Battery Building Advice (Tongxin motor)

[faphillips]

I have been experimenting with various battery options on my Nano Brompton including the standard 36v 7 Ah Li Po battery and a much smaller Bosch 2.6 Ah 36 v Li Ion powertool battery.

Both batteries take up too much room in the pannier bag, although I have also made a battery holder for the Bosch battery to go in the rear pocket. Sadly 2.6 Ah isnt quite enough for the range I require although I have considered mounting another battery in the other pocket which would presumably give me 5.2 Ah which is more than enough range. By the way can anyone tell me how I wire 2 batteries together so as to double the Ah but not the voltage - do I just join the 2 wires together at the controller? Do the batteries have to be equal size i.e. could I connect a 1.3 Ah and a 2.6 Ah?

The Bosch batteries recharge in 40 minutes from flat which makes them quite attractive although they do have to be removed and inserted into the charger which is a little inconvenient.

As this is a folding bike, battery options are also linked to luggage options and I have now concluded that what I actually need is a battery that will fit in a briefcase/laptop bag as well as a laptop and papers. The ideal shape size for such a battery might in fact be the shape & size of a small laptop.

The standard Brompton front pannier bag isnt really large enough to do all of this, certainly not with a 7 Ah battery and isnt really suitable to take to meetings so I therefore intend to modify a laptop bag or briefcase to mount on the Bromptons luggage block (probably using the Brompton luggage frame) with all the wiring connnectors built in.

Whilst I am a fairly competent DIYer I am not sure that making a battery is within my capability from the technical knowledge aspect and I therefore need some external assistance in the form of a company or specilaist who could do this for me. What I might also consider is having the existing 7Ah battery dismantled and reassembled into a new case although I dont know what cells are inside as the case is glued together. Alternatively a completely new battery of the right size, as light as possible of at least 5 Ah.

Is this battery proposal feasible i.e. a large thin battery and is there a company/specialist I could ask to build it? Are there standard battery cases or does this have to be made?

Any help much appreciated as always

Thanks

Francis

 

[Jeremy]

Taking apart a LiFePO4 battery similar to the one I have used (see the TongXin thread) would be fairly straightforward and would allow you to build a pack that was a better shape. The individual cells in these packs are flat and rectangular and are about 100mm wide, 135mm tall and 5mm thick. Each cell is rated at 5Ah, although the maximum continuous current per cell is only about 10 amps, which might be a little low.

To build a suitable 36V 5Ah pack you would need 12 cells, wired in series. The cells look similar to these (although these ones are a different size): Polymer Li-Fe-PO4 Cell: 8790160L 3.2V 10 Ah, 2 C Drain Rate (LFP-8790160L) - LFP-8790160L

If you wanted to make a pack that was similar in shape to a laptop, then you could arrange the cells as two layers, with 6 cells in each, to give you a pack that was about 300mm wide (making allowance for the connections to the top of the cells), 200mm high and about 10mm thick. The pack would also require some space for the battery management system, which is the small circuit board covered in yellow heatshrink sleeving in the pictures on the other thread.

A much more robust solution might be to go for a 36V 10Ah battery built the same way, as this would definitely source the current needed. The dimensions would be similar to those above, but the thickness would increase from 10mm to about 20mm.

If you contact Li Ping, the ebay battery seller that I purchased my pack from, you may well find that he will build you a custom pack. If he does, then it will come wrapped in silver duct tape and look fairly crude, but be assured that the cells and BMS system he supplies are fine. My 36V 10Ah pack cost around £150 delivered, including the charger.

My battery weighed 3.4kg before I fitted it into the custom made composite box. In the box it weighs 3.65kg. I would guess that a 5Ah pack might weigh under 2kg complete with BMS and case.

Hope this helps.

Jeremy

 

[faphillips]

Jeremy

Much appreciated. Sounds like just what I need and I will contact Li Ping -is he on eBay as pingping227 ?

I notice in your picture that the battery pack has 3 leads: pink, blue and black - why 3 leads. On my existing set up I use the same leads for discharge and charging using Neutric Speakon connectors which only have 2 leads. http://www.neutrik.com/client/neutrik/media/view500/Media_1560067473.jpg Could I do this with this setup because I want to try and have the same setup for all batteries so I have a choice.

Regarding my question about doubling up my Bosch batteries does the attached illustration correctly show how these should be connected?

Your help and assistance is very much appreciated. By the way the quality of your bike is excellent. Very impressed

Thanks again

Francis

 

[john]

You can sometimes get away with connecting batteries in parallel if they are identical but it is best to use schottky diodes like these Maplin > 30A Schottky Barrier Rectifiers
You just connect one to each battery, say the positive terminal, which allows current to flow out put not back in. This prevents any possibility of one battery discharging into the other.

John

 

[Jeremy]

Francis,

Thanks for the compliments about the bike. Li Ping is indeed pingping227 on ebay.

The reason for the three leads is that the battery management system contained within the yellow sleeve has a dual function. It has a charge limiting and cell balancing function when being charged, which is why there is a separate negative lead for charging. It also has current limiting for discharge, plus individual cell voltage monitoring. Should either the discharge current get too high, or any individual cell drop below a safe voltage, the pack will shut off power.

Battery management is pretty much essential with lithium technology, as without it the cells can easily be damaged from inaccurate charging, high discharge currents or most commonly of all, poor cell balance during discharge. It only takes one cell to drop below it's critical threshold voltage during discharge and it will be permanently damaged. The BMS low voltage protection system stops this from happening.

As has already been mentioned, it's a little safer to use diodes to connect a pair of packs, particularly if they are dissimilar. It's perfectly OK to directly parallel individual lithium cells though.

