New hub motor build-questions about boost controller(1.75kw pk DD hub 48v nom batt.)

[soldissimo]

Hi. this forum has has been a very helpful resource in building my ebike recently. Had prev experience with 500w hub motor on SLA batts years ago which was functional but now I'm using an eBay 1000w generic 48v front hub, put on one of my road bikes recently. 10ah lithium iron phosphate in parallel with a premade 13ah 48v bike battery (always same voltage I connect and disconnect separately to check). Have a cheap amazon watt, v, and peak w meter on it. Kit came with a 1250w peak controller without the option for regen, I swapped it to a 1750w peak trap controller with regen and wired that to a button to use separately to the physical brakes (mainly to reduce wear on pads rather than recover energy). I'm very happy with it, ,but considering an alteration/potential improvement inspired by this site- so registered to ask.

It accelerates up to 30-31mph and the motors rpm/v and back emf limit me there because my battery is at ~50v. I read with interest a thread about the hill climbing boost controller one of your members was running on a 36v pack. I want to use a similar 1800w boost converter set to 90v to give me 22amps 90v instead of 35a 50v I am getting now. My understanding is if I have relays activated by a push button that would switch the controller input to the higher voltage current limited supply I would be able to get a few mph more----Is this right? I already have a 12v system supplied by a 120w step down and many heavy duty auto relays I can use. Also my main question is what about on releasing the button, the sudden back emf would give me a sharp braking effect but would there be any harm to my batteries (or controller?) from suddenly having a higher voltage connected in its circuit, or would the relatively small size of that back current mean its not an issue? I don't want an early death to the batteries (or thermal runaway lithium flame up near me).

 

[soldissimo]

To be clear I mean, I'll get reduced torque because of reduced amps, but be able to push against the friction and air half as hard as before rather than not at all, so will creep up past 31 slowly.... I also saw a thread on reddit about adding another set of turnigy limply batts in series to achieve the higher voltage--is this better? Thanks

 

[amberwolf]

There are several potential problems.

The first is that your controller must safely support the voltage you want to use. If it is not designed for it, it could just blow up (potentially also taking out the hall sensors in your motor, any display you have connected, your throttle, PAS, or any other electronics connected to the controller).

The second is that if you switch the voltage while moving, the controller may reset, or even shutdown, depending on it's factory software and hardware design. If it does, you'd have to come to a complete stop to switch from one to the other.

The third is that the switching from one to the other, especially while in motion, could result in voltage spikes that could damage electronics (including controller and anything attached to it, the DC-DC booster, the battery's BMS, etc). This isn't nearly as likely as the first two, but is possible.

The fourth is that the DC-DC converter must be able to handle the full current and voltage and wattage of both input and output for the entire time it is being used, and should be rated for some significant amount higher than you'll use it at to give you a safety margin so it doesn't fail and leave you stranded (especially if it fails in a way that damages other stuff like battery and controller). Please note that just because a company rates it's DC-DC for say, 2v-48v input range, and say, 100V and 20A output, it probably can't do it's max ratings for all things at the same time. It might be able to 36v in and 100v out and 4A out simultaneously, or it might do 48v in and 60v out and 15A out simultaneously, but not 48v in and 90v out and 20A out, without cooking or blowing up. You'd need to find out from it's specs (which should include charts for this purpose) what current you can take out of it at what voltage difference and voltage output. Or experiment until you let the smoke out.

It should also be well-cooled to prevent overheating, as they are not usually very efficient devices and tend to make a lot of waste heat. Some are less than 50% depending on the input/output difference and current demand, meaning half the total power used is pure heat!

The fourth is that the controller will have an LVC to help protect your battery, which needs to be set for the lowest voltage your battery is "ok" with being run down to regularly. This is because you don't want to depend on the BMS to shut off to protect it every time--the BMS is set very low so it is a last-ditch shutoff to prevent damage or a fire. The controller LVC should be higher than that, so you're not pushing your battery as hard--this means you dont' get full stated capacity from it, but it will last you more cycles before you have to replace it. But the controller's LVC is related to it's max voltage, so if it is designed for say, 100v, it's LVC will be far above what your battery could normally output without the DC-DC, so when you aren't using the DC-DC, the controller wouldn't operate at all, as it would think your battery is dead.

Some controllers either don't have an LVC or are made for a very wide range of batteries, so they'd work (depending on what the LVC is for those that have a low one), but then you depend on the BMS all the time, which is harder on the batteries. Some controllers are programmable, so you could set it to the minimum the actual battery should go down to, and those would be your safest option as far as LVC / battery protection goes. (though depending on design might not be the best for other things).



Personally, I would just use a battery and controller and motor that goes as fast as I would ever want to be able to go, and then just use my throttle to go the speed I intend to go at any particular time.


If you want to see what effect a higher voltage will probably have on a particular system, you can go to https://ebikes.ca/tools/simulator.html, and read the entire page so you know what everything is for and how it works, then setup a generic system that approximates yours, and experiment with things until you see what each thing does in your circumstances.

 

[soldissimo]

Hi - thanks very much for this reply. I've played with the simulator and its prediction is that a 72v nom voltage will give me another 9kmph or so usable, but less torque which is my motivation to try raising the voltage when at higher speeds. I appreciate your comments on the wasted energy and potential damage from voltage spikes with tth buck idea-- what about smaller ah series battery? I liked the idea of adjusting the boost voltage to one when that the system can take and works best, but assuming the controller can take the volts I add is there anyone who has tried an additional small pack in series. Is there an issue with smaller batt in series whether switched in and out or not?

 

[amberwolf]

but less torque which is my motivation to try raising the voltage when at higher speeds.

Raising the system voltage is practically the same, regardless of what speed the motor is at when you do it. The only practical difference is what throttle amount you use between each voltage to get the same speed.

It's much simpler to just use the same voltage all the time.

what about smaller ah series battery?

A smaller Ah battery is simply lower capacity, and if it has the same cells as a larger Ah battery, it is also then only capable of lower charge and discharge currents. (if it has different cells, then it is dependent on which cells and what they can handle).

assuming the controller can take the volts I add is there anyone who has tried an additional small pack in series. Is there an issue with smaller batt in series whether switched in and out or not?

Sure--there are numerous threads about "boost" packs of various configurations...but it depends on your specific hardware, whether it works, and how well.

First, the pack must be the same Ah capacity and A current capability (or greater) as the main pack. If it's lower capacity, you get less range, max of whatever the smaller pack's capacity would get you. If it's lower current capability, then you have to limit the controller's A draw to what the smaller packs' capabilty is, or risk damaging it.

Second, the BMS FETs of *both packs* must be able to handle the full voltage of *both packs*, or else when one of htem shuts off for any reason, the full voltage of both packs will be across it's FETs and blow them up if they're unable to tolerate this.

Anything else in the system (DC-DC for lights, etc) must also be able to handle the higher voltage, or else only be connected in such a way as to *never* be able to experience both packs' voltage.