BMS with LCD?

Hi Guys,

I'm currently buiding an ebike for commuting to the train station. The distance I have to travel is really short, like 4km (2km both ways). My motor kit is the following:

https://endless-sphere.com/forums/viewtopic.php?f=3&t=58490

My battery will be 13s2p (total 54V 3Ah) with 18650s and vruzend modular kit. I would like to add a BMS to it, and something I would really like is to monitor cell groups voltage, not only when charging, but also when riding. I was thinking of using a regular BMS like this one:

http://vruzend.com/product/48v-13s-battery-management-system-bms/

Ideally, it would be nice to get cell voltage from the BMS, but I don't need a fancy bluetooth smart BMS ... I was thinking of just using multiple small voltage displays like this:

https://www.aliexpress.com/price/mini-voltmeter_price.html

I'm gonna 3D print the battery case, so I can easily design it to include those little displays. What do you think? Any suggestions? There's probably a simpler way to accomplish what I want. Please advise.

ic3wall said:

My battery will be 13s2p (total 54V 3Ah) with 18650s and vruzend modular kit. I would like to add a BMS to it, and something I would really like is to monitor cell groups voltage, not only when charging, but also when riding. I was thinking of using a regular BMS like this one: can easily design it to include those little displays. What do you think? Any suggestions? There's probably a simpler way to accomplish what I want. Please advise.

Click to expand...

1) The LED displays will discharge (and then destroy) the cells rapidly
2) Unless they all take exactly the same current (and they don't) then they will unbalance the pack and cause the problem you are trying to avoid.

ic3wall said:

I was thinking of just using multiple small voltage displays like this:

https://www.aliexpress.com/price/mini-voltmeter_price.html

I'm gonna 3D print the battery case, so I can easily design it to include those little displays. What do you think? Any suggestions? There's probably a simpler way to accomplish what I want. Please advise.

Click to expand...

One of these and a single voltmeter wouldl allow you to scan through the P-group voltages on demand, and switch it out of the circuit the rest of the time.

the led's would work, I do it on my 4s lifepo4 battery pack, so you can easily do it on a 13s. Power use would be inconsequential as long as you don't leave them on all the time. I use mine during charging to make sure nothing goes out of balance. I prefer seeing all the voltages of each group at one time.
With my 4s, I use a 4 pole toggle switch so I can turn the leds on and off. With a 13s you will probably have to plug and unplug the leds connectors when needed or maybe use a 3x 4 pole toggle switches.
Get the small leds that go down to 2.5 volts or near there, some don't go below 3 volts, and buy the ones that you can adjust the voltage. Some of the 1 dollar ones have a small screw in the back where you can adjust the voltage to calibrate it. It's very important, you will notice a difference. I bought some without the calibration and can't use those for precision work.

picture of balance led's on lifepo4

billvon said:

1) The LED displays will discharge (and then destroy) the cells rapidly
2) Unless they all take exactly the same current (and they don't) then they will unbalance the pack and cause the problem you are trying to avoid.

Click to expand...

1. I thought the power consumption was negligible
2. If you're right about 1., then I agree.

Buk___ said:

One of these and a single voltmeter wouldl allow you to scan through the P-group voltages on demand, and switch it out of the circuit the rest of the time.

Click to expand...

It would work, but it seems a bit old school, plus it doesn't show voltage of all cells at once.

Buk___ said:

the led's would work, I do it on my 4s lifepo4 battery pack, so you can easily do it on a 13s. Power use would be inconsequential as long as you don't leave them on all the time. I use mine during charging to make sure nothing goes out of balance. I prefer seeing all the voltages of each group at one time.
With my 4s, I use a 4 pole toggle switch so I can turn the leds on and off. With a 13s you will probably have to plug and unplug the leds connectors when needed or maybe use a 3x 4 pole toggle switches.
Get the small leds that go down to 2.5 volts or near there, some don't go below 3 volts, and buy the ones that you can adjust the voltage. Some of the 1 dollar ones have a small screw in the back where you can adjust the voltage to calibrate it. It's very important, you will notice a difference. I bought some without the calibration and can't use those for precision work.

Click to expand...

