Questions about installing an LED voltmeter between my battery and its charger

I was looking to hook up a little LED display voltmeter on my battery to monitor voltage as it charges, and also as I use it.
I was thinking the most convenient place to wire it would be on the charging connector, so it can always stay attached to the battery, even when I remove it from its craddle when I park my bike.
Can I safely connect it to the charging plug (in parallel) while that plug is also used by the charger to charge the battery? Basically I'd make a male coaxial plug that goes in the battery charging connector, run thick wires to a female coaxial plug where the charger would connect, and in that thick wire Id connect the small wires of my led voltmeter.

Questions:
1) Can I safely do that while the battery is charging?
2) Say my battery is low at 45v and the charger outputs 54.6v, what would my voltmeter display? 45? 54.6?
3) Can I safely keep it plugged in the battery charging port (with the charger disconnected of course) while I ride the bike to observe the battery voltage in real time?

A voltmeter can go anywhere that's in parallel with the voltage you're measuring.

Planet Indigo said:

2) Say my battery is low at 45v and the charger outputs 54.6v, what would my voltmeter display? 45? 54.6?

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Depends on the internal resistance of the battery and the current flow at the time. It'll start low and keep going up until it reaches the full voltage.

3) Can I safely keep it plugged in the battery charging port (with the charger disconnected of course) while I ride the bike to observe the battery voltage in real time?

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Yes, but remember it's taking power from your pack all the time it's connected, so if you forget its' on there and let it sit long enough it'll discharge the battery to empty (probably days, but depends on state of charge of the battery, capacity of battery, and how much power the meter takes; under some conditions it could be only hours).

Rather than a voltmeter, I highly recommend a full wattmeter. There are dozens (hundreds actually) of different ones; poke around the forums for discussions on the many brands/models, and their advantages and disadvantages. One simple search is just to look for threads with wattmeter in the title, or +watt +meter in case it's been entered as two words, but there are many other threads it's not part of the title. Prices range from a few dollars for the cheapies with basic functions to more expensive things like the Cycle Analyst (CA) that can do a lot more than just monitor power.

A voltmeter doesnt' tell you very much about a battery's actual state, but a wattmeter can, once you learn what each reading means, and most especially it can help you understand and even troubleshoot your whole system.

Perfectly safe, don’t worry.

The voltage displayed will be the voltage circulating in the battery in realtime. This means: when it is charging, the voltage will be higher than actual battery state, for it will fall lower if you unplug the charger. Then, the closer to full charge it gets, the smaller the differential. When you unplug the charger and the voltage doesn’t change, your battery is fully charged to the charger’s set voltage.

During discharge, it is the same principle of realtime display. It will show voltage sag when supplying current, and will rise back to actual battery state when at rest.

I hope it is clear enough to help.

Planet Indigo said:

Questions:
1) Can I safely do that while the battery is charging?
2) Say my battery is low at 45v and the charger outputs 54.6v, what would my voltmeter display? 45? 54.6?
3) Can I safely keep it plugged in the battery charging port (with the charger disconnected of course) while I ride the bike to observe the battery voltage in real time?

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I'm pretty sure your plan will work fine. However, I'd suggest getting something a bit more comprehensive that will monitor amps, watts, and volts during charge and discharge. I put an older version of this kind of meter on my bike, and I'm quite happy with it. You might want to consider going one step further and getting a Cycle Analyst from Grin..

https://www.amazon.com/Koolertron-Multimeter-Charge-Discharge-Temperature-Electricity/dp/B07D33JWHS/ref=sr_1_2?ie=UTF8&qid=1543116757&sr=8-2&keywords=coulomb+counter

I should probably let someone else answer your questions who can do so more authoritatively, but here goes:
1) Yes
2) It will be somewhere between the two voltages depending on your battery's resistance and, I think, how many amps the charger will put out. But it will tend to be closer to the battery voltage than the charger voltage. The voltage will climb as the battery is charged. It will also sag when loads are put on the battery.
3) Yes. In fact that exactly why I like the meter that I linked to. It can monitor the real time voltage, amps and watts being used and accumulated amps and watts used ass well.

A voltmeter alone CAN be enough, IF you know your system perfectly.

When I build a new system, I put a CA for a while. Collecting the datas does help tuning for optimal performance, and getting to know the system. After this testing and tuning period, I use a voltmeter alone. That is because I ride mountain trails and crash the bike once in a while, thus I like keeping accessories to a minimum to limit damages.

Planet Indigo said:

......

Questions:
1) Can I safely do that while the battery is charging?

