Alternatives to 1.5v alkaline batteries are hard to find for some devices.

Thread Starter

us202000

Joined Jul 5, 2011
25
Hi, All,

I have come across several devices that can only run on 1.5v alkaline batteries, not the rechargeable batteries, for example, some blood pressure monitors, pulse oximeters, and other devices that run small motors that would require either > 2A current, or near 1.5v per battery. The rechargeable batteries are typically at <1.3v, too low for these devices.

What are the alternatives ? I was hoping that buck/boost converters might work, but they may not, because they typically can handle current at 2A level, still not enough to drive small motors with load. Or the battery itself may have a current limit. I tested a boost converter today using a USB battery bank and it turns out that the battery itself has a current limit at 2A, not enough to keep a sprayer motor running which requires 4x1.5v alkaline batteries.

Any ideas? Thanks in advance.
 

Reloadron

Joined Jan 15, 2015
6,963
4 each 1.5 volt AA alkaline batteries can be 6 volts if the batteries are in series. A good high quality AA alkaline battery may be rated between 1500 mAh and 2000 mAh. You will get an hour of run time on a good day maximum. So are your four batteries in series or parallel because if in series as one would expect you need a 6 volt source capable of 2.0 amps plus some overhead so about 6 volts at 3 amps from your source. You never mentioned AAA, AA, C or D size batteries so I used AA purely as an example. You still want a battery source of power?

Ron
 

Papabravo

Joined Feb 24, 2006
19,284
The other problem with a boost converter is that they require more input current than the output current they can deliver at the boosted voltage. The immutable rule of DC-DC conversion schemes is: The output power will ALWAYS be less than the input power. Sometimes it will be much less. Do a back of the envelope calculation of how this might work assuming your converter is 85% efficient, and going from 1.2V to 1.5V

\( 1.5\text{ Volts }\times 2\text{ Amperes }=\;3\text{ Watts} \)
\( \cfrac{3 \text{ Watts}}{0.85}\;\approx\;3.53\text{ Watts} \)
\( \cfrac{3.53\text{ Watts}}{1.2\text{ Volts}}\;\approx\;2.94\text{ Amperes} \)

Things get real quickly at these levels.
 

BobTPH

Joined Jun 5, 2013
5,759
On the other hand, a buck converter can produce more current than it draws, so a 7.2V lithium battery with buck converter is a good alternative.
 

Reloadron

Joined Jan 15, 2015
6,963
The other problem with a boost converter is that they require more input current than the output current they can deliver at the boosted voltage. The immutable rule of DC-DC conversion schemes is: The output power will ALWAYS be less than the input power. Sometimes it will be much less. Do a back of the envelope calculation of how this might work assuming your converter is 85% efficient, and going from 1.2V to 1.5V

\( 1.5\text{ Volts }\times 2\text{ Amperes }=\;3\text{ Watts} \)
\( \cfrac{3 \text{ Watts}}{0.85}\;\approx\;3.53\text{ Watts} \)
\( \cfrac{3.53\text{ Watts}}{1.2\text{ Volts}}\;\approx\;2.94\text{ Amperes} \)

Things get real quickly at these levels.
No free ride. Good point.

Ron
 

Papabravo

Joined Feb 24, 2006
19,284
On the other hand, a buck converter can produce more current than it draws, so a 7.2V lithium battery with buck converter is a good alternative.
I thought we were talking about single battery solutions. Was I mistaken in drawing that inference?
EDIT: it looks that way since the TS mentions batteries, plural. Never mind.
 

Thread Starter

us202000

Joined Jul 5, 2011
25
4 each 1.5 volt AA alkaline batteries can be 6 volts if the batteries are in series. A good high quality AA alkaline battery may be rated between 1500 mAh and 2000 mAh. You will get an hour of run time on a good day maximum. So are your four batteries in series or parallel because if in series as one would expect you need a 6 volt source capable of 2.0 amps plus some overhead so about 6 volts at 3 amps from your source. You never mentioned AAA, AA, C or D size batteries so I used AA purely as an example. You still want a battery source of power?

Ron
I forgot to mention that the goal is to replace Alkaline batteries (2x1.5v or 4x1.5v) with rechargeable. What I found is that there is no easy solution: either voltage or current are not up to the level of 1.5 Alkaline batteries, but this is precisely some devices are designed to require so would not work with rechargeables.
 

Papabravo

Joined Feb 24, 2006
19,284
I forgot to mention that the goal is to replace Alkaline batteries (2x1.5v or 4x1.5v) with rechargeable. What I found is that there is no easy solution: either voltage or current are not up to the level of 1.5 Alkaline batteries, but this is precisely some devices are designed to require so would not work with rechargeables.
As has already been noted 4 rechargeables and a buck converter can replace 2 Alkaline batteries, 6 rechargeables and a buck converter can replace 4 Alkaline batteries. Not exactly ideal, but potentially workable.
 

Papabravo

Joined Feb 24, 2006
19,284
Alternatively, a single AA Lithium battery is good for 3.6 volts. So potentially two of those and a buck converter will replace two Alkaline batteries.
 

BobTPH

Joined Jun 5, 2013
5,759
Yes, those are lithiuum batteries plus a buck converter and charge controller. Not sure what the current capability is.
 

Thread Starter

us202000

Joined Jul 5, 2011
25
As has already been noted 4 rechargeables and a buck converter can replace 2 Alkaline batteries, 6 rechargeables and a buck converter can replace 4 Alkaline batteries. Not exactly ideal, but potentially workable.
Thanks! I was thinking to use 5 rechargeable 5x1.25=6.25v to replace 4 alkaline batteries at 6v. Is this doable?

Alternatively, I was also considering using a 2A 20v lithium trimmer battery with buck converter, although it might be an overkill, and a bit bulky.
 

Papabravo

Joined Feb 24, 2006
19,284
Thanks! I was thinking to use 5 rechargeable 5x1.25=6.25v to replace 4 alkaline batteries at 6v. Is this doable?

Alternatively, I was also considering using a 2A 20v lithium trimmer battery with buck converter, although it might be an overkill, and a bit bulky.
Yes! Buy the buck converter as a module. Make sure you can get the output voltage you want from the complete system. 0.25V might not be enough of a difference between input and output, and not all buck converters can run at high duty cycles (over 50%). Don't waste your time trying to design and build just one. Fine if you're going to make at least 100 of them.
 
Last edited:

Alec_t

Joined Sep 17, 2013
13,153
You might also get away with just two lithium cells and a couple of silicon power diodes in series for some applications.
 

Ian0

Joined Aug 7, 2020
6,293
A single lithium cell and a dummy battery perhaps?
I would think that most equipment designed for two AA cells which are about 3.3V when brand new, might be OK with 3.6V from a Lithium cell.
 
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