Battery charging recommendation for long life

[serisman]

I am in the process of making the switching from Lead Acid to Lithium Ion, and want to maximize the life of my new battery pack as much as possible. I would like some help in determining the best charging schedule to accomplish this.

My ebike currently has a conhismotor 48v 1000w rear kit on it. The controller has 63v capacitors, and pulls about 31A max.

With my HEAVY lead acid battery pack (~ 60lbs) I was averaging about 25 Wh/mile at approx 20 mph. I was using (4) 18Ah SLA batteries, that originally measured about 11Ah @ 10A draw (they were sitting around for about 1-2 years before I started using them). Originally, I was getting about 15 miles of range, but that dropped to about 5 miles a few weeks ago after putting about 700 miles on the bike. I re-measured the batteries and they were around 3 Ah (@ 10A). Since then, I have put them on a custom built desulfator for about a week per battery. This seems to have recovered a bunch of the capacity (haven't actually re-measured it again), but at this point I am done with SLAs for my bike.

I never really expected the SLAs to last very long. I only used them originally because they were sitting there feeling neglected and I didn't want to invest a lot of money on this hobby in case it didn't pan out. At this point, I am definitely hooked on ebiking so time for a real battery pack!

After doing a lot of research, I finally decided on and purchased the 50v 18.5Ah samsung triangle pack from cell_man:
http://em3ev.com/store/index.php?route=product/product&path=35&product_id=123

According to my calculations this should give me a range of 30-45 miles at around 20 mph, which is way more than I need most of the time. My normal commute is 15 miles round trip, but I have the opportunity to charge at both ends (and I have two chargers). I should use about 4 Ah for each half of the commute. Aside from my normal commute, I would like to be able to do a 30 mile (round trip) bike trail that I used to do under pedal power when I was in better shape. At the most, I would only do this once a week or so.

The chargers that I got from cell_man have 3 charge settings (100%, 90%, and 50%) (58.8V, 57.4V and 53.2V) (4.2V/cell, 4.1V/cell and 3.8V/cell)
http://em3ev.com/store/index.php?route=product/product&path=37&product_id=120

According to the following link, it appears the optimal charge range for longevity (while still retaining useful capacity) is 3.8 - 3.9 V/cell (37% - 63% capacity):
http://www.powerstream.com/lithuim-ion-charge-voltage.htm

So, I have a few options:

1. Charge to 90% (4.1V/cell), and re-charge (to 90%) every day (i.e. after using 8 Ah) or every other day (i.e. after using 16 Ah). Potentially charge up to 100% (4.1V/cell) immediately before going on the longer trail ride, but only re-charge to 90% afterwards.

2. Charge to 50% (3.8V/cell), and re-charge (to 50%) after each half of the commute (i.e. after using 4 Ah). Then, charge to 90% or 100% (4.1V/cell or 4.2V/cell) immediately before going on the longer trail ride, but only re-charge to 50% afterwards.

Option 1 will be (2-4x) less cycles (+), but they will be deeper cycles (- ?) and the battery will be sitting at a higher voltage for more of the time (- ?).

Option 2 will be (4x) more cycles (- ?), but they will be shallow cycles (+ ?) and the battery will be sitting at a storage voltage for most of the time (+! ?). This option will also discharge to a lower voltage than Option 1 (- ?). Worst case, I should still get 35% of the capacity by charging to 3.8V/cell which is still 6.475 Ah (more than the 4 Ah I need for each half of the commute). This should still leave 15-20% of the capacity in reserve and the cell voltage should still be above 3.7V/cell.

I am leaning towards Option 2 at this point, unless someone has a better suggestion.

Alternatively, does anyone know how to modify the voltages on the cell_man chargers? I would prefer to charge up to 3.85V/cell (~ 60% - 70% capacity) for the low end.

 

[999zip999]

If using fully charge if to use some day half chrge. In winter charge at stage 1. But recharge once a month.

 

[Gregory]

If I read #2 right I'd avoid playing around with just the last half of the energy. Headwinds, detours, cold weather will all conspire, and the controller trying to suck 31A from a near empty pack will eventually lead to your BMS finding the voltage of the cell with the highest IR diving low and shut itself off a couple of miles from home. Just not worth it.

Charge to 90% normally, store at 50% when practical to do so.

 

[dogman dan]

I agree, use the 90% setting most of the time, and charge every single chance you get. Batteries like shallow cycles better than deep ones, and leaving it slightly undercharged helps prevent one cell getting too overcharged.

Does the battery have a bms? If so, balancing the pack fully may require the 100% setting. So you might want to periodically use the 100% setting. But shallow cycles tend to leave the pack balanced, so you might not need to balance it daily.

If you will store the battery for days or weeks, then you might want to put off recharging past 50% until the night before you expect to ride again.

On the days you will go for long rides, don't hesitate to charge 100%.

 

[Teh Stork]

On the subject, the BMS delivered with this battery only equalize the cells to the same voltage level once you charge them to 4,2V. Therefore, do not just use the 4,1V setting on the charger - once in a while charge it completely full.

 

[serisman]

Thanks for all the feedback guys.

