12V battery load and fan and RPM's - Page 1
Forum section: electronics / beginners

author: Wavedave
author_rank: Newbie
timestamp: September 11, 2025, 07:08:57 pm
content: Hello. I am installing a new 12V computer fan in a clam shell vent for a compost toilet on a boat. The old fan "crapped out", no pun intended.I did an online calculation using a 12V (50AH) battery and a 12V fan rated at .08 amps.The fan will run continuous and eventuality, I will hook it up to a 12V solar panel on the boat for trickle charging.It states it will run the fan for 410 hours, using a 12V 50AH battery. I plan on buying a brand name quality 12V Lithium battery.My question is, does that mean the fan will run at the same RPMs for 410 hours or when the voltage slowly drops over time, the fans RPMs will also drop. And if the RPM's drop how fast or slow does that occur?Thank you for taking the time to help me. See photos.

author: jwet
author_rank: Frequent Contributor
timestamp: September 11, 2025, 08:52:09 pm
content: Li Ion have a very flat discharge and Lead Acid have a decently flat discharge curve. The RPM of the fan will be proportional to the voltage but the range of discharge is only about 20% which is what your RPM variation will be. You could put a 10v linear regulator in series with the FAN (LM317 or discrete- LDO would be nice at end of life) to make the RPM constant (relatively). Running a 12v fan at 10v or so is also a lot quieter and extends fan life and you get "most" of the fan CFM.A couple of other things-1. The stated capacity of a battery is generally at a rate of C/20 meaning for a 20 hour discharge. Your .08 A load is about C/600. The battery life will be significantly longer perhaps as much as 10X or more, which really means you could you could use a smaller batteries. If your cell is at or above a 30 hour rate, you can expect much longer life. Power sonic and some of the vendors have capacities at different discharge rates- your time is conservative- probably very.2. You want to have some mechanism so you don't over discharge the battery. The rule for 12v lead acid is 10v (1.65v/cell) and LiIon is similar (2.5v/cell). Though its heavier and larger, an AGM Lead Acid might be superior to a LiIon. They do well in lightly loaded floating charge applications like your solar deal. LiIon like to be discharged at real rates and precisely charged. You can have a little comparator circuit that trips a PFET or similar at the threshold. The circuit can draw .5 mA or so forever without hurting the battery. If you over discharge either chemistry, its hard on them and can basically ruin them.3. I don't know if you need to continuously run the fan. It would seem like if the toilet flushes reasonably well, you could run the fan for 15 minutes or so after each use. You probably want to have a vent stack of some kind to vent the gases that are hopefully downstream or at your holding tank- the holding tank may be vented off the stern. If the toilet is water operated and has standing water in and a trap, you won't have much sewer gas coming back up. (hopefully).Good luck.

author: Wavedave
author_rank: Newbie
timestamp: September 11, 2025, 11:39:50 pm
content: jwet. Thank you so much for the detailed reply. It is much appreciated and I learned some things!

author: jwet
author_rank: Frequent Contributor
timestamp: September 11, 2025, 11:48:45 pm
content: You're welcome, use the thanks button, its my perverse way of measuring my impact. Take care.

author: jwet
author_rank: Frequent Contributor
timestamp: September 13, 2025, 03:48:33 am
content: WaveDaveThis experiece was kind of funny, Its sort of kismet that this question came up and I happened across it. I'm a lifelong sailor, put myself through an EE degree rigging boats on the Chesapeake bay- all manor of electrical, plumbing, running and standing rigging and even some engine work. I then went on to a career in electronic engineering and worked in analog design and did a lot of work with batteries and portable stuff. (at Maxim now Analog Devices). Since retiring, I built my own house with a retired custom home builder friend that taught me a lot- did all the foundation, framing, plumbing, tile work, finish carpentry, flooring and electrical- the only things I contracted out were drywalling and roofing. If you have more questions, I'd be happy to help. I didn't actually know what a composting toilet was- is this designed for the marine environment or are you adapting it? Marine toilets are miserable things that give most boat owners a lot of grief. Anyway, take care and good luck with your project.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 13, 2025, 01:35:36 pm
content: On the boat I used to have, you took a dump or a wiz in a bucket and tossed it overboard, (natural and biodegradable waste).

author: golden_labels
author_rank: Super Contributor
timestamp: September 13, 2025, 03:30:16 pm
content: Would RPM even matter?Due to my condition I must deal with a lot of foul smell in my toilet. Yet I rely on building’s natural ventilation only. Of course I’m aware a boat will not get the same air flow as a chimney in a tall building. But it still feels like very little flow is needed and a drop in RPM isn’t going to affect that.If I were to add any electronics, I would rather consider a timer.BILLPOD:Yes, and in the past people did the same in their homes. Fortunately more convenient options are available.

