I also experience reduced range in colder weather / winter time.
Apart from increased loss / resistance in battery cells, I think there may be other factors that is seldom taken into considerations:

• Air density is higher in cold air than in warm air, and therefore give more drag.
• In wintertime the wind is often stronger than in summertime.
• We usually wear thicker clothes in cold weather and therefore has a larger surface to move air.
• Lubricant in motor, bearings, hub gear, chain etc. is colder / higher viscosity and therefore gives more resistance (loss).
• We humans tends to use higher assistance level when cold (?)
• More...?

List is not prioritzed and I have no idea which one is counting most.
Any comments on the list will be appreciated.

Regards from (cold) Denmark
Ole

 

[Fordulike]

Hi, I'm new here but have read this thread with interest, as I also experience reduced range in colder weather / winter time.
Apart from increased loss / resistance in battery cells, I think there may be other factors that is seldom taken into considerations:

• Air density is higher in cold air than in warm air, and therefore give more drag.
• In wintertime the wind is often stronger than in summertime.
• We usually wear thicker clothes in cold weather and therefore has a larger surface to move air.
• Lubricant in motor, bearings, hub gear, chain etc. is colder / higher viscosity and therefore gives more resistance (loss).
• We humans tends to use higher assistance level when cold (?)
• More...?

List is not prioritzed and I have no idea which one is counting most.
Any comments on the list will be appreciated.

Regards from (cold) Denmark
Ole

All very interesting, and scientific points. For me personally, and I didn't really think about this until reading your post, the 'lubricant' bullet point probably contributes to my greatest battery energy loss.

I have open gear grease packed around my BBS02 final drive cogs, which has a high viscosity, even during the warm summer months. This is great for preventing the grease from flinging off the cogs, but becomes increasingly more viscous as the temperature drops.

 

[OleP]

All very interesting, and scientific points.

Ha! - You pretty much hit the spot - I've been working with Electronics, R&D and for the last 30+ years and until my retirement, in the calibration business with Accredited calibration of various types of gauges. Most temperature, humidity and electronics.
I've only had my e-bike for 4 month now, so I hope the observed decline in range will recover next spring/summer.

Another possible bullet could be:

• Tire pressure drops a couple of PSI when temperature drops about 20 °C. Lower tire pressure gives generally increased rolling resistance.
• Wet (and muddy) roads increases rolling resistance (I think).

All of the bullets gives each only small additions to the range reduction, but summed up it should be mesurable.
(Please forgive my typos - English is not my native language)

 

[Gringo]

Hi, I'm new here but have read this thread with interest, as I also experience reduced range in colder weather / winter time.
Apart from increased loss / resistance in battery cells, I think there may be other factors that is seldom taken into considerations:

• Air density is higher in cold air than in warm air, and therefore give more drag.
• In wintertime the wind is often stronger than in summertime.
• We usually wear thicker clothes in cold weather and therefore has a larger surface to move air.
• Lubricant in motor, bearings, hub gear, chain etc. is colder / higher viscosity and therefore gives more resistance (loss).
• We humans tends to use higher assistance level when cold (?)
• More...?

List is not prioritzed and I have no idea which one is counting most.
Any comments on the list will be appreciated.

Regards from (cold) Denmark
Ole

I went out a few days ago on one of my usual routes and I was wearing my cold wet weather gear as it was 3 degrees, a little misty and a light wind. As the ride progressed the wind picked up and the mist turned to drizzle. Riding in sport mode this 25 mile route would normally use 3 of the 5 battery bars on the display but this time only 2 bars had been used.
I put this down to the fact that the battery was fresh off the charger, I was out riding within 5 minutes of the charge cycle completing so the battery didn't get a chance to cool down (Not that it felt warm)
I’m going to try and time my next charge to finish just before a ride, if that’s not possible I may put the battery in my airing cupboard for an hr before going out !