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Yes

Planet Indigo said:

2) Say my battery is low at 45v and the charger outputs 54.6v, what would my voltmeter display? 45? 54.6?

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The battery voltage (the lower of the voltages)

Planet Indigo said:

3) Can I safely keep it plugged in the battery charging port (with the charger disconnected of course) while I ride the bike to observe the battery voltage in real time?

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Yes, however it will not give you much information. As stated above an rc watt meter or another more comprehensive meter like a Cycle Analyst will give you more information to monitor your setup.

As an example of what can be done, in this picture you can see the volt meter I added to an inexpensive charger. I soldered the volt meter wires to the output wires of the charger. It is as safe as can be.


If your controller has a volts connector for your battery, it is safe to connect there as well.

This is the "voltmeter" device I have at the moment:
https://s.click.aliexpress.com/e/b4Li4atK

I also have a bulkier ammeter coming in the mail but that one doesn't have low/high voltage alarm settings, so I'd favor my voltmeter above in day to day usage since I can have it sound an alarm when it gets to 85% capacity while charging so I can stop the charge (to get more life cycles), or 15-20% when Im riding the bike to know tgat it's time to recharge.

Follow up questions:

1) If the voltage displayed during charge is higher than actual battery voltage, at what voltage should I set my alarm to indicate 85% capacity on a 48v battery? I was initially thinking 4.05v/cell, so about 52.6v, but if that 52.6v would lead to a battery at rest with lower voltage, then it wouldn't be 85%, would it?

2) Can I avoid this measurement inaccuracy by putting the voltmeter on the discharging plug instead (the end going to the speed controller) as it is charging from the charging plug?

3) Is it unsafe/not recommended to have something drawing current from that end as the battery is charging? It would be best to have absolutely 0 current from that end as the battery is charging? Does the bms get confused?

Then of course there is temperature of the battery.
My mistake was I would come in from the cold, and charge up right away without getting the battery a chance to warm up.
Also when I'd ride out into the the cold, the range would be a bit less due to the cold weather.

The battery is the most important component on the ebike, its the one that needs the most care, that is why it is not wise to buy from an unknown source online. If you buy from reputable sellers then you know they built the battery half decently. I am cheap, so I like to build my own so I know exactly how the battery was built.

If you buy them cheap volt meters on ebay, those would work, but I'd also hook up an inline current meter that is capable of what the battery puts out, 30A-50A. Real time data is good to have, but what is most important is to have the proper Low Voltage Cut-Off so when the battery gets low in voltage to a pre-determined amount, it cuts off the power so you dont wreck the battery. A mistake I made with LiPo's batteries in a ziplock bag as I like to describe them.

Planet Indigo said:

This is the "voltmeter" device I have at the moment:
https://s.click.aliexpress.com/e/b4Li4atK

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It is much like the one I stuck in the charger I showed.

I also have a bulkier ammeter coming in the mail but that one doesn't have low/high voltage alarm settings, so I'd favor my voltmeter above in day to day usage since I can have it sound an alarm when it gets to 85% capacity while charging so I can stop the charge (to get more life cycles), or 15-20% when Im riding the bike to know tgat it's time to recharge.

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Ok, a high low alarm is fine, but how will you know if your cells are balanced?

Follow up questions:

1) If the voltage displayed during charge is higher than actual battery voltage, at what voltage should I set my alarm to indicate 85% capacity on a 48v battery? I was initially thinking 4.05v/cell, so about 52.6v, but if that 52.6v would lead to a battery at rest with lower voltage, then it wouldn't be 85%, would it?

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If you put your meter between the charger and the battery, you should have no discrepancy.

2) Can I avoid this measurement inaccuracy by putting the voltmeter on the discharging plug instead (the end going to the speed controller) as it is charging from the charging plug?

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That could show you the battery voltage but will not show you any amp information during charging.

3) Is it unsafe/not recommended to have something drawing current from that end as the battery is charging? It would be best to have absolutely 0 current from that end as the battery is charging?

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A very small current like a volt meter will be ok. Discharging your battery by running your bike while charging could be a problem. However, your controller should have a connector for monitoring battery voltage so you shouldn't need to monitor at the discharge connector. And like I said, a volt meter connected at the voltage plug, or at the output wires of your charger will give you a battery voltage.

Does the bms get confused?

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Only if it is human. :wink: If you don't let the voltage required by the bms go balance the cells to start the balancing sequence, your pack will become unbalance and eventually cause you problems.

If you put a rc watt meter or a CA between your charger and battery, you will get all the information you need about charging your battery. With that information you will know how much usage you are needing to go where you go. After a while you will be able to know how far you can go without any meters.