Jeremy

 

[john]

Jeremy,

I sent you a PM yesterday about controllers.

John

 

[Manchego]

Dear Francis,

Very interesting reading about your battery experiments for the nano-brompton. I was also impressed with the neatness of Tony Castles small battery when i picked up my kit. I found the standard bag too small to carry battery, plus laptop, papers, waterproofs when i first got the bike so I immediately transferred the battery frame on to a Brompton Touring Pannier - much bigger capacity and I can get just about everything in it. The Brompton block bears up very well to the sometimes heavy load (with occasional tightening up)

I'm still using the N-B nearly everyday with no problems so far.

cheers

John

 

[kraeuterbutter]

hi faphillips:

as you i have also two of this BOSCH 36V packs..
different to you, i have the 1100mAh-cells, you have the 1300mAh-cells
but the size is 100% identically (18650 standard-cell-size), they also use the same plastic-housing from BOSCH

however:
i have dismanteled the BOSCH-batterypack
and - just for fun - cut open an old 750ml bike-bottle from my bike..

tada ==> ALL of the 20 cells of the BOSCH pack fit into the bikebottle !!
its not much space, but enough room between the cells to even use balancing-cables if you want)

so: i have 36V, 2,2Ah and only ~850g weight in the 750ml bottle
in front one single cable coming out

its will be a very stealthy setup, because nobody would think, that the battery is in the water-botle

until now i had no time to solder the cells
(because of the limited space, i have to solder always two cells inline which results in ten 2s1p packs, then parallel always two 2s1p packs to get 2s2p packs... and these five 2s2p packs i will wire for series)

will happen after i came back from holiday
will also try to get another 750ml bottle,
so i have one 36V 2,2Ah on the bike, and another 36V 2,2Ah in the Rucksack

greets,
reini

EDIT: just found out (from pics) that my bottle seems to be even smaller than 750ml... (nevertheless 20cells fit into it)

 

[faphillips]

Reini

Very interested in your project. Any chance of some pictures?

Which charger do you use?

Francis

 

[kraeuterbutter]

i use one of my chargers i already have for rc-modellflying:

https://ssl.kundenserver.de/s42221013.einsundeinsshop.de/sess/utn;jsessionid=1547ecee0706b42/shopdata/0020_Akku-Ladeger=E4te+=26amp=3Bslash=3B=3Cfont+color=3D=23000080=3E=26amp=3Bslash=3B+Battery+Chargers=3C=26amp=3Bslash=3Bfont=3E/product_details.shopscript?article=0020_MICROLADER=3Ci=3Epro=3C=26slash=3Bi=3E+=280680=29

its already ~10years old (bought it 10 years ago used)
it was from pre-lipo/fepo time *gg*

when 6 years ago lipos came i sent it in and they made an update to the software
when last year Fepo4 came i sent it in and they made an update for Fepo4

now the eprom is full with software, no further updates possible..
but i can choose for my own any possible charge-voltage
(4,2V, 4,1V, 4,16V, 3,6V, 2,5V, ......) so its open for batteries which may come next years..

pictures: unfortunatley i didn´t made pics, but after holiday when iam home again i will make some

 

[kraeuterbutter]

hey..
was yesterday in a bikeshop:

my bottle were the 20cells 18650 fit in, is a 500ml bottle

there were also 750ml and from looking at them i bet, that even
30cells fit into this..

so: 36V and 3300mah (or 4800mah when i use the Konion1600cells)

with 36V and 4800mah i guess range should be not that bad, and this all in a bottle nowbody would think that it is the battery

 

[frank9755]

You can sometimes get away with connecting batteries in parallel if they are identical but it is best to use schottky diodes like these Maplin > 30A Schottky Barrier Rectifiers
You just connect one to each battery, say the positive terminal, which allows current to flow out put not back in. This prevents any possibility of one battery discharging into the other.

John

John, or anyone else knowledgeable on electronics, reading this,
The Maplin site has four different 30A rectifiers and gives maximum specifications for three voltages:
recurrent peak reverse voltage
RMS voltage
DC blocking voltage
I'm not sure which one is critical for connecting two 36v e-bike batteries with a peak current demand of around 20A, and hence which version I would need? Anyone have any idea?

Frank

 

[Jeremy]

I'd go for the 50V or 60V ones, to be on the safe side, these are the MBR3050PT and MBR3060PT ones.

Its the DC blocking voltage that's the critical parameter, so the 45V ones would just about be OK, but it's possible to get the odd spike on the supply, so as they're all the same price you may as well go for the higher rated parts.

Jeremy

 

[frank9755]

Thanks, Jeremy - I'll do that.
You don't happen to know how you connect these little gems to the batteries? I think they have three prongs and a hole - do you know what goes where? I am assuming that I only use two of the connections but not sure which two!

Frank

 

[JamesC]

Thanks, Jeremy - I'll do that.
You don't happen to know how you connect these little gems to the batteries? I think they have three prongs and a hole - do you know what goes where? I am assuming that I only use two of the connections but not sure which two!

Frank
This thread deals with fitting a second 36v battery to the ezee Torq.

The diodes were from RS, but probably the same.

Connect the +ve of each of the batteries to an outside pin.
The centre pin contains the combined batteries through to the +ve side of the control system.

The diode from RS was metal backed for heat dissipation when mounted to a metal surface. BUT I was reminded by Ian and Tiberius to fit the special insulating pad (Maplin Part # CH03D) between diode and bike as the backplate becomes positive like an unshielded connection.

The additional battery is a great success and gives a comfortable 50 mile range.

James

 

[frank9755]

Thanks James,
Much appreciated. Glad yours is performing well!
Frank