Explain to me how discharge rates work with controllers

[ClintBX]

Hi EPFMs,

I'm shopping around for a new battery and one major objective is to get more range.

I'm keen to get a better battery than my current 36v 12ah golden motor battery. I just learned that my motor is 500w and that my controller is a 20-25 amp (though its just speculation and I have yet to test that with a multimeter)

I'm also informed that if this controller is 20-25 amp, a 20 continuous/40 max battery would be a good match.

I don't fully understand how that works. Could someone please explain to me how this continuous/max amp discharge correlates with the amp of the controller?

I really want to make sure that I don't end pairing it with a battery that harms either the controller or the motor. Or gets harmed by the controller or motor.

 

[DanGT86]

I'll have a try at explaining it to you since I like to explain things.

When you are talking about current, the controller will always demand what it needs from the battery. The battery will not damage the controller or force anything through it. The only way you could damage something is by using a controller that was requesting more amps from the battery than the battery could provide. This is why the batteries come rated for specific discharge rates. The battery rating is telling you the max it can support so any amount less than that is ok. When you buy batteries you are considering: Voltage V, capacity Ah and the safe discharge current amps. Generally, larger capacity (Ah) batteries will also be able to supply more current so they will be advertised with higher discharge ratings. Having more capability than you need is not a bad thing at all.

So:
-100 amp controller with 40 amp limited battery would damage the battery or just trigger the BMS to shut down the battery to protect it.
-40 amp controller with 100amp limited battery would damage nothing but your wallet since higher discharge batteries are usually more expensive.

Think of it like your local water supply. There is enough water for the whole city yet it doesnt flood your house when you fill a glass of water because the faucet is only asking for a little. Faucet would be the controller and the water supply would be the battery in that analogy.

So buy this biggest battery you want for long range just keep the voltage about the same unless you are trying to hotrod. There is a little wiggle room on voltage too but its best to leave that the same if you are not into tinkering with your setup.

Don't worry about the motor at all since it will only take what the controller gives it no matter what battery its hooked up to. Motors don't really have an exact rating. Its more of an estimate of power you can run without overheating it. You could run a brushless 36v motor at 136v if you wanted and it would just go faster. There are a few other things to consider in a case like that but the point is that you would probably know if you were going to do something harmful too it.

Hope that helps.

 

[dogman dan]

One of my favorite battery rules of thumb, is to cut stated c rates or discharge rates in half.

so say you look at a battery and it says 30 amps continuous. Sure, it will be able to do that 30 amps. But, the battery will be much happier, stay better balanced, run with less voltage sag under load, likely discharge more total watt hours/amp hours per charge, and potentially last longer if you cruise using only 15 amps continuous.

FWIW, 15 amps of 36v is about 500 watts, and will generally get you cruising at least 20 mph, possibly faster if wind or hill doesn't slow you. So very little of your ride is typically at your controllers max amps. You don't need it to go that fast, nor will the motor be able to draw the max amps, for reasons I still understand only vaguely. The motor is unable to pull it's full power once going 20 mph, unless the hill is as steep as a wall.

In general, a typical kit found on the USA market has a 20-25 amps controller, and a battery capable of 30 amps continuous will run it ok. But a larger battery to have longer range with the same cells will be capable of much more continuous amps. So a larger pack will always be better, provided carrying it will not be a problem.

As a general rule, a 10 ah size battery will run your type of kit. ( it will usually be able to run at 30 amps in that size) 15 ah will do better, less voltage sag, less strain on each cell. A 20 ah will have very good range, and should run your bike excellent.

 

[ClintBX]

Ok. So you can never have much amp. Only too little to do any damage.

So, is that discharge rate per hour?

If the battery is rated at say 20 amps continuous, is it 20 amps per hour?

Is it the same with the controller? Say my controller is 20-25 amps. Does that mean it can pull 20-25 amps per hour?