Based on the recommendations so far, I think I will do the following:

Charge to 90% (4.1V/cell) immediately before heading out in the morning, and then charge to 90% (4.1V/cell) again immediately before heading home. I should be able to replenish the 4Ah that each half of the commute takes in a little less than an hour, so it should be pretty easy to figure out when to start the charge. By not charging until the last minute, it lets the battery sit at a lower voltage for longer which I think should help with longevity.

I will also charge to 100% (4.2V/cell) a day before heading out on my extended trips. This will max out my range and allow the cells to re-balance if needed.

If I know I won't be riding for at least a week, I will drain the pack to about 1/2 and then charge to 50% (3.8V/cell) for storage.

 

[999zip999]

I would not drain to 50% but if after a ride and for storage and less than 80% or lower like after a short ride and leave it there. I would tink you would be riding a lease twrice a week. It should get 4-5 years out of it. At which time a better battery should be here.

 

[serisman]

So, I received the new battery pack yesterday. What a difference it makes!

With light pedaling, I was averaging between 25-30 Wh/mile (@20 mph) with my old (SLA) battery pack which meant about 4 Ah of energy used per half commute.

With a new 7-speed 11t freewheel I can now pedal moderately @ 20mph speeds. Combined with the new battery pack I only used 1.9 Ah (~15 Wh/mile) of energy for the first half commute today (~7 miles).

Most of the energy savings probably come from the reduced weight and additional pedal assist. Having substantially less battery weight and positioning it in the triangle (instead of above the rear wheel) definitely makes the bike feel normal again as well.

I highly recommend this battery pack.

 

[friendly1uk]

They like to be half charged, so charge till they have 2Ah more than half, then take your 4Ah and charge it to where you started again.
I have not seen anyone do this, but it seems logical. What I do see is people run between 3.85 and 4.1 which is probably not that good for them.
Why not just carry what you need. Use 100% of it and get the performance stated on the label. If I needed 4Ah there is no way I would carry 4 times more. That is 5kg you don't need.
Actually, My pack is 5Ah and goes from 3-4.2 which returns about 4.75Ah. It is good for about 10 miles with a 20mph target speed. If I want to go further, I can add another pack. No need to carry it all the time though.

I should get 160 cycles. That will do. You can do the math and see my expected costs. If I was carry twice as much would it last twice as long? I just can't answer that, so I can't work that way. I sure wouldn't carry 6kg when 1.5kg would do(I'm at 44v with hardcase packs so my numbers are a little different, but the facts remain the same)

 

[serisman]

I assume you are using LiPo? Otherwise, how are you able to carry only 5Ah and still be able to draw enough current? My controller draws 30-31 Amps max.

I looked into LiPo but decided against it for many reasons:

• I like to charge at work, and am not willing to subject the building to an substantially increased risk of fire.
• I don't really want a fire at home either.
• I do want the extended range (~30 miles) fairly often. And I don't want the hassle of swapping battery packs around.
• LiPo is not much cheaper (for the same capacity) by the time you add in all the extra costs of connectors and charger(s).
• There are lots of horror stories about buying from HobbyKing
• The cycle life is quite a bit shorter
• The cells appear to be pretty temperamental

The primary advantage I can see with LiPo, is that you can get by with a lot less capacity and still get a decent current draw. This would mean reduced weight, and it could be easier to find a place to mount it. Interestingly, I still see people recommending 10-15 Ah for a 30 Amp load in order to reduce voltage sag and increase the cycle life. So that puts it right back in LiCo/LiMn territory.

I understand that many people prefer LiPo. I researched it pretty heavily, but decided it wasn't the best choice for me.

Keep in mind, I am migrating from SLAs, so the 14 lbs of weight (in the triangle) is peanuts compared to the previous 60 lbs (above the rear wheel). I could probably cut that in 1/2-1/3 again by going with LiPo, but how much would I really notice another 7-9 lbs difference? At that point, I would also want to reduce the weight of the motor (another 15 lbs or so).

Regardless, the decision has already been made, and I am extremely happy with the outcome.

 

[serisman]

Also, you mentioned you expect 160 cycles, and you expect that to be cost effective. I am curious how you calculate that, because to me that seems barely better than SLA.

I could pick up (3) 5S1P 5000 mAh 30C Zippys for about $55 each (+ shipping). That is ~$200 (shipped). Running them in series, I would get about 277.5 Wh per full cycle, or maybe 225 Wh with a little headroom. At 160 cycles, that is ~36 kWh till the batteries are spent or $5.5 / kWh.

The SLA pack that I was using cost about $140 (shipped). It was (4) 18 Ah batteries @ $35 /each (shipped). I got ~700 miles at 25-30 Wh/mile out of that pack before deciding to ditch it. So, I pulled 17.5 - 21 kWh out of the battery pack, or ~$6-8 / kWh. I could have kept using it, it still had some life left in it (just needed to go through a desulfating phase).

My new battery pack should be much better than that. It cost me $700 + shipping (~$100). I will get about 860 Wh with a 90% charge/discharge profile. At an estimated 500+ cycles, that should be around 430 kWh or $1.86 / kWh. Even after 500+ cycles there should still be quite a bit of usable capacity left, so I could probably get at least double the kWh before the pack is no longer useful.

So, again... I am curious what your calculations look like.