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I'd like to occasionally be able to use a 1600W hairdryer for ~10 minutes. Maybe once a week, but only ever after a significant drive (and hence a well-charged starter battery.)

My starter battery is a 95Ah 12V lead acid battery. The inverter I'm looking at buying is pure-sine 2000W continuous (4000W peak.)

The alternator in my van (a Mercedes Sprinter) declares itself to be "14V / 180A", though I suppose that won't be a continuous 180A.

Inverter

Alternator

If I start the engine, switch the inverter on, and then use the hairdryer for 10 minutes, what is the expected outcome? Will it work? Will the van still start next time I crank it over? Will the lifespan of the battery be reduced, and if so how often might I expect to have to replace it?

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  • \$\begingroup\$ Depending on the alternator in your vehicle, It won't have any difficulty returning the battery to a full charge. Newer vehicles have MUCH higher outputs than those of the past. If you have a 180 amp alternator (180A * 12.6V = 2268 watts). These voltages are crude, alternators usually output somewhere between 12 and 18 volts depending on how fast they are being turned. In any event there is still the efficiency of your inverter. Either way, it the engine runs while you use your 1600 or so watts, it won't matter much. \$\endgroup\$
    – Wendall
    Mar 30 at 18:20
  • \$\begingroup\$ Oops, second question: Regarding the life of your lead acid battery. Batteries used in most vehicles will last the longest if we don't use more than 15% of their capacity before recharging them. In this situation you will not be depleting it at all, since it can generate power faster than you are using it. There are always a few other variables, the temperature of the battery, how well your alternator is working (they do have brushes in them) and how fast the alternator is being turned. \$\endgroup\$
    – Wendall
    Mar 30 at 18:28
  • \$\begingroup\$ You might be concerned about the current fed to the inverter. Drawing ~150 amperes takes heavy wire and rock-solid connections. Internal heating in the battery is also a concern-- they're meant to provide perhaps 100 A during starting, perhaps for 30 seconds, max, but will the alternator be turning fast enough to prevent damage to the battery? \$\endgroup\$ Mar 30 at 19:19
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    \$\begingroup\$ Could you put on safety goggles and open the side window fully while going along? \$\endgroup\$ Apr 2 at 10:00
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    \$\begingroup\$ You make a good point @mkeith. I just gave that as a ballpark figure. It actually maxes out at 2200W when the "turbo" button is pressed, and below that there's 4 different levels, ranging from 500W to 1850W. I'm going to put the inverter in and start at the low levels and see how we get on. \$\endgroup\$
    – Codemonkey
    Apr 5 at 8:52

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Alternator will supply up to 180 A only if you run the van motor at medium or high speed. In that case, with a proper installation, the battery will not be discharged.

If the motor runs at idle speed, the alternator output current will be much lower. Some alternators plates shows idle and rated speed current, for example 50/180 A, but we don´t have the data for this model. Assuming the idle speed current is 55 A, and the inverter current about 150 A, the battery will supply 95 A.

In this question you can see discharge curves of a lead battery for different currents. For a battery 95 Ah and discharge current of 95 A, you should use 1CA curve. A fully charged battery will drop to 12 V in 10 minutes, you will have about 60 % of capacity. You´ll find some information about battery life in the same link.

These are rough numbers, but with a simple multimeter, you can check battery voltage (without load) before and after a hairdryer trial. Search for relation between voltage and lead acid battery capacity and you can have real data.

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  • \$\begingroup\$ You have to let the battery sit for a while to get the true open-circuit voltage. The battery voltage recovers over a long time after a sharp discharge. \$\endgroup\$
    – mkeith
    Apr 2 at 18:53

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