Running case fans on D cell batteries

Question

I'm moving into my car and I may need to wire up ventilation that doesn't depend on the engine running. So I found these case fans. However, because I'm not going to be using the car battery, I need a power source. I've been looking at a few power stations such as this one, but nowadays I tend not to trust obscure sellers on Amazon; regardless of the review rating, and even though the price is more than I want to spend it's far less than the going rate for the same specs, which screams to me "to good to be true".

Alternatively, I thought about just using rechargeable D cell batteries. The fan's max draw is 12V at 0.16A. Which is 1.92W each, assuming max load which I am. If I have 4 fans running while I'm sleeping, and let's assume a 10hr sleep, that is 76.8Wh or 6.4Ah at 12V.

I found these rechargeable D cells on Amazon. They're 10Ah each. If I wire up seven of them in parallel that should give me 70Ah at 1.2V or 84Wh; for anyone that knows what the usable percentage of that will be, that would be helpful. So it would appear that with six of these batteries fully charged, I'll have the energy to fully power the fans at full blast all night long. Is this correct? -- assuming they could run at 1.2V, which they can't.

Now, I'm going to have to step them up. And since we're assuming max load, that puts me at 7Ah at 12V. Will stepping it up reduce the energy enough such that I need to add more cells?

And finally, I'd like to be able to charge them while the car is running, transparently, i.e. without having to remove them from the circuit. Can they be both attached to the fan circuit and wired up to be charged simultaneously? Is running the fans off them and charging them at the same time possible?

So to summarize the important questions:

1) Are 7 of these cells enough assuming connecting them in parallel and stepping up the voltage to 12V?

2) Can they be both attached to the fan circuit and a charger at the same time?

That's all I got I think. Any help would be greatly appreciated!

Answer 1

The Fans you have selected are quite reasonable: you can find more data on them here.

Arctic characterize the fans this way:

1. For your battery I'd suggest you use a sealed 12V 10AH motorcycle Lead Acid type battery instead of buying NiMH 1.2V D cells. While the bundle your point to has a charger, it's mains powered. Connecting this type of NimH batteries to your car system is complicated and you need a specialized charger. With a Lead acid battery you can simply charge the battery from your cigarette lighter or other utility connection and not have any problems. If you are worried about charge current (and someone is sure to bring it up) you could simply add a series 0.2 Ohm series resistor to ensure the charge current never gets too high. Plugged into your cigarette lighter or utility connector you will be able to operate the fans and charge the battery at the same time.
   Since you seem to be using Amazon as your source here is the type of battery I suggest. There are lots of options, but your should be able to get a battery in the $20-30 range that adequately meets your needs.

Update: Here is a datasheet for an SLA battery. Depending on whether you use liquid or Gel electrolyte will dictate whether they can be used in any orientation. Most are Gel based and can be used in any orientation. The batteries do have a vent, but it can only be popped due to abuse (typically high temperature overcharging). They are considered indoor safe.

2. Since you'd like to speed control the fans you need a speed controller. I'd suggest that you DO NOT get a PWM speed controller as some BLDC fans may operate poorly using these. Notice here in the Arctic data they simply point out the operation of the fan at several DC voltages. Since you've pointed to a 3-pin fan you might be able to use a computer style PWM controller, but you need to get one that always starts the fan on a high voltage and then reduces it. Otherwise if you turn the unit on and the speed is set low, there is a chance the fan will stall.
   The fans you've selected have an amazingly low start voltage (2.6V) so maybe you'd be OK, but I would be nervous.
   Arctic have the F8 4 wire PWM fan (available on Amazon) for almost the same price but they are just slightly smaller. Here's a link to search Amazon for suitable speed controllers.

If you have not yet committed to the fans, I'd suggest that you get 4 wire fans. These have a separate speed (PWM) signal, and are internally intelligent enough to ensure they start at whatever speed you set.
The really nice feature is that you can run the fans on 12V (or at least the 13.8 - 10V range you use an SLA battery) all the time, and simply send a digital signal to tell them the speed to run at.
As you can see from the Arctic data on the F8, you don't need to change the supply voltage (which is what the high powered 3 wire PWMs do) and can still reduce the speed and the power consumption.

Arctic also show an F9 PWM fan, and it's not that much more expensive, but I don't see it for sale other than singly on Amazon.

Answer 2

BLDC Fans are pretty cool, pun intended. The load current rises with voltage so they have like a resistive load with some ripple current.

Thus any power calculations must be adjusted for actual voltage used. 12.5/12*76.8W =80W...leading to

76.8Wh or 6.4Ah at 12V

An average car battery in good shape is is 50Ah with 12V avg (12.5 to 11.5) thus offers 600Wh. But often car batteries are fickle and have lots of CCA starting power but unknown Ah due to acid specific gravity in each cell and metallic particles in the acid shorting out a cell briefly. They can start one minute then die shortly after when due for replacement.

recommendation

So a spare lead-acid battery perhaps mounted securely in the trunk, is your best bet purchased cheaper from a scrap yard.

The charge connection can be a headlamp relay and switch in series between batteries to indicate charge current, energized by ignition voltage from the cig, lighter or equiv.

This avoids large load surges on your alternator diodes, a frequent cause of faulty alternators.

In general, the best battery is closely matched to your load requirements for several days to allow for graceful ageing and include a compatible chemistry profile charger. Boost regulators are possible but do not add capacity so series-parallel arrays near the 12V target are best.