Battery Questions

Satoshi Deguchi · Jul 19, 2023

Hello,

I have some battery related questions.

According to this website, connecting batteries in series increases the voltage and connecting them in parallel increases current. Does this mean if I connect two AA sized batteries in parallel, with a maximum discharge rate of 5amps, the maximum discharge rate will be 10amps?

Context:
I plan to modify one of my flywheel Nerf blasters using AA sized LiFePo4 batteries from the OutOfDarts shop. The biggest roadblock is finding AA sized (or any that aren't Li-Po) batteries that have 10+ amps of discharge to meet the demands of the Honey Badger motors (22amps for a pair according to a modder). I've made the decision to not use Li-Po due to my concerns about it's safety/stability.

audioguru · Jul 24, 2023

The low power LiFeP04 AA size batteries are too weak to power his high current motors:

Satoshi Deguchi · Jul 26, 2023

audioguru said:
The low power LiFeP04 AA size batteries are too weak to power his high current motors:

Those are the batteries, but I am looking into ways to increase the current so the motors can start up. I'll be posting a thread dedicated to questions in relation to that.

rjenkinsgb · Jul 26, 2023

Good quality protected 18650 cells are easy to find.

eg. A few seconds on google:

Samsung 35E 18650 3500mAh 8A -Protected Button Top Battery

Get the Samsung 35E 18650 3500mAh 8A protected button top battery for reliable power. Perfect for flashlights, doorbells, and more. Ensure safety and compatibility.

www.18650batterystore.com

Use cell holders and a separate cell charger; that avoids the other common problems due to trying to incorporate charge circuits in a device.

rjenkinsgb · Jul 26, 2023

To boost the available start-up current, you could connect a couple of these across the battery (assuming that is 4.2V max).

Each one gives an extra 8A peak current.

https://www.mouser.co.uk/ProductDetail/KYOCERA-AVX/SCMT32D755SRBB0?qs=qSfuJ%252Bfl%2Fd7f1WRUHFjPrg%3D%3D

Do be aware that when the gun is firing, the motors will be under some load and the current rather higher than idle - and that must be within the battery continuous ratings.

Satoshi Deguchi · Jul 26, 2023

rjenkinsgb said:
To boost the available start-up current, you could connect a couple of these across the battery (assuming that is 4.2V max).

Each one gives an extra 8A peak current.

https://www.mouser.co.uk/ProductDetail/KYOCERA-AVX/SCMT32D755SRBB0?qs=qSfuJ%252Bfl%2Fd7f1WRUHFjPrg%3D%3D

Do be aware that when the gun is firing, the motors will be under some load and the current rather higher than idle - and that must be within the battery continuous ratings.

Is this a capacitor? I theorized that using one to discharge enough current to get the motors up to speed could work but am unsure of how to wire it.

Diver300 · Jul 26, 2023

Satoshi Deguchi said:
Is this a capacitor? I theorized that using one to discharge enough current to get the motors up to speed could work but am unsure of how to wire it.

In parallel with the batteries.

You must not exceed the rated voltage or the capacitor will be damaged and may explode. The capacitors are 5.4 V like the ones in the link, you should connect the capacitor in parallel with one cell, or cells that are in parallel. Do not connect a 5.4 V capacitor across any series combination of Lithium cells.

rjenkinsgb · Jul 26, 2023

Satoshi Deguchi said:
Is this a capacitor?

It's "Supercapacitor", an electrochemical device that acts like a massive capacitor - Five FARADS for the one in the link;

ie. five million microfarads.

Two or three of those, in parallel with a 4.2V battery pack, would boost the peak current to the type of range you need.

Basic electrical definition:
One farad with 1A current (= one coulomb of charge per second) will change voltage (charging or discharging) at 1V per second.

With 5F and 5A load per device(shared over as many as needed), if the motors start in 1/10th of a second, the voltage would only drop 100mV during startup & the battery would top up the supercap while the motor idled.

danadak · Jul 27, 2023

One caution, at battery end of discharge, hence its DC ESR rises, would turnoff
of motor create HV transient and blow the supercap ? One might think of adding a
TVS diode also in parallel across motor winding or battery to protect cap.

Regards, Dana.

Satoshi Deguchi · Jul 27, 2023

rjenkinsgb said:
It's "Supercapacitor", an electrochemical device that acts like a massive capacitor - Five FARADS for the one in the link;

ie. five million microfarads.

Two or three of those, in parallel with a 4.2V battery pack, would boost the peak current to the type of range you need.

Basic electrical definition:
One farad with 1A current (= one coulomb of charge per second) will change voltage (charging or discharging) at 1V per second.

With 5F and 5A load per device(shared over as many as needed), if the motors start in 1/10th of a second, the voltage would only drop 100mV during startup & the battery would top up the supercap while the motor idled.

How do I trigger the release of the super capacitor when the motors start up?

How would I wire this up?

danadak · Jul 27, 2023

Why disconnect, leave them electrically attached to recharge. But you
would have to insure the back EMF does not rise above super cap
ratings.

Regards, Dana.

rjenkinsgb · Jul 27, 2023

Satoshi Deguchi said:
How do I trigger the release of the super capacitor when the motors start up?

How would I wire this up?

No "trigger".

They would connect directly in parallel with the two (?) parallel 18650 cells. Think of them as like extra cells, that share the load.

Satoshi Deguchi · Aug 1, 2023

How do I calculate the maximum discharge rate of a battery? The battery linked below doesn't have any specified discharge rate in amps. I need to figure this out in for motor selection.

12v Ni-MH
Ni-MH Battery Charger

Charging Rate: 500mA
Battery Capacity 2400mA

Also, would there be any issue in replacing the battery charger's connector with an XT-30 or XT-60 and doing the same to the battery?

danadak · Aug 2, 2023

Just a thought but one would think if a part does not have a spec of
concern find one that does .....? Battery construction, chemistry, all factors
in this. Point is spec has underlying (hopefully) support for desired performance.

Regards, Dana.

audioguru · Aug 2, 2023

Amazon sells cheeep low quality no-name-brand junk. I doubt the battery can produce the very high current you need.

Satoshi Deguchi · Aug 2, 2023

audioguru said:
Amazon sells cheeep low quality no-name-brand junk. I doubt the battery can produce the very high current you need.

I'll have to look elsewhere then.

Which sites do you suggest I check out for batteries? Looking for Ni-MH, Li-Ion, LiFePo.

Lightium · Aug 2, 2023

You can buy NiMH batts anywhere. Try hobby stores for lithium batts.

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