Help with voltage math pls

[NeilW]

Please help me understand the voltage drop  on a 12v battery when a connected device is pulling 200 mA per day. Assuming the bike isn’t started or trickle charged, how long before it is down to 11 volts?  
( assuming a Tenere 700 battery)

 

Thanks friends. 

[Rider 101]

Assuming a 10AH battery with a load of 200ma then the battery should be discharged in 50 hours.

Life = capacity/load 

         =10/.2

          =50 hrs

 

Maybe from that and assuming a linear voltage drop then dividing 12 volts / 50 hours will come out at 0.24 volts / hour

So I would guess to drop from 12v to 11v would be roughly 4 hours.

 

Bear in mind I am a fitter so could be totally wrong!

[NeilW]

Just now, Rider 101 said:

Assuming a 10AH battery with a load of 200ma then the battery should be discharged in 50 hours.

Life = capacity/load 

         =10/.2

          =50 hrs

 

Maybe from that and assuming a linear voltage drop then dividing 12 volts / 50 hours will come out at 0.24 volts / hour

So I would guess to drop from 12v to 11v would be roughly 4 hours.

 

Bear in mind I am a fitter so could be totally wrong!

I have been furiously googling and found the same as above EXCEPT I have an LED volt display coming off my battery and with this new device attached to the battery directly- and the manufacturer says it draws 100-200 mA, the display hasn’t budged from 13.1 in 48 hours. 
That kills our elementary math….

but thanks for trying

[Rider 101]

I read you original wrong.

200mA / 24 hours = 8.3mAH

 

Life = 10AH/.oo83AH

         = 1024 hours

 

So 12v/1024hrs = 0.012v/hour which is roughly 83 hours to drop 1v

 

That makes more sense.

Edited December 14, 2023 by Rider 101

[Tenerider]

The reason it doesn't drop that fast is that the battery won't be discharged to 0V. One should stop discharging at 10.5V in order not to damage the battery.

 

Edit: @Rider 101 has got it right, I'm still sleepy

Edited December 14, 2023 by Tenerider

[Rider 101]

23 minutes ago, Tenerider said:

The reason it doesn't drop that fast is that the battery won't be discharged to 0V. One should stop discharging at 10.5V in order not to damage the battery.

 

Edit: @Rider 101 has got it right, I'm still sleepy

You're just getting up and I am on my 2nd beer! ( 5:30pm )

[NeilW]

I appreciate the calculations but the led display hasn’t budged in 48 hours. Started at 13.1v and still at 13.1 v 48 hours later. 

[Rider 101]

Then the current draw must be very low.

[NeilW]

Just now, Rider 101 said:

Then the current draw must be very low.

100-200 mA

[Rider 101]

Well I guess at 100mAH then 1v drop would take 160 odd hours.

[Tenerider]

7 minutes ago, NeilW said:

I appreciate the calculations but the led display hasn’t budged in 48 hours. Started at 13.1v and still at 13.1 v 48 hours later. 

Most likely voltage drop isn't linear. I know for other types of rechargeable batteries this is the case, they keep a high voltage for a long time and then drop relatively fast.

 

Had some cups of tea now, and I think the calculation regarding current draws and capacity might have some errors as well as mine had.

 

A current draw of 200mA = 0.2A for 1 hour requires a capacity of 200mAh = 0.2Ah (it's simply current by time, hence the unit Ah=Ampere*hours). Per day it would be 0.2A*24h=4.8Ah

@Rider 101 's first calculation is right. It should last 50 hours.

 

Total voltage drop should then be 13.8V-10.5V=3.3V (only if the battery really has 13.8V when fully charged).

Under the assumption that we have a linear voltage drop (which is unlikely, see first paragraph of this post), it would mean 0.066V of voltage drop per hour.

[NeilW]

Just now, Rider 101 said:

Well I guess at 100mAH then 1v drop would take 160 odd hours.

We will see

[Rider 101]

Keep the clock running and let us know!

[Tenerider]

And disconnect the connected device when the voltage drops to 10.5V - battery needs to be recharged then in order to not getting destroyed.

