Series batteries question

I am making a simple arduino HoverCraft(with a pillow) project. Currently I have got to the chassis, fans, power, and spinning testing, so it's coming along nicely.

Now I have faced one issue though. I think this might be related to some wrongness in my power circuit.

I am using 2 rechargeable 9Volt batteries to give me beefy 18-19 volts of output. I need most of this power, because each fan I use is 12Volt PC GPU mini-fan. I connect my battery cases in series, then connect resulting + and - leads to an efficient DC-DC power converter I have, then I connect the leads of the power converter to fans, arduino, anything else that might need power. I have configured power converter to operate at 11.85Volts, to avoid harming my arduino pro mini too much.

The issue though, is very weird. I was trying to do a full charge test, but my 350mAh/each batteries got drained in under 5 minutes! I have looked at power drains from fans and arduino, and I go only above 60mA at most(or at least I believe so...)! The weird thing was, the second battery which hands out the positive lead, was hot like hell! And when I checked the charges of them with a multimeter, I got really confused. The first battery, which eats the negative charge, was almost full (8.8V), meanwhile the second one was at -0.26V! Wow, now that's what I didn't expect.

How I connect my stuff:
Series batteries as mentioned
Batteries 18-19V -> DC-DC 12V
DC-DC 12V+ -> 4x Fans and arduino pro mini
DC-DC GND -> common gnd for everything

What could be the issue?

Thanks in advance!

Hi, can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?
Also a picture of your project to see how it is assembled.

You could get anothet DC-DC converter and set it up for 7V to drive the arduino, or set it for 5V and feed it to the 5V in the pro-mini.

The size of your batteries and the size, spec of your fans would also be appreciated.

Tom..Hope to help.... :slight_smile:

Here is a mock-up I did on my “blueprint” that I used for cutting out the chassis for my project.

I also put a legend to make it more understandable.

Nothing too over-complicated.

EDIT: The fans are 12V PC GPU fans, 0.1Amp each, 40mm X 40mm.

Murloc992:
I am using 2 rechargeable 9Volt batteries to give me beefy 18-19 volts of output.

It might technically be 18V but "beefy" ? :slight_smile:

Hi, if those batteries are the "transistor" type size, like what you put in smoke detectors then there is no way they are going to supply almost 0.5A for very long
You need to do a bit more research on battery capacity and halt any hardware development until you have it solved.
The batteries you will probably need are a bit heavier in mass and ofcourse will effect your overall perfomance.

The first battery, which eats the negative charge, was almost full (8.8V), meanwhile the second one was at -0.26V! Wow, now that's what I didn't expect.

I don't think you have got the idea of batteries in series, both batteries have the same current flow, the fact that one is lower than the other, in fact reverse polarity means it was not at the same charge level as the other, or it is faulty.

Tom....... :slight_smile:

Yeah, seems that the batteries are not as "heavy" as they should be. I have taken another 9V I got in my guitar pedal, and boy, it's at east 2 times heavier and only has 250mAh and they're all NI-MH! And I was tracking the battery voltage during a full test for the fans:

The first battery behaves OK: 9.5V -> 8.6V with no flaws, very slow drain
The second battery does not: 9.5V->8.6V OK, but then it hits a sudden jump, and rapidly discharges like 0.1 volt each step and over 10 seconds it hits 2Volts...

Time to get new batteries... :slight_smile:

Thanks for the help!

It is vital that when you put batteries in series they are of exactly the same type.

The fact that one battery got hot and the other did not means that the one that got hot had a higher supply impedance than the other due to them being either of different types or different states of charge or the capacity of one being down due to having had more use.

Batteries are the same brand, and appear identical. Full charged capacity also is the same. But I have bought them off E-Bay, and it seems like they are different by their usage or are just defective. Moral of the story: do not trust E-Bay when buying batteries. :smiley:

Many cheap things off eBay from the far east are sourced from the reject bin of manafacaturers. That is why they are cheap.

So maybe you know any trust-worthy brands on E-Bay? Or shoud I just stick to electronics stores? :~

I would stick to an Electronics distributor, like DigiKey, Mouser or Farnell (Newark )

I was reading about 9V Batteries and apparently there are 9V LI-ION batteries. Are they worth the buck?

Also, how could "negative charging" I have experienced earlier with my faulty battery be explained? It goes from 9.5V to 0V and then 0V to -4V and so on.

Murloc992:
I was reading about 9V Batteries and apparently there are 9V LI-ION batteries. Are they worth the buck?

Also, how could "negative charging" I have experienced earlier with my faulty battery be explained? It goes from 9.5V to 0V and then 0V to -4V and so on.

Consider your circuit as just a glorified charger for that duff battery. Consider the load as just a means to connect the power source (the good battery) to the duff battery. Draw that out and you'll notice that the power source is being connected back to front.

ie, it it actively DISCHARGING the duff battery.