# Charging in series and cells



## Banham361! (Oct 29, 2017)

Hello Everybody

I am a newby and a little naïve so please be gentle.

I am planning a 72v motorbike conversion with 20 cells installed in series.......1st question is how do I charge 20 cells at the same time.

2nd question is has anybody had good or bad experience purchasing 186500 cells from Aliexpress?

cheers
Tony


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## Ziggythewiz (May 16, 2010)

You need a charger designed/programmed for the type and total voltage of the cells you use.


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## Guzel (May 21, 2017)

Banham361! said:


> Hello Everybody
> 
> I am a newby and a little naïve so please be gentle.
> 
> ...


Connect them in the series configuration. Keep the polarities same. Read the basic theory of voltage sources in series here (In case you don't know). Remember that in series the voltages are added (for same polarities). Finally, you'll need an appropriate charger providing the equivalent charge. The image below further clarifies it:


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## Banham361! (Oct 29, 2017)

many thanks for your help.


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## ga2500ev2017 (Jun 12, 2017)

Banham361! said:


> Hello Everybody
> 
> I am a newby and a little naïve so please be gentle.


Welcome! Happy to help...


> I am planning a 72v motorbike conversion with 20 cells installed in series.......1st question is how do I charge 20 cells at the same time.


Typically there are two types of charges: bulk charging and balance charging. With bulk charging a charging voltage is applied to the top of the battery at the full battery voltage. So in your example, you'd be charging to a nominal 84V. You would apply enough voltage to get to the required charging current (called the constant current phase) until the battery reaches it charged value. Then you hold the voltage at that value (known as the constant voltage phase) until the current drops below a certain level. The battery is then fully charged.
The charging current is called the C rate and is related to the capacity of the cells that make up the battery. So if you are charging 2500 mah cells, then the 1C rate would be 2500 ma for example.

The problem is that not all cells in a string have the same exact voltage or capacity. So while for a perfect battery each of the 20 cells in a battery charged to 84V should be exactly 4.2 volts, typically imperfect batteries will have a mix of cells with some that are less than 4.2 volts, some over 4.2 volts, but adding up to 84 volts total. The problem with that is that the two prime directives with lithium chemistries is that you never overcharge them (above 4.2 volts) and you never overdischarge them (under 2.5 volts). Either will damage the cells. This is where battery monitoring systems (BMS) and balancing chargers come into play. With both systems the voltages of each individual cell is separately monitored and independently charged (or if necessary discharged). Many BMS systems will check the individual voltages of each cell and will stop the bulk charger when the first cell reaches its maximum safe voltage. Of course all the other cells will be less. Other BMS systems will individually bleed charge from a full battery while allowing the others that are not full to continue to charge. And balancing chargers will charge each cell individually until it is full.

Yet another method is called bottom balancing. In this system all the cells are emptied to a known voltage (2.5 volts is typical) and then the entire battery is charged until the first cell is full (so it still requires a BMS to monitor). The final voltage of the battery is noted and in future charges the battery is charged to that exact voltage. Now that means that some of the cells are not completely full, so the final voltage may be 83.4 volts for example. But by guaranteeing that an equal amount of energy is put into each cell and stopping when the first cell is full it guards against overcharging. Also because each cell has an equal amount of energy, discharging all the cells together means that every cell in the battery will run out of energy at the same time on a discharge, thus protecting the battery from over discharge.

There are a bunch of threads on all of these topics throughout the forum. Hopefully this explanation and the terminology will help you in your search for understanding.


> 2nd question is has anybody had good or bad experience purchasing 186500 cells from Aliexpress?


I'd pitch a couple of alternatives. First there is an Ebay seller named alarmhookup that is selling thousands of 36V packs, with an attached BMS, for rock bottom prices. Another option if you are in the US is that Lowe's is selling their cheapest 24V 1.5 Ahr Kobalt tool battery for $10 each. With both options legitimate LG and Samsung cells can be had for less than $2 USD a cell, with an attached BMS. Just something to consider.

ga2500ev


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## Solarsail (Jul 22, 2017)

Banham361! said:


> Hello Everybody
> 
> I am a newby and a little naïve so please be gentle.
> 
> ...


Hi Banham361 - I purchased Panasonic 18650s from Alibaba, and was quite satisfied. I tested half of them and they all met spec. My only gripe was air shipping charges that added 25% to the cost. I don't mind if it were by sea. For more details please see:

http://www.diyelectriccar.com/forums/showthread.php/18650-13s10p-project-48v-x-34ah-188618.html


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## Solarsail (Jul 22, 2017)

Further to your question - how to charge

Let's say you are using the 3.4Ah batteries. So a 20s15p battery pack will connect a group of 15 cells in parallel, and 20 such groups in series to give you 72V nominal and 51Ah. These are to be charged to 84 volts and not to be discharged below 56V.

You need a balancer-protector, a DC-DC step-up CCCV charger, and a power supply. It is also possible to purchase a charger and power supply in one package.

