Battery mA Query.

Irving

Joined Jan 30, 2016
3,190
Well theres no easy way to determine battery life on a 1:59 intermittent use as its not one of the standard IEC60086-1 & -2 tests. However you should be able assess likely life from manufacturer data (see attached). Looking at the data, your figure doesn't seem unreasonable.

You'd be better of with a Lithium rechargable. The lower internal impedance (10mOhm v 125mOhm) and flatter discharge curve would give better results and lower run costs, at the expense of added complexity/upfront costs.
 

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dcbingaman

Joined Jun 30, 2021
807
Well theres no easy way to determine battery life on a 1:59 intermittent use as its not one of the standard IEC60086-1 & -2 tests. However you should be able assess likely life from manufacturer data (see attached). Looking at the data, your figure doesn't seem unreasonable.

You'd be better of with a Lithium rechargable. The lower internal impedance (10mOhm v 125mOhm) and flatter discharge curve would give better results and lower run costs, at the expense of added complexity/upfront costs.
Thanks for the information! I need 5Vdc for the Wireless UART module, and 3.3Vdc for the microcontroller and other circuitry. Plan to have two regulators LDO's to generate that. With 1.5V D Cells I need at least 4 in series to get the necessary voltage for the LDO's possibly 5 to deal with lower voltage as batteries discharge. I will look into using rechargeable Lithium's. They do have to handle extreme temperatures. We can have well over 100 in the summer and well below freezing in the winter. Thanks for the pdf's! They are very informative. Appreciate your help.
 

Irving

Joined Jan 30, 2016
3,190
I will look into using rechargeable Lithium's. They do have to handle extreme temperatures. We can have well over 100 in the summer and well below freezing in the winter.
Lithium (all variants) are fine from -20°C (~0°F) to in excess of 50°C (~120°F). A non-rechargable Lithium primary D-cell has an energy density over twice that of an Alkaline-Manganese D-cell, about 17Ah. And its less impacted by cold than Alkaline, giving >70% nominal capacity at -20°C compared to 50%. Its also gives 3.7v rather than 1.5v, so requires half the # of cells. But it is rather more expensive.

A rechargable cylindrical 38120 LiFePO4 cell, giving 3.2v nominal @ 10Ah is approximately the same size as 2 D-cells 38mm v 33mm dia,120mm v 2 x 62mm long, but has an energy density almost double the alkaline cells. Trickle charging from a renewable source, eg solar and/or windpower is relatively straightforward. 2 cells in series would give 7v down to 5.0v @ 80%DoD, and is ideal to power a buck/boost regulator to give a stable 5.2v supply. Alternatively, prismatic cells are a similar volume but may be a more convenient form factor...
 

Irving

Joined Jan 30, 2016
3,190
@dcbingaman
Thinking more about your issue, it occurred to me that trying to assess capacity in mAh is incorrect. To do proper comparisons you need to look at energy capacity in Watt-Hours (Wh) which takes into account the changing voltage and therefore the losses in the LDO(s) when batteries are 'new'.

So you have 5 D cells giving 7.5v (5 x 1.5) initially and allowing them to drop to presumably ~5.25v (5 x 1.05v, for 5v on a 250mV LDO). You say use 40mA for 1sec. I'm not sure if you were measuring battery current or device current, but lets assume, best case, the former, however even the best LDO draws a few uA when "off load", say 50uA (and 1.5% of output current 'on load", or 0.6mA, which we'll assume is already in your 40mA). Initially, therefore you use 7.5 X 40mA = 300mW for 1sec, and 7.5 x 50uA = 0.375mW for 59sec, a total of 322mW average, or 64mW per cell. But of course voltage drops all the time so by the end you are only consuming (5.25 x 40mA +5.25 x 50uA x 59) = 225mW or 45mW per cell. Looking at the trend for the constant power ratings for the Duracell MN1300 D-cell at 1.05v, the cell has a capacity of approx 18Wh @ a constant 50mW, so this should give you around 360h run-time.

