DIY Battery Pack

[Sjekke 87]

Prooblem : The battery of my Fibaro doorsensor drains fast

Reason : External garage not isolated, so in the winter is it’s very cold > drains the battery

 

My idea :

I want to make a EXTERNAL batterypack with several ER14250 or 26650 (also 3,6V but 9AH) batteries in Parallell 

Isolate the batteries in a leftover of PU isolation of my attic. 

 

Good idea or bad idea?

 

All remarks are welcome.

.

 

 

Edited September 3, 2018 by Sjekke

[Sjekke 87]

@petergebruers Maybe a question for Peter

[petergebruers 1,268]

I've been told some sensors use more power < 0 °C (which is out of spec) but I have never investigated this.

Your cell certainly performs worse when it is colder so that might indeed explain why you get less useful life from that cell.

 

Lot's of ideas discussed here:

 

 

I do not have any D/W sensor connected to a rechargeable battery... Oh, wait, I do have two running on a "rechargeable ER14250 3.7 V" but only since July. I don't expect those to last very long on one charge, because the measured capacity of that cell is only 260 mAh. That is less than 1/4th of an ER14250. But they have low internal resistance so they might be better at low temperatures.

[Sjekke 87]

47 minutes ago, petergebruers said:

I've been told some sensors use more power < 0 °C (which is out of spec) but I have never investigated this.

Your cell certainly performs worse when it is colder so that might indeed explain why you get less useful life from that cell.

 

Lot's of ideas discussed here:

 

 

I do not have any D/W sensor connected to a rechargeable battery... Oh, wait, I do have two running on a "rechargeable ER14250 3.7 V" but only since July. I don't expect those to last very long on one charge, because the measured capacity of that cell is only 260 mAh. That is less than 1/4th of an ER14250. But they have low internal resistance so they might be better at low temperatures.

 

Hi @petergebruersthx for the reply.

 

My sensor sometimes needs to work below 0 celsius but it keeps working but the battery drains extremly fast. Therefore My idea to make the battery external but isolated in PUR. The batteries don’t need to be rechargeable but can be normal ER14250. 

 

I’ll take a look at the link you provided.

 

Cheers.

 

 

[petergebruers 1,268]

9 minutes ago, Sjekke said:

The batteries don’t need to be rechargeable but can be normal ER14250.

They issue with the ER14250 is: as temperature drops, internal resistance increases. And that resistance is rather high to begin with. The sensor needs about 40 mA to transmit, if the resistance is too high, it'll drop below its lowest operating point. So it might not be really "empty" but it might be unable to supply sufficient current. As long as the device is sleeping, that is not an issue. But as soon as it has to report temperature or wakes up voltage drops -> transmission fails and sensor restarts.

 

To give you an idea of internal resistance, measured at 25 °C

 

Rechargeable ICR18650 advertised as 2200 mAh. Tested discharge capacity of these cells (I have 4) is 2300 - 2400 mAh. Internal resistance is below 0.080 ohm (that is the limit of my tester, it's probably a bit lower).

 

Primary ER14250 3.6 V "knock-off" from unknown supplier, advertised as 1200 mAh. Unusable in a Door/Window sensor. Internal resistance is between 15 and 30 ohm and that is too high. I do not have an original ER1450 but I've seen some graphs suggesting they might have about 10 ohm at 20 °C

 

You might think... 2 ER14250 in parallel give 2400 mAh so this would "beat" the 18650 rechargeable cell. But when you look at the internal resistance, that pair of cells would still have 7.5 - 15 ohm (versus 0.08 for the LiIon) which is OK at room temperature, but I guess it would fail near 0 °C

 

I would try (much) bigger primary cells or rechargeable ones because they have a lower resistance.

 

[Sjekke 87]

So noted!

 

tx