The balancer-protector will be for 20s, and the cutoff overcurrent will depend on what you require for your application. You can find a variety of such boards on eBay or AliExpress. The protection circuit will cut off the overcurrent if the discharge exceeds the current rating. Also cuts off if voltage of any cell goes above 4.2V during charge or goes under 2.8V during discharge.

The power supply would be DC 36V or 48V, let's say 1500 Watts. The charger will step up this voltage to a max of 84V (adjustable) and a charging current (usually 0.5C and adjustable). The charger first delivers 0.5C current (i.e. 3.4 * 0.5 = 1.7A), until the charging voltage maxes at 84V in which case the voltage stops rising. As the current diminishes, to let's say 0.1A, it will then turn off the current.

In other related threads I have linked to examples of these devices on eBay. Please look at some of the recent threads.


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## Banham361! (Oct 29, 2017)

Thanks guys

I was going to have the following battery set up:

40 x 18650 cells in parallel 3.7v 3500mah = 3.7v 140Ah x 20 cells 72v

Heavy but should have enough room!


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## Solarsail (Jul 22, 2017)

Banham361! said:


> Thanks guys
> 
> I was going to have the following battery set up:
> 
> ...


This would be 20s40p. That is over 10 kWh of storage - and is huge for a motorcycle. 
I would break that up into two or 3 or 4 parallel modules, such as four 20s10p in parallel. I don't think it is easy to find a balancer-protection board when you have 40 cells in parallel. That is a huge current rating, and off-the-shelf boards may not be available, while for 20s10p or 20s13p they are most likely available.


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## Banham361! (Oct 29, 2017)

Thanks I really appreciate you’re input and advice. I am more nuts and bolts than mathematics.

I am converting a 1961 Lambretta using a 5 KW BLDC Motor and 300a controller with Regen braking. It will be around 200lbs with the old engine, tanks etc taken out. I am mounting the motor and suspension by fabricating a subframe so that I don’t have to cut or weld onto a classic bike and can turn it back to stock if required.

Thing is I need a bit of range as I am approx 10 miles out of town, doesn’t have to be super quick but 55 mph would be good.

If I go for 20 number 18650 in parallel x 20(3.7v) do you think that is achievable?

Cheers


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## brian_ (Feb 7, 2017)

Banham361! said:


> I am converting a 1961 Lambretta using a 5 KW BLDC Motor and 300a controller with Regen braking.
> ...
> I am mounting the motor and suspension by fabricating a subframe so that I don’t have to cut or weld onto a classic bike and can turn it back to stock if required.
> ...
> If I go for 20 number 18650 in parallel x 20(3.7v) do you think that is achievable?


Using a rough value of 45 grams for a single 18650 cell, 400 of them would be 18 kg (or 40 pounds). If they were packed in a rectangular array (400 rectangular box shapes each 18 mm square by 65 mm long) the combination would occupy 8.4 litres... and much more (volume and weight) by the time packaging and wiring is considered. Is that a reasonable size for this scooter?

I assume that the battery will under the seat where the stock fuel tank is located. In photos it appears that there are two tanks... whatever the second tank might be, it would presumably need to be replaced by the battery.


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## Banham361! (Oct 29, 2017)

The second box is a tool box and a filter box in the middle. I?m ok for weight and volume but not so sure electrically capacity wise. Tony


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## Solarsail (Jul 22, 2017)

Banham361! said:


> Thanks I really appreciate you’re input and advice. I am more nuts and bolts than mathematics.
> 
> I am converting a 1961 Lambretta using a 5 KW BLDC Motor and 300a controller with Regen braking. It will be around 200lbs with the old engine, tanks etc taken out. I am mounting the motor and suspension by fabricating a subframe so that I don’t have to cut or weld onto a classic bike and can turn it back to stock if required.
> 
> ...


That would be 20s20p (20 cells in parallel to make a group, and a string of 20 groups in series). That will give you 20 x 20 x 3.6 x 3.5 = 5 kWh. If done right, 20s20p will weigh 5/0.25 = 20kg = 44lbs. The volume (batteries only) will be 5 x 2.2 = 11 liters. Cost (new cells LG) would be approx. 20x20x3.05 = $1,220 + 20% shipping charge + 10% enclosure + 15% power supply, charger, balancer-protection, monitor.

Range will be, assuming 25 km/kWh, 5x25/1.6 = 78 miles - probably 50 miles as it is a heavy classic.

300A is overkill I think, as it does not have to be super quick. That is 300x84/5000 = 5x the rating of the BLDC and could harm the BLDC. So go for 200A or 150A even 100A output from the pack. You will probably have difficulty finding a protection board rated at 84V 150A for the pack. So divide the pack into two 20s10p modules in parallel, and that is 75A per module, and you should be able to find an off-the-shelf 20s 75A balance-protection board (you will need two). About $25 each. If one module goes bad, you will still have 25 miles range. While if the 20s20p pack goes bad, you have 0 range.

