Battery amps and ah. Difference and testing

[Cyclomania]

So on most batteries it is stated an "AH". For example 13AH. 48volt 13ah as an example. This gives 624watt.

What is the difference between amps and AH when you are trying to find out how much max-current form a controller the battery can handle?

So if a controller has max current of 35, should my battery have amper hours of at least 35 to handle that load? Or does AH not matter here? Only amper?

Also, can I measure amps with a multimeter somehow? To know how much max current that controller can draw from that battery, without overheating that battery?

And can I also measure a battery's AH(amper hours) with the multimeter? (this could be useful when it is not stated for example)

 

[Cyclomania]

Also, it is a bit confusing that many controller have both "rated current" and "max current" stated. Is it correct to say that when most people say " I had a controller of 30amps" they talk about max current, and not rated current?

So a controller that pulls 30amps is basically "max current=30", right?

 

[4πr^2]

So on most batteries it is stated an "AH". For example 13AH. 48volt 13ah as an example. This gives 624watt.

Well, the 'hours' carries through as well, so technically 624 watt-hours.

What is the difference between amps and AH when you are trying to find out how much max-current form a controller the battery can handle?

Amps is current, amp-hours is capacity... which is just what it sounds like... a number of amps for a given time interval.

So if a controller has max current of 35, should my battery have amper hours of at least 35 to handle that load? Or does AH not matter here? Only amper?

No, that is not specifically necessary. Though you should only expect to draw that 35 amps for one hour before your 35 amp-hour battery goes dead. Or if you had the 13 amp hour battery mentioned above, you'd expect (13 amp-hours / 35 amps) = .37 hours -or about 22 minutes of use at full power. (In reality, these times will likely be a lot less because amp-hour capacity is generally tested with a low current draw and will drop when high current is drawn.

Also, can I measure amps with a multimeter somehow? To know how much max current that controller can draw from that battery, without overheating that battery?

If you have a multimeter with a high enough amp rating, then you could put it in series with your motor/battery combo and run the motor. The multimeter should read the current draw. Though most 'typical' multimeters are limited to about 10 amps, so you'd have to choose carefully.

And can I also measure a battery's AH(amper hours) with the multimeter? (this could be useful when it is not stated for example)

In general, no. Unless you have a fancy multimeter. Though there are several types/styles of power meter / power analyzers out there which can perform this function. Example:

https://www.amazon.com/Proshopping-Precision-Analyzer-Checker-Measurement/dp/B07Z8XJ71K

Also, it is a bit confusing that many controller have both "rated current" and "max current" stated. Is it correct to say that when most people say " I had a controller of 30amps" they talk about max current, and not rated current?

So a controller that pulls 30amps is basically "max current=30", right?

These are a bit vague terms. 'max current' could either be the current a 'smart' controller limits the output to. Or if the controller is not smart, could be a current where things start to get hot and may let out the magic smoke not too long after. Rated current tends to be more of a continuous operation current - though not always. Could be 'rated' for some short term use. The controller doesn't 'pull' current. It only acts like a switch. The motor is what actually 'pulls' current - or more correctly acts as the load in the system. Would be best to dig into the mfr. datasheet or do more digging for info on any specific controller. Though in general if it sounds too good, it probably is.

 

[amberwolf]

There's quite a few threads and posts that discuss various parts of this in detail, but general explanations are below:

So on most batteries it is stated an "AH". For example 13AH. 48volt 13ah as an example. This gives 624watt.

624 watt-hour. All of the unit being used carries through unless cancelled by something else in the math equation being used (this is true of any equation).

If you were using using only amps and volts, then you get only watts.

Ah is how many amps over how much time (hours in this case).

A is how many amps.

So Ah is capacity, and A is capability.

What is the difference between amps and AH when you are trying to find out how much max-current form a controller the battery can handle?

Since A is capability, it is the factor that matters in this case.

Some batteries don't list an A rating, only an Ah and a C-rate. For those, the C-rate (you can think of it as a Capacity-rate if you like) is multiplied by the Ah (capacity) to get an A capability rating. So if you have a 35Ah 2C battery, that's 2 x 35 = 70, or 70A.

Some of the C-ratings are listed as Max or Peak, some as Continuous; if it's not stated specifically as continuous it is safer to assume it is for a few-second peak rating

Also, can I measure amps with a multimeter somehow? To know how much max current that controller can draw from that battery, without overheating that battery?

A wattmeter is easier, because it will give you many measurements at the same time, which is more useful for these purposes, and generally also stores peak measurements, which are useful in this case as well (often too short to see on a multimeter display). Also, multimeters that can measure high currents above 10A or so are less common and more expensive, while wattmeters that do 100A are common and only $15-30.
Like this one: https://www.amazon.com/dp/B013PKYILS among many.

