Charging E-bikes from a 12v source - math check?

[Martinjmpr]

So I admit it, math and science were never my strong subjects in school. My undergrad major was history and I literally took the least amount of math and science I had to in order to graduate.

Anyway, we have E-bikes now and I'm curious about keeping them charged up on the road. On a typical Thursday-Sunday trip, it's not an issue - I charge them fully before we leave and that's usually adequate for a weekend. But for longer trips, I'd at least like to explore the option of being able to charge them either in the truck while driving, or while camped at a site that does not have shore power/120v AC power.

So I'm looking for a basic electronics/math check here. This is the label on the power supply for the chargers:



Input is 120v/240v AC, 50 or 60hz, 2A. Output 54.6v @ 2A.

So am I right figuring that 54.6v @ 2A should be 109.2 (call it 110) watts?

Which means that a 150W inverter running off of a 12v source should be able to charge these batteries with no difficulty, correct?

Then the next question is, my truck currently has a 400w inverter (that is only powered when the ignition is on) but from what I've heard, it is not a pure sine wave inverter. Would that be a problem trying to run a relatively low-powered (2A) charger like this one?

EDITED TO ADD MORE DETAIL: Charging a fully depleted battery seems to take 4 - 6 hours. I was thinking that if we ended up with a fully depleted battery, I could remove the batteries and plug the charger in and if we drove a full day to our next destination it should be able to fully charge one battery or half-charge two batteries. Charging at the campsite from the 230AH battery pack (2 x 6v golf cart batteries) of our trailer (which is connected to a 100W solar panel if we don't have shore power) would also seem to be possible. On a nice, sunny day the solar panel can put 5 - 6A into the batteries every hour for 8 hours or more, which is more than enough to keep it topped off and should have enough "left over" power to be able to send 2A to the charger without affecting the overall charge of the 6v batteries.

Does this sound right?

 

[2.ooohhh]

You need to account for the loss incurred when converting to 120v and then back to 54v. Also keep in mind charging multiple bikes will multiply your needs by the number of bikes you wish to charge. Not saying ebike charging is impossible, just might require a larger solar array than easily fits on most vehicles.


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[john61ct]

That seems odd. Input current should be less than output current

When the device is converting to/from different voltages,

need to switch to power (watts = V*A) to compare apples to apples

The plate specs mean little, need a wattmeter on each, measuring both the input side and the output side to see true efficiency losses.

These %s will greatly vary with the loads' level of demand.

Since we're talking DC in and DC out, an inverter in between will waste a lot.

NBD for alternator or mains, but wasteful when stationary off grid using solar, when every Ah counts.

There are boost DC-DC converters could be used, coupled with an HVC for charge termination, would be much better.

Best of all, a quality boost solar controller that outputs 48V nominal, probably high 50's in reality. e.g. Genasun.

In distant third, a cheap-chinese boost solar controller can also act as a DC-DC converter, but reliability is sus, buy a spare.

 

[john61ct]

If you mean factory-made OTS ebikes, yes in fact their systems are often proprietary and designed to require only using their gear.

Which would IMO not be a good investment long-term compared to making your own from quality "open hardware" components added to your pushbike of choice.

But to each their own, it is true if the ebikes only work with their shore chargers then that also forces you to use an inverter.