For monitoring, each module should have a bunch of double-pole selectors and a digital voltmeter so you can read the voltage of each of the 20 groups in a module. Also each module should have a digital ammeter and a fuse.

I didn't realize this was a scooter. In which case your range should be greater, maybe 35 km/kWh ideally, or up to 75 miles with a 20s20p pack. Each module can give you 35 miles range in flat paved conditions.

Build one 20s10p or 20s12p 75A module first, get it all to work, and see if you really need a second module. Also install the power supply and charger in the vehicle.


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## Banham361! (Oct 29, 2017)

Yes but originally it would have been Petrol 200cc capable of 55-60 mph. I?ll attach a pic!


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## Banham361! (Oct 29, 2017)

125kgs 200cc


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## kennybobby (Aug 10, 2012)

Banham361! said:


> ... using a 5 KW BLDC Motor and 300a controller with Regen braking. ...
> 
> Thing is I need a bit of range as I am approx 10 miles out of town, doesn’t have to be super quick but 55 mph would be good.
> 
> ...


Do you have a datasheet or specs on the motor, and what is your sprocket ratio and tire size, and expected total weight of the vehicle? You would need this information to determine how many cells you need to create a pack large enough to meet your performance requirements.

For example you have picked 72V, so a 5kW motor would be drawing about 70 Amps--but will it be making enough torque and at sufficient RPM in order to move you down the road at the speed you want? How much torque margin is available for acceleration? You could build a monster pack but if the motor is underpowered then it will be useless. What is the voltage rating of the windings, what is the temperature rating, can it handle 300 Amps, etc... There are too many unknowns at this point to start building a pack.


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## Banham361! (Oct 29, 2017)

Apologies it’s a VEC300-72 rated at 100a from Golden MOtor as attached:


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## Banham361! (Oct 29, 2017)

So is my best option to have 2 larger battery packs made up of series and parallel connections.


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## Solarsail (Jul 22, 2017)

As kennybobby says, there are too many unknowns to accurately calculate the acceleration and top speed, etc. I presume you spoke to some people at the shop and they recommended the 72V 100A motor for this scooter.

A Leaf Gen 1 weighs about 4000 lbs, with an 80 kW motor. So that is 20 W/lbs. The scooter at 125 kg = 275 lbs + 175 lbs passenger would need 9 kW to be very roughly comparable. So it appears the 72Vx100A motor may be underpowered. Note that the Leaf achieves full acceleration at 80 kW, and it is pretty quick. And when cruising medium speed on flat pavement the motor is operating at only 10 kW. So could it be that you do not need more that 7 kW to achieve acceptable performance? I have not compared the torque ratings - just the power ratings.

I believe that 100A will give you sufficient umph. But to be safe, design for 150A, if the protection board can be reasonably procured. Otherwise drop to 100A. 

For a 20s10p pack, 100A is about 3C, which is within the discharge limit of the cell. So I recommend as first try, to build a 20s10p 75A (or 100A) starter pack - so you may test the performance and determine if it is adequate. If you find the range or the acceleration or the top speed inadequate, then depending on how much more you need, you can build a 20s5p or 20s10p or 20s15p pack and connect it in parallel to the original pack, to get the desired performance.

As a 20s10p flat pack may not fit in the scooter bay, you may want to build that as two 10s10p sub-modules in series which would be easier to fit.


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## Banham361! (Oct 29, 2017)

Hi Guys

Things have moved on a little. I had donated a free small 106kg(233lbs)trials bike. I have 500 NCR18650b batteries with 2C rating. I was going to build 4 x 10s12p packs(2 in parallel then in series to get 72v)

Everything else remains the same, how do you think this battery setup will perform?

Cheers
Tony


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## Solarsail (Jul 22, 2017)

Banham361! said:


> Hi Guys
> 
> Things have moved on a little. I had donated a free small 106kg(233lbs)trials bike. I have 500 NCR18650b batteries with 2C rating. I was going to build 4 x 10s12p packs(2 in parallel then in series to get 72v)
> 
> ...


In other words, this is a 2s2p10s12p. Is that now a trail bike or trails cycle instead of a scooter? Are you happy with the cells? May I ask what you paid for them and what the shipping charges were?

This can give you 130 miles range under ideal conditions. First build two modules (2s10s12p) and checkout the performance, balancer, protector, etc., and then add the other two modules. You should theoretically have no issue doing your commute with just half the pack. If you have not purchased the motor/controller yet, would you consider 96V (2x 14s) instead of 72V - I believe 96V is more of a standard?

What cell balancer will you be using, and how will you be monitoring each group voltage?


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## Banham361! (Oct 29, 2017)

I was going to get balancer and monitor from eBay as convenient.

The bike is a Yamaha TW125 Road/Off-roader .

The cells cost $ 3.30 each + $ 210 shipping(I think I may get $ 300 tax bill as they used my business account to clear customs)

I got them this evening as I was leaving work so only checked a few but they look genuine. I’ll go through them on Monday a revert back.

I already have motor and controller so 72v it has to be.


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