Any lipo checker that is not powered only from first cell?

Hi,

I'd like to use a basic lipo checker while riding my ebike to be warned in case of a low individual cell voltage condition. Unfortunately it seems most lipo checkers/alarms are powered from the first cell only and thus shouldn't be used for extended periods as they would cause cells to go out of balance. Do you know if there exists any simple lipo checker/alarm that monitors individual cell voltages but is powered from all the cells at the same time (rather than being powered by cell #1 only)?

Thanks!

Im following as well, almost messed up lipo once due to this

Sent from my ALE-L21 using Tapatalk

You could add an active balance board, and that would keep everything at the same voltage, but would impose a small parasitic drain on all the cells.

If there is no lipo alarm which can be powered by all the cells, I'd just use a BMS then (this is for an ebike battery with Li-ion 18650 cells). Most BMSes also appear to draw less current from the battery than one of these cheap lipo checkers, not to mention a BMS draws power from all the cells, not just the first parallel group. I dont't really need a BMS as I'll balance charge using a hobby charger. Just wanted some way to monitor individual cell voltages while riding and have a low voltage alarm/cutoff but don't want to introduce cell imbalance with those cheap lipo checkers.

kilou said:

not to mention a BMS draws power from all the cells, not just the first parallel group.

Click to expand...

They don't all--many do but you must check that the one you want to use does so.

amberwolf said:

They don't all--many do but you must check that the one you want to use does so.

Click to expand...

Ok thanks for the information, I did not know that. Or would it be possible to mod a lipo checker so that it draws power from an external source rather than the one being tested?

BMS includes many functions. The least important is HVC and balancing, since that can be done while the pack is not in use, maybe disconnected completely.

The most important BMS function is cutting off use discharge based on the first cell / group to hit your desired LVC.

That can also be done at the pack level, with a DIY setup rather than a "canned" BMS, but it still counts as a BMS IMO.

But you then are better off with a bottom-balanced pack, takes more attention, less easy with a poorly matched or worn set of cells.

Unless getting maximum range is not critical, just set a pack-level LVC much higher than you would a per-cell one.

_____
Note that **you** need to verify how much imbalance is created by any device left on the cells while in use.

Lots of BMS have that flaw too.

Even if easily rebalanced while not in use, still leads to uneven wear eventually.

A little DCDC buck converter to provide lower voltage power from the full pack supply is another approach,

many ways to skin the cat,

just be sure to account for all possible failure modes, and KISS as much as possible.

Put a switch on first input wire to lipo checker.....just turn it off when not in use and it does not draw current.

pullin-gs said:

Put a switch on first input wire to lipo checker.....just turn it off when not in use and it does not draw current.

Click to expand...

A switch wouldn't really help in my application because I'd like to use it while riding so that it monitors the voltage continuously and beeps if one cell gets too low, in replacement of a low voltage cut-off of a BMS. I'd unplug it anyway when I'm not riding. But it would probably still drain too much current while riding to cause small cell imbalance I guess. Si I may be better off using a global voltage alarm and set it higher as @john61ct suggested. Too bad none of these lipo checker seem to drain all cells equally

Whether it would cause significant imbalance depends on the actual power draw, vs the capacity of the cells, vs the ride time.


If the unit draws 500uA, (0.5mA), and the ride is an hour long, then over that hour it draws 0.5mAh. If the battery is a 5Ah battery, then it has only created a 0.1% imbalance. (1/1000th of the capacity). Negligible, and easily fixed by a balancing BMS or "RC" type charger.

If the unit draws 1A, then under the same condtions, now it has drawn 1Ah from the cell in a 5Ah pack, and it's taken 1/5 of the capacity away, and *that* is a serious imbalance.


Somewhere around the forum is a mod for the old Celllogs, which aren't made anymore, to make them drain from the whole pack they're hooked up to. I don't have a link, but if you find that you mgiht be able to adapt it to other cell-checkers, after you reverse-engineer the one you want to use.

Just get a very robust balancing setup that can handle whatever the checker does while in use

and make sure it gets turned off the rest of the time.

Having great balancing capability will be a Good Thing in any case.

kilou said:

A switch wouldn't really help in my application because I'd like to use it while riding

Click to expand...