The 38120 lithium rechargeables are rated at 30Wh to a similar cut-off (80%DoD), so fully charged will give you almost 600h run time, with plenty of scope for recharging. Depending on your available recharge options, you could opt for much smaller cells, or (as I have done), run the 3.3v MCU directly off the battery (most current 3.3v MCU are quite happy from 3.6v down to 3.0 or even 2.9v), and boost the voltage to 5v for the transmitter as and when needed. This saves a whole cell (cost/space saving), makes charging/battery management a whole lot easier and makes the whole thing more energy efficient (saves an LDO and the losses in same), as long as you can find 30W over a 20-odd day period of charging... unless you're in a very strange situation (underground?) that shouldn't be difficult...

Happy to provide more input if needed...
 
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dcbingaman

Joined Jun 30, 2021
807
@dcbingaman
Thinking more about your issue, it occurred to me that trying to assess capacity in mAh is incorrect. To do proper comparisons you need to look at energy capacity in Watt-Hours (Wh) which takes into account the changing voltage and therefore the losses in the LDO(s) when batteries are 'new'.

So you have 5 D cells giving 7.5v (5 x 1.5) initially and allowing them to drop to presumably ~5.25v (5 x 1.05v, for 5v on a 250mV LDO). You say use 40mA for 1sec. I'm not sure if you were measuring battery current or device current, but lets assume, best case, the former, however even the best LDO draws a few uA when "off load", say 50uA (and 1.5% of output current 'on load", or 0.6mA, which we'll assume is already in your 40mA). Initially, therefore you use 7.5 X 40mA = 300mW for 1sec, and 7.5 x 50uA = 0.375mW for 59sec, a total of 322mW average, or 64mW per cell. But of course voltage drops all the time so by the end you are only consuming (5.25 x 40mA +5.25 x 50uA x 59) = 225mW or 45mW per cell. Looking at the trend for the constant power ratings for the Duracell MN1300 D-cell at 1.05v, the cell has a capacity of approx 18Wh @ a constant 50mW, so this should give you around 360h run-time.

The 38120 lithium rechargeables are rated at 30Wh to a similar cut-off (80%DoD), so fully charged will give you almost 600h run time, with plenty of scope for recharging. Depending on your available recharge options, you could opt for much smaller cells, or (as I have done), run the 3.3v MCU directly off the battery (most current 3.3v MCU are quite happy from 3.6v down to 3.0 or even 2.9v), and boost the voltage to 5v for the transmitter as and when needed. This saves a whole cell (cost/space saving), makes charging/battery management a whole lot easier and makes the whole thing more energy efficient (saves an LDO and the losses in same), as long as you can find 30W over a 20-odd day period of charging... unless you're in a very strange situation (underground?) that shouldn't be difficult...

Happy to provide more input if needed...
Thanks! I may consider your second idea of one cell for the microcontroller and one boost regulator for the wireless UART. Do you have any suggestions on a given DC boost regulator with very low power loss when off?
 

Irving

Joined Jan 30, 2016
3,190
Well I rolled my own, there are plenty of suitable chips. If you're looking for an off-the-shelf module its a case of finding one with a 'shutdown' input, or using a MCU-controlled MOSFET to turn the supply to it on and off. There's a few ways to do that.
 

Thread Starter

biferi

Joined Apr 14, 2017
342
Ok I just Watched a Video on Amp Hour and I think I have it Right?

They Talk about a Dabble A Battery that is Rated at 2,000mAH and this is also 2. AH I get this.

And they Say it can Supply 2. Amps for 1. Hour.

This is the Basics of Amp Hour.

So my Battery that is 12 Volts at 18 AH can Supply 18 Amps for 1. Hour if I Understand this Right?

And yes I do know not everything will use 18 Amps most things will use Less Amps Per Hour.
So the Amp Hour will be Different.

But this just Tells you what it could do not that this is what it will do.

Now do I have it Right?
 

crutschow

Joined Mar 14, 2008
31,146
Now do I have it Right?
Basically.
But as noted, the battery will likely not provide the rated AH when discharged at the 1 hour rate, as the rating is typically for a 20 hour rate.
Also for best battery life you should not discharge it to more than about 50% of its rating.
 

Audioguru again

Joined Oct 21, 2019
5,427
You said you have Dabble A batteries that might be AA Alkaline. The Energizer datasheet says they produce 2000mAh only with a 250mA load and the voltage drops as low as only 0.8V that is almost half the original voltage.

If you use a 2000mA (2A) load then they might last only a few minutes before overheating.
Look at the datasheet to see.
 

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