lipo alarm operating power

[E-HP]

I'm asking about the small 8S lipo alarms used to monitor lipo packs, and how they are powered. I received two new lipos yesterday and a couple of lipo alarms. I connected the alarms to the balancing leads and the cells were at a storage voltage and balanced within a 0.01V. I left the alarms plugged in, planning to charge the packs up in the morning and check that they are staying balanced in case there's a warranty issue.
At 3AM I hear both alarms going off. I unplugged them and decided to check more closely in the morning. What I found is that both packs had cell 1 and 2 below 3V, while the other 5 cells in each pack were at the storage voltage they arrived with. The packs are 5Ah, so I was surprised the alarms could draw so much power so quickly. I don't recall this happening with my other alarms or packs. Any ideas of what's going on, and if it's normal?

 

[fechter]

It's not uncommon for the alarm circuit to run off the first two cells, which would explain why the others were still charged. As far as draining quickly, it sounds like either it's defective or it's a very poor design. It's not that hard to design such a thing that only draws a few microamps on standby.
After charging the low cells back up, you could try measuring the current draw on the alarm (measure at the B- connection). My guess is it just has a bad design intended to be only attached when you are actively discharging.

 

[amberwolf]

Are they LED-based? Or LCD? LED units will use a lot more power than LCD, so they will drain a cell faster.

Some of the LED displays could take 10-20mA continuously, (or more) depending on how many segments are kept lit. Some could take that many mA *per segment*, but that's not likely on a small PCB design like these alarms. Let's assume 20mA. You don't say what time you connected the alarms, but we'll just use 9pm since it's on the other side of the clock from 3am, for six hours total. 6 x 20mA is only 120mAh, so it's likely that your alarms use way more current than that. (you can measure the current if you like, putting your multimeter on Amps in series with the alarm ground to the pack ground (so you're not testing individual cell loads which makes it eaiser to do the test. ).

Plenty of these things power off the first cell or two, unfortunately, so they tend to imbalance packs if left connected very long.

I don't know that I ever spec-tested either of the LED alarm types I have here.


I haven't tested the LCD ones either, but they've been around long enough that I'm sure someone has--the Celllog, and the Battery Medic.




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Lipo Battery Low Voltage Buzzer Alarm 1-8S

Lipo Battery Low Voltage Buzzer AlarmUse for 1-8S Lipo/Li-ion/LiMn/Li-Fe Voltage detection precision: +_0.01V. Unit voltage display range: 0.5~4.5V Total voltage display range: 0.5~36V 1S test mode voltage range: 3.7~30V Low voltage alarm mode for 2-8S Alarm set value range: OFF~2.7~3.8V...

www.avtechtrading.co.nz

says that this kind

uses about 15mA. Internally it appears to be the same as the first LED one that I have, that I posted above.

If the cells were at storage voltage, that could be half of their capacity or less--you can figure out about where they were at by checking the discharge curve on the spec sheet for the cells (if there is one) for the voltage they started at, vs what they're at now. Alternately, recharge the cells monitoring Ah or Wh to see how much it takes to refill each cell, and then subtract that from the total capacity, and you have a guesstimate of capacity they started at when the alarms were plugged in. It's probably 2.5Ah or less.

If storage voltage was very low, such that capacity was very low, then even the lower-end estimate of alarm current could drain enough of that away to cause a problem.