 

[izeman]

there is nothing like "amps per hour". A is current. it has no time component. you can imaginge that as a stream flowing.
power = current x volts. capacity = current x time.
so if our battery has a capcity of 20Ah, that means that it could in theory deliver 20A for one hour. or 40A for 30min. or 200A for 6min. IF it is capable of.
let's say your battery has a "c-rating" of ONE = 1c, and we use the example above (20Ah battery), that means the battery can output a maximum of 20A. if it had 5c this would be 100A.
second: the controller's output capability is rated. if a controller is rated 20A, that means that it can output 20A to the motor. if this is continous or peak is up to the seller of the controller. some rate battery current sustained, some rate phase current peak ...
but normally you would match your controller and motor and battery, so that you first look at the wanted motor output. let's say 1000W. than you choose a battery voltage for the wanted speed (more voltage with the very same motor means more speed). so you choose eg a 50V battery. this means that you need a 20A controller (20A x 50V = 1000W). and you battery needs to be able to deliver 20A as well. and as stated before: the battery can't be too strong. so a battery that can deliver 100A is always to be prefered, if you can afford it, and it's size is appropriate for your bike.
i hope to have answered all your questions.

 

[ClintBX]

Thanks Izeman. I've got a clearer understanding of discharge rates and how they relate to the controller's rating.

My other question is about the voltage. If I were to increase the voltage of the battery, would that put any additional stress on the controller or the motor or is the same deal where the controller can only take as much as its capable of taking?

 

[izeman]

the higher the input voltage of the controller, the higher the output voltage.
there are "little switches (FETs)" in the controller. think of them as valves of a water hose. those valves are opened as you apply throttle. the more water you pump in on one side, the more water comes out if the valves are fully opened.
but there are limitations. first one basically are the capacitors, and second one are the FETs (the valves) - there are other limitations as well, but they won't matter most of the time. those are rated up to a certain voltage. if you exceed this voltage, they are toast. up to 63v you SHOULD be save. better to stay some volts below that value (maybe 60V) if you don't want to open the controller and take a look inside .
so if you raise the input voltage, the maximum output voltage will be raised by the same factor. so if your bike was accelerating up to the motor's nominal rpm, then you will reach a higher speed with more voltage. if your input power (voltage and current combined) was too low before to reach full speed, then raising the voltage will not help you at all. you will need to raise current as well.
so basically as long as the controller can handle the higher voltage and you don't raise the current you have nothing to fear. the current limit of the controller which is important to not "overload" the FETs is still in place and will be the safe guard. if your bike will be faster afterwards? probably yes, but i can't guarantee from far far away
REMARK: please notice that i can't take ANY GUARANTEE that all i said can be applied 100% to your sitaution. so if you see white smoke while testing: don't blame me!

 

[izeman]

forgot the second part of your question: will it put additional stress on the motor? hmmm. it depends. most probably not too much. more stress yes, as total power raises. eg. you had 20A @50V before this is 1000W. if you raise it to 20A @60V this will be 1200W. so additional heat will be generated as efficiency is well below 100%. this will heat up the motor more than before.
can your motor handle that? i can't say - see post above.

 

[Alan B]

Additional voltage puts more stress on everything. Not only does it increase the voltage (which increases the capacity for speed), it increases the current as well. It is the dual increase of voltage and current that increases power even more.

The voltage itself won't stress the motor. Insulation on motor wires doesn't care whether the voltage is 36 or 48 volts. Insulation can handle a lot more voltage. But the current will increase, potentially a lot. Current makes torque and current squared makes heat in the motor. So it can quickly become a lot warmer.

This is why so many folks do it. It is fun and effective. And may lead to failures.

Good luck in your project.

 

[izeman]

how would rasing the input voltage raise the current at the same time???
total power raises. yes. but not current. if the current limit is set to 20A than that's fixed.

 

[ClintBX]

I don't plan to increase my voltage by all that much. I'm just thinking about upgrading my 36v to a 48v. I know my 500 watt motor can handle a 48v. I'm not sure what the actual specs of my controller are. All I know is that its speculated to be a 20-25 amp controller. I'm not sure how many volts it can take but seeing as it was paired with this motor, I can guess that it can handle a 48v too.

I have one more question.

I've been learning about watt-hours vs amp-hours and how WH is a better indicator of capacity than AH.

Say that I upgrade my 36v 12AH to a 48v 12AH, would I get more range from my ebike or does it not work like that?

 

[izeman]

Short answer: YES

 

[izeman]

Longer answer: as you said Wh takes voltage into the equation. So your 36v pack will have 432Wh and your 48v pack 576Wh.

 

[Alan B]

Short answer: Input power WILL go up. Motor Current WILL go up, increased power guarantees this. Speed will go up unless something limits speed, and with speed increase losses go up and efficiency goes down, as does range. The increase in battery energy will partially offset this.