You got some good suggestions. It would be nice to find a display that could show voltage of all cells. Something like this, that I could switch off easily:

https://hobbyking.com/en_us/hobbykingtm-battery-medic-system-2s-6s.html

Actually the HK Battery Medic looks pretty cool. I could build a 12s2p instead of 13s2p, and use 2 of those. It woud give me voltage during riding and charging. I woud need to charge with 2x 24v chargers though, or bulk charge with a 48V charger. Am I missing something?

I use to use those 3 in 1 LCD lipo meters (like the hobbyking) but to me they where very hard to see, I find the led's at 1 dollar a piece is cheaper in the long run and very easy to see. But they would both get the job done. Just the LCD will be hard to read unless your right next to them.
Since I been using the LED's I wouldnt go back to the LCD's, they are both very accurate.
On the LCD, just use something to cover the buttons, I have press them by accident and they will discharge your battery.

The BM LCDs can be hard to read--some batches are better than others, and it depends on your lighting conditions. They don't have backlights so you can't see them in the dark at all without adding your own lighting.

OTOH, the LED meters can be hard to read in direct sunlight while riding, and the really bright ones will drain the cells faster (even if only by a little) *and* they will also be more blinding at night, shining up from your handlebars.

So it depends on the kind of riding you do most which one you might want.


FWIW, there are actually BMS systems that have LCDs as part of them that display individual cell voltages. I think some of the Bluetooth ones discussed on ES can send that info to your phone or tablet. Sorry I don't have links to any; it takes some poking around at BMS threads to find them. :/

ic3wall said:

Actually the HK Battery Medic looks pretty cool.

Click to expand...

They're not always all that accurate, and various batches of them have different problems. If you use a known-accurate meter to measure each cell at a high and low voltage at the same time the BM is doing so, you can note down teh difference (if any) for each cell input on the BM, so you know how far off it is. I have one and it's useful, but I don't use it for measurements, just to get a sense of relative bulk balance of cells (within a tenth of a volt or so) at a glance.

There's a number of threads and posts about the BMs here on ES if you poke around:
https://endless-sphere.com/forums/search.php?keywords=%22battery+medic%22+accur*&terms=all&author=&sc=1&sf=all&sk=t&sd=d&sr=posts&st=0&ch=300&t=0&submit=Search
is one possible list of posts mentioning them.


I expect the same is true of most of the other multi-cell monitors. It's true of the little LED ones I have, that rotate thru the attached cells to display each one's voltage then the next, and so on.

ic3wall said:

billvon said:

1) The LED displays will discharge (and then destroy) the cells rapidly
2) Unless they all take exactly the same current (and they don't) then they will unbalance the pack and cause the problem you are trying to avoid.

Click to expand...

1. I thought the power consumption was negligible
2. If you're right about 1., then I agree.

Click to expand...

It depends on the unit. A lot of "mini voltmeters" are not self-powered, and require a separate power supply. For those, there should be no significant drain on the cells. For the ones that are self-powered (powered by the cell they are measuring) then it could be up to a few dozen mA to light up the LEDs, in addition to any other drain (MCU, etc), for each of the meters.

If they all take the same current it's not a big deal because you just lose that much capacity on a ride. If the current differs significantly between units, a small imbalance will occur on each ride. If you rebalance the pack each time ti doesn't matter, but if you don't, it'll build up over time.

How much time depends on the difference in load and the time they're on. If the worst difference is only 1mA, then in an hour's ride it's only 1mAh difference in capacity drain of the cells. It'd take a few hundred hours of riding to get a huge difference in capacity drain. If the worst difference is 100mA, then only 10 hours of riding makes a 1Ah difference in capacity between cells.

Something like this, that I could switch off easily:

https://hobbyking.com/en_us/hobbykingtm-battery-medic-system-2s-6s.html

Click to expand...

They don't have a switch (none of the multicell monitors I've seen do), so you have to put a switch in line with each cell that's attached to it, so it doesn't have any drain on any of them when not in use. It's easier to unplug them, but that wears connectors and wires over time, so switches are better for long-term installed-on-bike use.

Thank you amberwolf. I'm a bit disappointed about the HK battery medic, they looked like a very nice option, but after reading some of the posts you referenced, I realize a lot of people found them to be quite inacurate.

This is getting more challenging than I thought.

A battery that is 4p would make life easier.

999zip999 said:

A battery that is 4p would make life easier.

Click to expand...

Please explain.

Reviews for this one seem much better:

https://hobbyking.com/en_us/turnigy-dlux-lipo-battery-cell-display-and-balancer-2s-6s.html