And can I also measure a battery's AH(amper hours) with the multimeter? (this could be useful when it is not stated for example)

A multimeter cannot directly do this; you would require multiple meters set to different things and setup in different ways, and a timer, and sit there watching them all and noting readings down constantly, then when the pack is empty do some math on all of the things.

A wattmeter does all this automatically.

 

[amberwolf]

Also, it is a bit confusing that many controller have both "rated current" and "max current" stated. Is it correct to say that when most people say " I had a controller of 30amps" they talk about max current, and not rated current?

So a controller that pulls 30amps is basically "max current=30", right?

It depends on the way the specific controller manufacturer decided to rate their controller.

Some will have short peak current draws higher (maybe much higher) than their labelled rating, but only continously draw at most that.

Some will only draw just that rating, ever, max or continous.

Some are rated for the peak current, and continuous is unknown but will be lower.

Some rate both, where max is probably peak, and rated is probably continuous (but if they don't state it, you won't know wihtout testing).

Some are rated in unknown ways that don't match the labelling. :/

Some (kelly) are not rated for battery current at all, but max peak phase current, which is what the motor draws and can be much higher than the battery current (it's just how these things work; if you're curious there are various threads about that with explanations).

 

[Cyclomania]

A wattmeter is easier, because it will give you many measurements at the same time, which is more useful for these purposes, and generally also stores peak measurements, which are useful in this case as well (often too short to see on a multimeter display). Also, multimeters that can measure high currents above 10A or so are less common and more expensive, while wattmeters that do 100A are common and only $15-30.
Like this one: https://www.amazon.com/dp/B013PKYILS among many.

A wattmeter does all this automatically.

I have a blue one that is called wattmeter looking like the picture below. This one shows some type of amps when riding.

To get a correct reading of the amps should it be checked going up a steep hill? Or how is it best used?

 

[E-HP]

And can I also measure a battery's AH(amper hours) with the multimeter? (this could be useful when it is not stated for example)

A multimeter cannot directly do this; you would require multiple meters set to different things and setup in different ways, and a timer, and sit there watching them all and noting readings down constantly, then when the pack is empty do some math on all of the things.

A wattmeter does all this automatically.

The one you linked definitely does (lower right field), and if I recall correctly (I have one of those lying around), it retains the accumulated Wh/Ah when powered off. Most of the inline meters aren't capable of storing data, and lose that when powered off. You have to use a pen or paper clip to press that recessed button to reset to zero.

 

[Cyclomania]

And can I also measure a battery's AH(amper hours) with the multimeter? (this could be useful when it is not stated for example)

A multimeter cannot directly do this; you would require multiple meters set to different things and setup in different ways, and a timer, and sit there watching them all and noting readings down constantly, then when the pack is empty do some math on all of the things.

A wattmeter does all this automatically.

The one you linked definitely does (lower right field), and if I recall correctly (I have one of those lying around), it retains the accumulated Wh/Ah when powered off.

Ah so the one I linked is actually a pretty good one then? So mine will store the data I have accumulated going up a steep hill or something?

 

[E-HP]

Ah so the one I linked is actually a pretty good one then? So mine will store the data I have accumulated going up a steep hill or something?

I'm only familiar with the one that Amberwolf linked. I haven't used it in a while, but I think you just get what's on the main screen, real time voltage, current, and watts, and accumulated watt-hours. The button turns on a backlight, or resets the watt-hours if you hold it down long enough. The shunt is good for 100A, and a bit bulky.

The blue one looks like it would work fine, but not sure if it stores the data, so you might need to take a pic of it before shutting off the battery.

 

[amberwolf]

And can I also measure a battery's AH(amper hours) with the multimeter? (this could be useful when it is not stated for example)

A multimeter cannot directly do this; you would require multiple meters set to different things and setup in different ways, and a timer, and sit there watching them all and noting readings down constantly, then when the pack is empty do some math on all of the things.

A wattmeter does all this automatically.

The one you linked definitely does (lower right field),

Yes, all the wattmeters do, inline or otherwise; and they do multiple functions at once, making them very useful for EVs and testing batteries, etc. It's why I suggested using one for this thread's purposes.

Multimeters only do one function at a time, and don't have the ability to measure things over time (unless you get into fairly high end expensive units, unless there are new ones out there I haven't seen yet).

 

[john61ct]

There is no reliable way to know the actual C-rate performance of a given model cell when new without testing to destruction.

And your definition of timing for peak vs continuous and resisting V sage, and the thermal limits will all be arbitrary choices based on your use case.

So, unless you are a knowledgeable wizard with all the right tools, best you can do is accept the test results and recommendations of trusted experts across the various forums for a given battery model.

Power performance is inherently opposed to high storage density, getting lots of amps sustained for minutes rather than seconds will require a big payload in kg not just Ah.

Especially if you want that performance to last more than a few dozen cycles, reducing the C-rate demanded by increasing capacity is a good strategy.

Do not bother messing with secondhand (scrapped) cells when starting out.