Calling all math majors

[PCeBiker]

There is also a 48V LiFePO4 version, which may give better results.

https://www.amazon.com/gp/product/B0DJ9SKY2Q/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

I'll have to play around with that one. Probably wouldn't require any padding for a 48V bike.

Yeah, I don't get it?
My 48V meter seems to work perfectly,..

I know that LiFePO4 cells have less voltage per cell than Lithium-Ion cells, but wouldn't they just add more cells to bring it up to 48V ?
So what difference would it make?

Maybe the full charge voltage, and empty voltages are slightly different, so the meter is scaled differently?
(but I bet it's done with dampers and zeners, not a software update. )

 

[6zfshdb]

Yeah, I don't get it?
My 48V meter seems to work perfectly,..

View attachment 189450


I know that LiFePO4 cells have less voltage per cell than Lithium-Ion cells, but wouldn't they just add more cells to bring it up to 48V ?
So what difference would it make?

Maybe the full charge voltage, and empty voltages are slightly different, so the meter is scaled differently?
(but I bet it's done with dampers and zeners, not a software update. )

Good questions.

I'm not sure why they offer a separate LiFePO version. As you say, it might have something to do with the scaling.
Might be worth $19.95 US to find out.

 

[PCeBiker]

Good questions.

I'm not sure why they offer a separate LiFePO version. As you say, it might have something to do with the scaling.
Might be worth $19.95 US to find out.

I found this,..

That looks a lot like your 52 volt battery?
Maybe it's got your damping resistor?

I don't know what the LVC is though?

 

[spokewrench]

Good questions.

I'm not sure why they offer a separate LiFePO version. As you say, it might have something to do with the scaling.
Might be worth $19.95 US to find out.

LiFePO charges up to 58.4, is fully charged at 54.4, and is down to 20% at 51.

 

[PCeBiker]

LiFePO charges up to 58.4, is fully charged at 54.4, and is down to 20% at 51.

I don't get it?
Does it stop taking charge after 54.4V?
And the used and/or useful voltage range is only 3.4 volts?

EDIT: Actually, from what I understand, a LiFePO battery is a lot like a lead acid battery.
It's kinda like a half step between lead acid and lithium ion.

 

[spokewrench]

What I'm in the process of doing now is installing this gauge,..

So you installed a shunt. I don't feel so bold!
It looks like 750 micro-ohms. I imagine the resistance could easily vary by a lot of micro-ohms. That meter is going to lie to you and laugh at you when you aren't looking!

 

[spokewrench]

I don't get it?
Does it stop taking charge after 54.4V?
And the used and/or useful voltage range is only 3.4 volts?

EDIT: Actually, from what I understand, a LiFePO battery is a lot like a lead acid battery.
It's kinda like a half step between lead acid and lithium ion.

My voltages are for a 48 volt nominal battery. If it reads 58.4 with the charger on, it's fully charged. When you shut off the charger, it's 54.4. It stays pretty close to that all the way down to 20%.

 

[PCeBiker]

So you installed a shunt. I don't feel so bold!
It looks like 750 micro-ohms. I imagine the resistance could easily vary by a lot of micro-ohms.

Shunts and meters have been standardized to 75 millivolts since they were invented.

The better quality higher amp shunts need to be built to super high standards so maintain an exact resistance under wide temperature ranges.
(PS,.. you don't need a 500 amp shunt on an ebike, and if you do can you post a picture of the ebike? )

So you installed a shunt. I don't feel so bold!

You don't need a shunt with the 20 amp version. That's why it's cheaper.

They're all the same meter, but come with different shunts, or no shunt because because it's built in to the meter itself.

The math part works nicely because All shunts have a voltage drop of Exactly 75 mV at their rated amperage so they pair with a standard "voltmeter" to read full scale at 75 mV.
(We bent a lot of needles in electronics class in the early 80's. Just before digital gauges were affordable.)

An analog ammeter is just a voltmeter that has been standardized.

 

[PCeBiker]

So you installed a shunt. I don't feel so bold!

I could have bought the 20 amp version (without the shunt) but then I would have to cut my battery wire, and instead of installing the shunt across the cut, I would have to extend the heavy gauge battery wire up to the handlebar gauge, then back down to reconnect.