If you want to put a rc watt meter or a CA between your battery and your controller, you can monitor your wattage use while you ride. Also, after a while you will understand how much wattage you use going to the places where you go.

At some point you can put the watt meter of CA on the shelf because they require more wires in your system and that increases the chance of failure due to a bad connection. If you build your own e-bike, a connector failure is something you will someday encounter.


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e-beach said:

Ok, a high low alarm is fine, but how will you know if your cells are balanced?

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I was planning to do a 85% to 15% discharge most of the time and do a full charge to 100% say one in 20 charges. I read about this strategy in another post about increasing life cycles. Does that sound good to keep the cells in balance? Should it be one in 10 charges or 20 is a good number or is that strategy just plain bad?

A very small current like a volt meter will be ok. Discharging your battery by running your bike while charging could be a problem. However, your controller should have a connector for monitoring battery voltage so you shouldn't need to monitor at the discharge connector. And like I said, a volt meter connected at the voltage plug, or at the output wires of your charger will give you a battery voltage.

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Out of curiosity, what is the reason why running the ebike motor while charging is a bad idea? Is it because the motor draws more than what the charger puts in? Is it because the bms gets confused? (I like the idea of this guy getting confused... Sue me...) Or is it because the two currents "add up" and cause extra temperature increase?

If you don't let the voltage required by the bms go balance the cells to start the balancing sequence, your pack will become unbalance and eventually cause you problems.

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See point #1 regarding my 1/20 or 1/10 full cycle, but out of curiosity: at what voltage does the BMS balances the cells?

If you put a rc watt meter or a CA between your charger and battery, you will get all the information you need about charging your battery. With that information you will know how much usage you are needing to go where you go. After a while you will be able to know how far you can go without any meters.

If you want to put a rc watt meter or a CA between your battery and your controller, you can monitor your wattage use while you ride. Also, after a while you will understand how much wattage you use going to the places where you go

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Yes, I was planning to do that with that ammeter that I will receive soon, but since it lacks the high/low voltage alarm I also wanted the voltmeter to give me the audio cue I need while it's charging.
This is the ammeter I have coming in the mail. It doesnt save info between reboots though I think, so it might need an extra small battery wired in to keep it on, or good ole paper/pencil to keep track...
https://s.click.aliexpress.com/e/Hrr4XxK

At some point you can put the watt meter of CA on the shelf because they require more wires in your system and that increases the chance of failure due to a bad connection. If you build your own e-bike, a connector failure is something you will someday encounter.

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I used rc cars for a few years several years ago and I never had an issue with deans connectors. Are the xt60 plugs more fragile? That's what I've started putting on the ebike but it's never too late to switch. How about those bullet plugs that the charger and the dolphin case use? Are those more or less durable?

Thank you for the help by the way.

Planet Indigo said:

I was planning to do a 85% to 15% discharge most of the time and do a full charge to 100% say one in 20 charges. I read about this strategy in another post about increasing life cycles. Does that sound good to keep the cells in balance? Should it be one in 10 charges or 20 is a good number or is that strategy just plain bad?

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Depends on things like the state of your individual cells and how deep your daily discharge. The older the cells get the more likely they will drain at different rates and the deeper you discharge the pack the greater the chance of draining the cells to different levels so they will not charge uniformly when you are not balance charging them. If you want to balance charge the pack every 10 to 20 rides, learn how to meter the parallel groups so you can, after a long ride check them for consistency.

Out of curiosity, what is the reason why running the ebike motor while charging is a bad idea? Is it because the motor draws more than what the charger puts in?

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yes, depending on the wiring you might pull amps through the charger.

See point #1 regarding my 1/20 or 1/10 full cycle, but out of curiosity: at what voltage does the BMS balances the cells?

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It depends on the bms. They are set at the factory to balance when the overall pack hits specific voltage. For a 18650 12s pack, for example that might be 4.2v per cell x 12 cells in series or 50.4v. However it could be different then that.

I used rc cars for a few years several years ago and I never had an issue with deans connectors. Are the xt60 plugs more fragile? That's what I've started putting on the ebike but it's never too late to switch. How about those bullet plugs that the charger and the dolphin case use? Are those more or less durable?

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E-bikes bounce around a lot and with all the connectors on them some are bound to fail. I use bullet connectors on some connectors and just solder some wires together to avoid connectors at all. It really depends on your ability to cleanly and professionally put the wiring harness together. BTW check out this thread. https://endless-sphere.com/forums/viewtopic.php?t=96806

e-beach said:

Depends on things like the state of your individual cells and how deep your daily discharge. The older the cells get the more likely they will drain at different rates and the deeper you discharge the pack the greater the chance of draining the cells to different levels so they will not charge uniformly when you are not balance charging them.

