Math check: Power consumption of aftermarket car heated seats, KOEO.

[Nayov]

I run aftermarket heated car seats during my lunch hour at work with the engine off, key on. Other cars in the rotation I don't think about because they have 24F batteries and the 1 hour of usage never affects starting at the end of the workday.

Now though I've added a Fit to the mix with an appropriately-sized "clown car"battery, 151R. (I know it's upgradeable to a slightly larger size 51R, but the dealership installed a new NAPA 51R, so I'm going to ride it out until it dies.)

That battery has a 55 amp hour rating. Seats use 45 watts, so 3.75 amps (45/12). I found a calculator on line and it showed 14.6 hours (55/3.75) until the battery goes flat. Is that correct? Doesn't sound to me that it could even remotely last that long. I do run it for the hour and so far so good but if I had no jump box available I wouldn't risk it.

 

[Skippy722]

55Ah seems a bit high for that small of a battery, I found a NAPA 151R battery with a reserve capacity of 55 minutes? Usually only deep cycle batteries give Ah.

It should be okay. Reserve capacity is for how long the battery can supply 25 amps, at 80F, without dropping below 10.5V. If it is 55Ah, then theoretically yes it can do 14’ish hours, obviously less the colder it gets but still plenty for your use.

 

[dogememe]

I was under the impression you could divide reserve capacity by 2.4 to approximate Ah. I don’t think that small battery has 55Ah capacity!

 

[dnewton3]

While I do agree with the calculations Skippy722 showed, there are presumptions baked into that which may not accurately represent your sitation.
- it's not 80F if you're using the seat heaters; probably much colder which affects battery performance
- some engines needs more than 25A to turn the engine over; your little Fit engine probably takes way less than a BBC, obviously
- 10.5v may not be sufficient to fully engage the starter properly; some systems won't engage the starter properly even at 11.5v
Hence, the reserve capacity calculation Skippy722 shows is completely accurate, but it does NOT necessarily reflect the conditions any one car engine will need to start. It's just a means of "apples to apples" rating of batteries.

However, I do agree that a hour's worth of seat-heat is unlikely to cause a no-start condition under the circumstances you describe, PRESUMING the subsequent drive is sufficient to re-charge the batter to a good state each time. Don't forget that into your calculation. If you're only driving 5 min to/from home each day, after an hour depletion at lunch each day, eventually you'll use more than you replenish.

 

[Danno]

Alternators are made to maintain a battery charge, not bring a battery up from significant, continuous drains.
Are we looking at a shortened life for this component?

 

[JHZR2]

I run aftermarket heated car seats during my lunch hour at work with the engine off, key on. Other cars in the rotation I don't think about because they have 24F batteries and the 1 hour of usage never affects starting at the end of the workday.

Now though I've added a Fit to the mix with an appropriately-sized "clown car"battery, 151R. (I know it's upgradeable to a slightly larger size 51R, but the dealership installed a new NAPA 51R, so I'm going to ride it out until it dies.)

That battery has a 55 amp hour rating. Seats use 45 watts, so 3.75 amps (45/12). I found a calculator on line and it showed 14.6 hours (55/3.75) until the battery goes flat. Is that correct? Doesn't sound to me that it could even remotely last that long. I do run it for the hour and so far so good but if I had no jump box available I wouldn't risk it.

The 55Ah are only achieved at a 10-20 hour discharge rate. 0.1C is 5.5A (assuming 55Ah is right, seems high for that tiny battery), 0.05A is around 2.75A. So you can probably do it, once. My concern is thst an alternator doesn’t fully charge a battery, so unless you’re topping it off, you will remain at sequentially lower states of charge, and the battery will sulfate and fail faster…

You need to really determine the correct Ah rating of that battery. The higher the rate (current draw relative to actualmAh of the battery) the poorer it will perform and bigger an issue it will be.

 

[eljefino]

How did you come up with the 45 watts? If they're rated for that at 13.8 volts running they will draw less at 11.x volts sitting.

I'm going to disagree with dnewton, you won't eventually run flat. The further you discharge a battery the lower its internal resistance will be, and the faster it will replenish via the alternator. But, you'll be in a situation where you might be at 75-85% state-of-charge all the time which is not great for the battery's longevity. (This is probably where postal vehicles run, as they get 30-45 seconds of run time for every start.)

If you're running the heater from your cigarette lighter there's an upper design limit of 10 amps, probably less than that, accounting for the cheezy connection. Your load is like a factory car radio, which plenty of people waste time using. Plus, when summer comes, your battery will get fully charged. If you go for a drive on the weekend, or if you throw a charger on it once in a while, it will help prevent suflation.

The only potential flaw is if your honda uses some smart charging strategy that only charges to 14 volts when the brakes are on, but otherwise runs at a float voltage.

 

[spackard]

Aftermarket seat heater?
LiFePO4 battery 12.8V 6Ah, $27.99
3A charger, $19.97
Female cigarette lighter, bare wires, $5.65
Have to crimp two 1/4" female spade onto the bare wires.

It's a bit of a hassle to take it out to recharge daily.
You could money up on the battery a little to avoid daily recharges. A 12Ah is $54.99.