Exactly....turn it on when you ride, turn it off when done with ride.

kilou said:

But it would probably still drain too much current while riding to cause small cell imbalance I guess.

Click to expand...

Not true....they only draw about 50MA. For one hour, it would only draw 50MaHr of pack capacity. For a 10AH pack, it would take 200 hours to deplete pack (10,000Mahr/50MaHr = 200).
For a one hour ride, you would use about 1/2 percent of pack.
If you went with an LCD version, Quiescent Current is much less: <10mA
That is less than 1/10th of one percent!

pullin-gs said:

For one hour, it would only draw 50MaHr of pack capacity. For a 10AH pack, it would take 200 hours to deplete pack (10,000Mahr/50MaHr = 200).
For a one hour ride, you would use about 1/2 percent of pack.

Click to expand...

Is it possible to calculate the voltage difference that would result between the first cell (powering the lipo checker) and other cells after e.g. 5 hours of riding? Wouldn't that lead to a non negligible imbalance in the pack, easily fixed by s RC charger but which may affect the pack life in the long run?

General Lithium Ion/Poly voltage levels @ 1C discharge relative to percent of capacity avaiable:

100.00% 4.2
90 4.13
80 4.06
70 3.99
60 3.92
50 3.85
40 3.78
30 3.71
20 3.64
10 3.57
0.00% 3.5

Running an LCD voltage monitor for five hours will utilize (10Ma load on cell#1) 1/2 of one percent of total capacity of the #1 cell-bank for a 10AH pack.

Extrapolating, this comes to a voltage difference of about 0.0035 volts imbalance between #1 cell and the other cells in pack when pack is recharged to 4.2V 100% capacity.

Imbalances due to variances in cell manufacture and pack inter-cell conductor construction results in far greater imbalances.....thus the reason quality BMS function does provide (albeit very low current) balancing.

If 0.0035 is excessive for your requirements, you could avoid it entirely by simply getting an isolating DC-to-DC voltage converter (Buck variant for step-down applications) which gets input from entire pack, and provides 3V output to cell#1 placeholder on voltage monitor. The voltage reading for that will always be 3V, but the individual cell readings will be correctly monitored.
If you have an 8S pack, you will need a voltage monitor that can do 9S cells.

kilou said:

pullin-gs said:

For one hour, it would only draw 50MaHr of pack capacity. For a 10AH pack, it would take 200 hours to deplete pack (10,000Mahr/50MaHr = 200).
For a one hour ride, you would use about 1/2 percent of pack.

Click to expand...

Is it possible to calculate the voltage difference that would result between the first cell (powering the lipo checker) and other cells after e.g. 5 hours of riding? Wouldn't that lead to a non negligible imbalance in the pack, easily fixed by s RC charger but which may affect the pack life in the long run?

Click to expand...

In theory yes.

In reality, depends on many interdependent factors.

There are dozens of other factors that have a greater impact

but use common sense , do be checking for imbalances "often enough"

I am also looking for something reliable of that kind.
I found this product:
https://www.aliexpress.com/item/32962134925.html

The seller replied to my several questions about it:

"Dear friend
Thanks for your inquiry!
The total voltage of the battery to be tested must not be lower than 4.80V or higher than 27V, lower than 4.80V, and the screen is not displayed. Above 27V, it may cause LCD-6 damage.
The Li battery cell voltage must not be lower than 3.00V. Sorry, we don't know the refresh rate of the display now.
Thanks
Best regards!"

I guess we can assume that this unit is powered from the total 6S voltage connected to it? (The seller said it won't work if the voltage is below 4.8V which has to be the full 6S voltage)

Has anyone tried this product? The display is nice too and even looks less "Chinese".

I asked another seller of this product if he know the refresh rate, since I already got to see voltage meters that update only once in 3 seconds or so.
Also the product page says it doesn't measure below 3V which is ok for Li-Ion, but for LiFe not good (voltage sag during discharge can fall down to 2.5V when the battery is in the last quarter of it's capacity)

And what about this unit?
https://www.aliexpress.com/item/4000052302952.html

It's supposed to be also a balancer (albeit only at 50mA), but it's enough to assume that it doesn't drain only the first cell, right?
And it measures from 0V-5V, according to the seller. (I asked)

See PM for a dump of my raw notes.

Let us know what more you learn please