[NeilW]

It is now 10 hours after my last photo still 13.1v

 

for anyone curious, I am testing the SIZZAPP GPS locator/ tracker that has the constant draw because it is always alive and testing a new lithium battery at the same time. 
 

[Rider 101]

Maybe a trickle charger when parked in the garage.

[Hollybrook]

@NeilW did you measure 200mA (0.2A) or is that the rated current draw?  If it is the rated draw, it would be interesting to measure the actual power usage, as it appears to be less than that.

 

A 10AH battery would only last 50 hours...

[NeilW]

12 minutes ago, Hollybrook said:

@NeilW did you measure 200mA (0.2A) or is that the rated current draw?  If it is the rated draw, it would be interesting to measure the actual power usage, as it appears to be less than that.

 

A 10AH battery would only last 50 hours...

The manufacturer of the SIZZAPP says 150-250mA draw on a 12v battery. 
incident measure it myself. 

[winddown]

Bypassing the bikes circuity and connecting anything permanently to the battery is always a bit risky.  There are plenty of devices that can be placed between the Batt and device that will shut you off at 10.5.  Bear in mind though, 10.5V is an arbitrary cutoff, the battery will drop a lot more a lot faster if a heavy load is connected than a light 200mA load.

 

12V Battery Low Voltage Cut Off Switch

Adjustable voltage settings: With the press of a button, you can easily adjust both the turn-off and turn-on voltage values on our controller. This gives you the flexibility to customize the settings according to your specific requirements and the needs of your battery.

 

[Black99S]

Lithium battery has a completely different discharge profile from lead acid.

Edited December 18, 2023 by Black99S

[Black99S]

On 12/14/2023 at 12:31 PM, NeilW said:

The manufacturer of the SIZZAPP says 150-250mA draw on a 12v battery. 
incident measure it myself. 

Until you measure the actual current draw the conversation is based on quicksand and bullshit. Does the led stay on constantly or go out until prompted? What is the actual current draw?

 

[NeilW]

25 minutes ago, Black99S said:

Until you measure the actual current draw the conversation is based on quicksand and bullshit. Does the led stay on constantly or go out until prompted? What is the actual current draw?

 

Back up a bit. I have mounted a SIZZAPP directly to the battery. It is the on all the time. It is a GPS bike locator and also motion sensor alert. The led is plugged into my charger pigtail. It is the SIZZAPP that the SIZZAPP company says pulls 150-250mA continuously. 
So restating my original question, how long does it take for the SIZZAPP to pull the battery down 1 volt?according to the LED I lost 1/10 v in 5 days (before I went for a 300 miles ride)

 

[Black99S]

6 minutes ago, NeilW said:

Back up a bit. I have mounted a SIZZAPP directly to the battery. It is the on all the time. It is a GPS bike locator and also motion sensor alert. The led is plugged into my charger pigtail. It is the SIZZAPP that the SIZZAPP company says pulls 150-250mA continuously. 
So restating my original question, how long does it take for the SIZZAPP to pull the battery down 1 volt?according to the LED I lost 1/10 v in 5 days (before I went for a 300 miles ride)

 

Look at the discharge profile for the LiPo battery vs lead acid. 13.1V is 80% discharged for LiPo which you said you have installed. Still useful to confirm the actual battery draw of SIZZAPP regardless what the manufacturer states. a good Multi-meter is your friend here.

 

Edited December 18, 2023 by Black99S

[NeilW]

1 minute ago, Black99S said:

Look at the discharge profile for the LiPo battery vs lead acid. 13.1V is 80% discharged for LiPo which you said you have installed.

Yes it is a LiPO

[Tenerider]

6 minutes ago, NeilW said:

Yes it is a LiPO

Really a LiPo, not LiIon? I'd not run a LiPo on my bike, they are touchy little things. Use them in my RC crawlers, they are great for such applications, but imho not for situations where you don't regularly check them. A LiPo fire is not funny and not so unlikely as it may seem.

 

@Black99S thanks for the discharge curve, I wasn't aware that Pb batteries show an almost linear drop. As you pointed out, that's not the case for lithium-based batteries.