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So you are saying that even if I don't discharge very deep (only down to 20% or so), the cells will go out of balance before 20 cycles? How about discharging down to only 30%?
Is there anything short of getting specialized balancing monitoring equipment that I can do to avoid doing full charges on every cycle? I'm trying to really maximize life cycles.

Planet Indigo said:

So you are saying that even if I don't discharge very deep (only down to 20% or so), the cells will go out of balance before 20 cycles? How about discharging down to only 30%?

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It depends on the quality of cells and their capabilities, as well as how well-matched they are to each other.


If they are not very good quality, and/or are low-C-rate cells, they may not be capable of handling the current drawn from them in a particular usage. It causes heat in the cells (which ages them faster, making existing problems worse), and it causes different voltages in each cell, lower voltages in ones with worse problems during discharge, and higher voltages during charge.

This isn't an issue in the majority of bike builds, which don't use much power, or only use a burst here and there, but some use higher currents very often or constantly. That's where a wattmeter is handy, because you can see your peak and average usage (and realtime if you have it on the handlebars).

If they're not well-matched (most packs are not built from cells matched to each other at all), they may have very different internal resistances and capacities, which can cause imbalance quickly under some conditions. Even if it's only a little difference between cells, it can still happen if used hard.


In general, I wouldnt' worry about it unless a problem occurs (lower capacity each ride, for instance). When it does, just do a balance charge (the worse the problem is, the longer this will take, from hours to days to weeks, to restore full capacity).



For instance, I have had an 18650 LiFePo4 pack made of cheap cells that required balance every time. Eventually some of the cells actually failed and began draining the cells they were connected to in the same groups. I've worked on a few similar packs for others with similar problems, with various chemistries and "brands" of cells; the only "fix" was to replace all the cells in the pack with well-matched ones that are more than capable of the demands placed on them (not an option in any of those cases), or to balance charge them every time. (or not use them as hard, so the problems take longer to manifest)


The packs I use now are made of automotive-grade cells (EIG NMC), and I don't even need a BMS; they stay balanced despite my relatively hard usage.

Planet Indigo said:

So you are saying that even if I don't discharge very deep (only down to 20% or so), the cells will go out of balance before 20 cycles? How about discharging down to only 30%?
Is there anything short of getting specialized balancing monitoring equipment that I can do to avoid doing full charges on every cycle? I'm trying to really maximize life cycles.

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It is not the discharge that wares out a battery so much. It is giving it a full charge that keeps the life cycle down to manufacture specs. If you never charge below the manufactures discharge rate and you never give it a full charge, then you cells can last longer then the manufacture specs. You can deeply discharge a battery pack (but not beyond the rating for the cells you are using) and do very little to the life span of the cells. It is the top end charge that you want to be concerned about.

Yes, if you continually discharge 20% of capacity, and rarely balance charge, your cells, depending on the condition of each cell can get out of balance enough to stress the cells around a low cell enough to shorten the life span of other cells.

Every battery pack is different. If it is from a respected manufacture, you will ok if you only balance charge every 10 - 20 rides, that is until the pack starts to age. If you go with an unknown battery pack builder then all bets are off.

One way to go about this is to get a custom BMS that will balance charge under the manufactures ratings maximum charge ratings. As an example, If you are using 18650 cells that balance at 4.2v fully charged, get a BMS that will balance at 3.8v per parallel cell group and set your charger to charge to that voltage. (as an example, if you are running a 12s battery pack, then the charge should be to 45.6v. ) Combine that with a controller with a LVC that kicks in at above your fully discharged pack, and you will never go above or below the soc that you desire.

All this being said however don't expect miracles in longevity no matter what you do. If you get 2 years past what is expected out of a pack you are doing very well.

What is important is that you keep them balanced, would you have to balance everytime or once in a year is irrelevant.

Then, when you need to balance lico everytime, it is because something is weak. Poor cell quality, improper charging or discharging capacity limits, C rate abuse, too hot or too cold operation temperature...

Staying within safe cell voltage high and low is a general measure of safety and durability, but not the only one. Operating temperature is a much more important factor.

Then, using lico chemistry and trying to make it last is not a good policy. Lico is not made to last. Choose another battery chemistry that is more appropriate, if your priority is durability. Lico is to EV, what nitromethane is to combustion: made for performance, not for durability.