author: Wavedave
author_rank: Newbie
timestamp: September 13, 2025, 07:40:23 pm
content: That's definitely the route I'm going to take if the compost toilet isn't available or it's not working correctly

author: MrAl
author_rank: Super Contributor
timestamp: September 14, 2025, 03:18:34 am
content: Quote from: Wavedave on September 11, 2025, 07:08:57 pmSome notes...1. A straight up calculation shows 50/0.08=625 hours, but it's always a bit less than that. 400 would be a good target but that does sound low.2. The RPM's drop roughly as the cube of the voltage, but probably an exponent of 2.5 would be better.3. A lithium battery voltage will not drop that much at first, but then suddenly drop by a lot. You have to provide some kind of cut out protection or you could ruin the battery. That's the most important point because these things are expensive. You can also estimate run time better by incorporating the Peukert factor for that type of battery. As the battery ages though, the run time will decrease. Aging is related to temperature, current draw, and depth of discharge.4. You have to use a quality charger to recharge the battery. Also very important for long life and for safety.5. You could get a huge boost in run time by using a vent that allows free air flow when the boat is moving. If the fan is still in the picture too, it would use less power. If the fan is switched off, it will turn by the air flow which should not impede the air flow very much."Crapped out", kind of funny

author: Psi
author_rank: Super Contributor
timestamp: September 14, 2025, 03:32:42 am
content: You will probably find quite a difference in efficiency between different fans since they are usually not designed with efficiency in mind.So some testing of various fans might yield you some quite high gains in battery life with the same airflow.I would recommend a larger fan spinning slower and go with a brand that is well known to make good fans, noctua comes to mind.You also want it to have good bearings if it's going to be running a lot. So that's another reason to go with a good quality brand.I think noctua has full specs on their fans so you might be able to search for the best airflow vs watts.My gut says fans designed to be low noise will have better efficiency due to less energy lost as noise.

author: kite31
author_rank: Regular Contributor
timestamp: September 14, 2025, 03:50:53 am
content: Look also at AC Infinity fans. Some are ducted.

author: paulca
author_rank: Super Contributor
timestamp: September 14, 2025, 10:11:12 am
content: Depending on your local weather/climate....A PC fan pulling 12V @0.08A = 0.96W Lets call it 1W.My approach.. Multiply this out to get the kWh per 24 hour solar cycle.1W for 1 hour = 1Wh. So in a 24 hour period it will consume 24Wh.Multiply this by 10. 240Wh. Drop the h. You need 240W of solar panels.If you enjoy a lot of sunny days locally, your battery could be as small as 240Wh, smaller in the tropics. If you live somewhere like I do (Ireland) if you want the fan to run over January and Febuary you will probably need twice that.With a 12VLA or 4cell Lion equiv, 240Wh is only a 20Ah battery. Better 40/50Ah for the LA to not risk draining. A 50Ah LFP equivalent "drop in" would work on perfectly, with margin and it will last 10 years or more unlike the LA.It all seems to add up to be reasonable. Although a 240W solar panel is about a square meter/yard. If you have plenty of good weather, you could try skimping on a smaller panel.EDIT: If you have any other power systems on the board, such as an existing solar or even battery+shore power+alternator+generator setup, check the battery controller doesn't have a 12V utility output already. The Victron inverters used in many boats have a 12V lead acid charger output (Dseigned for charging the engine start battery independently) which can be used to float your LA from the "boat batteries" when needed.

author: Nominal Animal
author_rank: Super Contributor
timestamp: September 17, 2025, 06:21:12 pm
content: Quote from: Psi on September 14, 2025, 03:32:42 amSeconded,NF-A14 FLXin particular for example. They even give them 6 year warranties (when used in an enclosure).It does require you to have either an 140mm diameter opening with the fan, or a short section expanding to 140mm diameter. (These are easily 3D printed in e.g. ABS.) Running from (regulated) 12V directly, drawing max. 80mA / 0.96W, it will move 115 m³/h, or about 32 liters (over 1.1 cubic feet)per second. Even with ULNA adapter (11 dBA, quite quiet), it'll still move over 88 m³/h, or about 24 liters (or 0.8 cubic feet) per second. The airflow with ducting will be less than those figures (measured in free air), but still quite significant.You do need to ensure the fan will not be exposed to rain or very humid air (think shower or similar), because these ones are not intended for humid environments at all.The mains voltage and 12V dedicated ducting fans are, but they also cost much more if you aim for similar airflow and low noise operation.The 3-pin fans are easiest to control, because the third pin is just the tachometer return, two pulses per rotation, and you can just leave it flapping in the wind (or connect to ground via a 1k to 10k resistor). The LNA/ULNA adapters are just diodes or resistors that drop the voltage over the fan; the fan speed depending only on the supply voltage.It is not that much more effort to control 4-pin PWM fans, though. The fan speed is controlled by the duty cycle of around 25 kHz PWM open collector signal, pulled up to some voltage within the fan. Something likemy 12V PWM single fan controllerought to work just fine. Today, I'd probably redo the design using CH32X033F8P6 and have JLCPCB do both the PCB and the part assembly. That way, you could add more inputs to the microcontroller, say a microswitch or Hall effect sensor with a magnet, to detect e.g. door opening and closing, and program in timing patterns for the fan. Heck, even make them adjustable with another pot, remote one, where humans can fiddle with the timing and max power depending on the power of the chemical weaponry they deposit in the toilet. The CH32X033F8P6 has a ROM-based USB firmware uploader, so you could even reprogram it on the fly if you keep a laptop with you when you're on the boat.(It does really irk me when people insist on doing all kinds of tricks with diodes and whatnot to interface to fans, when the utterly robust, cheap, and simple method uses just one jellybean N-MOSFETs in SOT-23 (BSS138 or NX138AK) for tach, and another for PWM, and 2-5 pull-up and gate peak current limiting resistors. JLCPCB charges all of 0.02€ per NX138AK when assembled, LCSC 0.0254€ apiece in lots of 20, and Mouser 0.081€ in lots of 10, 0.038 in lots of 100, sopriceorcheapnessain't the reason for doing silly tricks here.)