What size inverter for battery charger?

I have an ebike battery. 48v 17.4ah. I want to be able to charge it in my car from the "cigarette lighter" port. How big of an inverter would I need to charge it with the car running? 200w, 400, or 1000w? Charger is 2.0a 54.6v

Thanks in advance!

2.0a times 54.6v = 109.2w but, there may be more to it than that. I'm no electrician.

Yes, a 200w inverter would cover it, but you need to check the current rating of your car sodket. If you draw the whole 200w (which for safety you must take into account) the current draw is nearly 17A, so ideally around a 25A circuit will be needed to avoid oveheating the wiring loom.
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Most cigarette lighter sockets have a 10 amp fuse. You'd need to replace that (if it's safe to do so).

WalkIntoTheLight said:

Most cigarette lighter sockets have a 10 amp fuse. You'd need to replace that (if it's safe to do so).

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I can answer this. No, not safe. Disregarding the wire's ability, the lighter socket is simply not good enough to deliver that current for long periods of time. I have a 12v spray rig that pulls a lot of power. Comes really close to melting a standard socket after a while.

mdmountainman said:

I have an ebike battery. 48v 17.4ah. I want to be able to charge it in my car from the "cigarette lighter" port. How big of an inverter would I need to charge it with the car running? 200w, 400, or 1000w? Charger is 2.0a 54.6v

Thanks in advance!

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The specs from the charger would be really useful, but simply multiplying the amps and volts yields 109 watts. Divided by 12v lighter socket yields 9.1 amps. Adding 20% for efficiency losses yields 10.9 amps.

That's pushing what you can pull from a lighter for sustained periods of time. I think the fuse it usually 15 amps, but the socket itself will start to heat up.

To answer the inverter question... looks like at least 150 watts. FYI, most car inverters are square wave or modified sine wave. Some chargers do NOT like this and will malfunction.

Here's one in your watt range, pure sine wave also.

https://www.amazon.com/BESTEK-300Watt-Power-Inverter-Adapter/dp/B07KQ4Q2L5/ref=sr_1_6?crid=2DIVE7V6XRPXS&dchild=1&keywords=150+watt+pure+sine+inverter&qid=1608915889&sprefix=150+inverter+sine%2Caps%2C171&sr=8-6

Run a properly sized pair of wires through the firewall to the starter / alternator circuit.

Use Anderson connectors.

Thanks for the info. It sounds like a dedicated hard wire to the battery is the safer bet. This question was actually for a buddy of mine with a full sized pickup. My Jeep grand Cherokee has. 115v 150w inverter built into the console. It's amazing that all modern vehicles don't have that as standard.

I'd get a 200 watt true sine wave inverter and have it wired directly to the car battery. Wagan makes a decent one for about $120. Cheap modified sine wave inverters don't work for running battery chargers. True sine wave inverters under 200 watts are very hard to find. Most car 12 volt sockets are rated for 120 watts. Changing the fuse to a higher amperage will not make the wires thicker to carry more amperage and doing so can burn the wires and cause a fire hazard. Don't do it.

Hooked on Fenix said:

I'd get a 200 watt true sine wave inverter and have it wired directly to the car battery. Wagan makes a decent one for about $120. Cheap modified sine wave inverters don't work for running battery chargers. True sine wave inverters under 200 watts are very hard to find. Most car 12 volt sockets are rated for 120 watts. Changing the fuse to a higher amperage will not make the wires thicker to carry more amperage and doing so can burn the wires and cause a fire hazard. Don't do it.

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Inverters are much easier to find now, with sine wave, and under that watt range.

mdmountainman said:

Thanks for the info. It sounds like a dedicated hard wire to the battery is the safer bet. This question was actually for a buddy of mine with a full sized pickup. My Jeep grand Cherokee has. 115v 150w inverter built into the console. It's amazing that all modern vehicles don't have that as standard.

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Yes, if you are willing to do that, that is the best solution. Don't forget to include a fuse in the wiring, sized to protect the wiring from the battery to the inverter. You choose the wiring and the fuse.

The e-bike battery is 835 W-h, so charging at 120W is going to take over 9 hours to charge from a fully discharged battery. The equation is 825 W-h/120W/80% efficiency = 8.6 hours. In reality a full charge takes somewhat longer than this, but the additional time only adds a small amount of capacity.

Charging at higher power levels can significantly reduce the charge time, but it's not proportional. Calculate the charge time to 80% of full charge. Charging from 80% to 100% of full charge typically takes 2-3 hours regardless of charger capability.

Some have suggested tapping off the alternator output. While I'm no expert, I'm pretty sure this is so that the charger can only operate when the vehicle is running. Charging with the vehicle not running could leave you without enough left to start the vehicle. In easy conditions, given the large battery typically found in full sized pickups, this might not be an issue if the battery is new and strong. But adding in battery age, size, and environmental conditions, you could easily run into problems.

Again, not an expert, but I think there's not a huge difference between tapping off the alternator or the battery if you are careful to ensure you keep enough charge to start the vehicle. In general, tapping off the alternator output should be more efficient if you bypass the losses between the alternator and the battery.

In today's vehicles, you might not have access to the actual alternator output. If the rectifier and regulator are integrated, by the time you get out of the physical alternator package, that might be the same circuit as the battery itself. Maybe some auto experts can chime in here.

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Another option is a Deep Cell battery in your car & using a pure sine inverter: that'll be plenty strong for your charging.
https://www.amazon.com/dp/B00H8N97E2/?tag=cpf0b6-20

..then you can pull the deep cell out of your car & throw a trickle charger on it at your convenience.

I use a DC charger & a deep cell to charge my Li-Po units in the field (at 200~300Watts),, couldn't be a better setup.
it's a similar concept to you bike battery charging.


Add: the only thing you have to make is a couple of cables to connect you deep cell to your inverter,, just use super flexible welding cable