Wiring help

I came up with a project idea that I need to wire some small fans together. I'm having an issue and am not sure what to do. The fans are all 12v 0.15 A. I also have some 4 AA battery holders.

I want to wire up 6 of the fans and have 4 battery holders. I was thinking of wiring 2 sets of the battery holders together and then wiring the 2 sets in parallel to get increased amperage at 12 volts. I'm very rusty when it comes to the calculations and I wanted to make sure that I'm not going to overload the fans, and if I need any resistors and what kinds.

I'm attaching a picture of what I currently have in my head for the wiring.

You've got your fans in series, which means each will only get 2V. You need to put them in parallel,

The 8 AA batteries in series will give you 12 V. Two groups 8 of in parallel is still 12 V and gives you twice the maximum current, or twice the running time for any particular current. As you only have 12 V, you can't overload the fans, but as Buk says, they have to go in parallel. The total current will be about 1 A so the switch needs to be able to take more than that.

Be aware that your fans will only run about 2 hours before the batteries are dead. The fans in total will draw a little less than an amp, so the drain on each battery bank (of 8 AA cells) will be about 500mA.

The graph shows the discharge curve for a number of AA battery brands at 500mA draw. The batteries all shown a bit over 1 amp-hour of drain before they die.

1 amp-hour = 1000mA-hour.

1000mA-hour / 500mA drain = 2 hours.

I would like the batteries to last longer than 2 hours. I should look into a rechargeable battery pack instead of the Alkaline cells. It has been 20 years since I did any electrical calculations so I wasn't sure about the effect of higher amps on the fans. I do have a 6V 15000 mAh rechargeable battery pack and can pick up a few more. Just have to figure out how to recharge as it didn't come with a charger. A model Ni3615T30P3S534416. If I can connect a USB cable to the circuit to charge it, this is what I came up with.

TRMerc said:

If I can connect a USB cable to the circuit to charge it,

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You need more than just a USB cable to charge a NiMH cell pack. That pack is designed to be fitted to a phone which has a charging circuit built in.

Also, if you intend to buy more, the prices from the place that one came from seem very high. Try these guys.

TRMerc said:

I do have a 6V 15000 mAh rechargeable battery pack

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Also, I think you're doing your math wrong. Those 3-cell packs are 3.6V 1500mAh (not 15000mAh); and if you put 2 in series to get 6V (7.2v) then they remain 1500mAh.

Put 4 in series to get 12V (14.4V) and they remain 1500mAh.

To get 6V @ 15000mAh, you'd need a 2s10p configuration, which is 20 packs @£22 = £440.

8 of these cells and 1 of these chargers would cost ~£75 and give you 14.4V @ 7000mAh.

I'd suggest a couple of other options:
Using 18650 lithium cells; three of those in series give a roughly similar (but slightly better) voltage range as the eight AA cells, starting just over 12V and down to aroudn 10V flat, rather than 8V for alkaline.

You can get holders to take various numbers of individual cells and multi-cell chargers are cheap. Good branded cells are typically 2.5AH or so.


If you want higher capacity, RC aircraft flight batteries are probably the next step up, eg. three cell 5AH packs are available on ebay.

The downside with going to any multi-cell lithium battery pack is they _must_ be charged using a proper balance charger that controls the individual cell volatges.
The RC battery packs have a small auxiliary connector that must connect to the charer in addition to the main power connections. That's used to measure and adjust each cell voltage.
Example:

I got the battery pack from a clearance store, so it only cost me $5. The store had it labeled wrong when I bought it, I guess. The holders I have are similar to the style that rjenkinsgb suggested. All I need is a battery type to use in them. Also, the 15000 was a typo, hit an extra 0 while typing.

I need a configuration that would last 8~10 hours, so cutting down the number of fans is possible. A heavy battery pack isn't an option as I will be carrying this for the entire time.

TRMerc said:

A heavy battery pack isn't an option as I will be carrying this for the entire time.

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6 x 18650 cells is ~300g.

Configured 3s2p it would give you 12.6V fully charged falling to ~10.8V and 5 to 6 Ah depending on the cells you chose.

Buk said:

6 x 18650 cells is ~300g.

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Yes, Li-Ion is remarkably light - I replaced the NiCd batteries in my cordless tools with Li-Ion and they weigh a fraction of what they did - yet have much higher capacity.

You need a circuit that detects when a Lithium battery cell has dropped below 3.0V then it disconnects the load. Some cells have a Protection Circuit inside that does it.

audioguru said:

You need a circuit that detects when a Lithium battery cell has dropped below 3.0V then it disconnects the load.

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This (from here) would do the trick: