Series Parallel Battery & Open voltage controller

Pete1961

10 W
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Dec 28, 2008
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Telford UK
As more amps = more torque & more volts = more speed, I came across an 'open voltage controller' - I thought of this, two nom 36v batts @ 15A max each, to be connected via relays either in series or parallel to give 72v @ 15A for high speed on the flat or 36v @ 30A for serious hill climbs. I can see problems with putting two BMS in series? Only charge each batt individually @ 36v. Idea may be better suited to Ni Cad or Ni Zn (if the latter ever takes off). I am thinking of a direct 'crystalyte' hub motor - above is just an example of the layout - could it be then applied to a light trike 'car' with a single rear driving wheel (a bit like the old Morgan or BSA models of the 1930s). Idea is to have high speed available without extra weight & huge cost of say 72v 30A batt. Also the motor would remain nearer to its natural speed when climbing the hill. Does the open voltage controller need any setting to a voltage range? or can it do what it suggests? I seem to remember that switching between 36 & 48v was once done when SLA was more common on ebikes
 
It depends on the specific controller, but there are many where you can set the LVC to something like 18v and there is no upper limit other than the parts ratings. You would then need some other mechanism to make sure the battery doesn't get over discharged.

There won't be much advantage with lowering the pack voltage. If you run 72v all the time but just limit how much throttle you give, it won't be much different than running 36v.
 
Only advantage I see is cost, a 72v 30A battery would be more costly than two 36v 15A. It was a cheapness idea to get both high amps when needed & higher cruising speed when on the flat. I have found that a motor only draws about 6A @ nom 36v on the flat once up to speed pulling a 200 Lb rider & 40Lb bike, even if that motor is capable of running at 20A max. My idea is to be able to shift voltages on the move (a bit like the old Xlyte 409 /4011 used first wye delta, & later a tapped winding).

I realise that above 20 mph wind resistance would become more of a factor. I do remember a guy called Solarbbq (Brett White) building a slingshot trike with a single rear hub motored wheel. He managed 50 mph, had a fared cabin, found it here
https://www.youtube.com/channel/UCQMsn_8_zaOWzGh2mQRYQ3g Seems they are now called electric reverse trikes
 
I have a 36/72 V setup
I have two 36v/12 a bosch batteries
a Grinfineon 36-72 40 amp controller

So I have the batteries in parallel for 36 v to controller . For 72v I have another wiring harness that series the 2 batteries for 72 v

So you need to switch wiring harness from batteries to controller to obtain 36/72 v

But as mentioned you can leave it on 72v and adjust your throttling to give good mileage on flats , and extra power for hills and high speed. I'm running a 9C rear and have no problems with hills and top speed 40-45 mph
 
Interesting, bionix 1954, you have done the set up. I wonder what the 'c' rate of the Bosch battery is? I presume there is no problem in seriesing up the two Bosch BMS controlled batts (I was told that two BMS controlled batts should not be put in series). I am thinking that 40A can be happily pulled from the parallel pack, but maybe not from the seriesed 72v (though there are now high c rate cells)
I have some tough hills round here. Contactor relays exist that can break some contacts & make others when energised. I do take the point re motors that have more turns around each pole make more torque but at a lower speed, & in terms of watts the whole thing comes out the same. Only thought is that the motor is not being lugged down so far from its sweet spot @ 36v. I see also GKN have made a 2 mech speed e car transmission - presumably to keep the motor more in the sweet spot?
 
There's a number of threads about putting batteries with BMS in series, that describe damage-prevention methods and why they may be necessary. Both I and Fechter have posted in some recently, so you can look thru our posts for them if you don't find them by title.



Regarding the proposal in the OP, as Fechter and Bionx1954 point out, there's controllers taht will do it.