I didn't want full battery power running to and from the handlebars.

I bought the 50 amp version with the shunt so I can install the shunt across the cut, then run tiny wires to and from gauge to feed it the 75 mV and the few mA that it requires.
(It also need positive battery power to run the gauge and Backlight.)

What would be Even Better is to use my analog gauge with my 20 amp shunt, that it is scaled to, but I don't know how to program it to count Watt Hours.

 

[spokewrench]

Shunts and meters have been standardized to 75 millivolts since they were invented.

It reminds me of the way a technician might pick up a resistor with insulated pliers and warn laymen to stand back because it was 33,000 ohms.

 

[PCeBiker]

I used to like grabbing both terminals of a car battery and doing the shock dance to freak people out.

 

[spokewrench]

What would be Even Better is to use my analog gauge with my 20 amp shunt, that it is scaled to, but I don't know how to program it to count Watt Hours.

https://www.amazon.com/DROK-Capacity-Voltmeter-Multimeter-Shielded/dp/B08HP5VD4M

In 1879, 24 y-o Edwin Hall discovered that he could count watt hours without cutting the wires from his e-bike battery.

 

[PCeBiker]

The math part works nicely because All shunts have a voltage drop of Exactly 75 mV at their rated amperage so they pair with a standard "voltmeter" to read full scale at 75 mV.

The thermopile for my furnace uses the same 750 mV standard,..

A thermopile is just a bunch of thermocouples piled up.


A thermopile needs to pump out enough power to operate the gas valve and thermostat, then the appliance can operate without electricity.

My furnace is ancient but if the power goes out, I stay toasty warm.

 

[Roamers]

I don't get it?
Does it stop taking charge after 54.4V?
And the used and/or useful voltage range is only 3.4 volts?

EDIT: Actually, from what I understand, a LiFePO battery is a lot like a lead acid battery.
It's kinda like a half step between lead acid and lithium ion.

Flat voltage curve is the beauty of LFP, in the RV world maintaining consistent voltage is a good thing. For that reason, voltage measurement is a poor method of monitoring SOC. Coulomb counters are more accurate.

 

[PCeBiker]

I'm going to get my Coulomb counter hooked up then, so I can monitor my batteries if I ever go LFP on my e-bike.

I guess I could pimp this ride with LFP, but I don't want to put a hundred dollar saddle on a ten cent horse

 

[Roamers]

I guess I could pimp this ride with LFP

Looks like it might fit in your bin, 400AHr LFP might be considered pimpin' and LFP is a safer chemistry than Li-ion. win - win!

 

[PCeBiker]

Looks like it might fit in your bin,

It might not fit in the bin, but I could probably drop it off at the scrap yard without having to pay?

400AHr LFP might be considered pimpin' and LFP is a safer chemistry than Li-ion. win - win!

It's safer than lead acid too, without the liquid acid. (my SLA batteries are a little more spill proof, than a regular car battery)

I think they're better at delivering huge amps without the voltage sag of li-ion too?

The only downside is the power to weight and volume ratios, compared to li-ion

 

[6zfshdb]

There may actually be an advantage to using that 500A shunt.

It would generate less and dissipate more heat under low current conditions. Therefore, there would be less variation in resistance. The question then becomes what is it's accuracy in the 0 to 40A range.

Looks like another experiment to fool around with.

 

[Roamers]

I think they're better at delivering huge amps without the voltage sag of li-ion too?

The only downside is the power to weight and volume ratios, compared to li-ion

Never did comparison to other Lithium chemistry; comparison was to lead acid in RV application.

Although I have no problem charging 400 AHr LFP in my van, bikes will get charged outside the van even though the charge will be from the van.

 

[Roamers]

There may actually be an advantage to using that 500A shunt.

View attachment 189475
It would generate less and dissipate more heat under low current conditions. Therefore, there would be less variation in resistance. The question then becomes what is it's accuracy in the 0 to 40A range.

Looks like another experiment to fool around with.

Guidance I recall seeing is up to 80% of rating. Probably would reconsider 500A shunt for 40A load. But I am not familiar with what you have shown.