Can I add another battery in parallel to have more battery life?

Thread Starter

rambomhtri

Joined Nov 9, 2015
574
Hi, I just fixed a small bluetooth speaker (fits in your hand) and I noticed it has a small battery of 3.7V 150mAh, really simple rechargeable battery with a small protection circuit and just a positive and ground.

My question is... Can I simply solder 2 batteries in parallel (stack them together) so I have 300mAh in total?

I guess the charge circuit checks if the voltage of the positive and ground reaches about 4.20V, and once there, stops charging. So it would simply take more time to charge, but it would last about double of the original design. Is there any problem I might have thought about?

Will the batteries stay all the time more or less at the same voltage?
What if not? (is it even possible)

Batteries charge really slow (compared to today's smartphones and stuff) so heat is not a problem.

I've done sometimes battery enhancements of devices, like putting a battery with more mAh, but never used 2 stacked batteries (although I've thought about that for years "what if..."). Oh, the reason why I want to stack 2 is because the area I have to install the batteries is about 25mm x 20mm. I can't fit any 300mAh battery because they measure 35mm x 30mm or something like that, and the only way to increment is to stack 25x20 batteries.
 

dendad

Joined Feb 20, 2016
4,496
Just make sure the voltage of the batteries are about the same to prevent heavy current flow when you connect them together.
If they are off a fair bit, connect a seriies 100R resistor in series with one of the batteries until they equalize.
Or you could just replace the battery with a single larger one. Maybe even a 18650 cell?
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
574
Both batteries are the exact same model:

1676981656744.png

Both are about 25x20x5 mm, and I chose these because, as I explained, the area I have to install them inside the speaker is roughly 25x20 mm, and I have some spare room in height, where I can stack 2 together.

My question is... how parallel batteries work?
If one starts discharging at a lower rate, will they compensate or equalize themselves?
When charging them, what happens if one reaches 4.20V while the other one stays at 4.16V?
 
Last edited:

cmartinez

Joined Jan 17, 2007
8,276
My question is... Can I simply solder 2 batteries in parallel (stack them together) so I have 300mAh in total?
Yes you could ... but then the batteries would self-discharge a lot quicker. You see, imbalances in their voltages would make one battery charge the other one until they reach a state of equilibrium. But then after a short while the imbalance would reappear due to differences in their internal resistance and other factors (such as temperature, etc), and the process would start again. This would result in their spontaneous discharge without their ever being actually used.

1677000009567.png

The proper way to connect batteries in parallel is to use a diode in line with each battery to prevent the aforementioned phenomena. Schottky diodes would be best. This results in the loss of a couple of hundred millivolts due to the diodes' forward voltage. If this drawback is a problem for you, you can also resort to using either pFets or nFets for the same purpose. This would drastically minimize said losses. This thread is a good example of what I'm talking about.
 

KeithWalker

Joined Jul 10, 2017
3,145
I have never had a problem with batteries connected in parallel self discharging. They may do that in theory, but in practice, it does not appear to me to be a problem. I converted an electric drill and screwdriver to Iithium ion batteries connected in parallel several years ago. They get very intermittent use. They hold their charge and perform perfectly when switched on.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
574
Yes you could ... but then the batteries would self-discharge a lot quicker. You see, imbalances in their voltages would make one battery charge the other one until they reach a state of equilibrium. But then after a short while the imbalance would reappear due to differences in their internal resistance and other factors (such as temperature, etc), and the process would start again. This would result in their spontaneous discharge without their ever being actually used.


The proper way to connect batteries in parallel is to use a diode in line with each battery to prevent the aforementioned phenomena. Schottky diodes would be best. This results in the loss of a couple of hundred millivolts due to the diodes' forward voltage. If this drawback is a problem for you, you can also resort to using either pFets or nFets for the same purpose. This would drastically minimize said losses. This thread is a good example of what I'm talking about.
Wow, actually never thought about that... good point, however I have a counter argument, see if you agree...

Now that I think about that, let's say we have 2 identical batteries in parallel, same voltage same capacity, battery A and battery B.
Let's suppose battery A self discharges in 700h, and battery B self discharges in 800h. By self discharge I mean they get to a voltage point where the device won't turn on. We have 3 scenarios:

1. We use Battery A alone. Well, the device will not turn on after 700h.

2. We use Battery B alone. Well, the device will not turn on after 800h.

3. We use both batteries in parallel. My guess is that, since battery B will have slightly higher voltage than battery A, B will charge sometimes battery A, and the device will not turn on in a mix between the lowest and the highest endurance battery. In this case, my guess is the device will not turn on after about 730h.

My point is, connecting batteries in parallel will increment the capacity, and the self discharge rate will be somewhere in the middle of the lowest and highest endurance battery. In other words, self discharge won't be faster, quite the opposite, will be slower than the fastest self discharging battery. And... considering of course all batteries are okay, you will have a pretty nice endurance (yes, worse than the best battery, but slower than the worst)
 

KeithWalker

Joined Jul 10, 2017
3,145
My point is, connecting batteries in parallel will increment the capacity, and the self discharge rate will be somewhere in the middle of the lowest and highest endurance battery. In other words, self discharge won't be faster, quite the opposite, will be slower than the fastest self discharging battery. And... considering of course all batteries are okay, you will have a pretty nice endurance (yes, worse than the best battery, but slower than the worst)
You are correct. The batteries will not discharge each other. They will act like one battery with a capacity of the sum of the two separate ones. You definitely don't need any diodes in the circuit.
 

SyedBasit

Joined Jan 1, 2024
1
Yes you could ... but then the batteries would self-discharge a lot quicker. You see, imbalances in their voltages would make one battery charge the other one until they reach a state of equilibrium. But then after a short while the imbalance would reappear due to differences in their internal resistance and other factors (such as temperature, etc), and the process would start again. This would result in their spontaneous discharge without their ever being actually used.


The proper way to connect batteries in parallel is to use a diode in line with each battery to prevent the aforementioned phenomena. Schottky diodes would be best. This results in the loss of a couple of hundred millivolts due to the diodes' forward voltage. If this drawback is a problem for you, you can also resort to using either pFets or nFets for the same purpose. This would drastically minimize said losses. This thread is a good example of what I'm talking about.
In this circuit how am I supposed to recharge them ? I will need a bypass
 

BobTPH

Joined Jun 5, 2013
9,160
@rambomhtri: Listen to @KeithWalker, he is the one giving you complete and accurate information. There is no problem paralleling two batteries, as long as the start at the same voltage they will them stay at the same voltage. They cannot do otherwise, since the parallel connection forces them to be at the same voltage. If their capacities differ, they will automatically divide the current proportonally.
 

drjohsmith

Joined Dec 13, 2021
852
Yes you could ... but then the batteries would self-discharge a lot quicker. You see, imbalances in their voltages would make one battery charge the other one until they reach a state of equilibrium. But then after a short while the imbalance would reappear due to differences in their internal resistance and other factors (such as temperature, etc), and the process would start again. This would result in their spontaneous discharge without their ever being actually used.


The proper way to connect batteries in parallel is to use a diode in line with each battery to prevent the aforementioned phenomena. Schottky diodes would be best. This results in the loss of a couple of hundred millivolts due to the diodes' forward voltage. If this drawback is a problem for you, you can also resort to using either pFets or nFets for the same purpose. This would drastically minimize said losses. This thread is a good example of what I'm talking about.
How does the op connect the charger with this schematic ?
 
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