The problem you may run into is trying to switch "on the fly". You can wire it up, as long as you have a switch that can:

--break all the connections before it makes any other connections

--break the connections under load without arcing across teh open contacts, at the highest possible voltage and highest possible current

--handle the max possible current thru the contacts while not switching, without heating up


But even if you do use such a switch, the controller may not operate normally after the switch until the motor comes to a stop. I've had various controllers that will not respond to throttle after a power glitch until I come to a complete stop, and sometimes I have to cycle power again while stopped to make them work. :/


Personally, if I had to do this, I'd use a block of some high current connectors (like PP75, SB50, or some form of XT or bullets), with two separate blocks keyed to fit only one way. One is plugged in to series the packs, and one is plugged in to parallel them. Neither can be plugged in wrong (keyed), nor plugged in while the other is, so there's zero chance of ever shorting something out. This could also be used as a security measure, as without the keyblock you can't operate the bike or trike. ;) Just don't put it somewhere exposed, or else someone could come along with something metal and short things out either accidentally or on purpose.
 
My batteries are each made up of 3 bosch bat838 36v power tool packs in parallel . so each battery is 36v@12a. These bosch pack do not have a bms. They can provide crazy amount of current.

Yes to change voltage I need to physically unplug one harness and plug in the other. But there is no need for relays or contactors , plus I can use a 36v or 72v charger . I have a grin universal charger so I can slow charge at 36v@ 5A or fast charge at 72v@5A41rt6wZFlPL__AA218_.jpg
 
A lot of good thoughts here, thanks. Sorry in the delay - internet has been down in my area. It seems that the 'shift on the fly' concept is the most difficult part to achieve. There have it seems been other attempts at these 'reverse trikes' BUT the main problem in my area is the transition period (a bit like when ICE vehicles took over from horse transport) Imagine having a 2 ton SUV crash into something like Brett's or the Bee trike
I am about to re wire a 408 to 13G phase & 36v nom 30A universal controller. Seen here, arrived quickly www.aliexpress.com/item/Greentime-36V-48V-500W-600W-30Amax-BLDC-Motor-Controller-Electric-Bike-Tricycle-Controller-Driver/2012487273.html?spm= Has self learn. Hoping to build a Vruzend pack to run it. This got me thinking aloud about the reverse trike. I will be going 2 wheels with above for now as we have dedicated tracks for them here.

BTW what cells are in the Bosch packs that they can manage without a BMS?
 
Bosch original packs used Sony konion 18650, they now have Samsung cells. The Bosch 36v Fat Packs were very popular in the early ebike days. Here in Canada a 36 v@4A cost about 200.00 cdn/ 145.00 us. I have being using them for several years, These packs last me about 3 years about 800 cycles. There are several threads about them on ES.
 
Thanks bionx, will study further when I have more time, seems there are many Sony Konion types. eu.nkon.nl/rechargeable/18650-size.html I am amazed they can run successfully with no BMS
Have bought Panasonic NCR18650PF from above.
 
Regarding the series - parallel, shift on the fly idea, it occurred to me that a large geared hub motor with freewheel clutch may be a better candidate for this as no back drive from the road wheels to keep back emf feeding into controller. An idea like the 'tacho generator' that monitors an AC washing machine motor's shaft speed - or indeed the lack of back emf pulses from the motor itself once it had stopped could tell the controller electronics that the motor shaft was stationary - the vehicle need not be though, & thus allow the shift. With this idea some of the problems of breaking poles @ max current go away. There are LV DC relays / contactors that connect some poles & break others on energising with quite wide pole gaps.
The electronics of the special (open voltage) controller to monitor the above & thus only allow the shift when motor is de energized are well beyond me but the idea seems viable. I just put this out there for others wiser to run with the ball

Also a BLDC motor like this (has planetary reduction) with a freewheeling output sprocket added would be a non hub way of driving the rear wheel. There is more scope for fitting the 'tacho generator' if it was needed, made by Shenzen Unite
www.alibaba.com/product-detail/DC-motor-electrical-motor-dc-motor_60366724091.html?spm=a2700.details.maylikever.3.60561cb81sQdqt

Two downsides to all above are; reversing not possible, & no regenerative braking
 
I'm sceptical a series/parallel switched system will give you any advantage over a permanent series configuration. The higher voltage controller should be able to output sufficient current to give you all the hill-climbing torque you need at low speeds by virtue of working likes a buck converter.
 
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