Battery advice please

[bmc]

I've got 40 miles in very hilly terrain out of my new conversion(Q100cst and 36v 10ah bottle battery). Battery was still showing 2 bars left, but I think that must be a false reading, as on the last few hills, indicator went down to one bar, but then came back up over the hill. Charger charges @ 2.5 amps and it took 3.5 hrs to charge, so I'm thinking there can't have been much more than 1 ah left.

Now my local regular exercise loop is only 10 miles so I'm thinking I could do this on 2..5 ah's. based on the above figures. I know D8veh will recommend lipo, and he's right( for ease of build and the price they are now),but at my age (72 and having senior moments) I wouldn't feel happy handling them.

I've seen these 18650 batts. (Link below) and 10 in series would give me 3.4 ah's and only weigh about 0.5kgs. but I don't know if the discharge rate is enough. A pack this size would great just for my local loop and also as a back up for the regular battery on a longer trip. But I could be talking a load of nonsense..............

http://www.ebay.co.uk/itm/Genuine-PANASONIC-NCR-18650-B-Li-Ion-3400mAh-3-6V-Rechargeable-Battery-TABs-Tags-/360924318426?pt=LH_DefaultDomain_3&var=&hash=item92bf8a9c57

I've seen others with 10amp discharge rate, but they don't have tabs, so wouldn't be so easy to build.

All thoughts and advice welcome

 

[Alan Quay]

It says on the spec that the max discharge is 6.8A, which is very low current on a pedelec. You would need at least 2, maybe 4 sets of 10 in parallel to get anything much out of them. Then you will need some kind of BMS, or else be very careful how you charge/discharge them.

I have also heard that many of the Panasonic cells on Ebay are counterfeit. I don't know how you could be sure they were genuine. Price would probably give it away, but I don't know how much they should cost.

 

[Deleted member 4366]

The price and UK seller indicates that they're unlikely to be counterfeit, but 10 cells just won't be enough. You need at least two in parallel, which means 20. That will work out at £160 for a 5aH battery. You still need a BMS, leads and connectors, which will push the cost up to £200. It doesn't seem like good value to me. You can get other cells that can do it as well. 22650, 26650, etc
http://www.batterysupports.com/panasonic-26650-liion-36v-50a-2650mah-cgr26650a-battery-p-29.html

 

[bmc]

The price and UK seller indicates that they're unlikely to be counterfeit, but 10 cells just won't be enough. You need at least two in parallel, which means 20. That will work out at £160 for a 5aH battery. You still need a BMS, leads and connectors, which will push the cost up to £200. It doesn't seem like good value to me. You can get other cells that can do it as well. 22650, 26650, etc
http://www.batterysupports.com/panasonic-26650-liion-36v-50a-2650mah-cgr26650a-battery-p-29.html

Those 26650's are twice the weight. What's the minimum discharge rate needed.? These look good http://www.batterysupports.com/sony-us18650vtc5-high-drain-30a-rechargeable-2600mah-lion-18650-p-338.html

Would 10 of them work as a 2.6 ah pack ?

 

[Deleted member 4366]

They'll work, but how are you going to balance them? I'd be surprised if you get that capacity out of them. Also, don't expect as much power because they'll be spending more time at lower voltage.

 

[bmc]

They'll work, but how are you going to balance them? .

I don't know D8veh...........What's the normal procedure for balancing an 18650 pack ?

 

[Deleted member 4366]

You have to either solder in a BMS or break the pack into two halves and use a balance charger for each half, which will require you to dolder on two sets of 5S balance leads with connectors.

 

[bmc]

You have to either solder in a BMS or break the pack into two halves and use a balance charger for each half, which will require you to dolder on two sets of 5S balance leads with connectors.

I understand what your saying, and both those ways are feasible, as I'm only working on idea's at the moment, and everything is very fluid.

It's possible I could try 10 cells, and if that's inadequate, double up to 20.

I know it may not be best value, but I don't mind (within reason), if I can achieve what I'm looking for.

If I go for an initial 10 cells, what would be the minimum discharge rate that would work with my motor & controller ?

As you know. you recommended this kit to me, and I've got to tell you , I could not be more pleased. For a cheap kit from China it is remarkably smooth and refined, and I'm just loving the whole experience.

 

[Alan Quay]

If I go for an initial 10 cells, what would be the minimum discharge rate that would work with my motor & controller ?
.

6.8 amps is too low. It might work, but is not worth having. 13.6 is borderline, perhaps if you are really light, on a light bike with a low geared motor and don't mind going slowly.

Unfortunately you cannot change the laws of Physics.

 

[Deleted member 4366]

Whether you use the 18650s or lipos, the charging method will be the same, so it would be better to get lipos.

Jerrysimon uses some sort of 18650s for his Brompton. You still have to balance charge them, but he only balances them occasionally. If you check his threads about building Bromptons, you'll find details there. Personally, I'd go for lipos which are cheaper and a lot more powerful.

In theory, you can get 10 amps out of those cells you mentioned above, but I can imagine massive sag and reduced capacity at that rate.

 

[bmc]

6.8 amps is too low. It might work, but is not worth having. 13.6 is borderline, perhaps if you are really light, on a light bike with a low geared motor and don't mind going slowly.

Unfortunately you cannot change the laws of Physics.

I am light 68kgs, bike is light (for an ebike) 15.5kgs, motor is Q100cst with 201 rpm winding. I average 16mph in very hilly areas. All I'm trying to achieve
is a pack built with the same cells as my bms bottle battery, but with 25% of the capacity. ie. 2.5ah instead of 10ah. so I'm not trying to change the law of Physics.

The question I asked was "What is the minimum discharge rate I need from these cells ?" as I don't understand yet how the discharge rate of one cell relates to a pack.

 

[Alan Quay]

No matter how many cells you have, if you put them in series, the current will not increase. The voltage will increase.

The point I'm trying to make is that 10 cells in series will only put out the same maximum current as 1 cell. 6.8 amps is not very much. Certainly not enough to get you to 16mph on a 201 rpm motor on the flat.