 

[ulrichw]

[...] I'm not sure how many volts [my controller] can take but seeing as it was paired with this motor, I can guess that it can handle a 48v too.[...]

I'd like to hear the experts weigh in on this: To me this doesn't follow at all. I would be inclined to think (without any direct knowledge of the matter) that controllers' components are more sensitive to voltage than motors' (i.e., most motors support a higher range of voltages than the controllers they may be bundled with).

 

[DanGT86]

Motors are way more tolerant of voltage changes then controllers. This could get complicated pretty quick but many of us are making general statements based on experience with common ebike components.

If you are dealing with the common "infineon" style controllers that are on most entry level to mid level bikes then you will commonly find them in a few ranges with regard to max voltage.
63v max
100v max
100v+

This is because the capacitors and mosfets common in controllers are available in these approximate max voltage ranges and are most efficient when selected for the voltage range you plan to operate them at.

Most 36v controllers on the market will run 36 through 48 without any modification. If the OP was trying to use a 36v kit to run 72v, I would tell them to open the controller and look inside to see what ratings it has on the components. Older controllers used to require changing the input resistors to optimize for drastic voltage changes. Any more it seems like most vendors are selling controllers that will run any voltage below the rating on the mosfets and capacitors.

 

[Alan B]

The motor's only sensitivity to voltage is from the insulation breakdown, which can take a lot of voltage. The motor is sensitive to heat, and that comes from current. Raising the voltage does increase power which will increase motor current (even if battery current does not change). Settings in the controller (provided it is adjustable) will allow you to control how much power / current the motor gets. What happens is pretty much up to the controller.

 

[larry_says]

Ok. So you can never have much amp. Only too little to do any damage.

So, is that discharge rate per hour?

If the battery is rated at say 20 amps continuous, is it 20 amps per hour?

Is it the same with the controller? Say my controller is 20-25 amps. Does that mean it can pull 20-25 amps per hour?

In short, to answer all your questions at once the answer is NO. Try and follow along here =)

hahaha listen to me! this thread is almost 10 years old lmao. are you following a long buddy?

I'm pretty sure you learned a lot since this post started. I'm out of here...

 

[bbosncali78]

I'll have a try at explaining it to you since I like to explain things.

When you are talking about current, the controller will always demand what it needs from the battery. The battery will not damage the controller or force anything through it. The only way you could damage something is by using a controller that was requesting more amps from the battery than the battery could provide. This is why the batteries come rated for specific discharge rates. The battery rating is telling you the max it can support so any amount less than that is ok. When you buy batteries you are considering: Voltage V, capacity Ah and the safe discharge current amps. Generally, larger capacity (Ah) batteries will also be able to supply more current so they will be advertised with higher discharge ratings. Having more capability than you need is not a bad thing at all.

So:
-100 amp controller with 40 amp limited battery would damage the battery or just trigger the BMS to shut down the battery to protect it.
-40 amp controller with 100amp limited battery would damage nothing but your wallet since higher discharge batteries are usually more expensive.

Think of it like your local water supply. There is enough water for the whole city yet it doesnt flood your house when you fill a glass of water because the faucet is only asking for a little. Faucet would be the controller and the water supply would be the battery in that analogy.

So buy this biggest battery you want for long range just keep the voltage about the same unless you are trying to hotrod. There is a little wiggle room on voltage too but its best to leave that the same if you are not into tinkering with your setup.

Don't worry about the motor at all since it will only take what the controller gives it no matter what battery its hooked up to. Motors don't really have an exact rating. Its more of an estimate of power you can run without overheating it. You could run a brushless 36v motor at 136v if you wanted and it would just go faster. There are a few other things to consider in a case like that but the point is that you would probably know if you were going to do something harmful too it.

Hope that helps.

So I Just bought a 52v 50ah 90-100amp peak and 10amp discharge. I have a 48v 25ah battery not sure about the controller but have seen a few videos of it handling a 52v no problem on YouTube and you can change it on the screen to 52vlts. But from what I understand your saying that won't hurt anything don't worry about going up 4vlts but the rest is ok to run?

 

[99zip99]

Hey bb what's it going in are you ride on flats or hills what's your terrain ? How much you weigh ?. 52v 50ah ?
Where did you get that from give us a link what kind of sales what company how much you pay does have a BMS ? More info. Then a battery please.